5 Volt Transformers. What are they for?

[BobW]

I've noticed that 5VAC is a common output voltage on power transformers in recent years. I never thought much about it, because 5V is a common DC supply voltage for logic circuits, so it sort of makes sense. But then I got thinking that a 5VAC RMS transformer output doesn't necessarily translate directly into a 5VDC power supply, or does it?

 

[schmitt trigger]

Well, if you consider diode drops and the regulator's overhead voltage, it could make sense.

 

[ronsimpson]

5Vac rms = 7 Volts peak.
4-Diodes in a bridge uses two diodes at a time = 0.65V x 2= 1.3V loss
5.7V DC. A LDO regulator could output 5.0V or 3.3V.

 

[Nigel Goodwin]

If you're seeing it as 'common', then you're probably misreading it?, 5V DC is EXTREMELY common now, and has been for a number of years, often with a USB socket for the output. 5V AC would be pretty rare.

 

[dknguyen]

There are lots of transformer voltages. You could ask that about most of them. Also, SMPS.

 

[tcmtech]

I have a few of them from old 4.8 volt NiCad cordless tools and toys. Seems ~5 VAC rectified plus a small resistor was considered close enough for very cheap nicd charging work.

For the 'fancy' chargers that had the charge in progress indicator LED a 6 VAC charger and a slightly higher value resistor in parallel with the LED worked fine too. Once the battery pack got up over ~6 VDC the forward voltage drop of the resistor would be too low to keep the LED lit and thusly you knew the battery was ready.

All other modern chargers I know of that are 5 volt are regulated DC output as others stated.

 

[BobW]

5V DC is EXTREMELY common now, and has been for a number of years, often with a USB socket for the output. 5V AC would be pretty rare.

Yes, I know. That's what I said; 5VDC is extremely common, but if you look at one of the big online distributors, you'll see that 5VAC is also a very common transformer secondary voltage. I'm not talking about wallwarts; I'm referring to plain old transformers intended to be built into equipment. For example, this Tamura 3FD-210.
https://www.digikey.com/product-detail/en/tamura/3FD-210/MT2096-ND/285632

My point is that it seems to be too much of a coincidence that 5VAC (as opposed to, say, 4.9 or 5.1) would be the ideal input voltage to a rectifier/regulator to get 5VDC out. I've built a couple of 5VDC supplies with these, because I happened to have a couple on hand, and I found that they were borderline when you take into account the diode drop. It definitely requires an LDO regulator, and a heck of lot of filter capacitance. Doesn't seem ideal to me.

 

[Nigel Goodwin]

Y

My point is that it seems to be too much of a coincidence that 5VAC (as opposed to, say, 4.9 or 5.1) would be the ideal input voltage to a rectifier/regulator to get 5VDC out.

It's not, it's considerably too low.

I've built a couple of 5VDC supplies with these, because I happened to have a couple on hand, and I found that they were borderline when you take into account the diode drop. It definitely requires an LDO regulator, and a heck of lot of filter capacitance. Doesn't seem ideal to me.

That's why they aren't used, I presumed he was talking about wallwarts, as most people call them 'transformers' - I've not seen 5V AC transformers (real ones ) used in equipment, but I suppose they could be used for feeding 3.3V powered logic circuits?.

As for the transformers been 'common', I've just checked on RS under 'PCB transformers', and out of 627 transformers only two are 5V AC.

 

[dknguyen]

It's not, it's considerably too low.



That's why they aren't used, I presumed he was talking about wallwarts, as most people call them 'transformers' - I've not seen 5V AC transformers (real ones ) used in equipment, but I suppose they could be used for feeding 3.3V powered logic circuits?.

As for the transformers been 'common', I've just checked on RS under 'PCB transformers', and out of 627 transformers only two are 5V AC.

I think I've seen 5V transformers in doorbells.

 

[Nigel Goodwin]

I think I've seen 5V transformers in doorbells.

The doorbell transformers I've seen certainly weren't 5V, but it's so long ago that I can't remember what they actually were? - but nothing was 5V back then

Just googled modern ones, seem to be 8V and 24V - it wasn't those back in the days I was thinking of.

 

[BobW]

As for the transformers been 'common', I've just checked on RS under 'PCB transformers', and out of 627 transformers only two are 5V AC.

Well, it's a common output voltage in the Tamura line, so there must be a reasonably common application for them. Your suggestion about using them for 3.3 VDC supplies makes sense. Maybe that's the answer.

I think I've seen 5V transformers in doorbells.

Possibly for some modern ones, but doorbells traditionally used 16VAC transformers (at least in North America) and have an air-gapped core so that they won't overheat if the output has a continuous short circuit.

 

[tcmtech]

Just googled modern ones, seem to be 8V and 24V - it wasn't those back in the days I was thinking of.

Around here they are or were 16 volts +- a lot.

 

[RODALCO]

Older type doorbell transformers often had a 3 - 5 - 8 Volts, centre tapped winding rated at 1 Amps. and often, high impedance and short circuit proof. To get a reliable 5 Volts dc supply you need at least a 6.3 Volts (marginal) but better use an 8 Volts TX.