How to Make an Electronics Workbench Variac (variable autotransformer)
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- Опубліковано 9 лип 2024
- BG273 - Showing how a benchtop Variac (variable autotransformer) is made. These are super useful on the bench. This along with an isolation transformer and a dim bulb setup can keep you safe and be effective at working on vintage electronic equipment. Especially helpful when working on tube gear such as amplifiers, preamps, radios, stereos, etc.
I acknowledge the DC isn't pure DC and you could enhance this with a full wave rectifier and a CRC or similar circuit. Just showing what he has been building. - Наука та технологія
I can’t tell how many times over the last 40 years my variac setup has saved me when powering up a device on the bench for repair. It’s a must, especially with vintage EQ repairs. Thanks for the video!
Thank very informative Last year I inherited a high quality variac from the estate of a relative. Your video has given me the incentive to bring this old workhorse up to date 👍
Great video. As usual, I learned something. Keep up the good work!
thanks for sharing this with us Mark
Great demo.
Good video. For mounting the transformer, at the back, you could use Double Side Rubber Sealed Deep Groove Ball Bearings. Or similar.
Good video, man.
Very nice looking front panel layout and labels
. Needs to be powered up with a isolation transformer
Great video! Thanks for sharing. Stay Safe :-)
Mark, thanks for all the great and helpful videos.
Have you compared the SRPP Preamp stage with a Mu-Follower stage?
I've been running a 6SN7 Mu-Follower for the past decade,
The top tube is essentially a constant Current Source load for the plate of the lower tube.
So far none of my other projects have beat it for overall sound quality.
I want to build something similar to that and include a dim buld, my question is what gauge/style of wire should I use for all of the internal wiring?
i would like to build one of these, could you post the type variable transformer and digital readout gauges i should get ? thanks.
Nice setup. The DC output is only half wave rectified. A bridge would be a better choice for that and make certain the bridge can handle the supply current as well. But A Variac is really nice to have to SAFELY test transformers when you have no idea what they are...
I'm wondering how to power that DIN Rail AC meter with 5V since the ones I've seen have only AC input?
Did you connect the indicator lamp for the output after the current pickup? It's a classical error resulting in a minor current reading. BTW, I could use one too but for 230Vac, unfortunately shipment from the USA is prohibitively expensive...
Have you got any part numbers for the Variac and the Voltmeter/Ammeter. I'd need a 240V version. Just want to make sure I get one that is nice and compact.
The currrent indicated on the digital ammeter varied with the position of the variac. I suspect it was registering the current in the lamp. The idle current of the variac should be too small to register on that ammeter. It’s a nice build, though. I like it.
I wonder how accurate that panel meter measures current compared to a Kill-A-Watt meter?
I wired my old Variac so it maxed out at the line voltage. Never needed it go go higher. I have a NOS 2.2 KVA Powerstat that I want to turn into something like that because it has no switch, outlet or fuse-holder. Only screw terminals. It's large and weighs about 15 pounds thought so it would be hard to mount vertically.
Current is measured exactly the same way as an amp-clamp. The wire feeds through a core with a winding that (usually) gives 1mA on the output for each 1A through the supply wire. Should be at least as accurate as a Kill-a-Watt
i agree with a lot of comments, this is not such a good design, and not such a good video,
but this is such a nice community of interested people just pointing out flaws, not being rude :)
please change the camera angle next time, to avoid 11:14 hand in the frame, thanks
Can you provide info about how to get one of these units from "that guy from maybe the Raleigh area" that sells them at ham fests? I'll get one and probably do some of the mods that you did, plus I'll eliminate the DC output. By the way, what do you mean by "a CRC or similar circuit?"
I'd like to hear your reasons why "an isolation transformer an dim bulb setup can enhance safety and be effective at working on vintage equipment."
How about a completely ISOLATED power source? You could feed this Variac or a standard 1:1 unit from it, or vice-versa (IF you use a 1:1 unit).
