0-30V, 0-3A PSU - Audioguru's version

[soldar]

This a followup to https://www.eevblog.com/forum/beginners/diy-0-30v-0-3a-again/

It seems that project was not completed successfully.

I am attaching from that thread what I believe is the final design.

Is that considered to be the correct and final design and confirmed to work correctly?

If so, is it possible to buy the PCB?

In this thread I would like to come to a final, confirmed working, design, including specs, schematic, list of parts and PCB design.

I am analyzing it. I see the op-amp on the right does the voltage regulation and it looks quite simple and straightforward.

The op-amp and 5.6 volt zener in the center provide the voltage reference. I think it would be quite possible to simplify that and get rid of the op-amp and provide a simpler voltage reference. It might not be so exactly precise but I think it would be more than enough for most applications.

The op-amp on the left does the current limitation control and Q3 lights the LED when current limit is active.

I do not see why the 10V zener that lowers the supply voltage to U3 is needed. Maybe it can be omitted?


Maybe I would add an "ON" LED, just to show voltage at the main capacitor.

[Kleinstein]

The circuit is an improved version from a rather common kit, that has quite some mistakes inside.

It looks quite reasonable, though one might still want the transistor from the original to protect against an spike on turn on / turn off. With the auxiliary negative supply this may happen under some conditions during turn on.
Another point might be a diode prevent to much reverse voltage to the output stage base / emitter current (add a diode like 1N4148 from output to right OP).

The 10 V zener is there to reduce the supply to the first OPs. Depending on the OP used, this may not be needed or 10 V Zener could be still too little. To high a supply to the OPs was the main problem with the original circuit from the kit. A reduced voltage looks like a good idea.

For the reference part there are a few options. The OP circuit can work in a way to keep the extra current through the shunt constant. So it's not such a bad solution. The OP could share the same reduced supply as the current regulator - so no special higher voltage OP needed here.

By using only 1 raw voltage and with a relatively high drop, the circuit is not very suitable for high power.  With some extra filtering one could at least improve a little on voltage lost: So something like 2 V lower AC could be sufficient. I am still not sure the circuit as shown would get to 30 V under all conditions (e.g. low mains and full load).

[kripton2035]

seems you can buy this kit @ banggood and change the components
https://www.banggood.com/0-30V-2mA-3A-Adjustable-DC-Regulated-Power-Supply-DIY-Kit-p-958308.html

[Kalvin]

Probably one could increase the available power by using an optional tracking pre-regulator feeding the power stage. The tracking pre-regulator will increase the output noise somewhat, but one can get higher output current with less heat. If the pre-regulator would be bypassed by a switch when a lighter load is connected, one could achieve low noise output at lower currents. There should be some  cheap and suitable dc-dc buck-converters available at eBay, which can be modified so that it will track the power supply output voltage by 2 - 3 Volts or so. This modification would extend the usability of this simple power supply without serious heat issues.

[Kalvin]

I would also consider adding a unidirectional 30V TVS diode across the output, which would protect the power transistors from typical voltage surges occurring with inductive loads and connecting/disconnecting the load. Adding a polymer fuse to output should not hurt either.

[Kleinstein]

I would also consider adding a unidirectional 30V TVS diode across the output, which would protect the power transistors from typical voltage surges occurring with inductive loads and connecting/disconnecting the load. Adding a polymer fuse to output should not hurt either.

A polymer fuse at the output ( between the emitter resistors and the feedback) does add some resistance and thus effect the response a little towards a little slower response.

AFAIK the board from the kit does not include the 10 V zener to reduce the supply to the not so critical OPs. So it would need better OP for all 3. The difficulty here is getting DIP8 OPs that can stand a high voltage (e.g. 40..44 V), without paying a premium price (like for OP27). Plain old 741 could work - though a little slow (especially slew rate for current limit -  so quite some delay before CC mode sets in).

The board I know also includes a layout blunder, that causes quite some ripple from the rectifier when in the CC mode (poor sense point for the negative side of the shunt).  This can be fixed with a cut trace and bodge wire.

[Kalvin]

The difficulty here is getting DIP8 OPs that can stand a high voltage (e.g. 40..44 V), without paying a premium price (like for OP27). Plain old 741 could work - though a little slow (especially slew rate for current limit -  so quite some delay before CC mode sets in).

One can use an op amp in SO-8 with suitable breakout PCB adapter board or a piece of a strip board. Of course the op amp will cost a few dollars, which will effectively double the original price of the kit

[kripton2035]

LTC6090 @ around 8€ each ...

[perieanuo]

LTC6090 @ around 8€ each ...

What's wrong with TL082 replacement?


Envoyé de mon iPad en utilisant Tapatalk

[Kleinstein]

There are good replacements like the mentioned TLE2141 or MC34071 - though not that easy to get in DIP8 form.  An OPA604 could also work - though a little on the fast side and high supply current which is not that attractive with the negative supply. Only for 40 V, but otherwise the TLE2021 could be an option too.

