Bench power supply upgrade

[Sidecar Bob]

I think I understand about replacing VD3 with the 5.1V supply but I'll come back to that after I understand some of the other parts.

1) If I remove VD3, VD4 and C7 shouldn't R19 (and the entire red section) be removed too?
2) R18 is the current limiting resistor for the LED. Did you mean R19?
3) I don't know what C8 does or understand why it needs to be removed. Please explain.
4) I don't know what R17 does or understand why it needs to be removed. Please explain.
5) I have been studying the drawing you attached to post #13 about R4 and R8. Should they be as shown in orange in the version of the drawing below?
6) If I power it from 24VDC and 5.1VDC will the output be more than 24VDC?
If not would the original values of R23 and R24 be OK?

 

[Audioguru]

1) If I remove VD3, VD4 and C7 shouldn't R19 (and the entire red section) be removed too?
2) R18 is the current limiting resistor for the LED. Did you mean R19?

Sorry, I did not see your tiny numbers clearly. Yes R17 and R18 must remain. Remove VD3, VD4 and VD5, R16, R19, C6 and C7.

3) I don't know what C8 does or understand why it needs to be removed. Please explain.

Sorry again. C8 filters the opamps positive supply at high frequencies and must remain.

4) I don't know what R17 does or understand why it needs to be removed. Please explain.

R17 picks up the output current in R6. R17 and C4 are a filter for stability.

5) I have been studying the drawing you attached to post #13 about R4 and R8. Should they be as shown in orange in the version of the drawing below?

Yes it is correct.

6) If I power it from 24VDC and 5.1VDC will the output be more than 24VDC?
If not would the original values of R23 and R24 be OK?

Opamp OP1 has a 1.5V voltage loss, R3, VT2 and VT1 have up to 3.5V additional losses therefore the maximum regulated output voltage at about 1A is 24V - 5V= 19V if those parts have minimum specs.
R23 and R24 are calculated for 30V at 3A so the upper 35% of the voltage pot and upper 70% of the current pot rotation will produce your maximum voltage and current but they will not be regulated.

 

[Sidecar Bob]

Please ignore the schematic I have been using up to this point. I found it on an Aliexpress listing for what I believed to be the same kit so I assumed it was correct. When you mentioned having trouble seeing the numbers clearly (they are a bit blurry, aren't they) I went searching online and found what I thought was a clearer version of the same drawing on this page
https://chinese-electronics-products-tested.blogspot.com/p/hesai-30v-3a-power-supply-kit-tested.html

I enlarged and sharpened it so we should be able to read the numbers a bit more easily (especially when viewed full size)

When I started modifying it I saw that it already shows R4 connected correctly so I studied my PCB and found that it is also correct so I won't have to change that after all.

It still shows R8 connected to the negative output. Considering that there is only 0.47Ω between that and VCC- would changing it really make any significant difference?
Would it hurt anything if I tried it with the 270K resistor and changed to a 1.5K if the adjustment doesn't work properly?

 

[Sidecar Bob]

Sorry, I did not see your tiny numbers clearly. Yes R17 and R18 must remain. Remove VD3, VD4 and VD5, R16, R19, C6 and C7.

Could I leave VD5 in place and connect the and power supply (with resistor per our earlier discussion) as shown in this drawing?

 

[Sidecar Bob]

6) If I power it from 24VDC and 5.1VDC will the output be more than 24VDC?
If not would the original values of R23 and R24 be OK?

Opamp OP1 has a 1.5V voltage loss, R3, VT2 and VT1 have up to 3.5V additional losses therefore the maximum regulated output voltage at about 1A is 24V - 5V= 19V if those parts have minimum specs.
R23 and R24 are calculated for 30V at 3A so the upper 35% of the voltage pot and upper 70% of the current pot rotation will produce your maximum voltage and current but they will not be regulated.

Would I still have to change the values if I set the adjustment on the 24V 3A switching power supply so that its output is 30.VDC?

