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Rosco Bodine
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Voltage Controlled Current Sink
The latest variant which has been examined for its possible usefulness as a parallelable , high current capable Mosfet pass element output module is
attached .
This is *not* a complete system but merely one 25 Amp output module "block" , showing the architecture needed for the control circuitry , which IMO
has distilled from other
arrangements which have been examined as possible ways
of driving a Mosfet as a linear device , and keeping it stable .
The control loop is a highly damped three stage active low pass filter which has something similar to a PID control signal processing inherent to the
drive signal applied to the power Mosfet . This is the one scheme which seems most likely to
achieve a stable control loop with a minimum of phase shift
which did appear much more potentially problematic when using simpler control loop schemes . Many simpler drive schemes were considered and some may
be workable with
much higher resistance current sensing shunts , where the
voltage drop may be tolerable there . But for sensing shunts
of a milliohm or less , as is desirable on low voltage platforms
where every tenth volt counts .....things get a lot more complicated given the amplifications involved for the sensing to control loop , which leads
to something like the attached schematic as a probable solution .
BTW I have the asc file for LT Spice from which the attached schematic was printed and will link it here if anyone wants to look at the spice sim
displays for the performance of the attached circuit .
The values and components shown are not necessarily finalized or optimized , but this is simply meant to be illustrative using some components which
should work .
[Edited on 16-1-2008 by Rosco Bodine]
Attachment: LTC1152 25 Amp VCCS modeled stable.pdf (19kB)
This file has been downloaded 1000 times
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12AX7
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So why does it have two ICs and numerous resistors, when mine has less? And "per module" he says!
Tim
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Rosco Bodine
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There's nothing really wrong with the one op amp configuration if you want to use a lot of them , maybe a half dozen or eight of them , ( maybe ten or
twelve  ) instead of just two . But you will have to spread out the task to a
larger array using higher resistance shunts to maintain stability . And you will have to work out some scheme there for limiting the frequency
response or the loop will "ring out" past the frequency where it goes into self-reenforced feedback induced oscillations . Anyway that's what the
sims indicate . I "fought the good fight" trying to make that simpler configuration behave , but it has too much high frequency gain for the loop ,
the response doesn't roll off steeply enough and early enough for stability with low resistance shunts . And trying various schemes of using just an
integrator or simple filtering didn't tame the loop response
either . Seeing that what was needed most for damping was some sort of increasing gain error amplifier signal dependant upon the slew rate of the
signal from the sensing shunt , and an assymmetrical amplification/attenuation output from the error amplifier
(slower on the accellerator / faster on the brakes )
I concluded that a stacked pair of op amps in lead lag with their outputs ganged to provide a derivative , and then
using the Mosfet driver amp as the integrator , would
produce the desired result .
The stacked op amps which are the shunt voltage monitor and amplification stage function as a lead-lag network
and a low pass filter , whose effective gain is frequency variable . The frequency variable gain is also asymmetrical . What happens is that if a
constant current
load is producing a constant shunt voltage reference , the
stacked op amps respond with an effective DC gain that is fixed . But that effective gain changes , increases in response to a changing shunt voltage
, not just proportionately but exponentially , which provides the
desired damping and low pass filtering . And because
the response is steeper in attenuation than in amplification , that makes the circuit settle quickly in
response to any transients ......a full power pulse spike
settles to DC with under 3 ms of ringing at the full 25A
and maybe 5 or 6% overshoot . That's a worst case scenario response .
IIRC the total phase shift at rolloff to 0 dB is something around 100 degrees which is grrrrreat 
[Edited on 16-1-2008 by Rosco Bodine]
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bio2
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........ you can get 7 volts from the +5 V and +12 V outputs -- the +5 V is considered the negative (GND) and +12 the positive.....
I found this in an article and tried it and indeed it worked on my ATX 2.
This 480W power supply delivers 36A@5V and 16A@12V.
