Doug's Relay Power Supply and LEDS

[James H Harmeyer]

Can anyone tell me if Doug's Relay Power Supply can power 10 or 12 Leds? I believe each Led will have to have a current dropping resistor to bring the 5 vdc down to around 3 volts. But, other than that will the power supply work for 10 Leds?

[sluckey]

Doug's relay power supply uses a 7805 regulator chip that can supply 5V at 1.5A if a proper heat sink is used. A typical LED will use .02 amps, so you could theoretically power 75 typical LEDs at that current requirement. 10 or 12 typical LEDs would be no sweat.

[James H Harmeyer]

Thanks Sluckey for the response and the info, I appreciate your input. But, now I wonder if it would be better to change out the 7805 for a much more efficient switching regulator (LM7805-1A), that doesn't need a heat sink. Or will a good heat sink on the 7805 be good enough to keep this working for a long time? I'm just trying to keep heat from being the cause of a failure.

[sluckey]

I would stay with the TO-220 case. Just bolt it to the chassis to provide an adequate heat sink. You'll need an insulator.

[PRR]

You don't need a regulator. Take your raw 7VDC, use a 270 Ohm resistor for each LED. 19mA per LED, 0.13 Watts per resistor+LED (1/4W Rs are ample), 1.5 Watts total heat for a dozen.

[James H Harmeyer]

Sluckey,    what kind of insulator are you talking about for bolting the regulator to the chassis and this will still provide a good enough connection to transfer the heat?

[sluckey]

Mica or silicon. Use heat sink grease too.

https://www.google.com/search?sourceid=navclient&aq=&oq=to-220+heatsink+insulator&hl=en-GB&ie=UTF-8&rlz=1T4GZAZ_enUS367US367&q=to-220+heatsink+insulator&gs_l=hp....0.0.0.17337...........0.

[Willabe]

Mica or silicon. Use heat sink grease too.

Use heat sink grease on the mica only.