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Bench power supply upgrade

My ancient Heathkit bench supply died a while ago so I bought one of the 30V 3A variable power supply kits like the link below that are all over eBay to replace/upgrade the insides of it with.

Sidecar Bob

Dec 19, 2021
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Sidecar Bob submitted a new Project Log:

Bench power supply upgrade

Ancient history: When I was in school about half a century ago someone put me in contact with a widow who wanted her late husband's electronics stuff to go to someone who would get some use from it. One of the items I bought from her was a Heathkit EU-41A bench power supply.
A few years ago it stopped working. I wasn't doing a lot of electronics at the time so I put it aside to see if I could fix it later.

Recent history: Last year when it became apparent that I was actually going to have a place to work on electronics I started thinking about the equipment I would need and I bought one of the 30V 3A "red" power supply kits that are all over eBay but before it arrived the weather had warmed up and I was working in the garden and the garage so I put it aside for this winter. One if the summer's projects was re-organizing the garage shop and when I came across the Heathkit it occurred to me that putting the new kit inside the old case should give me what I need.
I also have a digital voltmeter/ammeter that I bought for another project and didn't use so adding it to the front of the case will be a definite upgrade.

Here's the Heathkit before I start taking it apart. I will have to dig through my box of knobs to see if I have anything better or whether I need to order some.








I had been hoping to use the original transformer but the first thing I noticed when I took the covers off is how small it is.Then I realized the EU-41A's maximum output was only 15V...

Read more about this project log here...
 

Sidecar Bob

Dec 19, 2021
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Thread about using a switching supply instead of a transformer
https://www.electronicspoint.com/fo...power-a-bench-supply-kit.296410/#post-1823343

There are many fake products and fake reviews "over there".

Without a load, most 24VAC transformers produce 25VAC or more. Rectified it produces 34VDC. It also has a negative 5.1V for the opamps then they get 34V + 5.1V= 39.1V or more but their absolute maximum allowed supply is only 36V.
The circuit has so many voltage losses that its maximum output was only 25V at 2A, not 30V at 3A. Years ago when I fixed the project at electronics-lab.com I increased the transformer to 28VAC and added a second output transistor with a more modern driver transistor.

I used TLE2141 opamps that are rated at 44V and they needed only a 1.4V negative supply. I increased the power ratings of many resistors then they could survive. The new Chinese kits did not use my improvements.

With a supply of only 24VDC the circuit will not work without an additional 5.1V supply. With the negative supply your max output will be about 19VDC at 2A.

How much current would the 5.1V supply need and does it actually need to be exactly 5.1V? Could I use the insides from a 5V phone charger for that?
 

Audioguru

Sep 24, 2016
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The Chinese circuit uses a 5.1V zener diode to regulate the 5.1V. A 5V phone "charger" does not use a voltage regulator so the 5V will change often and might even be pulsing. The 5.1V current is only 6mA.
 

Sidecar Bob

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So maybe a slightly higher voltage supply that the zener can regulate to 5.1V?
 

Audioguru

Sep 24, 2016
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If you feed a higher voltage to a zener diode then it burns out. Instead you limit the zener diode current with a series resistor.
The datasheet of the zener diode part number shows its recommended current.
 

Sidecar Bob

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I haven't even thought about how zeners work in decades. Would I calculate the resistor by supply voltage - 5.1 divided by recommended current?
 

Audioguru

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Maybe your supply voltage is 12V then the series resistor has (12V - 5.1V=) 6.9V across it. The circuit needs 6mA and maybe the datasheet for the zener diode says it needs 20mA. Then the total current in the resistor is 26mA and Ohm's Law calculates the resistor to be 6.9V/26mA= 265 ohms but use 270 ohms which is a standard value. It heats with 6.9V x 26mA= 180mW so a 1/4W resistor is fine.
 

Sidecar Bob

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Makes sense to me. I was on the right track except I forgot to figure in the current it needs to supply.
I have boxes of adapters/chargers of various voltages so this sounds like a good use for one of the lower current ones.
 

Sidecar Bob

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Without a load, most 24VAC transformers produce 25VAC or more. Rectified it produces 34VDC. It also has a negative 5.1V for the opamps then they get 34V + 5.1V= 39.1V or more but their absolute maximum allowed supply is only 36V.
The circuit has so many voltage losses that its maximum output was only 25V at 2A, not 30V at 3A. Years ago when I fixed the project at electronics-lab.com I increased the transformer to 28VAC and added a second output transistor with a more modern driver transistor.

I used TLE2141 opamps that are rated at 44V and they needed only a 1.4V negative supply. I increased the power ratings of many resistors then they could survive. The new Chinese kits did not use my improvements.

With a supply of only 24VDC the circuit will not work without an additional 5.1V supply. With the negative supply your max output will be about 19VDC at 2A.

My 24V switching supply is here. It has an output voltage adjustment and when I turned it all the way up it produces 30.6V.

Would I still need the separate 5.1V supply with that?

I've set aside a 12V 200 mA supply to use for use if I do. Its open circuit voltage is 16V and a 30Ma load only pulled it down to 15V so I'd have to calculate the resistor value based on that.
I also have a 9V 100 mA supply that measures 12.8V o.c. and 11.2V @ 26mA so unless you tell me a reason not to I think I'll use it instead.

Now I need to finalize the layout and start modifying the case.....
 

Bluejets

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Easy enough to use small converters to give the voltages you require and forget resistors etc.
 

