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Old 11th January 2010, 08:40 AM   #1
ZippyTs is offline ZippyTs  United States
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Default What size fuse?

So I am going to make my own relay setup for my remote wire now that I will be turning on multiple devices and some cooling fans. For the amps and crossover are 500mA fast acting fuses a good size to fuse them at?

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Old 11th January 2010, 09:05 AM   #2
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If the fuse is only going to power the relay coil(s), 500mA should be okay. Install a reverse biased diode across the relay coil(s) to reduce arcing of the switch contacts.
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Old 11th January 2010, 08:49 PM   #3
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Most turn on leads fail at just above 100 MA current rating which is there max rating as I recall. So maybe a 1/10th amp fuse might be a better size / fit.

A reversed diode across the coil is used to absorb back EMF off the coil to protect the turn on circuitry from any reverse energy spikes that might fail the solid state driver devices. Many Bosch type relays used in car alarms come with this already attached saving effort and time. mine has these like this, and they are off the shelf items at most car alarm places.

If your worried about the relay contacts try a simple disk capacitor across the contacts. this usually improves contact lifespan somewhat, but with what they cost and the very low failure rates at low current operation this last step is most likely not needed..and I would only worry if your switching loads near or at its max contact current ratings.

My system bosch relay isolation setup has worked for years without issue and it uses no contact protection, just a back EMF diode as described above...
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Old 11th January 2010, 11:44 PM   #4
mavric is offline mavric  United States
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Zippy, just swing by a car audio shop and pick up a starter kill relay pack with the diode allready on there. If you need specs on wiring the relay, its easy.
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Old 12th January 2010, 12:17 AM   #5
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Most turn on leads to car amps and signal processors and the like draw under 10ma and typically 1ma or less, so taking the number of switched electronic devices and multiplying by 10ma each will give you a real world current draw idea.

As for fans, lights etc... well it would be nice to bench them and measure their current draw and add all the numbers together to get a real world figure to deal with and fuse them accordingly.
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Old 12th January 2010, 01:28 AM   #6
ZippyTs is offline ZippyTs  United States
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I have a bunch of relays already, but the one I am using does not have a reverse biased diode. So I will need to hook that up. I have many rectifier diodes and such, so I can use one of them? Also just to make sure cathode side will go to source correct?

This is the Relay I am using I was going to use it with out a base socket and solder right to the lugs. The reason I went with This one is so I could isolate the fans from the remote so I could fuse the fans at proper current
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Old 12th January 2010, 06:11 AM   #7
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The diode goes across the relay coil with the cathode on the positive end of the coil.
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Old 12th January 2010, 06:21 AM   #8
ZippyTs is offline ZippyTs  United States
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ok thanks Frank, just as I was thinking.
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