The easy way is to just reduce the maximum voltage a little.
Another option would be to add a resistor and zener to limit peak voltages on the OPs supply. So the OP would not see the peaks.

Just changing the OPs on the kit board only fixes some of the weak points. There is still only 1 power transistor, too small a filter cap, the high supply to the other 2 OPs that don't need it, the poor shunt layout, a poor choice for the negative supply of the reference OP. One could directly support an OP is SO8 case that is easier to get.
There is already at least 1 thread in this forum about this supply and suggested changes. So there might already be an improved layout around.

So it might be worth to make a new, revised board and not use the kit.

Currently PCBs seem to be not that expensive when ordered in China.
This circuit should fit on an 8x10 cm² size - especially with the power transistors externally.

[rdl]

Why not just reduce the output to 20 volts? I doubt many people ever really need more than that.

[Kalvin]

LTC6090 @ around 8€ each ...

What's wrong with TL082 replacement?

Check the maximum operating voltage seen by the op amp: For 28V AC RMS rectified voltage, the peak rectified voltage will be 28V * 1.4142 = 39.5V. In practice one needs to provide few volts more across the power transistor, so in order to get stable 30V output voltage the RMS voltage needs to be 35V or thereabouts. Now, the peak rectified voltage seen by the op amp will be 35V * 1.4142 = 49.5V. Add to this the -1.3V negative power supply voltage and you will get 50.8V across the op amp, which will exceed the maximum allowed power supply voltage of TL082.

Like others have suggested, reducing the transformer voltage ie. reducing the rectified peak voltage ie. reducing the maximum output voltage, one can use those cheaper op amps.

[Audioguru]

The original Greek kit and the Chinese copies used TL081 opamps that have a maximum allowed total supply of 36V and need a negative supply that is at least 4V then its maximum allowed positive supply is only +32V. Its max output voltage would be +29V if the driver and output transistors have minimum hFE and the output current is 3A. I removed R15 in series with the output of the voltage regulating opamp and the driver transistor that reduced the voltage 2.5V max to +26.5V. The maximum worst case voltage loss of the driver and output transistors is 3V so the worst case maximum output voltage of this old circuit would be only +23.5V at 3A. You would want less supply voltage for reliability.

To make the +32VDC supply plus including the 2V drop from the bridge rectifier you need 34V which is produced by a 24VAC transformer.

Texas Instruments still make the 44V TLE2141 opamp in a DIL-8 package. Digikey and others have thousands in stock. I recommended it years ago, why use a tiny surface mount opamp that would probably get too hot? The new 140V tiny opamp does not spec its maximum output current when the supply is less than +/- 70V.

Besides the Greek and Chinese kits not producing 30VDC at 3A and not being reliable, they might not be made anymore. 

[floobydust]

The original 0-30V PSU schematic has D10 and R15 which are needed.
You can put a 1N4004 diode across Q4 E-C to protect against output overvoltages (instead of TVS) and it also stops the PSU failing when charging batteries.

Lately, the kits are coming with wrong resistor R7 as 47R not 0R47 5W.

[soldar]

Even though it is nominally 30 V I do not need it to go so high and I am happy to call it 24 or 25 volt PSU. I already have a 24 V transformer, rectifier bridge, filter cap, power transistors and some other components and I would only need to make the control circuit. I want to make it as simple as possible but also design something reliable that can be trusted to work. nothing fancy but definitely reliable and protected.

It looks quite reasonable, though one might still want the transistor from the original to protect against an spike on turn on / turn off. With the auxiliary negative supply this may happen under some conditions during turn on.

Kleinstein, thanks for your input. Could you expand on this? I guess it is a matter of disabling the output for a second or two when first connecting the unit so that all voltages have time to settle? This sounds necessary.

Another point might be a diode prevent to much reverse voltage to the output stage base / emitter current (add a diode like 1N4148 from output to right OP).

Generally I do not have the problem of connecting voltage sources to the output of the PSU and I would consider a reverse biased diode in parallel with the output to be sufficient protection.  Again, can you expand on this?

  The 10 V zener is there to reduce the supply to the first OPs. Depending on the OP used, this may not be needed or 10 V Zener could be still too little. To high a supply to the OPs was the main problem with the original circuit from the kit. A reduced voltage looks like a good idea.

Well, the specified op-amps have a max supply of 44 V and I am planning on using a 24 Vac transformer which would not produce anywhere near that so away goes that zener.

For the reference part there are a few options. The OP circuit can work in a way to keep the extra current through the shunt constant. So it's not such a bad solution. The OP could share the same reduced supply as the current regulator - so no special higher voltage OP needed here.

This seems to me to be more than is really needed for this type os simple PSU.  A simple zener or a zener and a transistor should be good enough. I will study this more carefully.

  By using only 1 raw voltage and with a relatively high drop, the circuit is not very suitable for high power.  With some extra filtering one could at least improve a little on voltage lost: So something like 2 V lower AC could be sufficient. I am still not sure the circuit as shown would get to 30 V under all conditions (e.g. low mains and full load).

