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Old Jul 16, 2015, 04:49 AM
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Switch on battery alarms automatically with the rest of the system

Hi there,

I would need some help to determine the best way of having the battery alarms automatically switched on when I power the system.

The configuration is 2x5s batteries used in series, I have an Emotec safety switch between the batteries and the ESC. In parallel with the ESC I have a BEC which powers the receiver. Behind the ESC is a 3.2kW 170kv motor.

I need something that closes the circuit on the 2 negative wires of both alarms when either the BEC or the ESC is powered. Now, I remember from school that coil based relays don't mix well with electronics without some sort of diode protection. I tried instead a 2 channel arduino style Solid State Relay but realised that it's designed to switch voltages around 220V and when used with the battery alarms it skews the readings quite a lot.

Could someone advise an elegant solution and point me to a part that would do the job? it would be fantastic.

thanks a lot folks!
clr
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Old Jul 16, 2015, 05:42 AM
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Don't you only need it to switch when the BEC is active? I don't see any situation where the ESC would be live and the BEC not outputting 5 /6V.

If so just put a small Double Pole 5 or 6V relay across the BEC output and use the contacts to switch the two negative LiPo monitor leads. The output of a relay is completely isolated from it's input and it has very low resistance when closed - ideal for the job.

It is normal to put a diode across the coil of the relay to quench any back voltage spike when it disconnects that might damage the output of the BEC. I think this is a low risk given the small relay and robustness of a BEC in any case.

Sorry about the crude picture but it gives the idea.

Any small double pole relay with a 5v or 6V coil will work. The currents you are switching are tiny.

Something like this

http://www.jameco.com/1/1/57788-tq2-...miniature.html

John
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Old Jul 16, 2015, 05:46 AM
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Thanks a lot for that John. Yes, you're correct, the ESC and the BEC are always powered together.
Are there different specs for the diode? would any work?
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Old Jul 16, 2015, 06:26 AM
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Any general purpose power (not signal) diode will work at these low voltages and current. All it has to do is provide a path for any current created when the field of the relay coil collapses as it is switched off. The extremely common IN4001 or IN4004 will be fine. It's not at all critical. If you don't want to buy one, you will find 2 or 4 of them in any of the old style Transformer Wallwarts (plugpacks) that we had for powering battery devices before switch mode supplies became common. A new one might cost you 10cents however.

John

PS: It is critical you install it the right way round. Band to the 5v +Ve supply. If you reverse it, it is just a dead short across the BEC output.
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Originally Posted by eLongboard View Post
Thanks a lot for that John. Yes, you're correct, the ESC and the BEC are always powered together.
Are there different specs for the diode? would any work?
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Last edited by jj604; Jul 16, 2015 at 06:40 AM.
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