Pro trick: you can watch movies on Flixzone. Me and my gf have been using it for watching loads of movies lately.
@Gordon Payton Definitely, I have been watching on Flixzone for years myself :D
sigh.. another project im going to put on the list of things to build..love the compactness of it.. got a dim bulb set up.. but still looking for a good affordable isolation transformer... but it has to be relatively cheap..lol.. thanks for my new project..the wife isnt so enthused though..lol...cheers
I'm looking for an isolation transformer too. For some reason they're expensive, even used. I snagged a NOS 2KVA Powerstat on ebay for $60 a few years back. But is has screw terminals so I need to make a similar contraption for it.
@@acoustic61 Look around for 2 qty roughly identical filament transformers or 24 volt control xfrmrs you wire secondary to secondary.
Search eBay for "medical isolation transformer" They'll come in a big case with IEC sockets, but the transformer can be easily pulled, or regular sockets retrofitted into the case. Following is my opinion only, read more about it to make your decision: I float the ground pins on the outputs to TOTALLY isolate the device from the utility side. Others say to leave it connected.
It'll be a cold day in hell before I ever pause a video to read a bunch of safety shit.
my brother!
Man...oh man.... Critics coming from all angles.....and you seem like such a nice fellow. Haha.
Anyway ... Nice to see the videos, wasn't expecting much from you with recent work changes BUT things sometimes work out.
Stay safe and healthy...
Great video! I have a Lutron 2000W AC dimmer. I would love to know how they can do this in such a small package.
An AC Dimmer is a different animal. A switching diode varies how much of the AC duty cycle is available.
Found this old stuff in a forgotten WWII bunker, did you?
Can you please tell me what meter are you using? Most of the ones I've found use 110 vac as the power and input.
Yes. All the ones I can find are source powered and stop working under about 30VAC.
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Wouldn't a full bridge rectifier instead of a single diode give a better VDC output?
I would think so...otherwise it isn't DC, but just half of the AC waveform.
Yes, for sure.
Yeah thats pretty dirty DC, as in 120Hz DC. Good for little more than a ripple counter input! Bridge rectifier and a few thousand uF would go a long way.
11:55 there should be no current draw its an open circuit after the current transformer... could be capacitive coupling but 40ma seems a lot at 60hz nice vid thanks subbed
I can understand the dim bulb idea for current limiting for a short curcuit, but you mentioned 'you or your heart' carryiing the load
That unit is 7.5A, so you probably have a high wattage bulb, even if you only had a 100W bulb, that would pass about 800 mA, which is a fatal current at anything over about 55V.
Because you have an isolating transformer, just touching the live leg yould make the variac unit 'float' so not as bad as touching direct mains, but if you touch hot and neutral, that's going to hurt.
For sure
Is there a link to the person who sells these?
No, just a guy I see at some hamfest. His name is Alan but don't know much more.
@@Blueglow Would you sell one?
@@MrMattkas I have another, slightly different. email me. you can find my email on website
Anyone know where to find the digital meter?
They are for sale for silly cheap all over ebay.
If someone can link to a specific meter it would be great. I'm also having trouble finding one. The ones I have found work off the supplied voltage, not a wall wart.
Really like you videos, Mark. You have taught me A LOT.
I can't find one with a range of less than 60VAC.
What I ended up doing was buying a digital meter, part # PZEM-061. There are videos on how to modify them to read 0 volts on youtube. I first tried it with PZEM-21 like the videos show, but it doesn't work with newer versions of those meters.
variac's need 2 fuses.... do the math imagine its set to 12 volts.. fused for full rating and the output is shorted so secondary will be at 10 times rating before the fuse blows the secondary is only rated same as primary so it burned since then 2 fuses one fir the pri. and 1 for the secondary are standard for me...its cheaper to replace fuses
Agree with multiple fuses. In my setup I use push-to-reset panel mount breakers and can switch between 1A and 3A on the output (3.5A variac.)
