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radiance88
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Learning Electronics?
Hi,
I'd like to get a good understanding of how to create and modify electrical setups, starting from simple things like light switches and ramping up to
cool and interesting things like perhaps a home-made solar power setup or creating an arc furnace out of scavenged parts.
Unfortunately, I have very little understanding of how any of this works beyond knowing that it's electrons flowing through a wire. I'd like to get my
hands on some material that teaches both the theoretical fundamental concepts, while also having DIY stuff that teaches and shows me how to apply
them to the real world. I'd very much like to be able to tinker and build anything electrical to my heart's content and not be limited by my rather
large ignorance on the subject at the current moment.
So as you can see, I'm trying to get a fully round education with electronic setups, but unfortunately don't know exactly where to start.
Can anyone recommend some good resources or starting material that would help build the tinker in me?
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aga
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Theoretically Chemistry is actually Electronics, although the word 'electronics' means something different.
To get started in Electronics, you'll not go far wrong buying one of these :
http://www.ebay.co.uk/itm/130-IN-ONE-ELECTRONIC-PROJECT-LAB-...
35 years ago i got one for Christmas, although the case was wooden, and it had 20 less circuits, and no integrated circuits.
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WGTR
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I am much older now, but I got started in electronics when I was around 10. This was around the early 90's, back when thrift stores sold a lot of
electronics junk for very cheap prices. $10 could buy a shopping cart full of old radios and other junk. I picked up many old vacuum tube radios,
old stereos made from through-hole components, and many cordless phones when I was interested in finding crystals and surface mount components. I
even found a nice vacuum tube Pioneer stereo tuner, complete with a cool "magic eye", that I took apart  for the nice air variable capacitors. I got my first signal generator (a General Radio 1001A) and oscilloscope
(Tektronix 547) at a thrift store. This was also before you could "just Google it". One had to go to a place called a "library" in order to learn
things.
Times have changed, but some junk stores still sell old home electronics from the 80's. Those are good places to get useful components on the cheap,
if the individual electronics parts in the radios are old enough to be large, big enough for a beginner to harvest and use with hand tools. I
wouldn't recommend getting started with surface mount components, as this adds an extra layer of frustration right at the start. Of course, now there
is eBay, and a plethora of other online merchants that can sell you whatever you want, and have it delivered the next day.
Some of the first radios that I built used vacuum tubes and big, air-variable capacitors. The tubes were really cool looking devices, and their
filaments made them glow in the dark. I'd use gas regulator tubes for voltage regulation, because those would glow pink from the ionized gas. It was
really something, sitting in a dark room watching the tubes glow, tuning through the local AM radio stations with those big variable capacitors,
knowing that it was a radio I built myself. I built some crystal radios, that use just a half a dozen parts or so. Crystal radios aren't powered,
they use the power from the radio station itself. I remember picking up Radio Nederlands on my crystal radio shortwave set late one at night. It was
such a powerful signal that I had no trouble picking it up, even on the other side of the world. Back then there was no "internet" as we know it.
Listening on shortwave was like getting "secret" information directly from other countries. It was all very, very, cool.
If there was one thing I would do differently, it would be to go back and learn as much math, as early on as I possibly could. It wasn't until
college that I took Calculus. Once I did, I realized how much I had been missing in electronics. Even using complex algebra though, electronics
understanding comes alive. Resistors, capacitors, inductors, etc., can all be defined using complex algebra. For example, a pure resistance is R
+0j, a capacitance is 0 - (j/2(pi)fC), and a pure inductance is 0 + 2(pi)fLj. These can be added, multiplied, divided, etc., to determine the complex
voltages and currents at different nodes. Without math, one will be limited to using other people's cookbook designs. This is OK for a beginner, but
the lack of flexibility quickly becomes frustrating.
Here are a couple of decent reads that I have encountered:
![IMG_0871[1].JPG - 1.5MB](http://www.sciencemadness.org/talk/files.php?pid=387087&aid=36541)
They are pretty cheap, even when new. I still recommend, though, going to a library. If one doesn't have what you are looking for, then go to
another one. Colleges that offer engineering degrees often have a very diverse selection in their libraries. My particular one has several full
stacks of electronics books, from the early 1800's up until the present.
