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| DIY 0-30V 0-3A ...Again |
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| Jwillis:
I know this darn project has been done to death and I apologize for bringing it up yet again. I've poured over all the posts here and across the Net concerning this project and have found nothing related to my problem. I understand that it's unlikely to achieve the advertised output of 3 amps without over heating and possibly destroying the circuit.That would be more impressive than what I getting right now. With no load it does indeed supply 0-31VDC. The current control seems to work as it effects the voltage and the LED does change off and on. My problem is despite all attempts,checks ,rechecks and more rechecks I can't get more than 30ma.On load test at full voltage and full amps all it does is 30mA and drops the voltage right down to 0.8V.Current can be adjusted between 0 and 30mA with delicate movement of the potentiometer. The transformer is a Hammond 120VAC to 24VAC 350VA .Now this should be plenty to power this PSU. I purchased new TL081 op amps from Digikey and tried 3 different sets. No change on load I replaced the filter Cap from 3300uFwith 4700uF.No change on load Replaced all diodes including zener .No change. All diodes have checked OK Removed Q1 suspecting it may be faulty.No change on load. Changed Q2 from 2SD882 to a BD139.No change on load. Checked the Q4 2SD1047 and it checks OK. Replaced Q1 (9014) and Q3 (9015) .No change All resistors check within 1%. All diodes have checked OK All potentiometers work as they should. I've checked all voltages across pin4 and pin7 supplying the op amps .All around 32 volts. I checked the zener diodes and get the expected 5.1Vdc across D7 and -5.1Vdc across D8. I load tested pin6 on U2 and get 600mA at 31V which should be enough to drive Q2 with the (R15)1K limiting resistor.Which should give 31mA at 31Vdc. Checked all traces and compared to working PCBs and found no difference aside from pattern. Checked and rechecked orientation of components. Checked continuity of traces and all solder joints.All functioning. The only thing that seems odd is that RV1(100K Trim pot) doesn't do any thing . Despite all this I can't get more than 30mA. Frustrated and out of ideas. |
| Andy Watson:
Double check the value of R7. If it is correct, load the output to 30mA (or whatever the maximum attainable is) and report the voltage across R7, and at pins 2 and 3 of U3 (relative to 0V side of R7). |
| Kleinstein:
The circuit is known from Chinese kits with several known issues. The most important is that with a 24 V transformer the supply for the OPs U2 and U3 will be too high (e.g. nominally some 38 V, worst case maybe more than 42V) So expect these to blow. Even if the OP survive (lucky or using other OPs like 741 , TLE2081) one would not get 30 V out under a significant load. With a 24 V transformer its more like 20-24 V. The current rating is more like short time peaks and should be reduced to something like 2 A. To keep the original OPs in spec, the transformer should be no more than 18-20 V AC and thus some 16 V maximum output voltage under load. For the broken circuit, having 31 V at the supply to the OPs suggests the negativ (-5 V) supply is not working. This could be a wrong part (e.g. zener D7 the wrong way around) or a broken OP, drawing to much supply current. 600 mA out of U2 suggest it's broken. Without the negative supply Q1 blocks any significant current - a little odd to still get 30 mA. 30 mA would be more like the normal current limit out of U2. |
| IanB:
This may seem obvious, but if we take the transformer at face value, then with a supply of 24 V AC, the maximum output current at 33 V DC using a perfect rectifier will be zero amps. If we subtract some diode drops in the rectifier and the necessary voltage drops across the regulation circuitry, then the limiting voltage for zero amps out will drop down to 30 V or so. If you are actually managing to get 80 mA at 31 V DC I would say you are doing remarkably well. It is way better than zero which is all you have any right to expect. |
| Jwillis:
Oh wow! Thanks for the reply s --- Quote from: Andy Watson on January 05, 2019, 09:05:09 pm ---Double check the value of R7. If it is correct, load the output to 30mA (or whatever the maximum attainable is) and report the voltage across R7, and at pins 2 and 3 of U3 (relative to 0V side of R7). --- End quote --- Thanks Andy.I checked R7 and pis 2 and 3 on U3 as you said. R7 is 47.3 Ohms and under max load (a whole 30mA) it reads 1.667 Vdc Pin 2 is 1.672Vdc and pin 3 is 1.723Vdc --- Quote from: Kleinstein on January 05, 2019, 09:15:39 pm ---The circuit is known from Chinese kits with several known issues. The most important is that with a 24 V transformer the supply for the OPs U2 and U3 will be too high (e.g. nominally some 38 V, worst case maybe more than 42V) So expect these to blow. Even if the OP survive (lucky or using other OPs like 741 , TLE2081) one would not get 30 V out under a significant load. With a 24 V transformer its more like 20-24 V. The current rating is more like short time peaks and should be reduced to something like 2 A. To keep the original OPs in spec, the transformer should be no more than 18-20 V AC and thus some 16 V maximum output voltage under load. For the broken circuit, having 31 V at the supply to the OPs suggests the negativ (-5 V) supply is not working. This could be a wrong part (e.g. zener D7 the wrong way around) or a broken OP, drawing to much supply current. 600 mA out of U2 suggest it's broken. Without the negative supply Q1 blocks any significant current - a little odd to still get 30 mA. 30 mA would be more like the normal current limit out of U2. --- End quote --- Thanks Kleinstein. I don't expect much from this kit without modifications. But was hoping to get at least 1 Amp.So far I've retested those Zeners and they both read 5.1V and the orientation is correct.Double checked all the diodes for orientation. The Ops are all fine .I've switched them out several times with the same results. --- Quote from: xavier60 on January 05, 2019, 10:33:58 pm ---What value is R15? Is D10 fitted the correct way? --- End quote --- Thanks xavier. I recheck D10 and its the correct way. R15 is 1K ohms But I found another schematic where R15 is 100 Ohms and this person claims a much higher current output. http://electronics-lab.com/community/uploads/monthly_08_2014/post-107142-14279144694286.jpg given the extra power transistor I don,t doubt it. I was kind of leaning toward the idea that R15 may be to high but the circuit is identical to others that function fine. :-// |
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