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# how do i find the voltage of a transformer? Answered

i have 7 transformers, all shapes and sizes, and i dont know the voltages of anyof them. i need a 24v 2 amp transformer, how do i figure out the output voltages of these transformers?

1 is from a cable converter
1 is from a cd player/radio
1 is from a old cassette player (for ac input to run without batteries)
1 is from a new-ish rca (i think) alarm clock
3 i dont remember where they came from

can someone help?
PS: i have a multimeter

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## 8 Replies

Do you just need a transformer, or do you need a power supply which will deliver DC (possibly regulated DC) output? I'm going to assume transformer since that's what you asked...

IF you know which side is input and which side is output, and you know input was supposed to be 110VAC, determining voltage is easy -- plug it in and put the meter (in AC Volts mode) across the outputs. If you aren't sure, I would recommend putting a lower AC voltage -- say, about 6V, at limited current -- across one pair of leads and measuring the other; that will give you the ratio that the transformer changes the voltage by and which direction you need to connect it to step the voltage up or down. (Obviously, you want about a 5:1 reduction; 120 divided by 5 yields 24.)

As far as whether it can deliver enough current: The simplest test may be to connect the transformer outputs, with the voltmeter still connected across them, to the circuit you're trying to power. If the voltage dips more than slightly below the open-circuit voltage -- or the transformer heats up more than slightly -- this transformer can't deliver the needed current and you need to find another.

Recommended test setup for all of this, by the way, is to plug the transformer into a power strip that has its own switch AND its own circuit breaker or fuse. That lets you turn it on briefly for testing and off again if something seems wrong, without risking shocking yourself... and it gives you at least some hope that if the transformer melts down and shorts out, it'll just blow the power strip, not your house's fuses or breakers. _Ideally_, I would also recommend plugging the power strip into a Ground Fault Circuit Interrupter, as additional protection against shocking yourself.

(I built myself a short plug-in cord running to a box containing a GFCI, many years ago, as a generally useful safety device for times when I had to work on open circuits or plug outdoor devices into non-GFCI outlets.)

zack247 (author)2010-08-09

24v regulation could work too, but i need to make a power supply for my powerbook 190, since it didn't come with one when i got it.

frollard (author)2010-08-10

For a laptop I recommend just getting a proper (even if 3rd party) replacement. Laptops expect reasonably steady current and a transformer/wall wart won't do. Super cheap and (imo) reliable. http://www.dealextreme.com/search.dx/search.replacement%20powerbook

Re-design (author)2010-08-10

Oh - absolutely. Laptop batteries don't like DIY chargers at all. Belkin makes a universal charger that should work for you that works great on my HP laptop.

zack247 (author)2010-08-10

yeah. i saw a 16-24v adapter on dealextreme, but i dont have a paypal or visa. i dont know how to set a paypal up with my bank card.

orksecurity (author)2010-08-11

Paypal can be linked to a bank account, if you trust them with access to your account. (I mostly do, but I still haven't authorized them.) Or, if the bank card can function as a debit card, paypal will probably accept it as a credit card.

orksecurity (author)2010-08-11

That's a very good point. Lithium cells are a serious fire and explosion hazard if abused. I would _hope_ that the fact that you're providing the power to the PC and it's dealing with the charging process would protect you to some degree, but this may be one of those "if you have to ask, pay someone else to take responsibility for the mistakes" items.

orksecurity (author)2010-08-10

Agreed. Most laptops are better isolated from power glitches than most desktops (the battery charger provides some filtering and acts as a UPS), but any irregularities on the line which do get coupled through can disrupt computation, and not necessarily in ways that you will immediately notice. Paying someone else to make the mistakes on units that don't get shipped to you is probably worth the cost.