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smaerd
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I am still an electronics hack. The way I was introduced to it formally was in my college physics courses. I feel like that was the best way for me to
learn the fundamentals because not only was it, "this is a resistor, this is a capacitor, etc" it explained what capacitance was, what potential was
etc. Of course that sort of knowledge is not necessary to build circuits really but I found it valuable to know those things before going into
kirchoffs rules and before going into AC circuits (complex algebra).
So my recommendation is to go to a used book store and grab a calculus based college physics text-book for a couple dollars and read the electricity
and magnetism section. Work a few problems, check your answer, then use the internet as a resource if you get stuck. Again you don't need to know
gauss's law to build a simple circuit but having an understanding of the principles of electricity and magnetism is good and interesting  .
But yes for simple circuits knowing linear algebra, or even differential equations can help out big-time. These days there are soft-ware/free-ware
suites that take a lot of the real 'work' out of circuit design by employing the SPICE engine. I have yet to have any of those programs work for me so
I tend to do manual analysis, but your mileage may vary.
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Metacelsus
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I recommend TINA-TI (from Texas Instruments) as a good free circuit simulator. I used it to design my induction heater, and I found it to be pretty
accurate.
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Little_Ghost_again
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For high end sims use LT SPICE you can get it free from linear technology web site, there is a yahoo user group as well for it thats a must join kinda
thing if you want the most from it.
Join a forum like electrotechonline or one the others as most electronics forums are pretty much the same.
If you think chemistry is expensive then be prepared for a shock lol, eventualy you will wonder into microcontroller's and then you will find that
opinions on which micro is best are like which colour is better.
I have my faves but most likely you will be steered down the ATMEL route and arduino, unless you want to get really serious the3y are about the
easiest and by far the quickest way to do something micro without all the learning as most the routines are pre written.
Get high end serious and you need to look at stuff like energy micro (now silicon labs) and there CORTEX based GECKO range which is 32 bit and some
seriously powerful micro's in the range right upto Cortex M4 with 1meg program space, but forget those for now as they are BGA package and your
looking mainly for DIP stuff for now, get a DECENT bread board (maplin do a blue one thats good).
Get a half decent multi meter and a couple of cheap ones.
Get a pc power supply and use that as a general PSU for electronics. Dont get fooled into a full on lab supply at this point as you can do most things
you need in the first year with a pc PSU and a few voltage controllers or transistor circuits.
Use ebay and china for parts, use common sense and choose the sellers with reasonable feedback, its china so dont hold 6 week delivery against them if
thats all the neg stuff says in there profile.
USE LEAD FREE SOLDER ONLY FOR ANYTHING YOU WANT TO EAT AFTER!
Seriously the biggest tip I can give is get decent LEAD SOLDER, DO NOT use solder for plumbers as the flux will eat your little circuits up in no
time. Buy a half decent £45 soldering station and not a £10 soldering iron (thank me later).
Electronics is easier than chemistry and rarely does it ruin clothes or carpets. Resist the urge to get an oscilloscope until you know why you need
one.
Erm enjoy and have fun but please join a decent electronics forum they are a great help.
One other thing stay the fuck away from flyback transformers and MWO transformer and all other high or mains voltages until you have some decent
experience behind you, despite what some say the truth is more die per year with electric than by blowing themselves up! 50KV from a neon sign
transformer wont hurt a bit but you also wont get a second go at the cherry.
I guess as a comparison on here.................. As they say, if you have to ask how to make phosgene safely then dont make it. same with HV stuff,
if you need to ask your not ready.
I once got a small kick from a very old car ignition coil, non lethal but man not something you wanna do for fun, a MOT or flyback is another league
from that, it jumps towards you if your grounded and 2-3 inch gap is no problem, once its reached you rarely do you get to try it again.
Sorry for the ELF and safety stuff but so many people at the moment on electronics forums asking about jacobs ladders etc. Perfectly safe with
experience and caution, but you need both not one or the other
Whats your location?
[Edited on 11-1-2015 by Little_Ghost_again]
Dont ask me, I only know enough to be dangerous
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Fulmen
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I don't have any good online sources for you, but I'm sure you'll find them if you try. I think you should start with Ohms law and Kirchhoff's circuit
laws, They provide the basic understanding between voltage, currents and resistance. You should be able to calculate the current through a resistor at
any given voltage and to calculate the resulting resistance of resistors in both parallel and serial configuration.
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aga
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That is probably the Best advice given so far !
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elementcollector1
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Seconding these words of wisdom.
