0-30V Stabilized Power Supply

[MyTallest]

Just started gathering parts for this project. I thought I'd pass along an idea. I didn't have much luck in finding the right transformer plus I thought I'd like to keep the "box" fairly small. So I went for an external transformer. A lot cheaper and nicely packaged. 

http://www.amazon.com/gp/product/B0027BUS0M

The one they sent me was actually rated at 90W instead of 70W as advertised.

 

[audioguru2]

Just started gathering parts for this project. I thought I'd pass along an idea. I didn't have much luck in finding the right transformer plus I thought I'd like to keep the "box" fairly small. So I went for an external transformer. A lot cheaper and nicely packaged. 

You didn't buy a transformer that has a 24V AC or 28V AC output. Instead you bought a switched mode power supply with a choice of DC outputs which is no good for this project.

The modified project has a 28V AC or 30V AC transformer that is rectified and filtered and it produces +40.2V DC and -1.3V DC. Your project will not work when the positive unregulated supply is only +24V DC and with no negative supply.

 

[HISPID]

Thanks audioguru.

Operational amplifiers OP07CP won't be destroyed, because their maximum voltage + - 22V (total 44V), I attached screenshot from the specification.

Yes I agree that in the original scheme there are voltage losses, and some parts become hot. But with small corrections (replacement of some parts by more powerful analogs) it will work. And it works for many users who built the original scheme.

I have forgotten to mention that i have not polyester C4, C5. And i used their ceramic analog. Whether there can be a problem in it?? I think that isn't present...

View attachment 41573

 

[audioguru2]

Operational amplifiers OP07CP won't be destroyed, because their maximum voltage + - 22V (total 44V)

The first page of the datasheet from Analog Devices spec's a max supply of only 36V but later it says 44V so it is very confusing. I would not use more than 36V.

Yes I agree that in the original scheme there are voltage losses, and some parts become hot. But with small corrections (replacement of some parts by more powerful analogs) it will work. And it works for many users who built the original scheme.

It is wrong to overload parts. The original circuit cannot produce a regulated (no hum) +30.0V at 3.0A.

I have forgotten to mention that i have not polyester C4, C5. And i used their ceramic analog. Whether there can be a problem in it?? I think that isn't present...

C4 and C5 are simple filters. Their tolerance and type does not matter.View attachment 41574

 

[fly3rman]

Arghs. Crap. Mixed up Base and Emitter of 2n3055 and now somethin weird is happening, but its not working. Output of maximal 3V and extreme hot R1 and R2. But iam able (in a small range) to control voltage and current. extreme strange.
Test everything will be fun :/

I think i saw some picture of "as it should be" voltages and differente places. will look into that.

update: u1 and u2 (tle2141) do have 42.6 volts on pin 7. u3 does have zero.
when shorten the output, ~1.5 amper are flowing and 2n3055s heat up very well, so far working, but cant control current then. But when i set voltage to zero on voltage poti, its no current flowing anymore -> working. Led isnt lighten up.
I think i should replace u3, but maybe its something next to it.

Ideas please =)

 

[audioguru2]

Hi fly3rman,
When the 2N3055 transistors are connected with their base and emitters backwards, then the driver transistor Q2 takes almost the entire output load and probably burns out if the project has a load.

The positive supply pin 7 of U3 has a 10V zener diode in series so its voltage should be about +32.6V. Its output pin 6 is usually as high as it can go (about +31.3) then drops to reduce the output voltage when it regulates the output current.

U2 and the driver and output transistors are an amplifier with a voltage gain of about 2.68. Its input is 0V to +11.2V from the voltage-setting pot that is fed +11.2V from the output of U1.

R1 is supposed to be 2200 ohms/2W. With 42.6V across it then it dissipates 0.82W so it is very warm but not too hot.
R2 is supposed to be 82 ohms/2W. It should also not be too hot. 

 

[fly3rman]

@ audioguru, thanks for the input, you were right. It was the Q2. I put in a new one and now its working like it was before, but still not right.
I can:
Set voltage from zero to 11.1
Control current and also turn it off completely (led lights on)

Now some data:
- i used redwires pcb with latest values (see picture)
- power is supplied by 2x15 (series) 160VA transformer
- ac voltage is fine: 30v
- dc voltage after rectifier and supplied to 2n3055`s: 42v
- q3 is a bc327, collector:32v, emitter: zero aslong no c.c, base ~32v
- q2: zero to 11v on emitter (reacts on base voltage), 42v on collector, base zero to 11.9v
- negative rail is @  -1.3
- R1 is 2200 Ohm, R2 is 82 Ohm, both 2W, still getting extreme instant-fingerburn hot
- u1 pin7 voltage: 42v, pin6: constant 10.8
- p12: 0-10.9v, p10: zero constant, p5 10.9 constant
- p13:0-0.9v?, p11: zero constant, p6:0.9v
- u3: pin7: 32v, 6:zero to 32
- u2: pin7: 42v, 6: zero to 11.7
- trimmers seem to have no effect at all

It really seems that is dont get the voltage gain of 2.68 times. i start thinking about faulty opamps? they are hard soldered, so not fun to exchange them, but i wanted to put in sockets anyway ....
I hope you can provide some more help.