When connected as above the voltage drooped from 6.96V to 6.30V with a load of only 1.6A. Measurement was at the PS not the end of the load wire.No
higher load was tried for fear of damaging my brand new PS.
My question is what would be the current output when connected in this arrangement? Also how would the current limiting be affected?
7V suits my cell better than 12V if I could get a high current output but seems doubtful given the drop at such a low current.
Also I am wondering if the 3.3V and 5V outputs could be used in a similar fashion?
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bio2
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A quick up-date for anyone that might try this.
I applied a 3.6 ohm load to the 7V configuration and the unit tripped on
overcurrent (I assume). No damage ensued.
So it seems this hook-up is only suitable for "pilot" loads.
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-jeffB
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If you simply hook up a load between the +12V and +5V outputs, there's no way it's going to work. The PS is designed to source current from the
positive outputs, not sink it. I imagine your PS tripped on overvoltage on the +5V supply.
The only way you could get regulated 7V power out of the configuration you describe is if you put a fixed load between 5V and ground that drew at
least as much current as the load you're putting between 12V and 5V. In other words, if you want to draw 10A at 7V, you'd need to put a .5-ohm load
across the 5V supply first, drawing a constant 10A. Then, when you add your .7-ohm load between 12V and 5V, the current through both loads would be
10A, but the current out of the 5V output would be zero. I can't make any assertions about the stability of an arrangement like this, though.
(Actually, you'd need to draw even more than that, because IIRC most PC-type switching power supplies require a minimum load on the 5V line of
something like 10% of capacity. So you'd need to draw at least 13.6A through your dummy 5V load.)
Disclaimer: it's been a while since I've worked with power-supply designs, and I never quite got a 100% intuitive grasp of switching-regulator
function even when I was working with them.
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bio2
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......If you simply hook up a load between the +12V and +5V outputs, there's no way it's going to work. .........
Good thing you put in your disclaimer, lol. Read my post again!
The 1.6 amp load was applied for a couple hours with the droop as stated.
I did not imagine this or make it up so maybe think a little before stating absolutes.
The quoted statement was taken from a website detailing CPS units modifications for other uses. So I thought it's worth a try. I asked for input from
our local electronics experts in this thread because I could think of no immediate explanation.
BTW There is a continuous 1 amp load applied between 5V and ground for minimum load. A 5 to 10 watt load is adequate. The dummy load can be applied to
any winding, IIRC.
Funny, these last couple of days I keep being told that what I have experienced first
hand is impossible. I saw tungsten melt with my own eyes yet was told that my
flame temperature couldn't achieve this.
Guess there is no escaping the know- it- alls!
If I had a schematic of this PS then I could probably explain it, though it seems the
7V comes from subtracting 5V from 12V with the large drop being through the windings in series.
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-jeffB
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| Quote: | Originally posted by bio2
Good thing you put in your disclaimer, lol. Read my post again! |
I've read it several times, along with this one. Keep filling in more details, and maybe we can figure out what's going on. 
| Quote: | The 1.6 amp load was applied for a couple hours with the droop as stated.
I did not imagine this or make it up so maybe think a little before stating absolutes. |
My apologies if I came off as arrogant or dismissive. If I prefaced every statement with a thorough qualification describing all my uncertainties, my
posts would get too long for anyone to wade through. (There are probably some who say this has already happened.)
A "droop" from 6.97V to 6.30V corresponds to a 10-15% rise on the 5V line (from, say, 5.03V to 5.7V, assuming the 12V line stays stable). This is WAY
outside tolerance, and indicates that you're driving the PS well outside its normal operating regime. It's also not surprising, given that you're
apparently trying to sink a net 0.6A into a supply line that's supposed to source tens of amps.
| Quote: | | The quoted statement was taken from a website detailing CPS units modifications for other uses. |
Link, please? I'd just as soon knock out as many of my own misconceptions as possible on my own time, rather than airing them here one by one.
| Quote: | So I thought it's worth a try. I asked for input from our local electronics experts in this thread because I could think of no immediate explanation.
