Author Topic: Minimising power loss in a relay  (Read 11522 times)

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Offline David_AVD

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Re: Minimising power loss in a relay
« Reply #25 on: September 30, 2013, 09:17:41 pm »
It could be that they don't want the output to be possibly on at power-up ?
 

Offline dannyf

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Re: Minimising power loss in a relay
« Reply #26 on: September 30, 2013, 09:56:12 pm »
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a simple mechanical device

To some people, that can be fault #1.
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Offline HackedFridgeMagnetTopic starter

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Re: Minimising power loss in a relay
« Reply #27 on: October 01, 2013, 12:14:54 am »
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O.o what kind of client are you working for that a simple mechanical device is harder to explain than an RC time constant reducing drive current by some mechanism?
It's a reasonable question
In this job the main issues with the latching relay are that, it is different to what is currently installed and that it needs to go in a power meter and if it could be defeated that would then become my fault.

The only reason I am looking at reducing power is that the current version doesn't actually meet its own power usage specs, and I think I can bring it in to line by driving the same relay with less current, or even just speccing a lighter relay.
I will lay it out with two gpios and with a timer on at least one of them, then I can just do changes in the firmware.

Some rough power calcs.
For example to drive an engaged relay at 50% current.

If I use a series resistor then I am using 50% of the original power, split evenly between the resistor and the coil.
If I use a pwm duty cycle of 50% to give 50% current, then I am driving at full voltage for half the time which is 25% of the power.

It seems to me that the relay with its very large inductance and high series resistance is the perfect load for pwm.
So all seems good if you can bear the switching noise.
Any further comments about driving the relay with PWM are welcome, as that is how I am leaning.
 

Offline David_AVD

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Re: Minimising power loss in a relay
« Reply #28 on: October 01, 2013, 12:35:09 am »
Maybe add a small cap across the relay to suppress any whine from the coil?
 

Offline NiHaoMike

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Re: Minimising power loss in a relay
« Reply #29 on: October 01, 2013, 01:20:44 am »
Set the PWM to 22kHz or so, just enough to make it inaudible.
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Offline ve7xen

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Re: Minimising power loss in a relay
« Reply #30 on: October 01, 2013, 08:33:46 am »
It's a reasonable question
In this job the main issues with the latching relay are that, it is different to what is currently installed and that it needs to go in a power meter and if it could be defeated that would then become my fault.
And this is a good reason to go with your proposed solution. Thanks for humouring me :).
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