 

[Audioguru]

You can leave in the VD3 zener diode. The resistor feeding it needs to have the part number of the zener diode to match its current that might be 20mA or much more.

If you use a 30VDC power supply plus a negative supply feeding a resistor in series with the VD3 5.1V negative zener diode then the opamps will be close to their maximum allowed total voltage and the maximum regulated output voltage will be about +25VDC at 1A. The upper 17% of the voltage pot rotation and upper 70% of the current pot rotation will produce no regulation.

R8 wrongly connects to the output ground (the same error as on the original kit) where it will cause the offset adjustment pot R7 not to work since the TL081 datasheet shows R8 to be 1.5k connected to the negative supply voltage.
You show R8 wrongly connected to the input or output ground.

 

[Sidecar Bob]

OK, so if I set the switching supply's voltage to produce 24V maximum at the kit's output that should give the op amps a bit of headroom. If they feel warmer when I test it I'll just reduce the switching supply's voltage and live with less than 24V at the output.


Can you coach me about calculating better values for R23 and R24?

 

[Audioguru]

Voltage reference opamp OP3 has a 5.1V zener and a gain of 2 so its output is 10.2V.
The voltage setting opamp OP1 has a voltage gain of (56k/27k) + 1= 3.07 and since you want the maximum output to be 24VDC then the gain should be 24V/10.2V= 2.35. Then change R9 to 36k then the max output will be 10.2V x ((36k/27k) + 1}= 23.8V.

Resistor R6 senses the load current . The value of R7 is 0.47 ohms then its voltage is 0.47V with 1A of current. Replace resistor R23 with 200k ohms then the max regulated current will be 1.03A.

 

[Sidecar Bob]

OK, I understand about changing R9.

I would like the maximum current to be closer to 3A if possible. What value would R23 need to be for that?

What about R24?

BTW: Thank you for helping me learn about this. After 37 years of working on my own motorcycles and 22 years on forums about them I often find myself doing the same thing. Some forum members just give the new guy the information he needs but I usually prefer to tell him where to find it or teach him how to figure it out; When I see people I've helped to learn teaching others in the same way I have a feeling of great accomplishment.
I hope I'll be able to help others on this forum eventually.

 

[Audioguru]

If the output is set to 5V at 3A then the output transistor will melt with 78W of heat. If the output becomes shorted at 3A then the output transistor will melt with 93W of heat. That is why my fix uses two output transistors to share the heat.

3A x the 0.47 ohms of R6= 1.41V that opamp OP2 uses to regulate the max current. The reference from OP3 is 10.2V so R23 must reduce the 10.2V to 1.41v across the 10k current setting pot R21. Use 62k for R21.

The old TL081 opamp OP3 will not work properly if the 10.2V it produces is reduced by changing R24, because it does not use the negative supply voltage. Another reason I replaced the opamps in my fix with modern 44V TLE2141 opamps that work perfectly with a wide range of input voltages even with no negative supply voltage.

 

[Sidecar Bob]

If the output is set to 5V at 3A then the output transistor will melt with 78W of heat. If the output becomes shorted at 3A then the output transistor will melt with 93W of heat. That is why my fix uses two output transistors to share the heat.

The D1047 output transistor is rated at 100W. It will be mounted on the back of the case (1.6mm thick aluminum) with heatsink compound and a silicone insulating pad. There will be a fan blowing out on one side of it with one vent (series of holes) on the opposite side of the transistor and another on the front (I hope to add pictures if the modifications to the original case to the project log soon). If necessary I could add a finned heatsink on the outside of the case where the transistor is. Do I really need to be concerned about destroying it?
Re shorted output, I was planning to have a fuse on the mains power; Perhaps it would be a good idea to add another one in the output to protect against shorts.

3A x the 0.47 ohms of R6= 1.41V that opamp OP2 uses to regulate the max current. The reference from OP3 is 10.2V so R23 must reduce the 10.2V to 1.41v across the 10k current setting pot R21. Use 62k for R21.

