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I have a basic question which I wanted to run by some better informed people than me.

I'm planning on building a very simple circuit which takes a 12v feed from a broadcast camera and uses it to energise a reed relay. The battery supply to these cameras are high capacity lithiums which has me a little concerned. The current draw of the relay is only 12mA. Would this small load be a problem?

user1576488
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    IF the relay is rated for 12V operation then it will draw the correct rated current from a 12V supply regardless of the current capability of the supply. If the relay is rated for less than 12V operation then the 12V will need to be reduced to suit. More details = better answers. Usually :-). – Russell McMahon Jan 23 '13 at 16:58
  • More details are a good idea. A schematic would be great! –  Jan 23 '13 at 17:15
  • I should have elaborated on my question a bit more, but thanks. You've confirmed what I already thought. The relay is rated for 12v and the aux power supply from the camera is regulated to 12v. I just panicked a bit thinking that the low current drawn would be almost like shorting the battery, and I know that's not a great idea with lithiums! – user1576488 Jan 23 '13 at 17:15
  • A low current draw is essentially the exact opposite of shorting a battery, actually (no current draw being the ultimate antithesis). – Adam Lawrence Jan 23 '13 at 18:02

3 Answers3

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This is essentially the same question as Choosing power supply, how to get the voltage and current ratings?.

Short answer: it's not a problem, as long as these high-capacity batteries are the proper voltage.

Slightly longer answer: it's not a problem, but maybe you want to include a fuse or other device that breaks the circuit in the case of a fault so you don't melt wires, start fires, or explode batteries.

Answer by rhetorical question: Is it OK to power a small wall clock with a 2000MW nuclear power plant?

Community
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Phil Frost
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  • Thanks, just as I thought. As you can see I was just concerned about potentially blowing up a lithium battery. I will definitely include a resetting fuse in the circuit. – user1576488 Jan 23 '13 at 17:26
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The only way to know is to check the specs for the camera, which is apparently being used to only supply power in this case. I don't see how you think we can answer this question since we know nothing about this camera. Probably 12 mA is OK for a device like a camera, and if it is intended to source some power over a auxilliary port or something.

Olin Lathrop
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  • Sorry, I should have given more detail. I'm using an aux out supply from the camera which is regulated to 12v. This supply is intended to be used for a light so is rated at 0.5A. The relay I'll be using is also rated at 12v so I don't think there will be a problem. I just got worried about potentially shorting out the battery. – user1576488 Jan 23 '13 at 17:20
  • As long as the available current is at least what the relay requires, and it clearly is, there is no problem. – Olin Lathrop Jan 23 '13 at 18:25
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No - due to it's internal resistance, the relay will only draw as much current as it needs. As long as it's voltage is correct (i.e. not too high), then you are fine. If you want to be absolutely sure, then we need the part number and/or datasheet.

Oli Glaser
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