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Home made dual rail power supply problems

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joedevola:
Hello everyone.

I am a relative newb to electronics.  I like to build audio gear (amps speakers etc), have built half a dozen scratch built tube amps, active crossovers etc.  My 16 year old son has started to develop an interest in electronics (more mechatronics actually) so for his birthday I bought him some components, a good multimeter, a second hand scope and parts to build a dual rail lab power supply.  I used the schematic from Rod Elliot's site (Elliot Sound Products) here http://www.extremecircuits.net/2010/05/adjustable-current-limit-for-dual-power.html.  It is a pretty basic LM317/337 dual rail supply with a 24-0-24 transformer to give a variable dual rail supply up to about 25 volts.  That part of the plan worked.  Next I saw this schematic http://www.extremecircuits.net/2010/05/adjustable-current-limit-for-dual-power.html for a circuit to limit the current.  For the life of me I cannot get the current limit to work.  I have built it on perf board for the supply and it did not work, then I tried building it on a breadboard and got the same results.  Could someone check the current limit schematic to see if there are any errors and/or suggest a simple implementation of a current limit for a dual rail supply.

Complicating the issue is we have built the enclosure, have drilled a hole in the front face for the current limit pot and have stencilled in a label for it :( so simply not implementing the current limit would leave the supply with an ugly hole and also teach my son that to give up on a project is an acceptable outcome.

Any help appreciated!

Thanks,

Chris

w2aew:
I think the basic schematic looks OK.  One thing to be careful of is that the metal tab on most of these types of 3-terminal regulators (on the TO-220 package), including the LM317, is electrically connected to the Vout pin.  So, do NOT attach the the various regulators to a common electrically-conductive heatsink. 

If you can describe what isn't working, and some voltage measurements that you are getting while it is mis-behaving, maybe we can help you figure out what is going on.

Alan

joedevola:
Thanks for taking the time to respond.  I am about to step out for 3 days with work.  Will report voltages when I return.  Yes, tabs on TO220 devices are electrically isolated...  Voltage rails at filter caps prior to connection to regs is +/- 34 volts.

Cheers,

Chris

joedevola:
Thanks guys,

Still away with work at the moment, but if someone could help with an explanation of the circuit in question, it would really aid by ability to troubleshoot when I return.

Specifically, I am confused as to what specifically is happening with the voltages and currents around R1/R3 and R2/R5 and what the LM317/337 is trying to do with regard to the 1.25 volt reference.

From memory, I could get IC1 and IC2 to work independent of the IC3 circuitry (i.e. with just a 1.2R sense resistor it would limit current to 1 Amp).  I could also get IC3 to work independently, in that I could get it to limit from 10 to 60 mA when connected to a 10 volt supply.  From memory, I don't think I could get it working when connected across the rails, that is input +34 volts to output on -34 volt rail.  I see the data sheet has a 40 volt limit from in to out, but the original schematic call for +/- rails of "at least 22volts" and my supply is +/- 34, maybe a problem here?



Values:
R1, R2  1.2R
R3, R5  120R
R4  22R
P1  100R

Thanks for the help,

Chris

joedevola:
To me it looks like the IC2 setting 10-60mA will provide a bias voltage to the Adj pin of IC1 and 2 by the voltage drop across R3 and R5.  As they are specified as 120R, that would give an adjustment of 1.2-7.2 volts.  I would have thought 0-1.125 volts would have been the appropriate bias adjustment?

If my thinking is right, perhaps this amended schematic might work?



R1 and R2 are 1.2R to limit output to 1A. IC2 sets a current of about 15mA with R3 at 82R. R5 and R6 each drop about 15 volts to keep the voltage across IC2 at about 34 volts (+/- rails are +/- 34 volts) (or perhaps 15v zeners might be more appropriate here?). LEDs drop about 1.7 volts, so voltage divider dual potentiometer (R4) can set a bias of 0-1.7 volts to the adjust pins of the LM317/337 IC1/3.

Am I on the right track?

Thanks for the help so far.

Regards,

Chris

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