2 transformers using the same AC input possible

[amateur_24]

Hi
I have a problem with a psu am designing. I have forgotten about the drop out voltage the LM338 would have. Hence I don't have enough juice to gurrentee 24v out. It's a dual output psu am building. So I was thinking wiring my centre tap in series so I get the 48v. Then using a smaller transformer to feed the side which provides 3v,5v,9 and 12v via rotorary switch. Is it possible to feed all this off one IEC connector (c20) i.e. the ones a pc would use. The transformer sizes are 200va and 100va

 

[duffy]

If you are talking about the "power inlet" connector, it can almost certainly handle the few amps you are asking. Unless it's like an IEC 320-C8 and you are running at 110V, because that's only rated for 2.5A and you could pull 3A. But you are probably talking about an IEC 320-C14 (commonly used on computers) and these are good for 15A. Also, I surmise by your curious spelling of the word "center" that you are probably working with 220Vac, so it will be more like 1.5A max for you.

I'm more concerned about this 'center tap in series' thing - what do you mean?

 

[amateur_24]

Yes I'm in the uk our supply is 50hz@230v.
It has two sets of outputs ie centre tapped transformer. I thought if you wire them in series you get double the output voltage i.e. in my case I get 24*2 = 48v. My logic behind this design is because of the low voltages on the other side. It's linear regulator so if the input is much much higher then the ouput then the reg would get extremely hot indeed. So I would use a 3A plug top fuse right? I was imformed of this here:
https://electronics.stackexchange.c...t-in-series-to-get-twice-the-voltage-possible

 

[duffy]

Afraid I'm still not quite following this - do you mean to wire one side of the small secondary to the center tap on the larger unit and use a separate bridge and so forth? Could you provide a drawing? Even a crude sketch in PC Paint will do. Electro Tech Online has a wonderful image upload capability direct from your hard drive in the "Go Advanced" options.

A 3A fuse might be a little big (you will only be pulling 1.5A) and depending on how you have this wired it may be sensible to try and protect the smaller transformer with a considerably smaller fuse.

 

[duffy]

I'm familiar with the problem of minimizing the input-output voltage differential with linear regulator to reduce the amount of voltage they have to "burn off". Fortunately, a full-wave rectified transformer always puts out a higher DC voltage than the AC voltage it is rated for. Your rectified output will probably be more like 28 or even 30V.

Before you get too much further in this other design, try hooking up just that much (bridge and caps). Wire it up on the bench, use jumpers or whatever, make a mess - but give it some big-ass filter caps. Then take a DC voltage measurement. I think you will be pleasantly surprised to find it is more than you need, and even the loaded voltage will be high enough to compensate for the regulator drop.

 

[audioguru]

An LM338 reduces its max output current if its input-output voltage exceeds 10V.
A graph on its datasheet shows the max typical current limited to less than 1A and many of them will be lower.

 

[Diver300]

You can certainly run several transformers from one main input connector. In fact the C20 plug at 16 Amps is serious overkill and you could use a C14 which is rated at 10 Amps and is far more common.

A lot of audio amplifiers have two transformers run from one power inlet.

If you want 24 V DC out from a 24 V AC input, that should work because the peak voltage is 1.41 x 24 V = 34 V. From that you will lose the regulator drop out voltage, diode loss, reduction due to the smoothing capacitors discharging and reduction in the transformer voltage due to the current being very peaky. However, you have 10 V so you should be OK.

However, if you put the two 24 V windings in series, there will be over 60 V available, so the voltage drop will be around 36 V. If you want 5 A, you need to dissipate over 150 W of power in the heat sink. I think that would be a poor way of doing it.

 

[amateur_24]

Ah ok sorry guys. Maybe I haven't fully undestood my ac theory from class. Damn it there's a high possibility I'll fail this exam! To be honest I had it hooked up to a extension lead so I could turn it from the other side of the room just in case it blow up lol. I did notice the voltage was something like 30v. I thought that wouldn't be enough given I wanted 5a

 

[tcmtech]

5 amps at 30 volts will not be a problem for the 200 va transformer.

 

[audioguru]

5 amps at 30 volts will not be a problem for the 200 va transformer.

True if the output is 5A AC.
But if the 30V output is rectified to a peak of 42.4V and filtered then the transformer is overloaded with 212VA. A 200VA transformer will get too hot after a long time.

 

[duffy]

It's a 24V transformer, AG - the peak is 33.9V.