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Old 16th February 2009, 10:18 AM   #1
Atilla is offline Atilla  Norway
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Default Rectifier for lighting transformer

I've a couple of AC-AC switch-mode halogen light transformers around. I'm planning to use them to drive a few massive LED strings, they should be able to provide more than enough current for that.

I'm just not sure how to figure out what rectification method to use - DC rectification of SMPS source is a little bit of a mystery to me. A lot of the bridges I see are rated explicitly for 60hz operation, which probably means I can't just use them directly in my circuit. I'll probably need an ultra-fast schottky rectifier(or diodes) for that, correct? I'd prefer to use a monolithic bridge, one of those small round ones for PCB mounting, rather than discrete diodes.

Any tips?
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Old 17th February 2009, 07:54 AM   #2
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LEDs are diodes, i. e. rectifiers. All you need is a smoothing capacitor against flickering. But don't expect those transformers to be designed for big capacitive loads. You will have to rely on Trial-&-Error and be prepared to lose one or a few transformers during the tests.
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Old 17th February 2009, 10:20 AM   #3
Atilla is offline Atilla  Norway
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Hm, I'm a bit afraid that wouldn't work, for one because of the capacitive load and because of what exactly I'm trying to do. I was planning on LM317-based constant-current control, and running a simple PWM trough a power FET transistor for controlling brightness. So I'd need some proper DC, at least to power a couple of schmitt triggers.

As for the experiments with the transformers, I don't mind that - this is the fun part, as long as precautions are taken . A protection diode, resistor and a cap is all that takes to power a led on AC. (Low-voltage AC, i'm not plugging anything straight into the mains!)
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Old 18th February 2009, 03:43 PM   #4
Atilla is offline Atilla  Norway
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Would 3us recovery times be enough for a low-khz smps and current draw under 1A? What are the general rules for rectifying these - I've been trying to find some info, since I'm sure there's a lot I don't know and it might not be as simple as throwing a bunch of ultrafast elements in.
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Old 18th February 2009, 04:22 PM   #5
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3s is a very slow diode. You need ultrafasts or schottkys.

I believe it should be possible to rectify the output of one of those electronic transformers for halogen but I haven't tried. They usually contain a series resonant self oscillating type of circuit. It will probably not run with light load though.

There usally isn't any primary side storage on these so output will be pulsating with double the mains frequency. Might not be very useful.
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Old 18th February 2009, 06:58 PM   #6
Atilla is offline Atilla  Norway
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I did some reading and considering the cheap make of the things, I'll probably run into some issues. I'll try to figure out exactly what the layout is and if I can use them for anything useful. If not, I'm sure I can find a cheapo transformer with a 12-14V secondary somewhere.
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Old 23rd February 2009, 07:49 AM   #7
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Default Re: Rectifier for lighting transformer

Quote:
Originally posted by Atilla
I've a couple of AC-AC switch-mode halogen light transformers around. I'm planning to use them to drive a few massive LED strings, they should be able to provide more than enough current for that.
Take a look at http://www.qrp4u.de/docs/en/tube_smps/index.htm
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