Sciencemadness Discussion Board - Why is my choke overheating?

Why is my choke overheating?

elementcollector1 - 13-2-2014 at 18:42

Why is my choke overheating?
I recently built this (http://www.instructables.com/id/DC-Boost-Converter/), again, because the last one broke somehow. Seems like everything's wired up right, but when I connect a 6V battery, of all things the choke starts emitting the Magic Smoke. Plus, voltage reading was only ~1V when it should have been 150V. So weird...

bfesser - 14-2-2014 at 16:25

Since I happened to have the exact RF choke and other necessary components in my bins, I soldered up a replica of the Instructables circuit. The only appreciable difference is that I substituted an IRF830 (V<sub>DSS</sub> = 500 V) for the IRF510 (V<sub>DS</sub> = 100 V) that the author used, and that I added a heatsink, which proved unnecessary. The output fluctuated between ca. 25 V to ca. 60 V, when powered from ca. 4.5 V (3 × AA). Disappointingly, I didn't get any Magic Blue Smoke—haven't seen any in years. I have no use for this circuit, but was just hoping to find an answer to your question; I'm sorry I don't have one. Maybe it would help if you posted high-res. photos of the top and bottom of your board so I can check over the connections.

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[Edited on 15.2.14 by bfesser]

elementcollector1 - 14-2-2014 at 17:59

Quote: Originally posted by bfesser

Since I happened to have the exact RF choke and other necessary components in my bins, I soldered up a replica of the Instructables circuit. The only appreciable difference is that I substituted an IRF830 (V<sub>DSS</sub> = 500 V) for the IRF510 (V<sub>DS</sub> = 100 V) that the author used, and that I added a heatsink, which proved unnecessary. The output fluctuated between ca. 25 V to ca. 60 V, when powered from ca. 4.5 V (3 × AA). Disappointingly, I didn't get any Magic Blue Smoke—haven't seen any in years. I have no use for this circuit, but was just hoping to find an answer to your question; I'm sorry I don't have one. Maybe it would help if you posted high-res. photos of the top and bottom of your board so I can check over the connections.

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Heh. Fixed that problem only to run into some new ones. Soldered a second 0.01 uF capacitor parallel to the first (it was whining), and powered it off of 6V. Result? Highest recorded was 40V, but only for a fraction of a second. The voltage drops *very* quickly, on the order of milliseconds! I'm thinking this has something to do with the lack of a heat sink for the MOSFET.

I'm going out to get some electrical parts next weekend, and I guess I'll see if I can find a more appropriate 'FET - thanks for the suggestion!

bfesser - 14-2-2014 at 19:27

I split this out of the short question thread; I hope you don't minetd. I'm curious, what kind of load are you putting on the output?

elementcollector1 - 14-2-2014 at 19:34

Not at all.
There is no load on the HV output, just a multimeter...

Marvin - 15-2-2014 at 05:59

"My results were staggering when I was able to convert 4.5 volts to over 100 volts by accident."

[Mythbusters] There's your problem. [/Mythbusters].

You are copying someone who doesn't know what they are doing and building a boost converter without feedback regulation. With an undesigned circuit, operation can depend on how much the components can be stressed before they fail/behave oddly, minor characteristics, room temperature or the direction of the wind.

bfesser - 15-2-2014 at 07:52

Heh, I didn't expect the thing to do much at all. I just had some old components to waste and hopes of seeing some magic blue smoke escape. elementcollector1, is this another part of your <em>HλLF-LIFE²</em> gravity gun prop project (topics <a href="viewthread.php?tid=24731">one</a> & <a href="viewthread.php?tid=24226">two</a>

elementcollector1 - 15-2-2014 at 12:00

Technically, yes. Although, it was intended as a short question, you can merge it into there if you want.
So, how do I add feedback regulation?

Metacelsus - 15-2-2014 at 17:47

[This is assuming a 555 timer as the clock, which works very well for me]

Use an adjustable voltage divider to convert the output voltage into something more manageable, then feed it back through a transistor into pin 5 (control) of the timer.

Example (made for driving NIXIE tubes):
http://www.dos4ever.com/flyback/flyback.html

elementcollector1 - 15-2-2014 at 18:49

No 555 timer present in here, unless one or more of the components doubles as such...
I have a 555 timer, but no clue how to use it.
Adjustable voltage divider? And what counts as "manageable"?

Now that I look up the basic schematic of boost converters, mine seems a bit overcomplex. Do all the added resistors and/or capacitors help stabilize the voltage?

Marvin - 16-2-2014 at 06:18

The circuit consists of a free running oscillator wired to a boost converter. The simplest hypothesis assuming you use exactly the same components is that the extra voltage means the MOSFET is stuck on.

Using a 555 allows some control over the oscillator, and it's likely to work over a wider range of voltages. 4.5v may be too low. I'd stick with the circuit Cheddite linked to.

Metacelsus - 16-2-2014 at 08:33

Adjustable voltage divider: voltage divider with a variable resistor, so you can control the output voltage of the boost converter.

Manageable: small enough not to destroy components.

The Instructables boost converter posted in the other thread uses an oscillator made from two transistors. I recommend using a 555 timer instead, as it is easy to tweak its duty cycle and frequency, and can be modulated using the control pin.

For a MOSFET, I recommend the IRFP460, which is what I use. Be sure to use a good heatsink.

My boost converter (actually a flyback converter) is 12V to 350V, with some control over the output voltage. It's based on the link I posted, but with some modifications.

Marvin is right: if the choke overheats, the MOSFET is stuck on. This may mean too much V_GS, or a bad MOSFET.

Do you have an oscilloscope to test proper function of the oscillator?

If not, you can wire it up to a speaker for a quick qualitative test. (Boost converters generally use frequencies in the upper audio range. My dad can't hear 25 kHz, but I can

)

[Edited on 16-2-2014 by Cheddite Cheese]