power transformer rated at dual 18v at 2.5VA ....?

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JRE Productions

Well-known member
Joined
Dec 10, 2004
Messages
255
Location
Chicago, IL USA
I have a line on some transformers that are rated at:

115V

ta60E talema 50/60Hz

acco to EN 60742 70024K

2.5VA + 2.5VA

18V + 18V


How does the 2.5Va relate to real amperage to drive circuits? I looked around to figure out VA verses AMPs. I would be using these to drive a API style circuit. Looking for about 15 to 16vdc. So I am trying to determine how much current they will supply.


PRR stated in another of my posts 18x 1.4 for total available voltage, and that the diodes would use up some of the power. So I am pretty sure these will supply enough voltage to run the opamps at 16vdc or so. But how many opamps can I run!!!!


Thanks

Joe
 
These transformers are useful for specific projects - as they are really small - about 50mm (2 inches) in diameter.

But the power rating, quoted in watts (or VA) is simply the voltage X the current.

So each 18 volt winding can give 2.5 divided by 18 = 0.139 amps.

In common with small transformers the regulation factor is high (20%) - this means that the open circuit (no load) voltage will be 20% higher than at full load, so it will measure around 21.6 volts AC.

Remember that when full wave rectified and smoothed - the voltage will be X 1.414 so it could be as high as 30 volts. The DC current available is proportionally reduced - so only about 100mA will be available (from each winding). By the time a few losses are taken into consideration - even lighting a couple of LED's - there isn't much left!

So will this be enough?

If space isn't a consideration - certainly get a bigger transformer (15 or 30VA) - as these will cost only a little more......
 
Converting AC to DC with a rectifier feeding a capacitor filter causes heating in the secondary winding due to high peak to average currents when charging the capacitor at 120 Hz (2x line frequency assuming 60Hz input & full wave bridge rectification).

Designers usually derate the secondary current to approximately 60% with capacitor input filters. Add some losses in the transformer core itself, and I find 80ma available.

Of course, one could also look deeper at the copper wire size in the windings. It is about heating & core size is usually oversized with regard to gauss capacity. Average copper wire size in transformers is 700cm per ampere & some cheap designs go down to 500cm. If a 700cm wire design, I would guess it could be stretched to 100ma without overheating which means I agree with the other post.

The easiest test is to just try it & if transformer does fail, you simply screwed up.

Of course, just order higher VA capacity 'mini' Toroidal transformer(s) from Digi-Key in USA. Cheap at about $13 to $16 each.

My 2 cents
 
Ok, thanks. Yes these are really small. The price was right, but if it won't power at least 125-150ma, they really are not going to work for my application.

If anyone else was going to buy these, go ahead.

Joe
 
Good advice from amperex. Yes, these are wee tads. OTOH, it is now so common for transformers to be operated into cap input filters that some manufacturers rate them consevatively, and you can get away with some apparent abuse.
 
I recently used a smallish toroid, like the one you are talking about, and found them to actually hold up fairly well to abuse/overheating.

I used a talema, rated for around .5 amp(after rectification and smoothing) and drew nearly 1 amp from it continuously. this was strictly for a test I was doing but found that while the little thing overheated, it didn't waver from spec very much at all!

I expected a huge deviation actually but it kept working fairly well.

:thumb:
 
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