Variac for the Hobbyist - Page 1
Forum section: electronics / testgear

author: Bryan
author_rank: Frequent Contributor
timestamp: September 07, 2015, 11:09:03 am
content: Looking around Ebay for a Variac for the bench. Anybody have any suggestions. Seen these for relatively cheap, not sure how good they are though. Quality, safety ?http://www.ebay.com/itm/Variac-Variable-AC-Power-Transformer-with-Meter-0-130-VAC-Max-20A-TDGC-2KM-/121581613434?hash=item1c4ed4317a

author: alexanderbrevig
author_rank: Frequent Contributor
timestamp: September 07, 2015, 11:29:12 am
content: Just chiming in as I'm actually looking for a Variac myself. I guess I need to find some with 220V input, or use a step down before going to Variac...Looking forward to suggestions for Bryan (and myself)

author: alsetalokin4017
author_rank: Super Contributor
timestamp: September 07, 2015, 11:37:13 am
content: I've used a Chinese one that looked just like that and did not have any complaints. But .... well, you know how it is with Chinese products, YMMV and all that.My own Variac is a genuine General Radio 1kW unit with two fuses (line and output) but no meter, that I paid 50 dollars for at the legendary Mike Quinn's in the San Francisco Bay area 15 years ago. An equivalent unit from a reasonable seller would probably cost five or six times as much today.

author: Stray Electron
author_rank: Super Contributor
timestamp: September 07, 2015, 12:15:37 pm
content: They're very useful IMO. The most obvious use to to bring power back up slowly on old electronics in an attempt to reform the capacitors without blowing them. You can also use them to test electronics that uses a lower voltage AC source when you don't have the original power supply.I find a lot of high quality GR (General Radio) brand Variacs in military surplus equipment. You just have to make sure that they're not designed for 400 Hz.The main concern with a variac is that you have to understand that it is an auto transformer. That means it only has one winding instead of two so there is no isolation between the primary winding (and the main AC line) and the output. Therefore if something goes wrong or you touch a live terminal, you could be connected directly to the AC lineregardless of what you have the output set at. A lot of old TVs and such connected one side of the AC line to the chassis. Situations like that can be a hazard with an auto transformer and you need to use an isolation transformer with that type of equipment. But you can still use a variac and the isolation transformer together and get the benefit of both. You just need to treat the output of a variac as you would a live AC line and you'll be fine.

author: Marc M.
author_rank: Regular Contributor
timestamp: September 07, 2015, 12:53:52 pm
content: Something to consider when shopping for these sorts of items is isolation. An autotransformer has one side common to input and output therefore there is no isolation and one side of you DUT will be connected directly to Mains. Connect your earth grounded DRO ground to the hot chassis and .... arc/flash. So certainly from the repair side of things (where I came from many years ago), an a/t is normally used in conjunction with an isolation transformer so your DUT is fully isolated from mains. BTW, for those playing along, order doesn't matter: Mains->Iso->A/T->DUT or Mains->A/T->Iso->DUT. But of course, an isolation transformer is an additional cost if you don't own one. If this is the case, you may want to look at AC Power Supplies which usually (not always) have isolation transformers built in. Here in the states the B&K Precision 1655 and the Sencore PR57 were the 2 models you'd find in just about every TV repair shop back in the 70's and 80's. Only 3 or 4 amp output but how much do you really need? I was servicing huge 27" consoles that were still gas-state from the tuner to the tube and rarely did I have to break out the 10a Staco to power them. If you watch Fleabay you can pick up 1655's and PR57's in the $75 to $150 range and you get a 3-4a autotransformer, isolation transformer, and a large volt / amp meter. You also get uA leakage meter as an extra bonus (although the probes are often missing). Sencore also has an updated digital version of the PR57 which is the PR570, but they are still bringing $250+.Another point to keep in mind is the output voltage range. Many a/t have at least 1 additional tap that will allow an extra 10-20 volts on the output. The ability to run at higher voltages is useful for all sorts of things making it a very desirable feature. Some include the ability to switch between the taps, others are hard wired to one or the other. I think my good old Staco is switchable between 120 and 140 vac out while the B&K was hard wired to the 140vac tap. Personally, I always install a switch and keep them in 120v to reduce the chances of accidentally powering something at full over-voltage.If you do require higher currents then it's down to separate components. The a/t you list is the typical Chinese ones all over e-bay. I have no direct experience with any of these but they will follow the same rules as everything else. I'm sure it will work OK when you get it, at least in the nominal parts of it's curve. If you try to push it hard you will likely start discovering weaknesses in it's design. And of course you'll never find any replacement parts (with a/t's that normally means the wiper brush).Apologies in advance for any duplicated material covered in replies that happened while typing this for the past 2 hours (constant interruptions).