I agree. I am going to call it 25 volt and the power transistors and other heat-generating resistors will go on the back, outside the box. I'm thinking "outside the box" here.

[soldar]

The original 0-30V PSU schematic has D10 and R15 which are needed.

You can put a 1N4004 diode across Q4 E-C to protect against output overvoltages (instead of TVS) and it also stops the PSU failing when charging batteries.

Yes, easy enough. OTOH, putting that resistor there means a voltage drop so I would like to supply the op-amp with a couple extra volts supply. Maybe, do on the positive rail something similar to what is done on the negative rail to get a couple extra volts.  I will think about this. It may be worth it. I'd like to read opinions on this.

[soldar]

seems you can buy this kit @ banggood and change the components
https://www.banggood.com/0-30V-2mA-3A-Adjustable-DC-Regulated-Power-Supply-DIY-Kit-p-958308.html

It sounds like kits for sale are not to be trusted and I think by the time we are finished re-designing this it might look very different.

[soldar]

I removed R15 in series with the output of the voltage regulating opamp and the driver transistor that reduced the voltage 2.5V max to +26.5V. The maximum worst case voltage loss of the driver and output transistors is 3V so the worst case maximum output voltage of this old circuit would be only +23.5V at 3A. You would want less supply voltage for reliability.

Audioguru, thanks for your input.  What is your opinion on restoring R15 and getting a couple extra volts to supply the op-amp as I mentioned earlier?

To make the +32VDC supply plus including the 2V drop from the bridge rectifier you need 34V which is produced by a 24VAC transformer.

I am not too concerned with the final voltage I can get. If it goes up to 25 Volts I consider it enough. I am making this with used junk parts and I have a couple of 12 Vac transformers from halogen lights. Now they are being replaced with LED lights I have a whole pile of these transformers.

I am not interested in buying the kits as they sound like crap but I would be happy to devote some time to finish with a good and reliable design which others can build. I do not want it to go to so many volts or amps. Foremost I want it to be simple and reliable.

[rstofer]

How much does Q4 dissipate when Vout = 1V and Iout = 3A?

[soldar]

There are good replacements like the mentioned TLE2141 or MC34071 - though not that easy to get in DIP8 form.  An OPA604 could also work - though a little on the fast side and high supply current which is not that attractive with the negative supply. Only for 40 V, but otherwise the TLE2021 could be an option too.

OK, let us call this project a 25 V PSU. As I have mentioned I already have 24 V transformers so the voltage at the filter cap will not go above, say, 32 V. I realize that maybe with high current demand the output voltage may not be able to go all the way to the max, That is something I can live with. So something like 25 volts at 1 amp, 24 volts at 2 amps, 22 volts at 3 amps is something I can live with.

It sounds like the suggested Op-amps go to 44 volts so that should be plenty.

I'd like to come up with a totally revised design we can all agree is reliable and well done. If it can use cheap and easy to find components I would prefer that over getting a few more volts out.

[soldar]

How much does Q4 dissipate when Vout = 1V and Iout = 3A?

The answer is "it depends". It depends on the collector voltage, which depends on transformer and filter cap. It depends on the output voltage being output. Since the current is divided between two transistors each one is dissipating half but the total still has to be dissipated.

Very bad case: collector voltage 33 volts, output voltage 1 volt, current 3 Amp, total power dissipated: 96 W.  better have some good heat-sinks!

[soldar]

Why not just reduce the output to 20 volts? I doubt many people ever really need more than that.

Well, I would say 24 or 25 volts is good enough for general work.

Really, going much above 20 volts would require switching input voltages or generating a ton of heat. I would say 25 volts is about the limit with a single supply voltage and is generally enough for everyday work.

[soldar]

Probably one could increase the available power by using an optional tracking pre-regulator feeding the power stage.

No, that is totally out of the question. I am going for "really simple, cheap, straightforward and reliable".

[soldar]

AFAIK the board from the kit does not include the 10 V zener to reduce the supply to the not so critical OPs. So it would need better OP for all 3. The difficulty here is getting DIP8 OPs that can stand a high voltage (e.g. 40..44 V), without paying a premium price (like for OP27). Plain old 741 could work - though a little slow (especially slew rate for current limit -  so quite some delay before CC mode sets in).

So, assuming a 24 volt transformer, so about 35 volt max input voltage, what are the options for common, cheap, through-hole op-amps?

[rstofer]

How much does Q4 dissipate when Vout = 1V and Iout = 3A?

The answer is "it depends". It depends on the collector voltage, which depends on transformer and filter cap. It depends on the output voltage being output. Since the current is divided between two transistors each one is dissipating half but the total still has to be dissipated.

Very bad case: collector voltage 33 volts, output voltage 1 volt, current 3 Amp, total power dissipated: 96 W.  better have some good heat-sinks!

I must have missed that, the schematic in Reply #13 seems to show just one pass transistor.  The original schematic in the OP shows two pass transistors.  Either way, it's a lot of heat and that's why these projects turn south.

There is a reason why transformers for bench PSUs have multiple taps and relays.