control of AC voltage

This is the place for any magazine-related discussions that don't fit in any of the column discussion boards below.
Post Reply
DRCT
Posts: 13
Joined: Tue Dec 11, 2001 1:01 am
Location: U.S.
Contact:

control of AC voltage

Post by DRCT » Thu Jan 30, 2003 2:26 pm

Anyone know how to vary the voltage of an AC transformer using a 0-12vdc signal? I now use a 0-130VAC variac, but due to its location near high voltages I do not want to reach in and turn the knob. I also would also not like to use a motor control on the variac. I have heard of puting the load in series with the primary of a standard xfmr and applying the 0-12vdc signal to the secondary thus, varying the reactance of the xfmr. Any info?
Thanks

User avatar
Edd
Posts: 885
Joined: Sun Jan 27, 2002 1:01 am
Location: Dallas Tx
Contact:

Re: control of AC voltage

Post by Edd » Thu Jan 30, 2003 3:27 pm

DRCT:
I’m assuming this is to add to the safety factor of your not having to reach into a less than desirable location within a cabinet. Also you neglected the KVA rating of the Variac. Also whether the variac fed into an inductive load…not the best of a condition… to feed control circuitry using Triac or dual SCR control. Or the series saturable reactor /xformer technique would take a continual bit of DC power to keep it within the mid state range……all for a mere 10-20 % controlling variance as well as knocking the power factor.
I’d suggest the same thing that we used on Megawatt VOA xmitters in trimming in screen and bias voltages… the Variac had a ½ in shaft coupler and 2-3 feet lengths of phenolic and/or fiberglass rods and the knob went outside the cabinet, isolated, out of harms way. Now you might optimally have to realign the Variacs orientation, but would that be too simplistic of a solution ?….cheeep too !<p>73's de Edd
[email protected]attbi.com .......(Interstellar~~~~~Warp~~~~~Speed)
[email protected]...(Firewalled-Spam*Cookies*Crumbs)
;)

DRCT
Posts: 13
Joined: Tue Dec 11, 2001 1:01 am
Location: U.S.
Contact:

Re: control of AC voltage

Post by DRCT » Thu Jan 30, 2003 8:57 pm

Thanks Edd,
I appreciate your input. As for your questions: the variac runs at about 100VAC @ 1Amp. The load is resistive. I thought about the long shaft and knob but the variac and the cabinet it is inclosed in is several hundred feet away. I was thinking of couple of wires and a POT that could be run to the user at a control panel. I don't want the wires at the panel to carry 120VAC due to the danger to the user. 0-12VDC would be much safer.

toejam
Posts: 253
Joined: Wed Dec 05, 2001 1:01 am
Location: n.c.
Contact:

Re: control of AC voltage

Post by toejam » Fri Jan 31, 2003 8:25 am

if you are you just controlling 100 v0lts into a reistive load where is high voltage involved? if its somewhere in the box I would use an isolation transformer where the variac was and hook an extention cord from that transformer to the variac.

DRCT
Posts: 13
Joined: Tue Dec 11, 2001 1:01 am
Location: U.S.
Contact:

Re: control of AC voltage

Post by DRCT » Fri Jan 31, 2003 9:10 am

Thanks Toejam,
The variac controls the filament voltage (~6.3v) on a 50kv electrron gun. The variac needs to be located in the same cabinet as the HV and the control voltage at the user panel needs to be low ~12VDC.

User avatar
Edd
Posts: 885
Joined: Sun Jan 27, 2002 1:01 am
Location: Dallas Tx
Contact:

Re: control of AC voltage

Post by Edd » Mon Feb 03, 2003 6:51 pm

DRCT:
So much for the budget, still in the dark on the freq spectrum of the unit ? Could even be X-ray, Commercial Ultrasonic welding, or even a micro Shiva Star system ? On the in housing location, and as a separate shielded and multiple RFI filtering entity applicable digital control circuitry can still be swamped.
What is the deal on the fil voltages adj, (or % of variance if required)? As , naturally, our units fils were pre warmed via power relay sw prior to main B+ power up but the units almost ran continually with other day alternation. <p>73's de Edd
[email protected]........(Interstellar~~~Warp~~~~Speed)
[email protected]...(Firewalled-Spam*Cookies*Crumbs)

Post Reply

Who is online

Users browsing this forum: No registered users and 10 guests