Elements Collected:52/87
Latest Acquired: Cl
Next in Line: Nd
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Fulmen
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Agreed, I struggled with cheap irons for decades before realizing that even infrequent use merits a decent station.
Also, if you're making PCBs you should seriously consider a photopositive system. I made a small developer box from a bug zapper, combined with a
CAD-system like Eagle CAD you can make complex and detailed prints with little effort.
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WGTR
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Quote: Originally posted by Fulmen  | Agreed, I struggled with cheap irons for decades before realizing that even infrequent use merits a decent station.
Also, if you're making PCBs you should seriously consider a photopositive system. I made a small developer box from a bug zapper, combined with a
CAD-system like Eagle CAD you can make complex and detailed prints with little effort. |
I used the cheap $4 irons from Radio Shack for a long time. They took about 5 minutes to heat up, and didn't have any temperature regulation to speak
of. I burned the tips repeatedly. When I got a "real" job, I acquired a nice $400 iron. It warms up in 10 seconds, is temperature regulated, and
has a multitude of replaceable tips of all sizes. As for the Radio Shack type...never again.
I'm busy with other things for the foreseeable future; but if there is any interest, I will consider showing how to make circuit boards later on. I
use a dry-film photopolymer from Think and Tinker Ltd. They also supply the drill bits I use. The film comes on 100' rolls, but the cost is
something like $1 per foot. One roll is a lifetime supply for hobby use.
There are different ways of fabricating the board. Using negative artwork, the film can be used directly as the resist with ferric chloride etchant.
With positive artwork, the film is the mask for tin plating. The plating is the resist for an ammonia-copper sulfate etch. The latter option is best
when doing through-hole plating.
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Little_Ghost_again
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I use photo boards for final boards or boards that need top quality, But I do get very very decent results with using a HP laser printer and the toner
transfer method.
If using toner transfer then the following will save you hastle.............
Clean the copper extremely well and handle with gloves at all times, you need a polished surface that has a very very fine roughness for keying, I
use brasso for the first clean then I use IPA and acetone to clean off, after this I use a combination of fine rockwool with Diamtamaceous (sp?)
powder and rinse this clean with IPA again.
Use a decent modified laminator and as soon as you have done the final board clean do the transfer within 30 mins. Pre heat the PCB before and during
the transfer, DONT bother using a iron!!
It takes some practice but with toner I can get 10mil tracks easy. I have yet to find a better toner than HP origanal! Set printer to use best quality
and adjust so the printer uses max toner, the main thing is make sure the board is super super clean and NO FINGER PRINTS!!!
It can be frustrating to get right but the good thing is use a magnifying glass or microscope and check the tracks before you develop the board, its
easy enough to take the toner off with acetone and reclean if the tracks dont come out right, dont waste time trying to piss about with sharpie pen if
a track didnt stick! It normally happens if your board isnt clean enough or has too high a mirror finish (you need super fine scratches to key in and
make the toner stick well.
Take time preparing the PCB board before trying to get the toner to stick on it, I make many many boards this way and although it can take a while to
get right it can give great results!
I have also started to use silk screening but so far results are not great at really fine track widths.
While learning use perf/vereo board! its pretty good for quick boards and less hastle than making a PCB.
PRACTICE soldering and except that learning to solder takes time to get right, lead free solder is a bastard and I wouldnt use it until you solder
like a pro and by that time you wont want to use this evil nonsense anyway, I think in the states the brand you want is Kester? and in the uk use
Multicore electronics solder, most things can be soldered with 0.7mm solder, 1mm for wire soldering and smd is best with 0.5-0.3mm.
I have used some cheap chinese solder and depending on the batch it can be good, but while learning use the good stuff as its easier to get decent
results.
most of your non working boards will be down to poor quality solder joints, so dont skimp on decent solder or a decent soldering station (they are
pretty cheap these days).
Dont buy a fluke multi meter unless rich as you wont need it and the money is better spent elsewhere.
Use a digital meter to start with, analogue have there uses but while learning a DMM with auto range wont cost much and does a decent job. Dont bother
getting one that has 1000 functions you wont need, if you use 240V (or any mains voltage) then get and use a half decent meter with DECENT probes!!
Whats your location? if your in the Uk I can give you some decent ebay names to get stuff from.
Dont ask me, I only know enough to be dangerous
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radiance88
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Okay, I have no idea what most of you just said in this thread, but I got copies of the books that WGTR recommended, and am now looking into getting
some kind of circuit board I can play with as per Aga's advice.
