Variac and IsoTransformer

Matt M · Posted: Oct Tue 27, 2009 12:38 am

As a new member trying to setup a workbench, I have some questions about a variac and iso-transformer that I acquired. The variac is General Radio type 200B 115v 60hz 1A. The iso-transformer is Powertec with Primary 120v 60hz and Secondary 120v 100va. Are these sufficient to use for workbench radio restorations? I am attaching a photo of them (I hope).

[img]http://s808.photobucket.com/albums/zz2/MattM61/?action=view&current=CIMG7522.jpg[/img]

Dale Saukerson · Posted: Oct Tue 27, 2009 12:44 am

_________________
Dale Saukerson.
Diverted by scratch building

Matt M · Posted: Oct Tue 27, 2009 1:03 am

Thanks Dale.

Lou deGonzague · Posted: Oct Tue 27, 2009 1:43 am

The isolation transformer looks like one I bought from a Jukebox website. Is it the one rated at 100 watts. Put the variac first, iso second. When the power is applied there is a pretty good spike sometimes that would be tough on the small isolation transformer. Next thing would be to have an amp meter of 1 or 2 amps to monitor the current. A Kill-a-Watt is a good unit, check out ebay for one.

Johnnysan · Posted: Oct Tue 27, 2009 2:07 am

The variac is way underpowered at 1 amp. Sure, most radios draw less than 1 amp, but if you have a problem, current will far exceed that rating. You probably need a 3 to 5 amp variac.
The isolation transformer is the same story; it's rated at about .8 amp. If you wanted to make an AC/DC radio safe it would be fine, but not for repair work. You need one rated at about 5 amps.
Sorry for the bad news.

Matt M · Posted: Oct Tue 27, 2009 2:12 am

Based on archived threads, I had an idea these were not big enough (or safe enough) for my needs. I will be looking for another unit. Thanks guys for the advice.

Tom Herman · Posted: Oct Tue 27, 2009 3:24 pm

Chris108 · Posted: Nov Wed 04, 2009 1:23 am

Hi Matt,

The choice of variac and isolation transformer really depends on what you want to fix with them.

If you want to fix large radios or "hot chassis" TV sets, what you have will not do. But for five-tube AC/DC radios, or "three way" tube portables, you'll probably be okay until you get a chance to upgrade. Small radios do not typically draw more than 35-40 watts, which should be well within the capacity of the components you mention.

Most commercial transformers and variacs can tolerate significant overloading for a few minutes, so even if there's a problem with something you're working on, you'll have time to pull the plug. The variac and maybe the isolation transformer will buzz noticably when overloaded, so if you start hearing unusual noises, you need to stop and find out what's "making issues."

Two suggestions, tho. First, you need to fuse that set-up. At a minimum, a 1.5 amp, 250-volt, 3AG (normal blow) fuse and holder should go in line with the output of the isolation transformer--assuming the isolation transformer comes after the variac.

Second, it would be nice to have an AC voltmeter across the output. The dial scales on variacs aren't all that precise, and a meter will let you see more easily when there's an overload. The pointer won't go up when you turn the knob! There are zillions of old 0-150 VAC panel meters kicking around, and putting one to work on your isolation transformer will leave your DMM or VOM free for troubleshooting.

Lou deGonzague · Posted: Nov Wed 04, 2009 2:08 am

I agree with Chris. The GenRad is a high quality unit and could stand a mild overload. Most radios we work on are below 120 watts. Also most the time we are only using these items for short term testing like on power up. Don't go giving this set up the heave ho so quickly. Put a 1.5 A fuse on the output and if you can find a 0 to 1 amp meter or use the AC amp section of a DMM to monitor the current.

gmcjetpilot · Member Joined: 22 Jul 2009 Posts: 433

Agree you will be sorry getting the bare min rating. You want at least 250 watts or 2.5 amps at least. I have some 1.2 amp Iso Tans and they work. However a big Transformer or Variac will have less sag and better isolation (I think).

