Using transformer in reverse? - Page 1
Forum section: electronics / beginners

author: BeBuLamar
author_rank: Super Contributor
timestamp: November 20, 2023, 09:53:24 pm
content: I guess the best way is just to test it. I have step down transformer from 120V to 12V. The secondary is rated at 3A. Can I feed an AC voltage of about 6V-10V generated by a function generator into the secondary and get higher voltage out of the primary. I don't intend to power anything with it but rather create a variable AC voltage around 100V to test a meter.

author: iMo
author_rank: Super Contributor
timestamp: November 20, 2023, 09:59:16 pm
content: Yes, that will work fine.The frequency should not be too high as it is a 60Hz transformer, I guess.Also do not overload your signal generator.

author: BeBuLamar
author_rank: Super Contributor
timestamp: November 21, 2023, 01:08:00 am
content: Well it doesn't work. The function generator can put out more than 3V with 600 ohms termination. With the transformer I don't know what kind of impedance it is but even with the primary (using as output) not connected the voltage on the function generator can only get to 100mV.

author: bdunham7
author_rank: Super Contributor
timestamp: November 21, 2023, 01:18:19 am
content: Quote from: BeBuLamar on November 21, 2023, 01:08:00 amThe transformer is too large and the magnetization current is more than your FG can handle. Your FG doesn't seem very robust at 3V @ 600R. You need an audio amplifier that can put out some current.

author: BeBuLamar
author_rank: Super Contributor
timestamp: November 21, 2023, 01:19:10 am
content: Good idea. I will use my amplifier.

author: Thunderer
author_rank: Regular Contributor
timestamp: November 21, 2023, 01:35:36 am
content: Put some 10R in series with the output of the amplifier, so you don't blow it.

author: u666sa
author_rank: Frequent Contributor
timestamp: November 21, 2023, 05:45:52 pm
content: You can do something cooler. Instead of AC, use pulse.. That way you get much higher voltage. Thousands of volts. Back when I was about 10 I used to take those big power transformers from older lamp radios, and disassemble them. I would change windings and make it really high voltage so that sparks come out.

author: BeBuLamar
author_rank: Super Contributor
timestamp: November 21, 2023, 07:34:14 pm
content: I need an AC voltage in sine wave and about 60Hz to test the VOM. Pulses doesn't work in this case.

author: Nusa
author_rank: Super Contributor
timestamp: November 21, 2023, 10:19:58 pm
content: Quote from: BeBuLamar on November 21, 2023, 07:34:14 pmUm, given your US location tag, don't you have 120VAC 60 Hz at the nearest wall outlet?

author: Thunderer
author_rank: Regular Contributor
timestamp: November 21, 2023, 11:50:20 pm
content: ... and a death wish too?

author: Gregg
author_rank: Super Contributor
timestamp: November 22, 2023, 01:19:32 am
content: Proper solution = VariacAlternately a low voltage transformer can be configured in buck mode to lower the voltage.

author: Eraldo
author_rank: Regular Contributor
timestamp: November 22, 2023, 09:39:25 am
content: Just cascade 2 similar transformers for safety, and connect one of them to mains (the secondary of both sides need to be connected together and just connect one of the primary to mains and you have isolated mains voltage)

author: Vovk_Z
author_rank: Super Contributor
timestamp: November 22, 2023, 09:56:16 am
content: Quote from: BeBuLamar on November 21, 2023, 01:08:00 amYou need much less powerful transformer. Large own magnetizing current of a large transformer eats all power of your functional generator.You need something in like 1-4 VA rated.

author: radiolistener
author_rank: Super Contributor
timestamp: November 22, 2023, 11:18:23 am
content: Quote from: BeBuLamar on November 20, 2023, 09:53:24 pmYes, you can.But most of all your singal generator will be unable to keep 12 V / 3 Amps, so voltage can drop down and as result transformer output voltage will be less.In order to keep 12V / 3 Amps sine, there is needs power amplifier. You're needs power amplifer with at least 12 * 3 = 36 Watt output.

author: BeBuLamar
author_rank: Super Contributor
timestamp: November 22, 2023, 12:52:57 pm
content: The secondary winding is rated at 3A max but I think it shouldn't draw a lot of current if the other winding isn't loaded. With a transformer using in a normally manner that is if I have a step up transformer of 12 to 120V and even if the transformer is rate for 3A it should draw very little current if the secondary isn't connected (or connected to an insignificant load like a meter).

author: radiolistener
author_rank: Super Contributor
timestamp: November 22, 2023, 01:10:24 pm
content: Quote from: BeBuLamar on November 22, 2023, 12:52:57 pmIt depends on frequency. Since secondary winding has low inductance its reactance will be not enough to keep low current. In other words secondary coil has low impedance and will consume high current even if primary coil is not loaded. And even if inductance will be high enough, it still will consume reactive current and there will be some amount of power loss due to thermal loss in wire...

author: bdunham7
author_rank: Super Contributor
timestamp: November 22, 2023, 03:00:35 pm
content: Quote from: BeBuLamar on November 22, 2023, 12:52:57 pmI just did a quick test for you with stuff in front of me. I have a 12V lighting transformer (Class 2 enclosed) intended to drive 1 50W halogen lamp, so about 4.2A. It's not very big, but has adequate iron. Energizing the primary at 120V/60Hz I have a current that barely nudges the needle on the variac meter (analog, average-responding, the first mark is 100mA) and the open secondary voltage is 14.5V. If I leave the primary open and put voltage into the secondary, at 14.5V the magnetizing current is 1.25A as read by the meter. At 12V, where the transformer would be at full load, I get about 500mA magnetizing current. The magnetizing current is not sinusoid and so the average-responding meter can only give a general indication as to the magnitude of the current. Still, this should tell you that your FG is not going to drive a transformer of that size even with no load.

author: radiolistener
author_rank: Super Contributor
timestamp: November 22, 2023, 03:15:04 pm
content: you can try to use some 100W power amplifier for audio in order to amplify signal generator output for secondary transformer coil. They usually have low output impedance < 4 Ω, so probably can drive your secondary coil

author: Vovk_Z
author_rank: Super Contributor
timestamp: November 22, 2023, 06:38:10 pm
content: Quote from: radiolistener on November 22, 2023, 03:15:04 pmOr he just needs a much smaller transformer like here:Printed-Circuit-Board transformers