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Basic Electronics » Need help understanding a relay schematic

February 13, 2012
by Ralphxyz
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I want to control the relay from a pin on the mcu.

I "think" that I would want to use the NO (Normally Open) contact between 87 and 30 for the motor wiring.

Now what do 85/86 do?

I can not find a specsheet but I believe I ordered these resistors with a 5 volt coil.

I do not remember where I got them from.

There must be a common ground where would that be or is that what 85/86 illustrates?

Would this be a SPDT (Single Pole Double Throw) relay?



February 13, 2012
by dgikuljot
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Dear Ralph, I was puzzled by a relay diagram too before I took it into my electronics work area and played around with it. Basically you apply ground and positive voltage to pins 85/86. Polarity doesnt matter. So basically one pin will always have either grouund or positive. The other pin will either be triggered with ground or positive depending on your constant choice. So essentially pin 85 and 86 are the coil and when it gets energized the. Pin 30 disconnects from 87a which is normally closed and connects to pin 87 which is normally open.

I hope this helps but if you need more help let me know and I can clarify

February 13, 2012
by mongo
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Right! 85 and 86 are the coil of the relay. 30 is the common terminal of the relay contacts. 87A is the normally closed contact. 87 is the normally open contact. To operate the relay through MCU, I would use a transistor or a FET like the 2700's to disassociate the spike caused by the coil when De-energized. A diode across the coil to clamp the transients would also help. It would be backward biased so the anode would be on the negative or ground terminal side.

February 14, 2012
by Ralphxyz
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Allright, thanks everyone.

It makes sense now.

I actually intended to use some higher amperage MOSFETs but thought I'd use the relays as I had them.

The code for my project will work for either.

Maybe I'll do some opto isolation just to see how that works.

I'll have more questions once I lay out my project.

I start just turning on and off a couple of motors but then it gets complicated I am even thinking about using a encoder.

I have a tilt sensor from SparkFun that I'll also use, so this is gonna get interesting.


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