BTW There is a continuous 1 amp load applied between 5V and ground for minimum load. A 5 to 10 watt load is adequate. The dummy load can be applied to
any winding, IIRC. |
That emphatically was NOT the case for supplies I was looking at -- IIRC, I was looking at the Jameco catalog, back around fifteen years ago. At that
time, again only as best I remember, most of the multi-output switchers specified a minimum load on the +5V line to maintain regulation.
| Quote: | Funny, these last couple of days I keep being told that what I have experienced first
hand is impossible. I saw tungsten melt with my own eyes yet was told that my
flame temperature couldn't achieve this. |
What you think you see isn't always what's really there. And sometimes, what you're looking at isn't really what you should be watching. For
example, in the application you're describing, you mentioned only the voltage you observed across your load. Unless your load is known to be very
stable -- and I didn't see any description of what the load was -- you might also want to monitor the current through it, to see whether
you're getting what you expect. Most importantly, though, you should monitor the voltages of the 12V and 5V supplies with respect to the PS ground
line, because that's what the PS is trying to regulate. I think you may find that the 5V line is going even further out of regulation than my
simple-minded arithmetic above indicates.
| Quote: | Guess there is no escaping the know- it- alls!
If I had a schematic of this PS then I could probably explain it, though it seems the
7V comes from subtracting 5V from 12V with the large drop being through the windings in series. |
If you were dealing with a simple multi-tap transformer, maybe. But I think if you had a schematic of the PS you'd find that things are a lot more
complicated than this.
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bio2
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Here is the link and the complete sentence which is found about 3/4 of the way down the page just above the heading NEW.
http://web2.murraystate.edu/andy.batts/ps/powersupply.htm
.............As an aside, you can get 7 volts from the +5 V and +12 V outputs -- the +5 V is considered the negative (GND) and +12 the positive --
some geeks will use this combination to run their fans at a lower speed to reduce noise............
So it seems that this is an unregulated derived output intended for very small loads.
............If you were dealing with a simple multi-tap transformer, maybe. But I think if you had a schematic of the PS you'd find that things are a
lot more complicated than this..............
Yea, wishful thinking on my part as I expected as much. I've built a couple switchers
but am more at home with 500KVA UPS and Static switches. The ones with the SCR's
that are really the size of bricks. Not the "bricks" in the vernacular that are 100 amp.
The biggest one we ever built was 2K amp 480V/3Phase, special order custom unit.
The load I used is my little test, water electrolysis, cell which is very stable given
a stable temperature which needless to say will gradually rise too equilibrium with ambient.
Interestingly with a 4.78V input the cell functions (4 cells series) at 8amps+/-
(11% NaOH electrolyte) which is right at the theoretical minimum voltage. This measured with an el cheapo chinese meter so it's not 1/4% accuracy,
lol.
My venerable Fluke 87 won't boot up these last few days and I haven't got around to taking it apart yet. I'm hoping it's the rotary switch because
it's been giving trouble for some time in the same way.
What happens is when the full display test is initiated by turning the rotary switch on it then slowly blinks the full display on and off along with
the backlight accompanied by the relay clicking in about a one second interval. This is as far as
it gets, refusing to go to the proper input regardless of the rotary switch setting.
The previous symptoms for a few months prior were the same except after 2 or 3
tries returning the switch to off it would finally work normally.
Anybody have a clue what could cause this? Previous examination of the switch mechanism revealed no obvious defects like corrosion or dirt etc. in the
sliding contacts.
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-jeffB
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Good link, thanks!
| Quote: | .............As an aside, you can get 7 volts from the +5 V and +12 V outputs -- the +5 V is considered the negative (GND) and +12 the positive --
some geeks will use this combination to run their fans at a lower speed to reduce noise............
