Voltage doubling in simple rectifier circuit.

Thread Starter

RS welds

Joined Apr 2, 2021
6
I came to the forum looking for some help in solving a circuit problem that I am having. The basic circuit is taking 26v AC power supplied from an accessory circuit on a portable welder (not the main welding circuit) rectifying it with a bridge rectifier, and then using it to power a 24V DC motor.

My challenge is that when I add a capacitor in parallel after the rectifier, the voltage in the circuit goes up to 42V. I am using a Digital multimeter to check my voltages. Without the capacitor the voltmeter shows 24v after the rectifier. This seems to happen regardless of what size capacitors I hook up, (33 uF, 2000 uF) What I am suspecting is that I have some stray AC voltage that goes undetected on the AC side of the circuit.

After the it is rectified and capacitors are added it becomes measurable. Is this possible?

The 26V AC power supply shares a neutral wire with the 115V AC accessory circuit on the welder. This is making me wonder if I have some voltage somehow back feeding into my circuit.
 
Also if this is the case you won't notice any change just by adjusting the capacitance. You could add a cap of a smaller voltage rating and see if that drops your voltage. If so I'm leaning towards the rectifier being the issue.
 

Thread Starter

RS welds

Joined Apr 2, 2021
6
I am not that experienced with electronics. The bridge rectifier is a solid state 50A 1000V solid state rectifier with four wiring terminals on it, so I didn't connect any of the individual diodes myself. The terminals are marked +, - , and ~ and the fourth terminal is unmarked. My AC wires go to the ~ and the neutral goes to the unmarked terminal. The capacitors are connected in parallel between + and -.
 

Thread Starter

RS welds

Joined Apr 2, 2021
6
Without the capacitors in the circuit my digital multimeter shows 24VDC on the rectifier terminals. However when I hooked up a very old (vacuum tube) analog voltmeter it read around 40V with the needle waving. Connect the capacitors and the voltage on the analog meter and the digital meter sit right at 42V.
 

sghioto

Joined Dec 31, 2017
2,335
My challenge is that when I add a capacitor in parallel after the rectifier, the voltage in the circuit goes up to 42V
42 volts is about normal. The filtered dc output is appx. 1.414 x the AC voltage applied to the rectifier. With no load the output of the accessory could actually read around 29 volts AC.
 

Thread Starter

RS welds

Joined Apr 2, 2021
6
I think I am starting to grasp what is happening. 26VAC is the average voltage but the actual peak voltage in the circuit is higher than that. When it is rectified and a capacitor is added the capacitors smooth out the voltage at its peaks. My question now is how do I fix that?
I was creating a circuit to run a DC motor control which in turn runs a 24VDC motor (aluminum spool gun for welding) and a series of relays and solenoid valve. I don't want to be burning out components because the voltage is too high. I used 24V relays and a 24V solenoid valve. There is a 10A circuit breaker in the 26V AC supply. Any voltage regulation that I add needs to be able to handle about 10A.
 

crutschow

Joined Mar 14, 2008
27,002
I see two choices:
  1. Buy a switching regulator to reduce the voltage to 24Vdc ( a linear regulator would dissipate too much power).
  2. Buy a 24Vdc output power supply that runs from the main's voltage.
The first is cheaper but more complicated.

How much current does the motor with the solenoid and additional circuitry actually require?
Just because the 26Vac supply can handle 10A doesn't mean you need a 10Adc supply.
 

Thread Starter

RS welds

Joined Apr 2, 2021
6
In this project I have not done all the math that I should have in advance. I was building my circuit on my intuition of what should work based on some reasonable guesses. The largest load is the DC motor in the spool gun. I don't have any specifications on it from the manufacturer. Based on the size of the wires running to it I am thinking it will pull 4-5 amps. Their will be some loss in the motor control itself, the relays have a small current draw. I was able to test the AC current operating just the solenoid and it was 0.6 amps, that of course was with an input voltage of 38V (circuit voltage dropped with load). At 24V the current draw would have been less. That is a long explanation to say that I don't know the exact current draw in the circuit, don't have the specks on the motor to calculate it, but I am certain that it will be more than 1A or 2A that a cheap voltage regulator from Amazon can handle.
 
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