Measuring voltage of capacitors

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zaph1

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I've built a variable voltage DC power supply circuit using an SCR, rectifier and 160v 33uF capacitor. I've got a 7-150v DC voltmeter hooked up to display the voltage. It always reads 160v, even if I use my multimeter. If I remove the capacitor, I get true voltage readings. How big of a load do I need on it to get the correct voltage to display?
 

How big a load depends upon the circuit design of the power supply. And since you haven't posted that, we can only guess, and your guess is as good as mine.
 

In a DPM you get the peak readings of the phase controlled rectifier .
You actually need to get average value to read... Can you put a bleeder resistor ?
 

You need a load sutch that the capacitor can discharge during the mains cycle. One other way would be to trigger the SCR, AFTER the peak voltage of the mains cycle but you can only get up to 1/5 of the DC power output this way. With old fashioned Low Voltage, High current supplies (say 0 -24 V @50A), the SCR bridge would be followed by a large choke before the reservoir capacitor to smooth the wave form.
Frank
 

Here is a link to the SCR circuit that I am using. It's inline before the rectifier.
**broken link removed**

If there's a better, smoother way to get variable voltage, please let me know. I'm trying to combine two old projects into one. I want to remove the need for a variac and replace it with something more modern.
 

That link, links to a site that says its selling SCR power controllers. SCRs only work with DC, so for them to work with AC, you need a bridge rectifier, which I cannot see on the photographs. I believe the boards they are selling are TRIAC controllers that work with AC.
Frank
 

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