Efficient pre-regulator

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I'm starting to think about my next project where I want to power an AVR MCU and some LEDs efficiently from the 120V AC mains. I'm hoping to get above 90% efficiency.

I could use a transformer to get the line voltage down to something in the 10V-20V range, but transformers are big, heavy, expensive. Is there a way to pre-regulate the AC line voltage with high efficiency and with no transformer? I only need about 50mA.

Thanks for all inputs.

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There ARE off-line switchers. I think that On Semi and Linear Tech are two of several. One of the big challenges is that there is no line isolation. Thus, everything on the load side should be insulated from human contact.

Jim

 

Until Black Lives Matter, we do not have "All Lives Matter"!

 

 

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You also have all these tiny little 5 volt wall wart switchers that seem to be everywhere these days.

Tom Pappano
Tulsa, Oklahoma

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I should have mentioned that there ARE low-power off-line converters with load-side transformer isolation. Because of the higher frequency, the isolation transformer can be much smaller. They are usually not highly regulated by virtue of having to sense something on the primary side to detect changes on the load side. But, if you really are talking pre-regulator, that should not be a big deal.

Jim

 

Until Black Lives Matter, we do not have "All Lives Matter"!

 

 

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You could try a series capacitor. Think of it as a low-loss series resistor. The impedance is 1/wC, where w=2.pi.f or about 375. To drop 100V at 50mA, then, you would need about 1.3 microfarads. This would need to be a polyester or other non-polar capacitor rated in the 300V region, so it could be nearly as big as a transformer. Also, being a quasi-constant-current device, if the load became disconnected you'd see the full supply voltage. Also, you can't half-wave rectify this circuit - it must be full wave or AC. For simple applications like a night light using a couple of white LEDs at 5mA, though, a series capacitor is a good option.

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I would go for power-supply like this http://www.dealextreme.com/details.dx/sku.47417 plug-sized USB power supply.

You get insulation from mains and a stable 5V 500mA power source.

Markus

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I don't see where the dealextreme.com product claims to be isolated; it does say 5V @ 1A, though.

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devils4ever wrote:
Is there a way to pre-regulate the AC line voltage with high efficiency and with no transformer? I only need about 50mA.
With either tight or 10% regulation, no transformer (not isolated), and datasheet shows a 75% efficiency goal:
http://www.powerint.com/en/products/linkswitch-family/linkswitch-tn
Roughly (very) 50% efficiency with this AC-to-DC switcher:
http://www.powerconversion.com/assets/dch2_1246592741.pdf

"Dare to be naïve." - Buckminster Fuller

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I can't use wall worts and such. The power supply needs to be contained within one enclosure. These are all good suggestions. I'll start looking into them.

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tlucas wrote:
I don't see where the dealextreme.com product claims to be isolated; it does say 5V @ 1A, though.

Very true, this is not said explicitly. But I would be very astonished if it is not. And I'd expect the manufacturer to be sued by the family of the unfortunate person electrocuted by the earbuds of his ipod being charged by the device.

However, it looks like the OP needs the circuit internally to his device. One option would be to buy one of these, disassemble it and just use the circuit without the mechanics. There are even smaller varieties like this one: http://www.dealextreme.com/details.dx/sku.26753

Be careful with non-insulated circuits. You may end up at the receiving end of the lawsuit...

Markus

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More simple than the Power Integrations high frequency switchers; these switch at 100/120 Hz:
http://www.supertex.com/pdf/misc/inductorless__ics_SG.pdf
The SR10 datasheet states up to 75% efficiency:
http://www.supertex.com/pdf/datasheets/SR10.pdf

"Dare to be naïve." - Buckminster Fuller

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Quote:

I can't use wall worts and such. The power supply needs to be contained within one enclosure.

OOOkkkkaaayyyyyy--I understand the goal may be to develop your own power supply system.

Sometimes it just ends up easier/less expensive to use a wall-wart or similar. For example, they may already have UL/CSA/CE/whatever markings.

There is nothing that says you cannot mount the wall-wart (or whatever) >>inside<< your enclosure. Sometimes I've seen (and we also have done) a mains outlet to plug in the wall-wart--inside the enclosure.

Note that there are power supply modules with terminal connections rather than the mains plug or cord. Browse at www.jameco.com .

Lee

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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Take care with switcher power supply when your app has transients on the mains...
gchapman does you have experience with Supertex?

Regards,

Bruno Muswieck

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Hmmm. The SR10 looks promising. Also, I do like the idea of getting an off-the-shelf PS with UL ratings. I guess I might be able to mount it inside.

As an aside, does anyone know what it costs to get a UL rating?

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Quote:
As an aside, does anyone know what it costs to get a UL rating?

A lot! Depending on what you actual scheme is, you may not need one. That is one attraction of using wall warts, for example. If you will have a line cord going inside of a box you make, you will likely need "listing", but if your power supply inside the box is already a "recognized component", the cost might not be too bad. Worst case is if you have your own line voltage power supply design. Generally, if your gadget runs on 24 volts or less, you don't need listing on the gadget itself.

edit: Use ETL instead of UL- Less expensive and much faster

Tom Pappano
Tulsa, Oklahoma

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brunomusw wrote:
gchapman does you have experience with Supertex?
No.

"Dare to be naïve." - Buckminster Fuller