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

DaveO

I want to install small LED 'lamps' to each room to provide minimum lighting in the event of a power failure ( a regular thing in our country ). My 12vDC system runs off a UPS, so was thinking that using LEDs the best option due to the little current they draw.

I have found a local source of small 4 LED bulb lights ( typically used in cupboards, etc ) which supply enough light for me to see my way around in the dark. They are powered with 3 small AAA batteries ( 1.5v each ).

My thinking is to replace the batteries with an on/off switch connected to my 12v supply and a 7805 regulator to drop the power to 5v.

Which of the following would you consider the safer option ( in terms of wiring heat / potential damage / danger, etc ), considering the wiring will be running from my supply to each room through the ceiling :

A. run the 12v from supply to the room, then have a wall mount box containing the 7805 and on / off switch ( one box in each room ), or
B. have a single 7805 at the supply source location, and run 5v to each room with only the on / off switch the wall mount box ?

Also, for the 7805, I have read conflicting information :

1. that the 7805 can be used as is to drop the 12v to 5v,
2. that one should add 2 capacitors to the step-down circuit.

Which is the correct method ?

Grumpy_Mike

Quote
1. that the 7805 can be used as is to drop the 12v to 5v,

Wrong
Quote
2. that one should add 2 capacitors to the step-down circuit.

Right but not for the reason you quote, the capacitors are for circuit stability, they stop it oscillating.

There is no difference in safety, 12V and 5V are equally safe. Remember a child's train set has 12V exposed rails.

Local regulation however is better to minimise line volts drops.

CrossRoads

Pesonally, I would connect the LEDs in string of 4 or 5 with a current limit resistor and run them from 12V if that is an option.
Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
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DaveO

Thanks for the reply Grumpy_Mike.


Quote
1. that the 7805 can be used as is to drop the 12v to 5v,

Wrong

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 ?


There is no difference in safety, 12V and 5V are equally safe. Remember a child's train set has 12V exposed rails.

Local regulation however is better to minimise line volts drops.


Very true - I have a 12v slot car track in the lounge as we speak.

"better to minimise line volts drops" - are you saying better to run the 12v and avoid the drop to 5v until the last possible point, or have only 1 7805 at the source ?

Sorry to ask, but your reply confused me a little.

DaveO


Pesonally, I would connect the LEDs in string of 4 or 5 with a current limit resistor and run them from 12V if that is an option.



Hi CrossRoads

Thank You for the idea. I had considered that, but a few factors ruled that out :

1. the LED units, although not too expensive, would become so if I wanted to add 3 or 4 per room.
2. the additional light provided would not be required - 1 unit is enough.
3. they would drain my UPS faster.
4. would be more of an eye-sore in the room than a single 4 bulb LED unit.

Regards
Dave

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