Ideally the resource I'm looking for is a bit of both DIY and electronics theory, where the two go at more or less an equal pace. I don't know how
much of electronics theory is actually applicable to a hobbyist, I would hate to learn and memorize a ton of complicated formulas and theories only to
find out that I needed to know much less. Essentially I just want to break apart things and make new things with them.
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IrC
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http://williamson-labs.com/
This site has a wealth of information aimed at helping one learn electronics. You should get a good proto-board since with one you can quickly build
circuits, work out problems and then consider making circuit boards of a more permanent type. One option other than making a PCB is to use perf
boards. These along with wire wrap wire and a decent wire wrap tool can make building many circuits easy. For very large complex circuits (especially
digital with many traces) one must consider becoming skilled at making a proper etched PCB.
http://www.ebay.com/itm/2860-Points-PCB-Solderless-Bread-Boa...
http://www.ebay.com/itm/830-Tie-Points-Solderless-PCB-Breadb...
http://www.ebay.com/itm/4-pcs-5-x-7-cm-DIY-Prototyping-Perf-...
http://www.ebay.com/itm/New-JDV-HSR224-Tel-Hand-Wire-Wrap-Un...
http://www.ebay.com/itm/JDV-PRODUCTS-Strip-Wrap-Unwrap-Modif...
http://www.ebay.com/itm/30-AWG-Kynar-Wire-Wrap-UL1423-Solid-...
http://www.ebay.com/itm/30-AWG-Kynar-Wire-Wrap-UL1423-Solid-...
For quickly building a low to moderately complex circuit these tools work well if one considers the time involved in working out the artwork,
exposing/etching/drilling in order to create a more permanent PCB. One soon learns to weigh the complexity and time when deciding which method to use.
Of course when the frequency involved gets high or in circuits where there will be appreciable current flow (look at factory boards where the trace is
wider) the perfboard will not be a good idea. However for a large number of projects over the years this method has worked out well for me. At the
very least the methods listed above work well for one just beginning to learn electronics. One can rapidly build circuits and test ideas, reusing the
parts over and over. Also helps to acquire a supply of various components to build and experiment with. You can go a long way at reasonably low cost
using these methods.
"Science is the belief in the ignorance of the experts" Richard Feynman
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Fulmen
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For experiments a solderless breadboard is invaluable. For PCBs there are many options from acid-resistant pens and transfer decals to photopositive
and toner transfer. Toner transfer is cheap and require little equipment, but in my experience it does require a fair bit of experimentation. For the
occasional board photopositive is simpler to master.
Other equipment? A digital multimeter of course, you cannot do electronics without being able to measure. An oscilloscope is even better, but they
aren't cheap so you might want to hold off on that until you know this stuff is for you.
As for theory you need to cover the basics like Ohms law first. The math isn't that complicated, it's more about understanding concepts. Once you
understand the relationship between voltages, currents and resistance you'll have a foundation for further knowledge. Capacitors will probably be the
next logical step, after that you'll probably figure out the road ahead yourself.
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phlogiston
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+1 buy a breadboard.
If you buy one book, buy "The art of electronics" by Horowitz and Hill.
-----
"If a rocket goes up, who cares where it comes down, that's not my concern said Wernher von Braun" - Tom Lehrer |
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aga
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Buy the 130 in One toy/kit.
You'll learn by Doing, which tends to be much more fun than learning by reading.
The book might say : Wire A to junction X, Wire C to junction Y - the light comes on.
You can then see what happens if wire A goes to Y instead, and safely.
For 20 quid i cannot think of anything more fun, informative or useful for you.
3 years after i got it's ancestor (120 in One) for Christmas i had my first design published in Electronics Today International.
Yes, i re-invented the syn-drum.
At that time all music synthesisers were focused on keyboards.
Aged 15 i failed to even think to get it patented, and was ecstatic with the 175 quid ETI paid me, and having my name in the article published in
their magazine.
[Edited on 12-1-2015 by aga]
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radiance88
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Ok, so update - I was trying to get one of the 300-in-1 lab kits locally online, but it looks like the one item in my area already got sold. There was
another one being sold around for 200 USD.. I can buy a hotplate with that kind of money.. so I just passed on that one too. Importing one from the
States doesn't fare much better considering shipping costs.
