Diagnostic help

[svracer6724]

I'm a newbie when it comes to electronics and have recently put together a power supply kit- Elenco Model XP-15K http://www.elenco.com/product/productdetails/power_supply=ODU=/variable_voltage_power_supply_kit=NTcz. Unfortunately when I apply power the unit only outputs ~22 volts steady. The potentiometer has no effect at all. Since i have no practical experience with diagnosing the problem, I'm looking for direction as to what the problem might be. I have inspected the pcb looking for misplaced parts, parts installed backwards (diodes, caps), checked my soldering - all seems to be in order. I have determined that the potentiometer actually works with my dmm. I also replaced the LM317 regulator. Here is a link to the instruction manual; the schematic is on page 12. http://www.elenco.com/admin_data/pdffiles/XP-15K_REV-P.pdf.

Any ideas as to my next step(s). Thanks in advance for your help. 

Bill

[MosherIV]

Is D7 the right way round ?

[svracer6724]

Yes, all the diodes are oriented per the pcb.

[KeepItSimpleStupid]

Look at ground and (2) of the LM317
Look at ground and (3) of the LM317
Look at difference between (1) and (2) of the regulator - should be 1.2 V or so.
Look at difference  between (3) and (2) of the regulator - If ~0.6 assume D7 is in incorrectly.
Check the - end of C4 to ground. Should be -1.2V .

Make sure D7 is oriented correctly.  Remove D7.  One side is fine.  Check with diode function.   You can operate the PS with D7 removed.

[rstofer]

How did the Troubleshooting part of the manual work out.  All the voltages correct?

[svracer6724]

Look at ground and (2) of the LM317  = 21.58v
Look at ground and (3) of the LM317  = 22.14v
Look at difference between (1) and (2) of the regulator - should be 1.2 V or so.  = 0v
Look at difference  between (3) and (2) of the regulator - If ~0.6 assume D7 is in incorrectly.  = .39v
Check the - end of C4 to ground. Should be -1.2V . = 0v

I removed one side of D7 and checked with dmm, diode is good. Rechecked the above; all were similar except now the - end of C4 reads -1.36v

Voltage still reads ~ 21.5 with D7 removed and pot has no effect.

Still trying a few other things.

Thank you for responding.

[svracer6724]

Thank you for responding.

Per the troubleshooting guide I checked the following:
cathode of D1 reads 22.18v
AC voltage across P1 & P2 = 17.40v
AC voltage across P2 & P3 = 17.28v

Voltage at output of D1 = 22.25v
Voltage at output of D3 = 22.23v

Under the heading "Poor Regulation"
says to check AC ripple at the input of the regulator. Not sure how to do that.

[Andy Watson]

Voltage at output of D1 = 22.25v
Voltage at output of D3 = 22.23v

Is that not one and the same node?

No matter - it feels about right.
What is the voltage at the output of D2-D4 ?

Under the heading "Poor Regulation"
says to check AC ripple at the input of the regulator. Not sure how to do that.

If there was significant ripple present the voltage you measured at the cathode of D1 would probably be lower. 22.2V is approximately the peak voltage you would expect after the rectifiers - so there is little or no ripple present.

[KeepItSimpleStupid]

Remove a leg of R4 and short The ADJ terminal of the LM317 to ground.  The output should be 1.2V.
Still leave D7 out.

There are then a couple of possibilities at this point since a lot of stuff is eliminated.

1. Reduce the 150 ohm resistor to ~120 ohms. You can tack solder a 470 ohm resistor across the 150 ohm one.    See if 1.2 V appears between 1 and 2 of the regulator.  That voltage has to be there for the regulator to regulate.  This 1.2 V must be there at all regulated voltages. i.e. . as built.

2. The regulator needs a minimum load.

Usually, you can do either one.  Try like a 1K 1/2 W resistor for a load.

[jitter]

Could you please post a photo of your circuit?

You may think you have checked everything but I can tell you, Murphy gets you every time...

[singapol]

Could you please post a photo of your circuit?

You may think you have checked everything but I can tell you, Murphy gets you every time...

svracer6724,
                     it's not your fault but with elenco. This is not the usual way a LM317 is connected ie. output starts at 1.2V to whatever the transformer voltage. Elecon states 0-15V and for that it pumps a negative voltage into the adj. pin. the elenco schematic is wrong I think about the orientation of D5 and D6. they should be reversed  as in TI's LM317 datasheet. http://www.ti.com/lit/ds/slvs044x/slvs044x.pdf

See page 11 of the diode...cathode to ground as elenco's version will shunt negative voltage to ground. 

[KeepItSimpleStupid]

singapol:

The datasheet shows a slightly different design.    Elenco is supplying a -1.2 or (approximately 2 diode drops) reference to the ground of the regulator.  The OP/TS measured -1.36 V with D7 removed.

TI's design allows that (-1.36 V) voltage to be trimmed so you can get 0V out.

[svracer6724]

It's going to be a couple days before I can get back into the shop. I'll try the additional suggestions and post a photo as soon as I can. Thank you.

Bill

[svracer6724]

Could you please post a photo of your circuit?

You may think you have checked everything but I can tell you, Murphy gets you every time...

Here are the pics you asked for.  Let me know if you see something that I'm missing here.

Thank you

[svracer6724]

Remove a leg of R4 and short The ADJ terminal of the LM317 to ground.  The output should be 1.2V.
Still leave D7 out.

----The voltage is 1.253v

There are then a couple of possibilities at this point since a lot of stuff is eliminated.

1. Reduce the 150 ohm resistor to ~120 ohms. You can tack solder a 470 ohm resistor across the 150 ohm one.    See if 1.2 V appears between 1 and 2 of the regulator.  That voltage has to be there for the regulator to regulate.  This 1.2 V must be there at all regulated voltages. i.e. . as built.

-----I soldered the 470 ohm resistor as you instructed and the voltage read 1.258v  between legs 1 and 2.

2. The regulator needs a minimum load.

Usually, you can do either one.  Try like a 1K 1/2 W resistor for a load.

Not sure what to do at this point... maybe take up another hobby? lol

[jitter]

Thanks for the photos.

The solder joints look reasonably neat, but some have only partial wetting. I would suggest you reflow all those and try again.

[singapol]

Not sure what to do at this point... maybe take up another hobby? lol

Look at similar circuit :
http://www.eleccircuit.com/wp-content/uploads/2013/01/0-60-volt-dc-variable-power-supply-using-lm317-lm337.jpg

Compare with your elenco. I would remove C4 and see what happens.

[KeepItSimpleStupid]

Remove a leg of R4 and short The ADJ terminal of the LM317 to ground.  The output should be 1.2V.
Still leave D7 out.

leave the 470 ohm resistor in.  Put R4 back and remove the short.  It should work.

If it works, put D7 back.

[DimitriP]

I'm almost convinced that pins 2 & 3 are shorted together. Or at least the picture didn't  convince me they are not


 

[svracer6724]

OK, so I figured out what was wrong. When I first put this kit together, the nut holding the 2kOhm potentiometer seemed to be damaged and I had to really torque it on. In doing so it appears that I pulled one of the legs enough to cause a break in its trace.   I soldered a wire from the leg past the damage on the trace and it works as designed now. Don't know if its really usable now but I'm happy to have located the problem.

Thank you all for your help and guidance.

Bill

[KeepItSimpleStupid]

Good job!