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Topic: 12v or 5v - whats safer (Read 1 time) previous topic - next topic

pgmartin

DaveO: if you are using a UPS, try not to waste current while doing a voltage conversion. Depending on the current used, the 7805 will waste energy producing heat.

Grumpy_Mike

Quote
Are you saying the 7805 should not be used to drop 12v to 5v, or only when used with 2 capacitors in the circuit to stabilise it ?

only when used with 2 capacitors in the circuit to stabilise it.

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are you saying better to run the 12v and avoid the drop to 5v until the last possible point,

Yes, that way any volts drop getting to the regulator is eliminated. If you regulate before your long run then you might not get 5V at the end.

pluggy

#7
Jun 06, 2011, 11:15 pm Last Edit: Jun 06, 2011, 11:21 pm by pluggy Reason: 1
If you only want 1 per room, I'd find the specs for the LEDs (foward voltage, recommended safe current) and feed it with 12volts through a series resistor of suitable value. LEDs aren't rated by voltage, using a suitable resistor you could run them off 200 volts if you chose to (not recommended).  Personally I'd wire 3 in series with 12 volts and a smaller resistor (it would use the same amount of electricity for 3* the light .  Buying small (5mm) high brightness LEDs is cheap in comparison to the effort involved in positioning and wiring them.  They can be had for pennies apiece on ebay.  Running 1 LED from 12V is no more wasteful than using a 7805 to step down the voltage.  

Carefully drill a 5mm hole (so as not to damage the surface) where you want the LED and stick a single LED in the hole from the back and hold in place with blutack/tape/hot melt/whatever and its barely noticable unless its on and dark in the room.

DaveO


If you only want 1 per room, I'd find the specs for the LEDs (foward voltage, recommended safe current) and feed it with 12volts through a series resistor of suitable value. LEDs aren't rated by voltage, using a suitable resistor you could run them off 200 volts if you chose to (not recommended).  Personally I'd wire 3 in series with 12 volts and a smaller resistor (it would use the same amount of electricity for 3* the light .  Buying small (5mm) high brightness LEDs is cheap in comparison to the effort involved in positioning and wiring them.  They can be had for pennies apiece on ebay.  Running 1 LED from 12V is no more wasteful than using a 7805 to step down the voltage.  


Hi Pluggy

So you're saying it may use less ( or very similar ) power to have, say 5 LEDs 2v 20mA in series with a single 100 ohm resistor connected to a 12v supply, than to use a 7805 to drop the 12v to 5v and connect a single LED with a 150 ohm resistor connected to the 5v ?

DaveO


Quote
Are you saying the 7805 should not be used to drop 12v to 5v, or only when used with 2 capacitors in the circuit to stabilise it ?

only when used with 2 capacitors in the circuit to stabilise it.

Quote
are you saying better to run the 12v and avoid the drop to 5v until the last possible point,

Yes, that way any volts drop getting to the regulator is eliminated. If you regulate before your long run then you might not get 5V at the end.



Thanks again Grumpy_Mike.

Any links you can point me to for the correct diagram and capacitor selection ?

I hadn't thought about the loss over the long run. Good point there.

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