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Old 17th January 2009, 08:26 AM   #1
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Hi there.

Been lurking around the forums for a bit but decided to sign up to post a question. Well, actually there are 2.

Here goes:

1) I want to connect a diffused red led to the mains (in parallel with 2 paralleled transformers) so that when I turn on the switch, the led lights up. I don't want to put it after the transformers as I want to keep the transformed current as clean as possible.

I put a 1W 1M resistor and that seemed to work fine but the light was dim. I put a 20k 0.6W resistor and that started to burn. (For experiments sake i put a 2W 470R resistor but that blew up spectacularly. Too much current I understand.)

From my calculations, I need either a 10k or a 20k resistor at 5W.

What I want to ask is. Is this worth the trouble? Or is it even considered safe or even good practice?


2) Was it wise of me to build a power supply out of a metal chassis? Do I ground it by screwing the earth line to the chassis because isn't it possible that ground may actually be energised, therefore causing possible harm to the user?


Cheers,
firethorn.
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Old 17th January 2009, 09:19 AM   #2
luka is offline luka  Slovenia
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First of all you made a BIG jump going from 1M to 20k, don't you think, 100k would be much better and even then usege 2x 47k in series, both 5w?

diffused red led, this is normal led right? what you could do is, use what I know as "start up" circuit for smps's. It is just 2 resistors, zener and transistor, and make constant voltage "regulator", don't know if this is really regulator, but it does regulate. what this will allow you is to take any high or low voltage DC and regulate it down to what you need. You probably won't go for it but anyway...

This is not smps right? normal 50/60Hz PSU?

Quote:
build a power supply out of a metal chassis
Aluminium or steel? About ground...I don't know, I don't even have it at home, nothing I can do or say
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Old 17th January 2009, 07:04 PM   #3
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If you are using toroid power transformers, it's possible to just add another winding to supply the low voltage for your LED.

As far as grounding your chassis: yes, you should do this. Connect the safety ground wire (green or green/yellow, usually) to the chassis. If you have a fault and the chassis becomes 'live' ,you want the mains fuse to blow. Don't take chances with grounding and safety- it's ok to blow up some LEDs and transistors, but you don't want somebody to get injured. Do your research and get it right.
Check the PSU area at Decibel Dungeon or the threads here by Nuuk- they're excellent.

Cheers
John
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Old 20th January 2009, 11:57 PM   #4
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luka: I'm not using an SMPS. Using a EI laminated core. But I get what you're hitting at. Step it down then use that power.

John: Thanks. I understand now. =)
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Old 21st January 2009, 12:11 AM   #5
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http://www.marcspages.co.uk/tech/6103.htm
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Old 21st January 2009, 12:15 AM   #6
luka is offline luka  Slovenia
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Mains led souds just what you are looking for
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Old 22nd January 2009, 02:38 AM   #7
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when putting a LED across AC it is usual that there be a protection diodes in series, other than using trick circuits I've seen that use a just a series capacitor of specific value. This is because LED's aren't designed to withstand Peak Inverse Voltage (PIV) that you are subjecting it to,...I think,...but don't quote me!
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Old 22nd January 2009, 02:50 AM   #8
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I think a very simple solution (as stated above) would be to take some several feet of Cat5 cable or magnet wire and wrap it around your toroid... through the hole round and round. Start with a dozen turns and measure the voltage when the tranny is powered. Depending on your LED, I reckon you want about two volts. Not enough? Take more turns. Too much? Take a few off.

It works.
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Old 22nd January 2009, 10:31 AM   #9
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Actually I'm using an EI laminated core.

No matter. I think it would be better if I just used a neon light for this one. After that, it's back to leds cause I'll be using a toroid for my next version. So much smaller and not that much more expensive. And simpler... I think.

Thanks for all the help. =)
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Old 29th January 2009, 08:16 PM   #10
Sponkii is offline Sponkii  Denmark
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I don't how a LED i going to affect your output current much.

it draws so little current that is it barely noticable..
depending on LED it is from 10 to 30ma og 0.01 to 0.03A..
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