DC Power Supply

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!

#1 Buy a small, cheap DVM that runs off an internal battery.
#2 Is there a capacitor after the diode. If there is not, then using one will get the output voltage up nearer the peak of the input voltage. A high load current will require a physically large capacitor (which may defeat the "small size" you require. You need to do the maths, knowing the test current, you can compute a capacitor value. It will be load dependent of course.
There are safety issues here, it would be safer to use a transformer since it provides isolation from the mains, which a variac does not do.
If you post more details, like the current required for the tests, it would help.
It is possible that a small off line switch mode supply may be better, it could provide the isolation, as well as the required voltage, and short circuit protection,
cheers,
Richard