Variac upgrades and testing
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- Опубліковано 29 вер 2024
- Adding a digital volt meter to a variac, adding a missing ground strap, and insulation testing of the chassis powder coat.
The display I installed is available from MPJA.com
www.mpja.com/Pa...
Great post! Your video production is a pleasure to watch. I hope you do many more. Sub'ed, thumbs up and shared.
Cheers, Martin.
The ground straps may have been left off just to isolate the secondary winding. Making the variac an isolated transformer on the output.
I'm sorry, but that comment is dangerously incorrect. This standard type of variac does not provide any isolation, it merely varies the voltage. Disconnecting the ground does not "float" the output in any way, and it reduces the safety of the device under test.
I plugged a receptacle ground tester into mine and under 60v it's says the ground is fine but anything higher it says it has a bad ground. I even scratched of the powder coat off around the ground lug & it still does it, any idea why?
THANKS
You can usually mod the two-wire devices to three wire and run the power to the module from the line. This means good brightness across the board and voltage measurement down to a few volts. As you have an autotransformer variac, there's no issue taking the supply to the module from the primary and the sense voltage from the secondary.
+Smudger Dave That's a good suggestion. I didn't take the little DMM apart, but yes with a bit of probing it's probably not hard to figure out the power vs. sense terminals.
I would like to have fitted a meter with both voltage and current displays, but that just wasn't going to fit without some real modification to the case.
+Corgitronics Search for Ebay item 141386550382 or "AC 80-300V 0-50A voltmeter ammeter" As I recall, the black wire is live/hot and the red is neutral/cold. Remove the surface mount diode behind the two upstanding power resistors. Connect a 1N4007 cathode to the pad away from the big film capacitor. The anode of the 1N4007 becomes the sense wire. Mine is good down to about 20V - below that, it is inaccurate. Also, you might consider ten turns through the current shunt to bring the sensitivity up to 0-5A
@@smudgerdave1141 ok, I know it’s been 6 years, hopefully you’re still around UA-cam. Is there any chance you could point me to the volt/ammeter you’re talking about here? I’m wanting to do this mod as you described.
You deserve to have a lot more subscribers. This channel is really going to grow, I'm sure.
About variacs by the way, there is no galvanic isolation between primary and secondary, correct? So if I want to use an isolation transformer together with a variac to have galvanic isolation between my device under test and mains, should I hook up the variac to mains and the isolation transformer to the variac, or the other way around with the isolation transformer going to mains? Or does it not really matter?
Thank you for the compliments.
I am not any expert on the use of isolation transformers... I don't have one, as I am not convinced that it would provide any safety for the tests that I have been running.
* I run all of the power on my bench through a GFCI (RCD) circuit. This _should_ limit the amount of current leakage though other devices like USB (or me) to ground. *
By default, a typical variac will _not_ provide any galvanic isolation, as the grounds are usually coupled. For info on isolation transformers, check out videos by Toddfun.com or Connor Wolf.
As for the use of an Isolation transformer, I can't provide any advice. If you really know what you're doing, then you'll know how and when to use an isolation transformer. I've seen a lot of bad advice, and a lot of contradictory advice about isolation transformers.
My solution was to put a little aside each month and purchase a differential probe (Pintech).
If you don't know the proper use of an isolation transformer, then you should discuss your situation with a properly qualified electrician (in your country/jurisdiction).
Messing with line voltage (mains) is nothing to be taken lightly, it's easy to get killed even with just the residual power in the capacitors of a PC PSU ( there was the unfortunate death of a teenager in my city just last year).
As you can see I have gone to great lengths to remain at distance from mains power. I run the tests in a hands-off situation... not getting any closer than a properly rated standard line/mains switch. I even use insulated spade connectors and shielded banana plugs (that many despise) just to avoid any issues if a connection comes loose.
** I've been hit by line shocks, it wasn't any fun 40+ years ago... I'm sure it hasn't gotten any better. ** PLEASE KEEP YOUR DISTANCE FROM MAINS/LINE POWER **
If you are a still a student, I wish you all the best on a curriculum in Electrical Engineering. It has been my passion, if not always my vocation.
This makes me wonder why the ship with such an unusable meter and position marked to begin with.
Thanks for the video. It solves the problem.
I used the cheap meter that takes 8-12vdc to run, that way you get all the way to zero volts
I just bought a similar unit so thanks.
Very helpful!
Made in China! woo-who! They advertise these unit now (or some of them) as Variac Isolation transformers, I guess that's why.
Yeah, this does not provide any isolation from mains.
How can you modify a variac transformer so it could have an input of
That would require a very different variac, and probably a step-up transformer to make it all work. I don't think a regular variac could, or should, be modified.
I just ordered a variac like this,if I mail it to you,would you be willing to install a digital meter ,and check the ground and so forth,I would gladly compensate you ,thanks
I know it’s weird of me to offer, but if you need help I would be glad to offer assistance
THANKS
Hi, I wanted to ask about this grounding to the chassis. I bought this Chinese variac the SC-5M. I has onolt one input receptacle on the unit. I looked on the inside like you have and I noticed the the ground from the AC wire that you plug into your wall is connected directly to the input ground socket, which if I understand this correctly would mean that the input receptacle is fully grounded, but there doesn't seem to have any additional wire grounding the chassis. Why would they sell a variac in the US or Canada if there was a real danger of being electrocuted if you touch the chassis? Now my question is that want to use this variac to bring down my line voltage in my house to 117 volts instead of 125 volts to run my vintage Fender Blackface Bandmaster amp with the line voltage it was originally in the 60's to run at. My power transformer does get pretty hot running on 123volt range. Is there any danger hooking up an amp like this to one of these variacs? My amp does have a 3 prong plug on it for proper grounding. Any advice would be appreciated. Thanks Michael Newell from Niagara Falls, Ontario, Canada
Depending on how the outlet is attached to the chassis it could provided a mediocre ground connection, but probably not. The usual approach would be to find a screw inside the chassis and remove any paint from around the hole and connect a ground wire with a serrated lock washer.
The reason you were able to buy it is the importer did not check the product, or it was bought on ebay/aliexpress,etc. I believce this type of product should be UL Listed, it might have the mark but that is certainly easy enough to forge and that often happens with this sort of product. If it has the UL mark, you can check the listing and complain to the seller if it's fake: www.thayerlightinginc.com/is-it-ul-certified/
I have just dug out the variac from this video, and it does not have a UL mark (there are various certifications from UL), and I bought it from a reputable supplier in the U.S. that sells lots of equipment like this to high schools and colleges.
@@Corgitronics, well after reading that post below where the guy says that he did exactly what you just said and with the meter was indicating that there was not a good ground at higher voltages. I'm just going to have to pay the $30.00 for shipping this boat anchor back to the guy in Vancouver for a refund.
@@michel333100 That's the safest option!