Any disadvantages using a 200VA trafo for a 4 W circuit?

[h_a]

Hi!

I want to use an EI-transformer for my power supply, unfortunately I can only get a 200VA variant with 2x30V secondary. My circuit dissipates only about 4W, so I would need a hint wether this ridiculous oversizing wpuld do any harm? Of course I want to improve things and not make them worse.

With kind regards, Hannes

[richie00boy]

No harm at all.

[pinkmouse]

I used a 300VA 55-0-55 for the HT in a valve preamp. Is that overkill enough?

[N-Channel]

So I guess a 2MW Power Supply is enough to power a flashlight?

[h_a]

Thanks for your replies!

Well I thought there could be an advantage of using a transformer with a rating, say only 25% above the needed wattage.

Something like delivering the power less spiky?

I want to use this EI, since it should be better against noise coming from the mains.

With kind regards, Hannes

[wine&dine]

Originally posted by h_a
I want to use this EI, since it should be better against noise coming from the mains.

Why would that be so? I'd rather think a small transformer with its high stray inductivity should filter out more noise ...

I'd rather implement a decent mains filter if noise were an issue. A dead PC power supply can be the donor for all required components.

Look for a PSU with "passive PFC" = a large (~10 mH) input choke, ideally one with two separate windings (common mode). Together with the smaller EMI choke and maybe uprated X capacitors this will keep a lot of junk out.

[peranders]

Originally posted by h_a
I want to use this EI, since it should be better against noise coming from the mains.

Can you explain this? I think it's the opposite you see.

The electrical bill, have you thought of that? 200 VA EI => 5-10 W in losses.

For 4 W load pick a 10 VA instead, or 2 pcs. 5 VA, 2 x 15 VAC

[h_a]

Can you explain this?

In short: not really. I read here in this forum a couple of posts, regarding transformers for small loads. Well, the idea I got was that toroids are more efficient, therefore smaller in size for a certain VA rating and have a far smaller stray field than EIs.

On the other hand toroids are worse regarding noise due to their large bandwidth as a consequence of their large capacitance between the windings. This capacitance is far smaller for EIs and therefore they should be better to avoid noise from the mains. In addition they should be softer and do not deliver the power so spiky.

Shielding between the toroid's windings improves in regards of capacitance but not to the same degree.

Of course I haven't seen any measurements, which is a pity.

Since I will build an external power supply I thought I would go for an EI, which I can get only with 200VA.

Is the loss increasing with increasing VA rating although the load is so small?

Where do you buy your transformers?

With kind regards, Hannes

[wine&dine]

Originally posted by h_a

Where do you buy your transformers?

Cheap (surplus, new old stock): www.oppermann-electronic.de
Cheap-Middle (new + surplus), smaller selection: www.pollin.de
Expensive/new: www.rsonline.de

Put in a filter to do the filtering, and pick a nice small transformer.

[wine&dine]

Originally posted by h_a
[... EIs] should be softer and do not deliver the power so spiky.

Softer = higher internal source resistance. At such low power levels, you might just as well add some external resistance yourself. This implicitly creates an extra RC filter BTW.

You can also improve things a bit by using the good old choke on the secondary side (LC filter). Be sure to understand the behaviour (or simulate it) - you don't want to create unnecessary resonant circuits there ...