Matt M · Posted: Nov Wed 04, 2009 1:26 pm

I appreciate all of the knowledgeable feedback. Until I can afford to upgrade (maybe Santa will be kind this year), I am going to try to use the setup that I have for a couple of AA5s. I can do my larger radios after I upgrade the equipment.
To start using the variac and iso transformer I have (in pics above), I need to understand how to hook them together. Each unit has a male power cord attached. I am guessing that I will need to attach the output of the variac to the primary of the transformer. Then take the male output cord of the transformer and change that to a female end. Also splice a fuse into the transformer output power cord. Does this sound correct?

K1PJR · Member Joined: 29 Aug 2009 Posts: 32 Location: Milford, CT

Matt,

The primary of the transformer plugs into your wall socket and the variac plugs into the secondary of the transformer. The fuse should be connected to either primary lead but make sure it is attached to the hot side of your line cord. You would have to know the amp ratings for the transformer in order to use the correct fuse rating. I'm assuming the amp rating is higher for the variac in which case you wouldn't have to fuse it. If the variac is less thn the transformer I would fuse the variac also.

Phil

Matt M · Posted: Nov Thu 05, 2009 5:14 pm

Phil,
Thanks for the reply. Now I am even more confused. These units were previously owned and used by a deceased hobbyist and I am having difficulty understanding how he used them. The transformer (rated at .8a) has a male power plug connected to the secondary. The primary is open but the contacts have solder residue on them.....something was connected to the primary at some point. The variac (rated 1a) also has a male plug on it. If both have male plugs then it appears they were not used together....which is what I am trying to accomplish. So would I remove the power plug from the transformer secondary and move it to the primary? And then connect the variac to the secondary?

Thanks,
Matt

K1PJR · Member Joined: 29 Aug 2009 Posts: 32 Location: Milford, CT

Matt,

Yes, reconnect the plugs. One to the input (primary) of the transformer. You can wire the secondary of the ISO directly to the input of the variac or use a female plug at the secondary of the ISO and a male plug on the input of the variac and a female at the output of the variac which would go to the radio.

As someone mentioned you could also attach an ammeter and voltmeter to measure current and voltage. I can send you a schematic of my set up but I don't know how to post it here. If you can send me your email to my in box I can scan and email a schematic. Your call.

Phil

Matt M · Posted: Nov Thu 05, 2009 5:29 pm

Phil,

PM sent

Don_S · Member Joined: 02 Dec 2007 Posts: 900

Most designs of variable isolation transformers I have seen show the variable transformer connected to the wall outlet and the output of the variable transformer fed into the primary of the isolation transformer.

Chris108 · Posted: Nov Fri 06, 2009 12:36 am

Hi Matt,

Although your variac and isolation transformer will certainly work regardless of which one is connected to the line first, there was a logical reason why it was suggested that you connect the variac to the line first and the isolation transformer second.

Whichever transformer comes first, has to supply a certain amount of current to make up the losses in the second transformer. While these losses are not very high, a 100-VA, 120-volt isolation transformer does not handle as much power as a 1-amp variac. Consequently, if you connect the isolation transformer first and the variac second, there will be less capacity (of the isolation transformer) remaining for radios. By putting the variac first, all the capacity of the isolation transformer is available.
_________________
Chris

"Engineering without numbers is just opinion."

Lou deGonzague · Posted: Nov Fri 06, 2009 12:42 am

Chris has it right, put the variac first.

K1PJR · Member Joined: 29 Aug 2009 Posts: 32 Location: Milford, CT

I have a Tripp-Lite ISO-250 (2.2amp) and a Superior Electric 5amp variac. I plug the variac into the ISO because that is how it is shown in R. McWorthers book.

I checked the house voltage vs. the output at the variac and its the same, 120v. I don't have a technical background but maybe it works in my case due to the ratings on the ISO and variac.

Phil

Matt M · Posted: Nov Fri 06, 2009 1:55 am

Based on the numerous threads I have read on this subject matter, my intention is to place the variac first and the iso transformer second. The question I have is how to accomplish this. Both units have male power cords on them, indicating they were not previously used in conjunction with each other.