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Old 28th February 2008, 04:27 AM   #1
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Default Powering LED from AC line

OK I know it can be done but I did something wrong. I have a LED with 3V f_voltage and 30mA current with an idea to power it directlu from the AC trafo supply (not the mains 240V).

I pinched the supply from the transformer AC line (~25V), on the junction with the rectifier bridge. From there the scematics goes like this:

~ - 1N4004 diode - cap 100uF 25V + leg - 380R resistor -
AC supply |
~ ------------------ -- cap - leg- LED cathode - o - LED anode

It worked for a while and then the cap blew up.

The idea is that with the half bridge rectification the LED can see only half of the supplied AC voltage. Therefore I thought that the 25v capacitor should be enough. 380R resistor is to provide desired voltage drop from 12.5 to 3V.

Am I doing something wrong here?
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Old 28th February 2008, 04:56 AM   #2
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If that's 25 volts RMS, and it's wired the way I think, you're probably getting over 30V on the cap- got a meter to measure it?
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Old 28th February 2008, 05:01 AM   #3
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Default Re: Powering LED from AC line

Quote:
Originally posted by DECKY999
The idea is that with the half bridge rectification the LED can see only half of the supplied AC voltage.
No, half-wave rectification doesn't give you half the voltage. You get the same voltage as you would with a full-wave rectifier, except it only sees that voltage every 1/60th of second instead of 1/120th of a second for a full wave bridge.

If your transformer's outputting 25 volts RMS, then the cap will see 25 x 1.414 or 35.35 volts. Your cap was only rated for 25 volts which is why it blew up.

As for the LED's 3 volt forward voltage, that doesn't mean you have to drive it with 3 volts. That's just the voltage across the LED when it's conducting. So you don't have to drop your voltage down to 3 volts, you just have to limit the current.

So, the first order of business is to use a higher voltage cap. I'd recommend 50 volts.

Next, if that 25 volts is from the transformer's datasheet and not what you've actually measured, then you should do some measuring. The specs rate the voltage for a given load current. Since you're not going to be drawing but a few milliamps through the diode, you'll likely end up with a higher voltage.

So build the circuit with just the diode and the cap and measure the voltage across the cap to get a starting point.

Although the LED's rated at 30mA, you don't need to run that much current through it. Just a few milliamps is sufficient.

So your LED circuit will be a resistor in series with the LED and those two wired across the cap.

The value of the resistor will be the voltage you measured across the cap, minus the LED's forward voltage, divided by the amount of current you want through the LED.

So if for example the voltage across the cap is 35 volts. Subtract 3 volts leaving you with 32 volts. Divide that by 0.003 and that gives you 10,666. Use a 10k resistor in series with the LED and you'll end up with about 3mA through the LED.

se
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Old 28th February 2008, 05:03 AM   #4
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I did not measure it - but I assumed that the diode will "give" only half wave to the rest of the circuit - i.e. ~60% of the RMS voltage -> hence the cap voltage of 25V should be enough.

I can put a 50V cap if that is the only problem.
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Old 28th February 2008, 05:07 AM   #5
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Default Re: Re: Powering LED from AC line

Quote:
Originally posted by Steve Eddy


No, half-wave rectification doesn't give you half the voltage. You get the same voltage as you would with a full-wave rectifier, except it only sees that voltage every 1/60th of second instead of 1/120th of a second for a full wave bridge.

If your transformer's outputting 25 volts RMS, then the cap will see 25 x 1.414 or 35.35 volts. Your cap was only rated for 25 volts which is why it blew up.

As for the LED's 3 volt forward voltage, that doesn't mean you have to drive it with 3 volts. That's just the voltage across the LED when it's conducting. So you don't have to drop your voltage down to 3 volts, you just have to limit the current.

So, the first order of business is to use a higher voltage cap. I'd recommend 50 volts.

Next, if that 25 volts is from the transformer's datasheet and not what you've actually measured, then you should do some measuring. The specs rate the voltage for a given load current. Since you're not going to be drawing but a few milliamps through the diode, you'll likely end up with a higher voltage.

So build the circuit with just the diode and the cap and measure the voltage across the cap to get a starting point.

Although the LED's rated at 30mA, you don't need to run that much current through it. Just a few milliamps is sufficient.

So your LED circuit will be a resistor in series with the LED and those two wired across the cap.

The value of the resistor will be the voltage you measured across the cap, minus the LED's forward voltage, divided by the amount of current you want through the LED.

So if for example the voltage across the cap is 35 volts. Subtract 3 volts leaving you with 32 volts. Divide that by 0.003 and that gives you 10,666. Use a 10k resistor in series with the LED and you'll end up with about 3mA through the LED.

se
Thanks - I understand my stupidity now.
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Old 28th February 2008, 05:18 AM   #6
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Default Re: Re: Re: Powering LED from AC line

Quote:
Originally posted by DECKY999
Thanks - I understand my stupidity now.
Stupidity? Nah. You simply didn't know. If you were stupid, instead of posting here asking for help, you'd still be sitting there blowing up caps.

se
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Old 28th February 2008, 05:46 AM   #7
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Default Re: Re: Re: Re: Powering LED from AC line

Quote:
Originally posted by Steve Eddy


Stupidity? Nah. You simply didn't know. If you were stupid, instead of posting here asking for help, you'd still be sitting there blowing up caps.

se
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