View attachment 41585

 

[audioguru2]

Hi fly3rman,
You used Redwire's pcb but my parts list. They have different resistor values but  they should work properly even if the parts are mixed up.
Your output amplifier has a gain of only slightly more than 1 so maybe R11, R12 or RV2 are broken or wrong values. Maybe C6 is shorted.

Here is Redwire's schematic simplified:

View attachment 41586

 

[fly3rman]

Hi fly3rman,
You used Redwire's pcb but my parts list. They have different resistor values but  they should work properly even if the parts are mixed up.
Your output amplifier has a gain of only slightly more than 1 so maybe R11, R12 or RV2 are broken or wrong values. Maybe C6 is shorted.

Here is Redwire's schematic simplified:

Hi audoguru,
I have no idea how you could find that problem but you was right, again! I actually double/three time checked all the parts, but this one slipped through ...
R12 was a 56 Ohm instead of 56K Ohm. No wonder it wasnt working.
Now it does freaking work. Voltage goes up to 30+something, i successfully trimmed it down to 30. Shorted Amper measurement was showing 5 Amp, then i stopped turning up =). Trimmed it down successfully to 3Amp!

Zero Voltage is something of 0.33mV. Cant trim it down, but will recheck values around RV1. But actually that Voltage is ok for me first.

Now i will build in the additional fine potentiometers. Do some heatshrink ...
And  then i will think about heating problems, my heatsinks are way not enough ( 3,5'K/W, http://www.segor.de/#Q=SK 63-37,5SA 3S)
For thins i thinking about follwing:
placing these heatsinks inside on the aluminium backplate (se picture), and outside another one big heatsink, conencted with screws/nuts and silicon heatpaste. Maybe a problem: the whole backplate gets hot? Other idea: on the bottom inside on the airholes i just place a 40x40 or 50x50 airfan, maybe controlled with a temperature-sensor.
I posted some pictures of my work so far.


Thanks man, you rock!View attachment 41587

View attachment 41588

View attachment 41589

 

[fly3rman]

One more picture:

View attachment 41590

 

[audioguru2]

Hi fly3rman,
With the output set to a low voltage or is shorted and the current is set to 3.0A then each output transistor has 40.6V and 1.5A which is heating of 60.9W. Your little heatsinks are rated for only about 10W each.

The 2N3055 is rated for a chip to case max thermal resistance of 1.5 degrees C per Watt. Thermal paste is about 0.2 degrees per Watt. 60.9W will heat the chip to (60.9 x 1.7)= 104 degrees C above its case temperature. The max allowed chip temperature is 200 degrees C but 180 degrees is safer. If the ambient free air is 30 degrees C then the heatsink must have a thermal resistance of at least (180 - 30 - 104)/60.9= 0.76 degrees C per Watt which is impossible without having a huge heatsink and a fan.

The original project used only one 2N3055 transistor that tried to dissipate 91.6W. Impossible!   

 

[fly3rman]

Yep, i came also to the same calculations. There are heatsinks with 0.8K/W, but they are too huge and its not really possible to mount the 2n3055 on it.

I will later check my local dealer, ive got an idea ....


Today i was playing around a little bit with the Powersupply and burned some leds. Why: If you set the current to ~10mA and turn off the supply, u get a short, huge peak of current which kills your leds. My slow multimeter shows up to 30-40mA for a setting of 10mA. Because the multimeter is slow and averaing, i think its way more for a short time.
At the moment ive got like 3000uF only on my caps, so if i got up to 12000uF the peak will be longer and higher.
Is there any solution to keep up the current control "till the end"?

Also possible solution: Switch for output and switch for the supply itself. Or maybe i get a multiple-pole-switch (never saw it with more than 2) switch turn off the output and 220v input at the same time. One switch and 2 relays would be an idea too ...

 

[audioguru2]

Hi fly3rman,
The original circuit used TL081 opamps that have a problem of "phase inversion" when an input voltage got within 3V of the negative supply. So when the power was turned off the negative supply voltage dropped and the output went as high as it could go. So transistor Q1 was added to short the drive to the output when the negative supply began to drop when the power was turned off.

The new circuit uses modern opamps that do not have the phase inversion problem so Q1 was removed. The output should not go high when the power is turned off.