I think that must be a typo. Did you mean "Use 62K for R23"?
If 3A output would be dangerous for the D1047 maybe we should aim for 2A or 2.5A maximum output.

The old TL081 opamp OP3 will not work properly if the 10.2V it produces is reduced by changing R24, because it does not use the negative supply voltage. Another reason I replaced the opamps in my fix with modern 44V TLE2141 opamps that work perfectly with a wide range of input voltages even with no negative supply voltage.

I only asked about R24 because you mentioned that it needed to be recalculated along with R23 several times.

 

[Audioguru]

Which burns out first, a power transistor or a fuse? If the current is set to 3A and a load reduces the output voltage to a short or any low voltage then the output transistor will get very hot.

62k is 62 thousand ohms. 62K is 62 ohms, 10%. All resistors on the schematic are lower case k.

With a modern opamp I would change R24 but the TL081 is old then it will not work so I changed R23 instead.
The reference opamp OP3 produces an output of 10.2V. If the current pot is at 10k maximum and is in series with 62k for R23 then their current is 141.6uA . The 141.6uA x the 10k pot= 1.416V across the 0.47 ohms of R6= 3.1A. If you use 68k for R23 then the max output current will be 2.78A.

 

[Sidecar Bob]

Which burns out first, a power transistor or a fuse? If the current is set to 3A and a load reduces the output voltage to a short or any low voltage then the output transistor will get very hot.

That's what fuses are for
BTW: I just posted the pics of the case modifications with the vents.

62k is 62 thousand ohms. 62K is 62 ohms, 10%. All resistors on the schematic are lower case k.

I understand that but you said R21 (the current setting pot) and I wanted to clarify that you really meant R23 (which you did in the last paragraph where you consistently said R23

With a modern opamp I would change R24 but the TL081 is old then it will not work so I changed R23 instead.
The reference opamp OP3 produces an output of 10.2V. If the current pot is at 10k maximum and is in series with 62k for R23 then their current is 141.6uA . The 141.6uA x the 10k pot= 1.416V across the 0.47 ohms of R6= 3.1A. If you use 68k for R23 then the max output current will be 2.78A.

Understood.

So I need to
- Correct the connection point for R8. I think I'll try it with the 270k in the kit before I change it to 1.5K because I looked at several data sheets for the TL081 and found anything from 0Ω (connecting the pot's wiper directly to VCC-) to 100KΩ in them.
- Change R9 from 56k to 36k. I don't have any on hand so could I use a 33K or 39k instead or do I need to find something that ads up to 36k?
- Change R23 from 56K to 68k. Or maybe I could put a 47K and a 33K in series and lower the maximum current a bit more?

 

[Audioguru]

I do not see pic's of your case.
I'm sorry I made a typo about R21.
Texas Instruments invented the TL081 so I use their datasheet. ST Micro is a copycat and use 0 ohms for R8 which causes a huge change in the input offset adjustment which makes it difficult to set the offset trimmer R7.
When the resistance of R9 is too high then the opamp OP has too much gain then if the voltage pot is set high there will be no voltage regulation.
You can increase the resistance of R23 to 80k (47k = 33k) to make the max output current be 1.13A, then the max heat in the output transistor will be about 34W.

 

[Sidecar Bob]

The pics are in the project log (scroll down to the bottom).

We all make typos. I think it proves we are human

I imagine using a much lower resistance (like the 0Ω in the ST datasheet for example) would indeed make it hard to set the trimmer. That's part of why I'm reluctant to change from 270k to 1.5k.

You can increase the resistance of R23 to 80k (47k = 33k) to make the max output current be 1.13A, then the max heat in the output transistor will be about 34W.

Is 1.13 another typo?

You said R23 should be:
62k for max 3A (post #30)
68k for 2.78A (post #32)
200k for max 1.02A (post #28)

I calculated the maximum current based on your explanation in post #3s and came up with the same results you did for those resistor values.