Audioguru

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Two of the old TL081 opamps and a transistor in the Chinese power supply kit need a regulated -5.1V at about 26mA or more. Since their maximum allowed total supply voltage is only 36V then the positive supply for them must never exceed 36V - 5.1V= 30.9V. Then the maximum loaded output from this power supply kit will be about 24 or 25V.
 

Sidecar Bob

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Considering that the original supply was only 15V and 750 mA I'll be happy if the upgrade gives me 24V.
I'm still not clear about whether I'll need the 5.1V supply for that or where to connect it.

Here's the schematic I found for the kit
Schematic.png
 

Audioguru

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The original kit and the Chinese copy have some errors:
 

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  • Chinese PS kit errors.png
    Chinese PS kit errors.png
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Sidecar Bob

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I've been involved with electronics for half a century but for the last 3 decades it has been mostly things like using LEDs as indicator lights on motorcycles and for many years before that it was almost entirely designing crossovers for speakers so I've forgotten most of what I learned in school all those years ago.

This was supposed to be an inexpensive and easy kit build to get me back into soldering on PCBs and at the same time replace an ancient & no longer working piece of equipment. Then I realized that I didn't have a suitable transformer and that buying one would cost more than some of the variable voltage power supplies sold as complete units and the project started to get complicated.

I think I've gotten in over my head with this so please treat me as you would a complete novice when trying to help me figure it out.

Audioguru:
1) Looking at both schematics I see that there is a connection to the AC at the input of the bridge rectifier and I don't understand why but I have a feeling that I won't be able to do that if I power it with a DC supply instead of a transformer. Is that why you say I need the separate 5.1V supply?
2) I thought the 5.1V supply had something to do with one of the 5.1V zeners in the circuit but I can't see how to connect it to either of them.Does this mean I need to get another 5.1V zener?
3) As for the errors you describe I've looked at a few pages about building these kits (I'm about to watch some videos about building them too) and they seem to work so is it possible that the errors are present in the drawings but the boards supplied are correct?

Bluejets: I think you mean that I could use a three terminal regulator like a 7805 to produce this mysterious 5.1V but again I have no idea where I would connect it or how.
 

Audioguru

Sep 24, 2016
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1) The input to the bridge rectifier is where a 21VAC transformer powers the circuit. Then the circuit gets +27VDC and -5.1VDC.
2) Fix the errors of R4 and R8.
3) Recalculate the value of R26 to match the current rating of zener VD6.
4) Recalculate the value of R24 so that there is voltage regulation when voltage and current ore set at maximum.
5) Remove zener diode VD3 and replace it with an external regulated -5.1V supply.
6) When overloaded parts burn out, replace them with larger parts.
 

Sidecar Bob

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1) I understand that the bridge rectifier is where the 24VAC transformer powers the circuit and I understand that the output of the bridge will have about 30VDC.
What I don't understand is the purpose of the portion of the circuit I have turned red in the image below. I have never seen a connection to the AC side of a bridge rectifier to the DC circuit before.
2) I have no idea what you mean by "the errors of R4 and R8".
3,4) I really do not understand how this circuit works so I have no idea why the values of R24 and R26 would be wrong.
5) Is this what you were talking about when you described calculating the value of the resistor in series with an unregulated power supply of higher voltage?

I know you are trying to help me but I'm only getting more confused.
Marked schematic for question.jpg
 

Audioguru

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1) A 24V transformer will produce 25V or more when it is no fully loaded. Then the positive supply to the opamps will be +34V or more. With the -5.1V negative supply then the opamps will get a total of 39V or more which will blow up the opamps since their maximum allowed total voltage is only 36V. That is why I used a 44V opamp in my fixed circuit.

R19 and C7 feed AC to the rectifiers VD4 and VD5 to produce a negative DC voltage that is clamped to -5.1V by zener diode VD3.

2) My previous post shows part of the datasheet for the TL081 opamps and the errors for R4 and R8.

3,4) The values for resistors R23 and R24 are calculated for an output of 30V at 3A that will blow up this wrong circuit.
You want a lower supply voltage that the opamps can use and a lower maximum output voltage and current that are regulated.
With a 20V transformer and the resistor values shown, if you turn up the voltage and/or the current then the output opamp will have its output saturated as high as it can go and not produce a regulated voltage.
In my fixed circuit I used a 28V transformer, 44V opamps that work fine with a 1.4V negative supply and calibration trimpots to set the maximum outputs to 30.00V and 3.00A.I also used two output transistors to share the extreme heat.
 

Audioguru

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I see that the Chinese copy corrected the error on the original schematic connecting R19 to the correct negative-going terminal of the bridge rectifier.
 

Sidecar Bob

Dec 19, 2021
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I think I'm starting to understand.

I would like to power this with a 24VDC 3A switching supply that has a voltage adjustment that can be turned up to a maximum of 30.6V. If I leave off the bridge rectifier and connect the DC supply to where the output of the bridge would be and then supply 5.1V to the red line that from another supply as in this version of the drawing would that work?
If I'm understanding you the total voltage on the op amps would be between 29.1V and 35.5V depending on how I adjust the switching supply so I could chose a safe voltage there that gives me an adequate output (I'll be happy with 24V or more).

Schematic with two DC supplies (1st version).jpg
 

Audioguru

Sep 24, 2016
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VD3 MUST be removed be replaced with -5.1VDC, not the +5.1V that you show. Remove VD3, VD4, VD5, C7, C8, R17 and R18.
R4 still connects to the wrong place and R8 still has the wrong value and also connects to the wrong place.

If you do not recalculate then change the value of R23 and R24 then when you turn up the voltage and the current there will be NO regulation above about 80% of the pot rotation when the output of this circuit is more than about 24VDC..
 
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