View attachment 41595

 

[fly3rman]

Hmm.

Okay still it does happen, and i do use TI Sampled tle2141, so they should be fine.
Can anyone else who build the supply test this behaviour? Redwire maybe?

I dont think its something in the circuit like the multimeter?

I will test further later today. Will see what happens on U3 pins.

EDIT: Btw it does only affect current control. It does not overcome the set maximum voltage. But if voltage is to max and current is limited, i still has the effect. Ive uploaded a video showing the effect, also very interresting is the flickering effect, no idea where thats from.
http://share.ovi.com/media/fly3rman.videos/fly3rman.10007

Also at ON of the supply there is kind of the same effect: current limit jumps in after a flash of led (so its conducting for a short time)

EDIT2: testing with dc trafo now, i think the effect isnt there with it. Must be the big transformer, maybe i DO need the full 12000uF wrong
EDIT3: Exchanged u3, no effect.

 

[audioguru2]

It looks like you have the output set for current regulation and the output voltage of your project increases when the power supply is turned off.
Then the output current also increases.

I think it is because the negative supply becomes zero volts quickly so D9 cannot keep holding the output voltage down.

You might fix it by increasing the value of C3.

 

[fly3rman]

Hi.
Ive increased C3 to 47uF + 100uF. Same behaviour. Increased to 47uF + 470uF same. 47uF+2200uF is way better. But for this is took reduced the C3 for 2200uF, only a slight increase of voltage/current on OFF.
Couldnt i feed C3 from C1?
With 2200uF on C3: Negative Voltage of -1.5 on negative lane and it does not get higher than -1.1 while OFF process. So thats not the problem i think.

Also nice: fast off/on doesnt produce this behaviour, because caps are still loaded then. So how to delay the output ON after ON of powersupply itself? Its not an option to fully go down with the potentiometers before on/off of the supply.
The effect is still only there if i use current limit, not voltage. IF Voltagelimit is to zero, no flash of led on start or off.

Could be in next design update.
Iam also thinking a on/off switch for the output is a must. But mechanical it would be a problem, because we want an OFF state for Output for each ON of the supply.
Also maybe q1 should find his way back into the circuit because of the dont-have-to-wait-cap thing:
"It is a great advantage in experimental work to be able to kill the output of a power supply without having to wait for the capacitors to discharge and there is also an added protection because the output of many stabilised power supplies tends to rise instantaneously at switch off with disastrous results."
If someone provide me a reasonable R15 value for the REV2 Circuit, i could implement the Q1 into my actually cirtcuit for testing if whished


Is actually the ON behaviour (no limiting for a short time) normal? Cany anyone test it pease?
Btw i have to replace the rectifier bridge (600v, 4A at the moment, getting too hot).

 

[fly3rman]

Hey guys, sorry for putting pressure on this, but i/we do have fixing of this problem as a task on sunday ;]
Can someone give me some input if he has the same problem and if it would help to reimplement q1 again and how to impelent it into the REV2 circuity?
I need clean on/off behaviour on my supply.

 

[audioguru2]

The problem of the output regulated current increasing when the power is turned off is caused by the -1.3V supply dropping too fast. In addition to increasing the value of C3 maybe the value of C2 should also be increased and the power rating of R2 should be increased.

You can add Q1 to short the output of U2 when the power is turned off like in the original circuit but it will be tricky to sense when the negative supply drops low enough to turn on Q1. R15 is not needed. But the max output of a TLE2141 is 80mA so there will be plenty of current in Q1!

 

[fly3rman]

The problem of the output regulated current increasing when the power is turned off is caused by the -1.3V supply dropping too fast. In addition to increasing the value of C3 maybe the value of C2 should also be increased and the power rating of R2 should be increased.

Ive replaced C2 and C1 with 470uF. The negative voltage goed down to -1.6V now.
But the behaviour is different from just drop too fast. It does oscilate. In my case it does go up to +3V, and after some quick flashes of connected leds it goed down  again to -1.2 again and is slowly dropping.

I am also testing Q1 solution right now. Used parts:
Q1 BC548
R13 10kOhm
R14 1.5kOhm
I quicktested, the transistor is always on :/ But i also have to put D10 into. R14 value must be corrected too i think because of difference from -5.6V to -1.3V. I might use a potentiometer for testing.

 

[audioguru2]

C1 must be a huge value to reduce ripple on the output. Therefore with a small load current it takes a long time to discharge. But the negative supply discharges quickly then U3 loses its negative supply and cannot hold the output voltage down through D9 when the power is turned off.

R13 andf R14 are a simple voltage divider. You can calculate values where Q1 is turned off normally but turns on when the negative supply reduces a little.