So unfortunately I won't have the quick and fun learning experience from one of those kits (they really do look fun and informative!), but I did have
the fortune of actually finding a shop here that stocked breadboards, and small DIY kits which consist of parts and not-so-explanatory, 1 page
instructions.
Another bump in the road for me is that to actually use any of them, I have to solder together one of their variable power supply kits first, despite
not knowing anything about them, or knowing how to solder stuff properly.
So, tomorrow I'm heading out and buying a breadboard, multimeter, a power supply kit and an FM radio kit despite not having soldered a damn thing in
my life. May God (and Youtube) help me.
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Magpie
lab constructor
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There is no need to buy or build a power supply, unless, of course you just want the experience. If you can find a junked computer you can salvage a
dc power supply for nothing. I did this using my junked Gateway PC. It's 145w; those of today are probably of higher power. Mine is shown in the
picture below. It supplies both 5vdc and 12vdc.
The single most important condition for a successful synthesis is good mixing - Nicodem
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WGTR
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If you feel like you need it, some of us here could probably help you get the kits put together. Feel free to post pictures with your questions.
If you are making a small little toy FM radio transmitter, then all you probably need are some batteries and a battery holder. No power supply
needed. What voltage does it require? It may work with a 9V battery.
Can you identify what model of meter you're thinking of buying? We could help steer you clear of something that won't be useful to you. Same
question about the soldering iron. Solder should be rosin flux-core solder, designed for electronics work (no acid core solder that is used for
plumbing). It has the flux inside the solder. A good solder wire diameter is 0.6mm (25mi) for the type of stuff you're thinking of.
Here's a basic setup:
There are safety glasses, small pliers and clippers, de-soldering braid (for removing solder), a spool of flux-core solder wire, an acid brush (for
cleaning), a soldering iron, a bottle of liquid rosin flux, and a bottle of alcohol (ethanol preferably, but anhydrous isopropanol OK, or acetone).
Normal solder melts at about 183°C. It joins two parts together by forming an intermetallic layer between the two parts and the solder. This
happens easily when the parts are clean, but there is always an oxide layer on the metal parts. The flux inside the solder cleans these oxide layers
off, allowing the solder to bond to the metal parts. Sometimes if the parts are very dirty, the flux inside the solder is not enough. In this case,
extra flux needs to be dripped onto the joint. Flux is your friend. When in doubt, flux it. The alcohol and the brush are used to clean off all of
the flux afterwards. Flux is sticky and tends to get everywhere, so be forewarned...
Soldering is an art, and I was put through a 1 week training course when I was first hired. The basics are pretty easy, though, and I'm sure you'll
do fine with the Youtube tutorials.
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The Volatile Chemist
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Thirding it. The reason why I'm unable to solder stuff is because my $10 soldering iron's coil arced while I was using it, and fried/severed both
power wires...
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Hawkguy
Hazard to Others
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Hey if you're working with electronics for the first time, try dissembling/ modifying basic electronics with the help of a book which tells you what
everything is... Its actually kinda enjoyable, and feels a bit like actually DOING something... Playful stuff is cool too, like making a shocker from
a disposable camera, or a radio controlled fireworks detonator from whatever...
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woelen
Super Administrator
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You can simply play with electronics and try to build all kinds of equipment, but for me the real fun has been in understanding electronics. The
following basic things you really have to understand:
- voltage, current and power, relation to other SI-units
- Ohm's law
- resistors, linear
- capacitors, inductors (understanding them qualitatively is not hard at all, if you want to do computations on them, then you need some math, using
derivatives, integration and maybe some differential equations). There are shortcuts, however, such as computations of cut-off frequencies and
resonance frequencies, if you don't have the calculus-knowledge but still want to have some quantitative insight in circuits.
- basic operation of diodes
- basic operation of transistors, the concept of amplification, different types of transistors.
Additionally, if you want to build electronics, using somewhat higher level components than what is mentioned above, then try to understand the
following:
- operational amplifiers (opamp), ideal opamp, real opamp as you can buy it
- digital logic (AND, OR, NAND, XOR), both mathematically and as components which you can buy.
From a practical point of view, if you want to experiment with electronics, then I would checkout eBay, chinese sellers. There is a lot of very cheap
electronics, which is good, certainly the basic stuff. As a starter, I would buy the following:
- resistors (E12-series, anything from 1 Ohm to 10 MOhm, 10 pieces each). Cost is just a few dollars including shipping from Chinese suppliers.