So it seems that this is an unregulated derived output intended for very small loads. |
I think the more important point is that the "geeks" using this combination to run their fans at a lower speed are probably also powering the rest of
their PC with the PS, which means there's always more than enough load on the 5V line to accept the extra current being shunted in from the 12V-5V
load. It's still "regulated", in the sense that it won't fluctuate any more than the difference in the regulated 12V and 5V lines, but you've got to
make sure that you're drawing enough current OUT of the 5V line to maintain regulation.
I'm also vaguely remembering that many multi-output switching PSes, especially cheaper ones, base their other voltages off the 5V reference instead of
setting each independently. If I'm remembering correctly, this means anything that causes the 5V supply baseline voltage to rise or fall will also
cause the other voltages to rise or fall. I don't know if this is still true.
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bio2
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An interesting tidbit on my cheapo chinese 480W PS (but everything seems to be made in China these days). They used to burn these things in for a few
hours in the days of quality electronics.
Anyway, I had adjusted the electrolyte concentration for 9 amps at 5V and was doing
some fiddling around taking measurements. I turned the unit back on when much to my surprise it was drawing 27 amps. Wow I thought, what is causing
this, must be a short in the cell somewhere.
So a couple seconds later smoke started pouring out thru the fan so I quickly switched it off only to discover I had hooked it up to the 12V wire
(rated 16A) Damn, so much for integral current limiting and temperature protection.
I took it apart and found no melted components only some discoloration and warping of the insulating sheet under the board. Much to my amazement when
I tried it again
the little POS still worked.
It had previously tripped a couple times at a little over 16A but 27A and it kept running, go figure. Yea, yea, I know should have had a fuse in line
but the best I could do without a trip to the store was 10A which was too small.
Wonder how many years were taken from it's miserable life?
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12AX7
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The current limiting is usually done with a current transformer at the output transistors. So, you were drawing about 12V * 16A = 192W, which is well
under the 480W limit (probably around 6A peak through the transistors, for what it's worth), but well over the rating of the 12V rectifier and filter
circuit. Lucky for you, the rectifier did not fail (or at least fail shorted).
They put dinky ass wire in the coils in those power supplies. Try drawing a full load at 5V some time....then don't try it again. :rolleyes:
Tim
P.S. We still don't have a :rolleyes: smiley! :rolleyes:
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dann2
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Hello,
Pity the '7v' outputs do not work. I had always presumed they would.
Some Links:
http://www.webx.dk/oz2cpu/radios/psu-pc1.htm
http://www.qrp4u.de/index_en.html
@ 12AX7.Will the constant current moule of the circuit that you give a page or two back work on its own.
If you put a simply voltage divider (a pot) at the input (In) will the CCM work from as a simple circuit for constant current out from a PC power
supply.
What MOSFET would you recomment for output MOSFET.
Cheers,
Dann2
Dann2
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Rosco Bodine
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| Quote: | Originally posted by 12AX7
P.S. We still don't have a :rolleyes: smiley! :rolleyes: |
Hey are you a whining and complaining....
argumentative person ???? It's soooooo
not politically correct
to use evil symbology having connotations of sarcasm ......
like the roll eyes icon !
Didn't they beat that attitude problem out of you during
conditioning and retraining ??? Are you reverting to
your former condition as a cretan and miscreant having
little antisocial toad like attitude problems in need of adjustment !!!! 
You even dare to mention the censored and banned
infamous roll eyes icon and think you can summon it
at will ??? 
This symbol has been reserved for use by only "specially priviliged and esteemed" members
To your having the idea ......
that you would even imagine yourself worthy ,
I can only say
http://uk.youtube.com/watch?v=UW32D_Y4Bhg
Now just watch the shiny object , swinging back and forth ,
and listen to the music , .........back and forth , your eyes are getting heavy , you have only thoughts of peace and love ,
there is no roll eyes icon , there is no roll eyes icon ......
there is no roll eyes icon .....you feel well and your thoughts are free .....you feel well and your thoughts are free
[Edited on 17-1-2008 by Rosco Bodine]
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12AX7
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| Quote: | Originally posted by dann2
@ 12AX7.Will the constant current moule of the circuit that you give a page or two back work on its own.
