transformer nameplate

[wakoko79]

Hi, I have a transformer, it reads:

24V-0-24V 3A

From what I know, it is a center tap transfromer and it will give a halfwave sine (with Vpeak = 24V) if you probe on ground and one 24V pin, right?
But what does 3A mean? 3A max (peak) current draw? Please enlighten me.. google failed me >.>

thanks!

 

[ericgibbs]

Hi, I have a transformer, it reads:

24V-0-24V 3A

From what I know, it is a center tap transfromer and it will give a halfwave sine (with Vpeak = 24V) if you probe on ground and one 24V pin, right?
But what does 3A mean? 3A max (peak) current draw? Please enlighten me.. google failed me >.>

thanks!

hi,
The 3A means it can supply a maximum current of 3Amp rms while maintaining 2*24V rms

If you attempt to draw more than 3Amps the 24V will fall to a lower value, also the transformer will start to overheat.

 

[wakoko79]

hi,
The 3A means it can supply a maximum current of 3Amp rms while maintaining 2*24V rms

If you attempt to draw more than 3Amps the 24V will fall to a lower value, also the transformer will start to overheat.

So the 24V there is in RMS?
So, if I connect it like this (to make a full wave rectifier):
View attachment 61934
would it be able to give me 6A rms?

I'm making a linear voltage regulator for my project, and I included in my specs that the voltage regulator will have the Vout=23Vdc and Iout(max) = 6A.
Will one transformer be enough? Or should I add more? I mean, I have a load expectation of about 4A (estimate).

UPDATE: I understand now since I already watched the youtube video that's on the sticky. I couldn't access youtube before since I'm at school. Thanks anyway mate. =)

 

[Reloadron]

would it be able to give me 6A rms?

No, as Eric covered, the maximum current you will get is 3 amps at 2*24 volts. So the max current for that transformer is 3 amps. If you are asking if you can parallel two identical transformers like the one you have the answer is yes. However, it would be wiser to just buy a transformer rated to do what you want to do.

Ron

 

[audioguru]

The transformer voltage is too high so the voltage regulator will waste a lot of power by getting extremely hot.
The transformer cannot supply anywhere near 6A.

The peak of the 24V RMS sinewave is 34V. The main filter capacitor is briefly charged each half-cycle with an extremely high current that causes a 1V drop in each rectifier. The power rating of the transformer is 3A x 24V= 72VA but the power comes from the 34V peak voltage so the actual maximum DC load current is 72VA/34V= 2.1ADC.

 

[kinarfi]

The transformer voltage is too high so the voltage regulator will waste a lot of power by getting extremely hot.
The transformer cannot supply anywhere near 6A.

The peak of the 24V RMS sinewave is 34V. The main filter capacitor is briefly charged each half-cycle with an extremely high current that causes a 1V drop in each rectifier. The power rating of the transformer is 3A x 24V= 72VA but the power comes from the 34V peak voltage so the actual maximum DC load current is 72VA/34V= 2.1ADC.

Interesting bit of info, I hadn't considered that the peak current allowed lowered when using peak voltage vs RMS, Kudos Audio.
Seldom run things at max, but good info for if I do.
Kinarfi

 

[Reloadron]

From the nice people at Hammond comes this useful information regarding transformers.

Ron

 

[cachehiker]

If the nameplate rating hasn't been exaggerated:

3A * 24V = 72VA from one winding that's supplying power 50% of the time.

3A * 24V = 72VA from the other winding that's supplying power the other 50% of the time.

72VA + 72VA = 144VA.

24VAC * √2 = 34 VDC.

144VA / 34 VDC = 4.2 ADC.

34 VDC minus 1.0 VDC drop through each diode equals 33 VDC at 4.2 ADC.

I've performed dozens of temperature tests to verify this although it won't hold true unless the transformer is on a bench in a 25°C ambient environment. The windings would probably reach something between 100°C and 140°C and the core would get plenty hot as well.

Once you put it in a box, you'll need to derate it, possibly by as much as 50% if it's a small box with little or no ventilation. You may need to derate it even more to prevent a cheap plastic box from softening or a metal box from burning one's fingers.

