Low output voltage problem with power supply

[fazeka]

Hi all,

Scratching my head with this problem I am having with this power supply. It's Peter's Green Pre PSU PCB and I am using it with a Triad VPT36-690 power transformer. Output of the Triad unloaded is about 21VAC.

From what I researched with this PSU, the 2.7k rail resistors gives 18VDC and 2.2k gives 15VDC. I wanted something on order of +/-~16VDC so I figured I'd start with 2.43k resistors. However, I only get +/-12VDC on the output...

What am I missing here?

Cheers,
Chris

 

[scott2000]

Is this the power supply???

 

[fazeka]

Yes sir, but I am not running 48V so that part of the circuit/PCB is not populated.

 

[scott2000]

Is there a load connected to the supply?????

 

[fazeka]

Do I need a load to check voltage? Without load, wouldn't the output be HIGHER? Sorry if this is a stupid question, I'm still learning.

Is there a load connected to the supply?????

 

[scott2000]

I'm not familiar with how this regulation works  in this regard so I'm trying to better understand by getting a picture of what your set up is.... and to add more info for someone else who may find the extra info useful in the troubleshooting.

I know current is important in dealing with voltage drop but, with regulation, I'm not clear yet how that works......

Maybe the regulator needs some kind of load on it for the R values chosen??

Hopefully someone can step in to explain this or explain what you may be experiencing....

 

[JohnRoberts]

Hi all,

Scratching my head with this problem I am having with this power supply. It's Peter's Green Pre PSU PCB and I am using it with a Triad VPT36-690 power transformer. Output of the Triad unloaded is about 21VAC.

From what I researched with this PSU, the 2.7k rail resistors gives 18VDC and 2.2k gives 15VDC. I wanted something on order of +/-~16VDC so I figured I'd start with 2.43k resistors. However, I only get +/-12VDC on the output...

What am I missing here?

Cheers,
Chris

The first step is to measure voltages to help understand what is wrong.

1) confirm there is adequate unregulated voltage to support the desired regulated output. (DC at C1 and C2)
2) confirm linear operation based on measured voltage at VREG com and VREG output.
3) If com voltage correct, and adequate unregulated V, check resistor values (R1,R2,R3).

Look at a data sheet for 317 to see details.

JR

 

[mjrippe]

You likely need a higher voltage secondary on your power transformer.  The schematic shows a 15-0-15, or 30v secondary.  You are measuring 21VAC on yours.  21v x 1.4 is about 29VDC after the rectifier or 14.5+ and 14.5-  The regulator needs the input to be about 3v higher than the output in order to regulate.  I bet if you put a load on it, the voltage would drop below 12v.

 

[fazeka]

Hi,

Thanks for the replies JR and mjrippe. 

mjrippe, that's what I was thinking too. I found it odd that a 18-0-18/36VAC transformer (I believe that's the specification) is putting out 21-0-21 or 42VAC. Wouldn't a 18-0-18/36VAC transformer show numbers closer to that than 21-0-21/42VAC? Do you think it would hurt the rectifier and/or regulators if I wired the xfmr secondary in series (i.e., 42V)? Just to see if I would get ~16-17VDC on the output?

Cheers,
Chris

 

[PRR]

> I found it odd that a 18-0-18/36VAC transformer ....is putting out 21-0-21

Why odd?

EVERYTHING sags. Transformers are rated at Full Load. Small transformer sags 10%-20%. Your 21V no-load is a 17% un-sag from 18V full-load. Working exactly as expected.

JR's advice works for me. If the water is weak at the faucet, check it where it comes into the house. In fact since you don't have to drill pipes to connect meters, or mop the floor, go ahead and read ALL voltages. Then think about what they mean.

 

[mjrippe]

Hi,

Thanks for the replies JR and mjrippe. 

mjrippe, that's what I was thinking too. I found it odd that a 18-0-18/36VAC transformer (I believe that's the specification) is putting out 21-0-21 or 42VAC. Wouldn't a 18-0-18/36VAC transformer show numbers closer to that than 21-0-21/42VAC? Do you think it would hurt the rectifier and/or regulators if I wired the xfmr secondary in series (i.e., 42V)? Just to see if I would get ~16-17VDC on the output?

Cheers,
Chris

Ok, that is different.  You said 21VAC out of the transformer earlier, not 21VAC per side.  If you have 42AVC end to end then there is something else going on.  As PRR said, measure voltages (AC *and* DC) everywhere.

 

[fazeka]

OK, all. Got an update, somewhat overdue (was out on vacation).

I basically had my secondary wired in parallel. Should have been wired in series. :

Everything much better now. ;D  I measure +17.38 and -17.28 VDC on the output, using 2k61 resistors for R1 and R5. Assuming 1/10 volt difference between the rails is negligible?