Reed relay in a car


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  1. #1
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    Default Reed relay in a car

    I'm using it on the output of an 18F4550. With the tiny coil on a reed relay, do you still need a diode in parallel with the coil? I want to use this in an automotive application and I have very little experience with reed relays. Do I need to be concerned about vibration? The PCB will be mounted in the passenger compartment but housed in a plastic case. The contact current will be less than 15mA.

  2. #2
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    Default Re: Reed relay in a car

    Some reed relay have built-in diode, some don't... but still you need one. I wouldn't use them in automotive application.

    Out of curiosity... Why do you want a relay for so such small current?!?
    Steve

    It's not a bug, it's a random feature.
    There's no problem, only learning opportunities.

  3. #3
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    Default Re: Reed relay in a car

    It's a long story. Quite frankly, I'm just frustrated with the whole thing. You commented on it - reference this post: http://www.picbasic.co.uk/forum/showthread.php?t=17476

    What it boils down to is my laziness to add a few extra components (mostly limited by PCB space) and spend $1 more. I won't rehash the thread above but this is where I'm at now.

    My original solution of using the capacitor with PWM is what I'm back to testing for the final solution. The only reason why I wasn't able to use it in the first place was due to the fact that I'm cheap and didn't want to use an analog input to know the true temperature. I maxed out the frequency but I still needed a fairly large capacitor to smooth out the PWM signal. With the larger cap, the voltage takes too long to drain down and taking temperature measurements with the OEM computer isn't viable.

    However, I realized I could simply use the dutycycle to know approximately where I was at with regards to the actual temperature. The temp can be + or - 2 degrees so using approximate values for the duty cycle is not a problem.

    My reasoning for using a relay (reed relay perhaps) is due to a voltage divider. I was going to use a voltage divider that was connected to +5 and -5 100% of the time. I would use the relay to supply the voltage to the thermistor input. Since I wouldn't need a capacitor, I could instantly capture a measurement. The other solution is to just buy a voltage regulator with two outputs where at least one has an enable input and adjustable output.

    So for now, the PWM output and using the duty cycle to approximate the temp is working.

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