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0-30V Stabilized Power Supply


redwire

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Guest liquibyte

Yeah, Eagle.  File > Export > Image @ 300dpi.  The resolution works well until you get to a bigger schematic and then I usually take it down to 150dpi otherwise it ends up huge.

I'm having a discussion over at eev about the mosfet schematic.  I think we have found two solutions.  Mine requires a zener from the source to the gate to keep VGS below 20V.  I'm going to try both and see how it goes.

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Guest liquibyte

Do you have anything that you can load this at 30V @ 3A?  Out of curiosity, I'd like to see how it behaves at the full 90W.  I'm still trying to simulate the mosfet circuit but if yours works well through the full range, I think I'll just go with that.  I'll try hooking it up tomorrow and post some results if I can get pics of it.

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Guest liquibyte

Hi liquibyte
I have been involved in this project for nearly two years.  I built the Picmaster version from March 09 and it worked fine for a while.  Recently while charging a 34Ah battery the power supply failed - voltage went to 42V and zero current.  What do you suspect has blown?

I split your post off from that thread because I wanted that one to be a showcase for people to post their completed projects.  I don't want it to grow into a discussion about it because we have two huge assed threads for that now.  I'd be tempted to merge them but since they're so long and have been going for so long it wouldn't make much sense now.

As for your question, I'm not sure but maybe someone might chime in for what to test for better than I can.  Have you done any testing?  You built it so you should know its quirks better than anyone.  I'm going to take a guess that it's either the BD139 or one of the op amps (U2?) because the voltage went to the full voltage of the rectifier/filter circuit.  I could be wrong so wait for a few more opinions, there's folks here that know way more than I do.
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Hi Red Baron,
I have Picmaster's schematic, revision 2.10 dated 10/10/2010, which is almost the same as mine. It uses TIP3055 output transistors instead of 2N3055 ones. The TIP3055 has an epoxy case, not metal and is rated at 90W max instead of 115W max so one might be shorted C to E due to excessive heat or maybe moisture from humidity got through the epoxy.
If the BD139 or the voltage regulation opamp shorted then the output voltage would be less than the unregulated positive supply. 

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Guest liquibyte

If the BD139 or the voltage regulation opamp shorted then the output voltage would be less than the unregulated positive supply. 

Now that I think about it, that does make sense.  Having a shorted C-E on one of the pass transistors would allow the full unregulated voltage.  Meh, I'm working on three hours sleep.
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Hi liquibyte
I have been involved in this project for nearly two years.  I built the Picmaster version from March 09 and it worked fine for a while.  Recently while charging a 34Ah battery the power supply failed - voltage went to 42V and zero current.  What do you suspect has blown?


A 34Ah battery?  Was it for a riding lawn mower?  If you are going to use the power supply at its limits for long periods of time you may want to consider upgrading to a couple of 2N5886 or MJ11016 output transistors mounted on a large heat sink with a small fan blowing across it.  A  2N3055 with a case temperature of 125 C  is only good for about 50 W.  A 2N5886 at 125 C is good for about  82 W.  For a 30v,  3 amp power supply operating at very low voltage and high current, the output transistor may consume 115 W.  Two 2N3055's are borderline depending on the cooling.
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Do you have anything that you can load this at 30V @ 3A?  Out of curiosity, I'd like to see how it behaves at the full 90W.  I'm still trying to simulate the mosfet circuit but if yours works well through the full range, I think I'll just go with that.  I'll try hooking it up tomorrow and post some results if I can get pics of it.


No, let me do some thinking on what I could use to load it up.  I do have a headlamp that is 12V and about 4.5A. 
  Perhaps I can find a small electric heater.
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Guest liquibyte

Got any old stove eyes laying around?  Two small ones and one big one gets you right at 11 ohms in parallel.  You can also cut them down but it's kind of trial and error.  Make sure you have them up off of anything that can melt or burn because they get hot at the 3 amps.

