woody Posted March 9, 2005 Report Share Posted March 9, 2005 Hi y'allCan someone help please:Made a current limiting PSU from circuit in the L200 spec sheet as below.Can't get the current limiter to work and am thinking I'm frying the OpAmp. :'( It gets 30 volts but its own spec sheet says 22 volts max.Is there a solution or am I barking up the wrong tree?ThanksWoody Quote Link to comment Share on other sites More sharing options...
audioguru Posted March 9, 2005 Report Share Posted March 9, 2005 Hi Woody,The spec sheet for the premium LM741 lists a max supply of plus and minus 22V which equals a total supply of 44V. The lower quality "C" version is listed at a total of 36V. It should be fine in your 30V circuit. All of its spec's are listed with it operating with 30V.The opamp's output voltage should go lower as the voltage across the current sensing resistor increases due to load current through it.Because of the limited range for the 741's input common-mode voltage, it won't work if the circuit's output voltage is within about 4 to 5V of the input voltage.A TL081 or TL071 opamp would work better in this circuit because they have an input common-mode voltage range that includes their positive supply voltage. Their pinout is the same as a 741. Quote Link to comment Share on other sites More sharing options...
woody Posted March 9, 2005 Author Report Share Posted March 9, 2005 AudioguruMany thanks - I will try the Op Amps you suggest and let you know the resultWoody Quote Link to comment Share on other sites More sharing options...
woody Posted March 11, 2005 Author Report Share Posted March 11, 2005 Ho AudioguruI swapped the 741 for a TL071. I get 30.6 Volts out regardless of volts pot setting. When I put a load on the output (66 Ohm) the volts drop to 0.9 and current is 13 mA. (At least it looks like Ohms Law is working!)Any idea what's wrong pleaseWoody Quote Link to comment Share on other sites More sharing options...
audioguru Posted March 11, 2005 Report Share Posted March 11, 2005 Hi Woody,If your P2 and R4 are correct, it sounds like you have burned away the collector of the L200's internal pass transistor, and its driver transistor can't pass much current.Isn't your L200 bolted tightly to a big heatsink with thermal grease?Isn't your R3 0.1 ohms, not 2.2K?Do you have a solid 30VDC well-filtered supply source? I have seen regulator circuits where the main filter cap had broken its connection and the regulator IC had a hard job dealing with the enormous ripple. Quote Link to comment Share on other sites More sharing options...
woody Posted March 13, 2005 Author Report Share Posted March 13, 2005 Hi AudioguruI replaced the L200.It is bolted and electrically insulated on a large heatsink.R3 is a 0.1 Ohm 10 Watt resistor. The main filter cap is 6800 uF and is doing its job.Still no joy.L200 pin readings are :(1) 33v (2) 1.23 - 1.38v (3) 0v (4) 0 - 1.23v (5) 0.9vTL071 readings are:(2) 0.9 - 1.23v (3) 0.8v (4) 0v (6) as 2 above (7) 33vReadings taken with a 66 Ohm load and swinging the pots fully.I must have done something stupid! I have checked the tracks again !!!!! while writing this have discovered micro break in track connecting pin 3 of L200 to the rest of ground.Now all is fine and lesson learned.Thank you for staying with me on thisWoody Quote Link to comment Share on other sites More sharing options...
audioguru Posted March 13, 2005 Report Share Posted March 13, 2005 Hi Woody,Good, you fixed it.The L200 must really like its ground. Quote Link to comment Share on other sites More sharing options...
woody Posted March 13, 2005 Author Report Share Posted March 13, 2005 Hi AudioguruBeen putting this through its paces. When I use the pot to reduce voltage under load (250 mA) from max, it cuts to 14 volts and 130 mA very soon after turning the pot. When I turn the pot fully up it stays at 14 v. The only way I can get it back up is to switch the whole thing off and back on again. Is there anyway I can unlatch this hangup, please? >:( L200 is running cool.I presume its the TL071 that is dictating?Woody Quote Link to comment Share on other sites More sharing options...
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