# Need help with repairing basic DC power supply

#### Dave Boothney

Joined Jan 18, 2019
6
Hi,

So I have a construction company, and recently I purchased a welder that I have wanted for some time (They don't make it any more but it's one of the best 120V welders ever made as far as arc purity is concerned.)

I notice a common problem with this welder is slow wire feed speed. I took the gearbox apart and there is no problem there, there are no restrictions in the guide liner.

However, when I put a volt meter on the motor, at the minimum wire speed setting it is only delivering .8 volts, and at maximum setting it only put out 2 volts. So I disconnected the motor and used a laboratory power supply, the motor is a 12V motor and draws .4 amps under heavy load. (Almost stall)

I have found some variable 12VDC power supplies on Ebay, but I want to put the original dial from the welder on the potentioemeter, I can remove the pot from the power supply and put it in the pot that's on the welder so the welder looks stock, but I am unsure if the pots are the right size for the dials that are on the welder.

Basically, I would like to diagnose the DC power supply. It has a transformer, the diodes it goes into put out 32 volts DC. From there my knowledge of variable DC power supplies ends. What could cause a 12VDC power supply to lose ten volts? Enclosed is a picture, I would be happy to answer any questions. Also on the circuit board are the electronics to control the voltage and heat of the arc itself, it uses a solid state relay.

#### Attachments

• 160.2 KB Views: 15
• 147.5 KB Views: 14
• 179.3 KB Views: 15
• 177.4 KB Views: 15

Joined Jul 18, 2013
25,977
.4 seems quite low at stall, does it perform OK on the 12v, rpm wise ?
Can you post a pic of the motor?
Max.

#### cpgos

Joined Nov 26, 2018
21
Could you replace the wire feed motor with a variable resistor, with a maximum value of ~100Ohms. Monitor the voltage and current and then gradually increase the current to establish whether or not the power supply can cope with a heavier load.

Best regards.

#### Dave Boothney

Joined Jan 18, 2019
6
.4 seems quite low at stall, does it perform OK on the 12v, rpm wise ?
Can you post a pic of the motor?
Max.
Sure thing. Sorry about the late reply I've been very busy. Yes the RPM is quite fast at 12 volts...

#### Attachments

• 138 KB Views: 7

#### Dave Boothney

Joined Jan 18, 2019
6
Could you replace the wire feed motor with a variable resistor, with a maximum value of ~100Ohms. Monitor the voltage and current and then gradually increase the current to establish whether or not the power supply can cope with a heavier load.

Best regards.
I'm sorry I don't have that...

Joined Jul 18, 2013
25,977
As a last resort you could pick up one of the PWM (<$5.00) controllers on ebay and even if you have to purchase a matching pot, not a lot to lose. Max. Thread Starter #### Dave Boothney Joined Jan 18, 2019 6 As a last resort you could pick up one of the PWM (<$5.00) controllers on ebay and even if you have to purchase a matching pot, not a lot to lose.
Max.
I think you are absolutely correct on that one. It's a great welder for what I paid plus 5 dollars more to fix the feed problem. Thanks again and cheers!

#### cpgos

Joined Nov 26, 2018
21
I'm sorry I don't have that...
Ok. The intention was to confirm independently whether or not the power supply is working. Some car bulbs may be an alternative way of connecting a suitable load.

Best regards

#### Dave Boothney

Joined Jan 18, 2019
6
Ok. The intention was to confirm independently whether or not the power supply is working. Some car bulbs may be an alternative way of connecting a suitable load.

Best regards
I understand, I think... But the .8-2 volt output was measured while the motor was connected (But no load on the motor, isn't the motor considered a load?)

#### cpgos

Joined Nov 26, 2018
21
My understanding from your original post is that with the motor connected the power supply outputs only 0.8volts, see below. It seems to me then that the first step is to check the power supply output with no load and with a known good load. The result of this check will give a an indication of whether the problem is with the power supply or somewhere else.

If the motor is electrically driven through some kind of amplifier the voltage measured at the motor may be very different from the power supply voltage.

However, when I put a volt meter on the motor, at the minimum wire speed setting it is only delivering .8 volts, and at maximum setting it only put out 2 volts. So I disconnected the motor and used a laboratory power supply, the motor is a 12V motor and draws .4 amps under heavy load. (Almost stall)
Best regards.