Transformer \ regulator question

[Agent24]

If I had a transformer which provides say, 12v output at 500 mA and I connect a 5v regulator, eg: 7805, since the output voltage is less than half the input, I can get about twice the current, 1 Amp at 5v - right?

[mariushm]

Re: Transformer \ regulator question

No, you'd get 5v at about 450-480 mA. Linear regulators chop the voltage down, dissipating the difference aka making heat.

7805 is particularly bad because it also has poor efficiency.

Switching regulators could do more than 500mA, but also themselves are about 70-90% efficient.

[mariushm]

Re: Transformer \ regulator question

Also, pay attention to what transformer you have and how that 12v 500mA is defined

If it's 12v AC rms then 12v is the equivalent of DC voltage after rectification, ignoring the losses in the diodes.

If it's not AC rms then it's 12v peak to peak, meaning your actual dc voltage will be much smaller after rectifying the voltage and the current output will be different - the maximum power the transformer outputs remains the same VA = volts x A

But you have to keep in mind the bridge rectifier (or the four diodes) that convert the AC to DC - at any moment two of those diodes conduct, so you have a loss of 2 x 0.3-0.7v when rectifying the voltage.

See this page for more about ac / dc : http://www.kpsec.freeuk.com/acdc.htm

[Agent24]

Re: Transformer \ regulator question

OK, but in general, if I used a switchmode regulator, I would be able to get a higher current out than in, along with the drop in voltage?

Is there an easy way to calculate how much would be provided for a certain input or how much would be drawn for a certain load?

[budm]

Re: Transformer \ regulator question

12V transformer with 500mA AC, the max current from the Fullwave rectifier circuit will be about 300mA DC continuous from the transformer.

[mariushm]

Re: Transformer \ regulator question

I've edited my previous comment a bit, because it was somewhat incorrect.

You can check the datasheets of various switching regulators to see how well they can do the conversion.

For example check this one: http://www.digikey.com/product-detai...-5.0-ND/363828

and the datasheet is here: https://cdn.badcaps-static.com/pdfs/...8809810ac8.pdf

The datasheet says it can do up to 92% efficiency, but at page 6 there's a small graph efficiency vs input voltage, which tells you that it's kind of 85% efficient with 12v input.
Keeping in mind that you won't get perfect parts and exact resistors and so on, assume it's 80% efficient.

Then, 12v x 0.5 A = 6 watts ... 80% efficiency means 6w x 0.8 = 4.8 watts ... so your switching regulator might do 4.8w / 5v = 0.96A

[Agent24]

Re: Transformer \ regulator question

Hmm, that's even worse going by that transformer datasheet...

12v @ 0.5 A I chose just for an example, but I don't know if this is going to work. Maybe I need a bigger transformer

I have actually a 16v @ 1.5A transformer and I wanted to get 12v at about 1.65 - 1.7 Amps.

[Th3_uN1Qu3]

Re: Transformer \ regulator question

I have actually a 16v @ 1.5A transformer and I wanted to get 12v at about 1.65 - 1.7 Amps.

That should work with a switching reg, the difference is small. It won't work with a linear tho.

[Agent24]

Re: Transformer \ regulator question

I guess I'll just have to try it out and see...