The USB power supply floating about inside is a major flaw in my view. You can buy small switch mode PSU modules on eBay for peanuts, they are designed for mounting inside equipment and come in a proper Aluminium chassis.
It’s interesting that you mentioned your intention to install an enclosure around this unit. I was ready to caution you (and more so your other viewers) to the dangers of leaving this exposed. Remember we are dealing with line voltage here and as such should always be enclosed to prevent things from falling into it potentially causing sparking and risk of fire. As techs we work with lethal voltages and exposed chassis all the time but for test equipment we need to be more prudent with how they are displayed. As an electrician I am always thinking a little more about these things.
Beautiful variac but that un-level receptacle on the front would really bug me. That would have kept me from purchasing this machine at all.
Without a case, it's a Scariac LOL..
My cat would try to get behind that panel and one of my grandsons may try to hide something there. I also have an open back setup and monitor the area when I am doing electronics.
Just a couple of thoughts on this.
1. That thin wires just soldered on to mains power and onto the 5VDC just "flapping around in the breeze" can't possibly be legal is it?
I know for sure that it would not be in my country. I would rather have preferred a mains plug on the backside where it was plugged into.
2. All terminals that is mains should be covered with heat shrink, not just some of them.
Other than that it looks ok, and a must for every bench.
Add a isolation transformer behind it and a dimbulb on top of it, and you good to go.
Btw, don't count on the dimbulb saving your heart if you get electrocuted, it will not!!!
Nothing about this is "legal" as in UL certified. Never claimed it to be a commercial unit, instead a home brew unit made by a local guy
@@Blueglow not legal as in "UL", but as in: It causes a fire and burns down your house, and insurance will not cover it .
@@geirendre In the 1970's thru 2005, I owned a shop. I made sure my technicians never left anything unattended, and certainly not running overnight. Every bench was independently powered and isolated, etc. and totally shut down when not in use. My insurance guy and landlord wanted it that way... so did I.
That is just about the crappiest DC output I've seen. Pointless.
agreed. would be easy to drop in a full wave rectifier and cap bank.
@@Blueglow That, and add a voltage indicator for the DC. I wouldn't be surprised to see that thing put out 200+ VDC
@@Blueglow the problem there would be you would need bleeder resistors across the cap bank and that would throw out the current meter as it would display the extra load also stored energy well you know,..... i suspect the guy who built it was trying to keep it as safe as possible while adding another option at the cost of a diode.
Also, the DC output is connected directly to the AC line. (As is the AC line.)
No ground isolation. Don’t use a circuit like this to isolate a DUT from sensitive test equipment.
Not even a full bridge rectifier? No attempt to smooth? Not even a single cap? Wow. This approach will damage everything on my bench.
I can see that a guy working only on tube gear could use this. If you do, make sure you know exactly why you’re using an isolation device.
You maybe need to be more careful about calling these units a “variac”.. Variac is a registered trademark of ISE inc..
Its like saying Kleenex :-)
No different to people referring to a circular saw as a 'Skilsaw' (or more often 'Skillsaw') even though most of the time they're using a Makita, DeWalt, Milwaukee or any brand other than Skil.
Yes.. it is one thing to refer to it by that name, in a non-commerial enviroment, but to brand the item with the name is another thing.. the owners of the kleenex or skilsaw brand name would not be happy if another business were selling tissues or electric saws using their brand name.. The variable transformer unit, the subject of the video, was clearly branded “variac” !!
@@andrewcourt5156 It's weird that you would go out of your way to protect the trademark of a company to which you, presumably, have no relationship. Variac has become a generic name, just like kleenex, etc.
Posting a brief message on You Tube is not really “really going out of my way”... it was just that breach of trademark was high on my mind at the time given that only a couple of days earlier we, my employer, had just bankrupted a rival business for using our trademark.