If you put a simply voltage divider (a pot) at the input (In) will the CCM work from as a simple circuit for constant current out from a PC power
supply.
What MOSFET would you recomment for output MOSFET.
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For computer PSU scales, a few IRFZ44 or so (commonly found in automotive apps, e.g., car amps, 12-120V inverters), or something in an IRFP, like an
IRFP064 or so (same idea, but bigger package (TO-247) --> more dissipation (about 100W, vs. 50W for a TO-220 like the IRFZ44, et al) in a single
unit). Rosco seems to be fond of the IRF3703, which is along the same lines.
It sure will work alone. The input voltage (above common) sets current according to the relation shown.
Tim
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Rosco Bodine
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Textbook circuit , top left corner of page .
Look at the shunt resistance value .
It could have something to do with how well
this circuit works .....or doesn't work .
Attachment: Pages from AN106 Op Amp circuit collection.pdf (51kB)
This file has been downloaded 773 times
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Rosco Bodine
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and here's another one
Bottom right on the page
Attachment: Pages from AN105 Current Sense Circuit Collection.pdf (89kB)
This file has been downloaded 1795 times
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bio2
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I still not sure I understand why the 12V X 27A = 324W on my 480W PSU didn't trip on overcurrent. Were you implying that the drop thru the too small
CT coil wires is the reason.? The toroids didn't have burnt windings and the source of the copious smoke
, strangely, could not be determined.
As I mentioned it had previously tripped OK at slightly over it's 16A rating.
Thanks for the 5V at full load advice as I was intending to pull the full 36A from it.
Probably would have set the load and walked away as the coil wires enamel smoked off, lol.
Don't you love this cheap shit? What ever happened to name plate ratings being meaningful?
Edit; OK I see now the effect of the 1 ohm resistor value.
But, shouldn't the circuit be capable of protecting itself from
a dead short condition or is this a separate circuit using a
crowbar type arrangement?
Also I would have expected the AC input fuse to have blown
from a near double rated load applied for a few seconds.
It didn't.
[Edited on by bio2]
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-jeffB
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| Quote: | Originally posted by bio2
I still not sure I understand why the 12V X 27A = 324W on my 480W PSU didn't trip on overcurrent. Were you implying that the drop thru the too small
CT coil wires is the reason.? The toroids didn't have burnt windings and the source of the copious smoke
, strangely, could not be determined. |
Again, I am not an EE, I do not have a schematic, etc, etc. But if a cheap PSU bases its regulation on the 5V line, I wonder if it might also base
its overcurrent cutoff on that line alone? It would give me the shivers to think so, but I can also imagine that you could shave quite a bit off the
cost that way.
The most "copious smoke" I've seen come off any type of electronic equipment has come from blown electrolytic capacitors. You can easily blow them
with overvoltage, but if you hit them with too much ripple current, I'd think you could overheat them as well. If that's what happened, you
might well be left with an open-circuit capacitor and an unfiltered 12V line. A DC meter would still show it producing the expected voltage and
current, but it would have a nasty AC component superimposed. Any chance you could put a scope on it while it's "still working" with a significant
load?
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12AX7
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Circuits I've seen add the voltages together and regulate them jointly.
What I mean by the CT is, all the power goes through the output transistors and CT. How it's split up after the transformer doesn't matter, be it 12V
at 50A, 5V at 100A or the correct loading on each output.
Nameplate ratings are just that, ratings; the nameplate didn't specify what, if any, the current limits are!
The coils may not be burnt, but they may be a bit "carmelized". You probably saw volatiles from the varnish on the coils, or maybe the goop sometimes
applied around them.
Capacitors are easy to check, they split open on top.
Tim
[Edited on 1-18-2008 by 12AX7]
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Xenoid
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| Quote: | Originally posted by -jeffB
Again, I am not an EE, I do not have a schematic, etc, etc. |
@ -jeffB
There is a circuit for an AT supply near the bottom of page 3 of this thread. There is a link to a circuit for an ATX supply about halfway down page
4.