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Guest liquibyte

I have no idea if I'm really doing this right but I seem to be getting results from LTSpice.  I was trying to model the spike and then tune the turn on curve of the transistor to fall after that range.  I have no idea if this would work in the real world or not but it sure does look nice.  I've attached the .asc file zipped up if anyone wants to give it a whirl.

post-107142-14279144766598_thumb.png

post-107142-14279144766974_thumb.png

soft-start.zip

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Guest liquibyte

Here's the mosfet solution that I've been working on.  It seems to work except that LTSpice says that the gate is seeing the pulse (V[n003]).  Changing the delay is just a matter of changing the value of R1.

Edit:  I'd been measuring the gate to ground and should have been measuring gate to source.  I got a little help with that and now it shows the correct plot.  So, we have two working solutions now.  This one may be a little better because of the voltage drop across a traditional transistor.

post-107142-14279144767571_thumb.png

mosfet-fixed.zip

post-107142-14279144767984_thumb.png

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I did some testing with the TIP141 and 5 amp load and nearly burned up the TIP because I didn't have a decent heat sink.  I put the Tip on medium sized heat sink and noticed the temperature rise to about 125 C which was hotter than anything else on the board, even the output transistor heat sink.    I wonder if I miscalculated something. I thought operating more or less like a switch would not generate that kind of heat (except at start up).  I was thinking that the voltage drop when fully on would be about 1.2V at 5 amps or 6 watts.  I'm definitely burning more than 6 watts.  The other thing I noticed is that the turn-on delay was nearly triple or about 3 seconds.

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Guest liquibyte

I did some testing with the TIP141 and 5 amp load and nearly burned up the TIP because I didn't have a decent heat sink.  I put the Tip on medium sized heat sink and noticed the temperature rise to about 125 C which was hotter than anything else on the board, even the output transistor heat sink.    I wonder if I miscalculated something. I thought operating more or less like a switch would not generate that kind of heat (except at start up).  I was thinking that the voltage drop when fully on would be about 1.2V at 5 amps or 6 watts.  I'm definitely burning more than 6 watts.  The other thing I noticed is that the turn-on delay was nearly triple or about 3 seconds.

That's definitely not a good thing.  You wouldn't happen to have a PFET to test my theory out with would you?  I haven't fully finished the simulations yet because I want to test using a load as well but I've just now figured out how to work some of the basic stuff so it may take a little time.

Edit: For example, here's the results with a 3A load.  I may be modelling things wrong but things definitely changed when I added the correct load for 3A and a 3A current source.

post-107142-14279144768333_thumb.png

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Guest liquibyte

I rearranged some values and redid the plot.  If I'm doing this right, the power looks good for the zener and the mosfet so there doesn't seem to be any issues there.  I put in the filter cap too because it's part of the circuit right before the mosfet and I wanted to make sure it didn't alter the results.

post-107142-14279144768534_thumb.png

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Guest liquibyte

I have a boatload of NFET's but I don't think I have any PFET's.  I'll look around to make sure because I have everything else I'd need to test this and it would be a shame if I didn't at least have one. 

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Guest liquibyte

I think I'm starting to get the hang of this after doing a bit of reading.  Here's the results with things set correctly if I'm reading everything right.

Edit:  After getting some feedback and doing a bit more research, I think I've finally got something that gives fairly good results.  I'll have to wait to test it but hopefully this is a solution that works.

post-107142-14279144768905_thumb.png

post-107142-14279144793514_thumb.png

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Guest liquibyte

Redwire, can you give us a screenshot of pin 7 to pin 4 of U1 and U2 with 5V/div @ 500ms/div?  I'd do it myself but no DSO.

We've been discussing the shots you did here and here and the consensus is that this is RFI at the switch contacts or something like that.  What I'm really curious about, and I can't take a picture of, is what the two op amps are seeing at power up across their power pins.

The consensus of many people that work in the industry is that this design is a fail and that it should be scrapped and redone.  I've heard this several times from several people.  I'm not ready to give up on it myself but that could just be my OCD talking.  I think it works very good other than the surge problems.

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Guest liquibyte

Excellent, that's what I wanted to see, thanks.  I'm trying to get a few more eyes on the problem and was asked about the voltages across the voltage pins for those op amps.  One of these days I'm going to get at lease a cheap DSO.

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