I calculated for 77k I got 2.494 amps and for 76k I got 2.52A, both close enough to the 2.5A I would like to aim for but I'm not sure I have resistors that will add up to those values. I do have the ones that add up to 80k and I calculated 2.41A for that.
I did all of the calculations several times to make sure and got the same result each time.

BTW: What's the weather like where you are?
We got a bunch of snow here in Central Ontario on Wednesday night and then we had to leave for appointments &c too early yesterday morning for me to clear the driveway first and got back too late to do it afterward so I had to run the snow blower for most of an hour this afternoon. I really should go out and shovel off the back deck now but I think I may leave that for tomorrow....

 

[Audioguru]

Opamp OP3 has a 5.1V zener diode and a gain of 2 so it produces the reference voltage of 10.2V that is fed to R23.
If R23 is 200k then in series with the 10k pot of R21, the current is 10.2V/210k= 45.5uA which produces 0.485V across R21 and is fed to opamp OP2. OP2 also senses the voltage produced by output current in R6. Then the max current is 0.485V/0.47 ohms= 1.03A.

There was a little snow in Mississauga last night but hey it's winter.
Right now there are many antivax idiot protestors in smoky and noisy trucks and farmers tractors in downtown Toronto. There are also many police and counter antivax normal people. I got my 3rd dose 3 months ago.

 

[Sidecar Bob]

Right, but wouldn't a value of 80k for R23 mean a max current of 2.41A?

Yeah, and isn't it amazing that the only people that are surprised when it gets cold and snows in the winter are the people that report the weather?
I don't want to get political but when I heard that truckers were protesting the government requiring that they be fully vaccinated to work I remembered all the years I worked in factories (& occasionally helped out in shipping & receiving) and thought I wouldn't want an un-vaccinated driver delivering where I worked during a pandemic.
You must have been among the first to get Covid shot #3. My wife and I have had our first 2 shots long ago. About the time they started giving the 3rd Covid we had our flu vaccines and shingles shot #1 (both shots on the same day so both shoulders hurt and the next day is a write off). Shingles #2 has to be given within a specific window after #1 so we couldn't have shingles #2 until a couple of weeks ago (along with the pneumonia shot, again both on the same day so both shoulders hurt). It is best to wait a couple of weeks after any other shots for Covid #3 so we should have Covid #3 soon.
My wife is responsible for compliance with whatever restrictions are current at her church so she has been following the medical and health unit reports &c since the beginning. She says it looks like the Omicron variant will probably be the end of it. From what I've heard case numbers have been dropping since last month and the percent requiring hospitalization is much lower than with the other variants (except among the non-vaccinated).
Hopefully the light we think we're seeing at the end of the tunnel won't turn out to be a train's headlight......

 

[Audioguru]

The total of 200k for R23 plus the 10k of the current-setting pot R21 is 210k but only 10k of it goes to the current adjustment opamp.
The reference of 10.2V/210k= 48.5uA. The 10k pot at max x 48.5uA= 0.485V. The current opamp output switches and regulates when it has 0.485V across the 0.47 ohms shunt resistor R6 which calculates to 0.485V/47 ohms= 1.03A.

The chief medical officer does not mention the numbers of antivaxers needing hospitalization or dying in the ICU of hospitals anymore, because the numbers of vaccinated people equals the number of antivax people. The antivax people do not see that the totals of each group are 10 to 1 so hardly any of the many vaccinated people need hospitalization.
Also, a fewer percentage of vaccinated people get the Covid virus compared to the antivaxers.
Doctors and nurses say that have never seen a person with 3 doses and no other medical problem in the ICU but some of the fully dosed people do get the virus and have no symptoms.

 

[Sidecar Bob]

Yes but I want 2.5A maximum. If my calculation is right 80k should do that.

 

[Audioguru]

2.5A x 0.47 ohms (R6)= 1.175V. 1.175V/10k (R21)= 117.5uA.
The voltage across R23 is 10.2V - 1.175V= 9.025V. Then R23 must be 9.025/117.5uA= 76.8k ohms, use 75k.