author: Marc M.
author_rank: Regular Contributor
timestamp: September 07, 2015, 01:01:29 pm
content: Quote from: Shock on September 07, 2015, 12:34:30 pmNo, you can't do that because the actual output voltage is a function of both the wiper position and thecurrentrunning thru it.

author: Kjelt
author_rank: Super Contributor
timestamp: September 07, 2015, 01:04:09 pm
content: There are two types: isolated and non-isolated (auto transformer), make sure you know what you want and label it appropriately.

author: Bryan
author_rank: Frequent Contributor
timestamp: September 08, 2015, 04:40:54 am
content: Quote from: Marc M. on September 07, 2015, 12:53:52 pmThanks for the info, much appreciated. Looks like the B&K and the Sencor are still commanding a pretty penny on Ebay, more like 3-4 hundred, but could be slim pickings right now.. Does make sense to pay more for isolation protection.

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: September 08, 2015, 04:51:58 pm
content: As others have posted you can get a separate isolation transformer. There are a lot of those on eBay too. I just sniped a 1000VA TrippLite for $100 delivered. Note that many of these require a modification, as they often have the secondary winding grounded. See this video for a decent explanation:

author: LabSpokane
author_rank: Super Contributor
timestamp: September 08, 2015, 06:15:27 pm
content: http://www.testequipmentdepot.com/bk-precision/power-supplies/1655a.htm?ref=gbase&gclid=CJGr8uqE6McCFciFfgodicIP9wI got one of these for just over $100 off eBay. It's great since it's a combo isolation transformer and variac.Variac's are great for making sure that things really do respond properly to fluctuations in line voltage before fielding them. Not everything does what it says on the spec sheet.

author: rsjsouza
author_rank: Super Contributor
timestamp: September 08, 2015, 09:08:43 pm
content: I gotthis Mastech variaca while ago. It is well built and works very well.

author: MarkL
author_rank: Unknown
timestamp: September 09, 2015, 12:20:05 am
content: Quote from: Bryan on September 07, 2015, 11:09:03 amLooks like the same model I bought a couple years ago that caught on fire:https://www.eevblog.com/forum/testgear/2000va-variac-(variable-ac-transformer)/msg306146/#msg306146Spend your money on something safer and of better quality.

author: mtdoc
author_rank: Super Contributor
timestamp: September 09, 2015, 03:58:10 am
content: Quote from: MarkL on September 09, 2015, 12:20:05 amI've got one of those. Works fine. No fires yet...For my very occasional need, I can't justify spending much more. But I'll definitely keep the fire extinguisher close by....

author: Electro Fan
author_rank: Super Contributor
timestamp: September 09, 2015, 04:17:49 am
content: If you want to discuss Variacs, call this guy in California:David Riddle800-544-3746818-994-2449Among other things, he is Mr. Variac. He will answer questions about the history (General Radio), present, and future of Variacs; he builds/refurbishes them with great attention to detail. Very knowledgeable and a good guy. He might not have the least expensive Variac but if he can't find/refurb/build the Variac you want you will at least come away with a clearer idea of what's what with Variacs.Variac Trademark (from Wikipedia)From 1934 to 2002, Variac was a U.S. trademark of General Radio for a variable autotransformer intended to conveniently vary the output voltage for a steady AC input voltage. In 2004, Instrument Service Equipment applied for and obtained the Variac trademark for the same type of product. This word has become generic for hand-variable autotransformers in general.

author: tomlut
author_rank: Regular Contributor
timestamp: September 12, 2015, 12:14:39 pm
content: Closer to home, Gammatron in South Australia supplied me a very nice 10A 240V single phase panel mount variac for a project in 2009. Still going strong. They don't have any listed on their website at the momenthttp://www.gammatron.com.aubut it might be worth giving them a call.Their variac product catalogue is attached. Don't expect eBay pricing though.