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Author Topic: strange diode reading with 5v relay  (Read 1169 times)
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Hello,

I am using a 5v reed relay with a coil resistance of 250ohms, and contact ration of .5 amps and nominal current 20mA.

I have the same relay on a breadboard and in a perf board and it seems to be soldered together the same way. When it is in the breadboard, the relay and diode on its coil works. On my multimeter I get a normal reading...(when I put the multimeter on ohms, one side gives a reading, the other does not) however this same circuit in my perf board, when I test the diode, i get a reading on both ends...meaning when I flip the + and - on the multimeter i get a reading of like 200ohms either way...when I take the diode out of the perf...the reading is normal again...the diode works.

Any idea what could be causing this?
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ok further testing and i noticed its as soon as i ground the relay that the diode give the funny reading. perhaps i am doing something different with my grounding in this circuit. how separate are you supposed to keep the ground from a 12v psu and the regulated current from a 7805? can the arduino and other components take the ground from the 12v psu? or do you make a separate ground from the 7805 center pin? ground is so confusing!
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when I test the diode, i get a reading on both ends...meaning when I flip the + and - on the multimeter i get a reading of like 200ohms either way...when I take the diode out of the perf...the reading is normal again...the diode works.
The coil is shorting out the diode for your meter.

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how separate are you supposed to keep the ground from a 12v psu and the regulated current from a 7805?
To the extent that they must be connected together, that separate.

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can the arduino and other components take the ground from the 12v psu?
Yes it has to, you have no choice.
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so all my grounds are connected....do you have any idea why the coil is shorting out the diode in one circuit and not the other?
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oh shoot...i was looking at it wrong. the diode stops working on the side of the relay that goes to the arduino. is there an order of the pins? like relay pin connects to diode, diode to arduino? or some other order?
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is there an order of the pins? l
No it doesn't matter so long as topologically it is the right circuit.
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thanks so much for the responses...but do you have any idea why this could be happening? or any other trouble shooting approaches? i've tried different relays, different diodes...it's just any time the diode is connected to the relay on both ends it stops working...if i just have the anode connected to he coil, it works...if just the cathode is connected...it works...but as soon as both are connected...it stops working...but in the same breadboarded circuit..no problems.
« Last Edit: October 06, 2012, 05:19:53 pm by treebykooba » Logged

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I have the same relay on a breadboard and in a perf board and it seems to be soldered together the same way. When it is in the breadboard, the relay and diode on its coil works.
If it works in one situation and not the other then it is not wired in the same way even though it "seems" to be. Otherwise physics falls apart and that will never do.

As you have the board only you can find out what is wrong but it is wired up wrong in the circuit that doesn't work.

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any other trouble shooting approaches?
1) Use test equipment and be comfortable with what it is telling you.
2) Trace a signal from origin to the final component, where you fail to find it your mistake is between there and the last good point.
3) Never make a measurement without having an expectation of what the answer should be.
4) Get an oscilloscope and learn how to use it, electricity is invisible an oscilloscope is your eyes.
5) When something simply can not be happening it isn't. You are misinterpreting something.
6) Most important to learn - all the mistakes are yours - even after 45 years I can make a mistake wiring up.
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if i just have the anode connected to he coil, it works...if just the cathode is connected...it works...but as soon as both are connected...it stops working...but in the same breadboarded circuit..no problems.

Well that leaves the state where you connect both anode and cathode but in the reverse direction that they were connected. If a diode is installed backwards across the coil terminals, it will act like a direct short when the coil voltage is applied, relay won't work and lots of current will be attempted to be pulled from the voltage source, possibly damaging whatever the voltage source is. If that's not it, then test to be sure you don't have a shorted diode.

 Also I really like when people post their wiring in schmatic form, just as our software gurus demand to see the code before they try and help people with their software problems.

Lefty
« Last Edit: October 06, 2012, 06:24:51 pm by retrolefty » Logged

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Yeah and I just noticed mine is broken too... Mebbe shouldn't have used it on those Nuts... So much.

Bob
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