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bio2
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........Nameplate ratings are just that, ratings; the nameplate didn't specify what, if any, the current limits are!.............
OK, I hear you but in the electrical world I came from the nameplate rating was
the maximum allowable continuous input or output.
This, is of course, notwithstanding SF (service factor) etc. And for motors or transformers anyway as long as temp rise was not exceeded the nameplate
could be trusted. A long time ago NEMA changed FLA to RLA for their own reasons.
Having built power electronic equipment from 20 amp up to many thousands of amps the ones we built were tested at 110% of rated load (nameplate) for a
minimum
of 8 hours before leaving the shop.
Needless to say this was not cheap consumer apparatus. I would hate to be the owner or specifier of a 15KV motor knowing that I could not absolutely
trust the nameplate ratings.
Then again, it seems the world has changed as, the now, global manufacturers struggle to compete with junk made with the cheapest materials assembled
by unskilled labor that is designed to last only as long as the warranties duration.
Remember years back when you put a NEMA starter alongside an IEC starter.
What's wrong with this picture, lol?
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dann2
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Hello,
@Aqua_Fortis_100% (quite a handle)
The circuit that you posted some time ago (page 8 of the KClO3 via water.......) has some Portuguese written towards the bottom:
Obs: Os dois.......................isolades.
What does this mean?
Sorry my Portuguese is not too hot.
I will guess it says that you need to isolate the trannies?
Cheers,
Dann2
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Xenoid
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@ Dann2
I guess it just means the two transistors are attached using plastic insulators around the screws and mica washers between the transistor and
heatsink.
I used a 5K pot. instead of the 4.7K pot, I found the TIP41 got very hot at the high setting, may need to increase the 100 ohm resistor. I will mount
the two transistors on seperate heatsinks when I get around to building it.
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Aqua_Fortis_100%
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@Dann2 ,
The notes on the bottom should be next to this:
"PS: the two trasistors should be fixed on individual metallic heatsink and properly insulated"
Yes is exactly the same that you and Xenoid stated..
Note, As I've said early this circuit WASNT made by me.. Was a reply from a friend which know much more than I about e-stuff, and this was when I
asked at a local eletronics forum..
I tried and worked well to regulate the current...
But as Xenoid said the TIP41C gets too hot within short space of time, so both transistors need of a heatsinker...Specially the 2N3773 or similair
'brothers' which need of a large heatsink.. Dont let the two heatsinker contact each other and/or touch the PC PSU..
Among the 4 transistors said there, I've used 2N3773 , but you can use others as 2N3055 , MJ802 , MJE13007 ..
The 2N3055 isn't realiable because that is very weak comparing with others and gives less current range.. The best said by the author is the MJ802
which is the strongest..
Yes , Xenoid is again correct.. Mica insulators are very good to do the job, authough I cann't buy it at the momment..
@Xenoid , about 5K instead of 4,7K pot , what max. current you get with your circuit? Seems very promising
Although I cann't got very high currents , since my PSU is quite old and poor (250W) and I dont want to "abuse" it..
The only problem I get when doing tests with my early chlorate cell was that the current wasn't that great.. I wished at least 7-8A , but the max. I
got was 4,6-4,7A(because I will use that in a 1000-1500mL cell) .. I believe that is because of resistence created by poor connections and cell
desing.. I will try that after finishing the treatment with my new anodes and soldering all connections and diminishing the anode-cathode spacing to
increase current..If gets too much current then I should only manually control the current on trim-pot and its done , probably without mess..
Ah, I've also put a PC cooler in front of the heatsinkers to help it to avoid overheating..
[Edited on 19-1-2008 by Aqua_Fortis_100%]
[Edited on 19-1-2008 by Aqua_Fortis_100%]
"The secret of freedom lies in educating people, whereas the secret of tyranny is in keeping them ignorant."
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