- capacitors (100 pF, 1 nF, 10 nF, 100 nF, 1 uF and maybe also 330 pF, 3.3 nF, 33 nF, 330 nF). Also dirt-cheap from eBay.
- a bunch of opamps (TL071, TL072 are decent ones, which are good for most electronics projects and cost less than $0.50 per piece).
- a set of transistors (BC547 and BC557, 10 each). For these sets you most likely also will have to pay at most a few dollars.
- a set of diodes (1N4148 and 1N4006, 20 each). These really are cheap.
- some power regulators (e.g. 7805, 7806, 7812).
- a set of LEDs (different colors, 3 mm transparent glass-like ones are cheap)
- some digital logic circuits (74HC series, lookup wikipedia for type numbers), e.g. NAND gates, AND gates, OR gates, some more advanced building
blocks like flip-flops, latches and so on.
- some special function chips to get an understanding of modern electronics, where special functions are put in a single chip. Nice examples are:
> So-called 555 circuit. This is a versatile thing, allowing you to build timers, oscillators, and many other circuits, from which you can learn a
lot. Buy a few of these, cost is just a few dimes per piece.
> LM335/LM35 temperature sensors
> Hall-effect sensor (lookup eBay)
You also need a breadboard/experimenters board and you need a good soldering station. These are discussed before. A decent digital multimeter, which
allows you to measure voltage and current also should be included. You can buy these as well from eBay, from some chinese seller. No need to spend a
lot of money on that. A basic symmetric power supply (e.g. 2x 15 volts, capable of delivering a few ampere will be OK for most experiments).
I did an experiment with electronics, just for fun and scientific insight, which gives you an idea what kind of components you need and how these
things can be described mathematically. It is a very simple piece of electronics, made on a breadboard, but the behavior and math behind it is quite
interesting:
http://woelen.homescience.net/science/math/exps/rossler_chao...
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radiance88
Hazard to Self
Posts: 64
Registered: 18-12-2014
Location: underground volcano fortress
Member Is Offline
Mood: a little less evil than usual
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Ok.. update:
I went out and bought myself a multimeter, breadboard, solder sucker and two kits - a power supply one and an AM/FM radio one. We already have a
soldering iron and solder at home. My multimeter is digital and has auto-range and also measures temp, which I intend on using on some experiments.
I bought the power supply because it seemed pretty essential given the fact that the other kit ran on 12 volts, and the 12-volt battery they were
selling needed some other expensive thing to charge it (according to the guy at the counter, said they didn't have anything else), so this is what I
have. btw my country uses 220 volts.
The parts for the power supply are down below:
And this is the diagram that it's supposed to follow:
I'm still trying to figure out how to wire the transformer and the switch. The transformer has one side with 6 wires saying 12V, 9V, 6V, 4.5V, 3V, 0
while the other says 220V, 110V, 0. So I'm guessing that the zeros are negatives.
The switch looks like complicated little piece of equipment. The bottom has an inner row of pins with holes in them, 2 of them. The second row outside
of that has 12. Not sure what goes with what.
I'll read more on this circuit later when I get out of work. It includes no manual so I have to figure out how to wire everything based on the diagram
alone.
I'll try to see if I can experiment with making it on a breadboard because I don't want to solder everything together only to realize that I screwed
up.
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woelen
Super Administrator
Posts: 8014
Registered: 20-8-2005
Location: Netherlands
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Mood: interested
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If you want to connect the transformer to your 220 V wall outlet, then connect the 0-connection at the left (this is the primary side of the
transformer) to one wire of the outlet and connect the 220 V connection to the other wire of the outlet. Leave the 110 V wire unconnected.
The secondary side has many taps with different AC-voltages. By means of the switch, you select one specific tap and the voltage of that tap is
rectified by the circuit with the diodes and capacitor. The LEDs and 1 k resistors give a visual clue which tap is selected and the 1 k in parallel to
the capacitor is a bleeder resistor, which assures that if the power is disconnected, then the energy, stored in the output capacitor slowly is
dissipated and it also assures, that if the selector switch goes from a high voltage to a lower voltage while there is no load connected, that the
output voltage slowly goes to the lower voltage. Without the resistor, the output voltage could remain at a high voltage for a very long time if the
supply is switched off or switched to a lower voltage.
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