transformer -> rectifier -> 7805 -> switching a relay

[sbirkenstock]

Hi everybody,

I bought a little 230V to 6V transformer.
This one with 2.8 VA:
https://www.conrad.de/de/p/block-vb-2-8 ... wE%3AG%3As

I am rectifying it with 4 * 1N4007 and I receive 13,5V.
Actually the transfomer puts out 10,5V AC. Not 6V. Guess because there is no load???

I use a 50uF cap to stabilize the voltage after the rectifier.

Then I hook a 7805 on.
It produces 5V DC just perfect.
Again I put a 50uF cap after the 7805.
The input voltage on the 7805 drops actually to 12,2V.

Then I attach a relay, 65 ohm over the coil.
The relay works, switches nicely.
But the voltage on the input of the 7805 drops to 8,15V DC and the output of the 7805 drops to 4,1V DC.
The transformer still delivers 10V AC. Shouldn´t that be enough that the 7805 still produces 5V DC?

Any hints on this?

I would like to use the transformer for up to 4 relays simultaneously.

Best regards,

Stephan

[sluckey]

I use a 50uF cap to stabilize the voltage after the rectifier.

Replace with a much larger cap, 1000µF or 2200µF. Any better?

[sbirkenstock]

Thanks sluckey!

You are right, I only had 220uF caps, but this did the job already.
Actually, before and after the 7805 was drastically improving the values.
With 220uF on the input and 220uF on the output of the 7805, even with 2 relays the voltage stays at 4.95V. Good enough!
I do not understand why really, I thought the cap is helping only during "power up" / switching the relay on.
Maybe because the input of the 7805 still has ripple?

Another question for me remains, why the 6v transformer delivers 10.5v.
I also tried another transformer that is supposed to be 7.5v, this one delivers about 11.5v.

The voltage goes slightly down if the 2 relays are on, but not much. Like 10v instead of 10.5v.

Does it possibly mean it will go down to 6v at maximum load?

[R.G.]

Welcome to the wonderful world of specifying transformers.

When a transformer maker says "6Vac @ 1A", as an example, they mean literally that. When fed >exactly< the specified input voltage (220vac for you) and loaded with >exactly< the maximum load, the transformer will produce the specified voltage +/- the tolerance on the spec sheet. Lower load? You get a higher output voltage.

Put another way, they design the transformer so that it will supply at least the specified voltage at the specified load. Since transformer coils have resistance, this means designing the no-load voltage so that the output voltage will sag down to the spec under the maximum load. Tiny transformers, like the 2.8VA one you're talking about are the very worst of the worst cases in this respect.

The filter cap after rectifying the output voltage is critical, and in both directions. Rectifier diodes charge up the first filter cap to nearly the peak of the secondary sine wave voltage. So at no load, the output cap will charge up to 1.414 times the >open circuit, no load< secondary voltage. Load it and this drops off a lot, because the transformer winding resistance lets the voltage sag. It's a quirk of rectifiers that the diodes charge the caps to the peak, but the caps have to support the load until the next rising part of the sine wave comes along. So a first filter cap that's too small will let the voltage sag between charging peaks, with lots of ripple. You need enough first-capacitor capacitance to keep the ripple under control. But a huge first capacitor makes the current from the rectifier come in short peaks, so again, the voltage can sag more because the transformer resistance sags the voltage with the high current pulses.

There are ways to calculate this all out; post back if you want to hear more about it.