# How to appropriately control the voltage

Discussion in 'The Projects Forum' started by bsayles100, Mar 14, 2016.

1. ### bsayles100 Thread Starter New Member

Mar 14, 2016
1
0
Hello,

My primary job function is to test and maintain MV and HV Electrical Power Systems Apperatus. So i have very little expertise with electronics. What I'm trying to achive is to design and build a very small veriable DC Suppply as portablity and durabilty are my chief concerns, This will be primarily used for powering and testing High Voltage breaker control circuits (Usually 125V DC).

Right now I am currently using a Veriac to supply veried AC voltage to a small seperate box houseing a simple 30A rated full wave bridge rectifier.

I have two main problems:

#1 - I would also like to add a digital voltmeter on the line side of the switch.

My problem is, the circuit is designed to have veried voltage and the voltmeter needs a constant 6-20V DC to supply power. is there something i can use to convert AC or DC voltage to a contsant lower output? I was thinking maybe a transister but im not sure.

#2 - Right now when i plug the rectifier into a 120V AC wall socket i get about 110V DC output. Is there anyway to electronicly bump my output voltage up to 125V DC without using some kind of bulky specialy made transformer?

Any help would be great, thanks!

2. ### wayneh Expert

Sep 9, 2010
12,382
3,238
Huh?! How did you get assigned a job that you are so badly unqualified for?

Yes. A transformer can reduce the AC voltage if needed, and then you use a rectifier to make DC, and then for any DC source you can use a voltage regulator to provide whatever constant voltage you desire.

Something is wrong. Fully rectified 120VAC should be ~170VDC.

Last edited: Mar 14, 2016
3. ### AnalogKid Distinguished Member

Aug 1, 2013
4,691
1,297
Depending on the rectifier configuration and the meter's averaging circuit, it could read the average value of a non-filtered full or half wave.

ak

4. ### crutschow Expert

Mar 14, 2008
13,496
3,373
Only if it's capacitively filtered.
Yes, a meter set to DC will likely measure the average value of a rectified sinewave.
For a full-wave unfiltered sinewave, the measured value would be .636 / .707 ≈ 0.9 of the RMS value or 108Vave for a 120Vrms input.

To power the meter, you can use a wall-wort with a DC output.

Sep 9, 2010
12,382
3,238
D'Oh!