In our typical application (in a non-flame rated plastic enclosure but with thermal protection) we would pretty much be derating it to somewhere around the 2.1 ADC figure given by audioguru.

 

[cachehiker]

From the nice people at Hammond

I'm going to have to run those numbers through the standard transformer model we use (well, the one I use, nobody else around here seems to bother with the math) at some point. To me, they seem to have a 20% safety margin built in.

 

[Reloadron]

I agree with Eric in that it is a 3 Amp transformer as in 24 V @ 1.5 amp * 2.

Ron

 

[cachehiker]

What are the transformer's dimensions anyway?

That would pretty much decide it. I have several 56VA and a couple of 72VA transformers kicking around. Most of them are labeled 16V @ 3.5A CT or 20V @ 3.6A CT. I don't have any labeled XXV-0-XXV XA. It's unlikely that the core is made of a more exotic electrical steel than they are and you can only deliver so many Amp turns before it saturates. If its cross sectional area is the same, it's 72VA. If it's double that, it's 144VA.

 

[Sceadwian]

From what I know, it is a center tap transfromer and it will give a halfwave sine (with Vpeak = 24V) if you probe on ground and one 24V pin, right?

I hope this is a miswriting rather than a misunderstanding. It will not show a halfwave sine, it will show a full sine wave of 24V peak, and so will the opposite pin it will just be 180 degrees out of phase with the first pin relative to the center pin.

 

[audioguru]

The 24V-0-24V transformer is 48V RMS center-tapped.
The RMS voltage of each output is 24V. The peak voltage of each output is 34V.

 

[duffy]

If you are asking if you can parallel two identical transformers like the one you have the answer is yes.

NO - don't do this. Read up on "circulating currents" in transformers. Even a small imbalance in the voltage output of two transformers wired in parallel can cause a large "circulating current" between them. Look at page 6 here -
https://www.electro-tech-online.com/custompdfs/2012/03/7400DB0701.pdf

The safe way is to run them through bridges first, and parallel the DC side.

 

[Reloadron]

duffy, the link is a far cry from what I mentioned. The link specifically says: "Equal Impedances—Unequal Ratios— Same kVA". Additionally. that link deals with using a tie breaker in a sub station (on page 6) with 13.2 KV primaries for an AC distribution for something like a large manufacturing plant. I also went on to say it would be wiser to just buy a single transformer that would do the job. The transformers in the link example are not identical, they have unequal ratios due to the tap settings used in figure 3. Because of the configuration one transformer has a 480 VAC secondary and the other a 492 VAC secondary. In that case the tie breaker should not be used and the transformers are not identical.

My post was aimed at the transformer the OP has and in that situation while paralleling the secondaries to get more current is not the best solution I see no problem with doing it. The ideal solution, as I also pointed out would be to get a larger transformer suited for the application.

Ron

 

[duffy]

Yes, one is putting out .48 kv and the other .492 kv - the whole point is that no two transformers are ever going to be perfectly identical, and even a small difference like this results in a circulating current that is 21.7% of the full load current (page 7). I know this is for a big transformer, but small ones have the same problem.

And the other reason not to tell someone to put transformers in parallel is because they can get the phase wrong.

Face it - transformers in parallel is giving bad advice, and trying to obscure this fact is just making it WORSE.

 

[Reloadron]

OK, I didn't see it in this case as bad advice. However, I do agree with the point you are making. In my reply I did go on to say the best solution was to get the right transformer. I also should have pointed out the phasing and that the OP should measure the secondary voltage of both transformers. I wasn't trying to obscure anything. I also agree that using two rectifiers is the way to go as you stated.

Ron

 

[colin55]

a 3 Amp transformer as in 24 V @ 1.5 amp * 2.

No so.

 

[DerStrom8]

No so.

How about a bit of an explanation, rather than just leaving it hanging?

If you want to disagree with someone, usually it's a good idea to give a little proof. Know what I'm saying?

 

[colin55]

a 3 Amp transformer as in 24 V @ 1.5 amp * 2.
No so.

It's ok. The comment has been removed, once the posted realised he was wrong.