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
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
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
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
h_a said:
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.
Can you explain this? I think it's the opposite you see.h_a said:
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
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
h_a said:
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.
h_a said:
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 ...
Thanks a lot for your reply! Danke schön!
However I need 2x30V secondary and this seems to lead only to at least 160VA transformers (at my distributor). If I'm willing to buy at Schuro, I could get for about 40Euros a 65VA toroid with electrical and magnetical shielding - or for less the industrial version without (30VA rated then).
That's the same price as the EI I can get from my favoured distributor.
Well maybe I could fit the toroid into my enclosure, so I would save money for the external psu enclosure...dunno if this is clever in case of a phono preamp?
I have a few more questions:
Is high filtering with large capacitors as efficient as using a choke (regarding noise and ripple)?
Are the transformer losses always the same, regardless of the load? So are the losses for a 200VA always dissipated, also for only the 4W load?
Would be great, if somebody could help with these points!!
Thanks a lot!!
With kind regards, Hannes
However I need 2x30V secondary and this seems to lead only to at least 160VA transformers (at my distributor). If I'm willing to buy at Schuro, I could get for about 40Euros a 65VA toroid with electrical and magnetical shielding - or for less the industrial version without (30VA rated then).
That's the same price as the EI I can get from my favoured distributor.
Well maybe I could fit the toroid into my enclosure, so I would save money for the external psu enclosure...dunno if this is clever in case of a phono preamp?
I have a few more questions:
Is high filtering with large capacitors as efficient as using a choke (regarding noise and ripple)?
Are the transformer losses always the same, regardless of the load? So are the losses for a 200VA always dissipated, also for only the 4W load?
Would be great, if somebody could help with these points!!
Thanks a lot!!
With kind regards, Hannes
>I need 2x30V secondary
Is that "raw" or after electronic regulation? 2 x 24V will give you ~2 x 32 V after the rectifier. For that, you could e.g. take 2 transformers type "Hahn 12" from Oppermann - 2x12V 130mA, 33mmx33mmx27mm, EUR1.85 each.
>this seems to lead only to at least 160VA transformers
Yikes ... methinks you've got the wrong source, Luke ... 😀
>electrical and magnetical shielding
Nice to have shielding, not required though. Use input filter and a good regulator.
>Is high filtering with large capacitors as efficient as using a choke (regarding noise and ripple)?
What's your definition of efficient - Volts ripple per Euro spent? In other words, it depends. Look at the old valve circuits - back then, large capacity (+high voltage) electrolytics weren't around, so chokes were used to improve filtering. If you use a choke before the first cap, you also reduce current peaks a lot (but at low power, as I said, you could use a resistor and lots of capacity). We don't know your load requirements yet ...
>Are the transformer losses always the same, regardless of the load?
Obviously not the I^2 * R losses (copper), but you will have to re-magnetize the huge core at every cycle, so the iron losses will be there.
Is that "raw" or after electronic regulation? 2 x 24V will give you ~2 x 32 V after the rectifier. For that, you could e.g. take 2 transformers type "Hahn 12" from Oppermann - 2x12V 130mA, 33mmx33mmx27mm, EUR1.85 each.
>this seems to lead only to at least 160VA transformers
Yikes ... methinks you've got the wrong source, Luke ... 😀
>electrical and magnetical shielding
Nice to have shielding, not required though. Use input filter and a good regulator.
>Is high filtering with large capacitors as efficient as using a choke (regarding noise and ripple)?
What's your definition of efficient - Volts ripple per Euro spent? In other words, it depends. Look at the old valve circuits - back then, large capacity (+high voltage) electrolytics weren't around, so chokes were used to improve filtering. If you use a choke before the first cap, you also reduce current peaks a lot (but at low power, as I said, you could use a resistor and lots of capacity). We don't know your load requirements yet ...
>Are the transformer losses always the same, regardless of the load?
Obviously not the I^2 * R losses (copper), but you will have to re-magnetize the huge core at every cycle, so the iron losses will be there.
Hi,
as the toroid gets bigger it's efficiency improves. The regulation is a rough indicator of improving efficiency.
A big toroid will usually waste more than a small toroid, when just ticking over, but the power consumed in this loss is quite small.
200VA may be 93% efficient when on half load. That's equivalent to about 7VA wasted. I would guess that the zero load loss will be less than this (that's the temperature rise you can feel/measure). Do the same for a small toroid/EI and you find they run a LOT hotter indicating a higher proportion of losses/wasted heat, small EIs are especially bad for temp rise.
Don't worry about near zero load losses unless you are trying to save the planet.
A choke input filter reduces hum much more efficiently than a capacitor input filter.
BUT, at a cost. The expense of the choke and the very restricted range of output currents that allows the choke input to work correctly.
An RCRC filter is much better than an RC filter.
An RCLC filter is better again.
LC betters all these.
as the toroid gets bigger it's efficiency improves. The regulation is a rough indicator of improving efficiency.
A big toroid will usually waste more than a small toroid, when just ticking over, but the power consumed in this loss is quite small.
200VA may be 93% efficient when on half load. That's equivalent to about 7VA wasted. I would guess that the zero load loss will be less than this (that's the temperature rise you can feel/measure). Do the same for a small toroid/EI and you find they run a LOT hotter indicating a higher proportion of losses/wasted heat, small EIs are especially bad for temp rise.
Don't worry about near zero load losses unless you are trying to save the planet.
A choke input filter reduces hum much more efficiently than a capacitor input filter.
BUT, at a cost. The expense of the choke and the very restricted range of output currents that allows the choke input to work correctly.
An RCRC filter is much better than an RC filter.
An RCLC filter is better again.
LC betters all these.
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