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Thread: Relay issue

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    Relay issue

    I am adding a relay to an amp. It's a 9v relay here is the datasheet.

    http://www.mouser.com/ds/2/315/mech_...2y-1076009.pdf

    The relay will not switch. I've tried 2 different relays...same model. They do switch with a 9v battery.

    Relay coil current is 22.2ma per the datasheet.

    Here's how I have it wired.

    +31vDC - 1k 1w - relay coil - ground

    31 - 9 = 22v
    22v/.022 = 1k resistor

    The voltage on top of the coil is 31v when the other end of the coil is lifted. When grounding the other end of the coil (low side switching) the coil voltage is + 9.15v.

    So I'm quite confused as to why it isn't working. My calculations seem correct. Help!

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    Supporting Member eschertron's Avatar
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    Your math seems good. I'm wondering if the 'holding' current is not much lower than the 'turn on' current. Any specs on that?

    edit: OK, so I looked at the data sheet. all the current, resistance, etc., looks pretty much in line with 22mA and 9v for all conditions. The data sheet says it will take 200% of rated voltage (indefinitely?). so downsize your resistor until it pulls in. Or use a regulator to get your control voltage. I'm still thinking the 1k resistor is 'choking' the relay.

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    Supporting Member The Dude's Avatar
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    How are you measuring the 9.15V- to ground or directly across the coil? Your math does look good. I'm wondering if your low side switching has some resistance or voltage drop that you are not counting in the calculation. If you haven't already, measure voltage directly across the coil.

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    Thanks.

    I'm measuring across the coil. It's the DARNDEST THING!!!

    I have the flyback diode in place. Anode to low side. I cut it out but same results. Baffling.

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    Supporting Member eschertron's Avatar
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    with the 9V battery, did you have any issue with matching polarity of the relay coil to battery? Some relays/bases have a snubber diode built in and require correct polarity to work.

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    don't forget the joker g1's Avatar
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    Good point. The schematic shows pin1 +, pin 16 negative.

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    Supporting Member The Dude's Avatar
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    I don't know what's in that relay, but if there were a built in snubber and the wrong polarity voltage was supplied, wouldn't you read a diode voltage drop across the coil instead of 9.15V?

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    Agreed dude. The chinaman is not the issue here.

    But seriously... I have the +v at pin 1. So it is backwards from how they show it on the datasheet. It'd be quite odd if it works but I'll swap it tomorrow.

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    Supporting Member loudthud's Avatar
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    Some relays use low cost metals in the magnetic path which become magnetized the first time the relay is tested at the factory. They call this a "feature" because it takes less Voltage to pull in the relay when the same polarity is used as when the magnetic path was magnetized. This comes from the old school "If you can't fix it, feature it."

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    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personel.

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    It was the POLARITY! Interesting. I guess current has to flow the correct way to cause the magnetic field to work correctly. Thanks all!

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    "Thermionic Apocalypse" -JT nickb's Avatar
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    Quote Originally Posted by lowell View Post
    It was the POLARITY! Interesting. I guess current has to flow the correct way to cause the magnetic field to work correctly. Thanks all!
    Well that's one to watch. I see the pin out diagram does show the polarity on the coil terminals. I guess it must have a 'helper' bias magnet inside.

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    Quote Originally Posted by nickb View Post
    Well that's one to watch. I see the pin out diagram does show the polarity on the coil terminals. I guess it must have a 'helper' bias magnet inside.
    Maybe. It could also be as simple as having an internal flyback catch diode. Many small relays do have these diodes. It's odd that it is not shown on the datasheet if so. @lowell: it would be interesting to see if your multimeter says there's a different coil resistance one way versus the other. I would expect a diode to be cheaper than a small, precision magnet.

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    Amazing!! Who would ever have guessed that someone who villified the evil rich people would begin happily accepting their millions in speaking fees!

    Oh, wait! That sounds familiar, somehow.

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    Supporting Member Jazz P Bass's Avatar
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    This from Omron: "A Relay equipped with a diode that absorbs coil counter-electromotive force will not operate properly if the positive and negative voltages applied to the coil are connected incorrectly.".

    I believe the 'bar' on the relay indicates that it is a polarized coil.

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    Oddly enough it reads the same in both directions. Resistance as well as semi. 410ohms/517mv.

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    Lifetime Member Enzo's Avatar
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    Do this experiment:
    take a plain old 1N4007 diode, and clip a 410 ohm resistor in parallel with it. Oh 390 ohms is a close standard value. Now try measuring the diode. I think you will find the low resistance obscures the diode function.


    In my days working on pinball machines, we drove the solenoids - the coils that worked the moving parts to slap the ball - with DC voltage and transistors. Those solenoid coils would destroy a transistor in an instant, were it not for a reverse diode across each coil. COnsidering a coil might be 550 turns of #24 wire, and so very low resistance, I had to convince them they would never detect a shorted diode with an ohm meter.

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    Yes makes sense. I blindly followed RGs instructions and failed to simultaneously use my brain.

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    Lifetime Member Enzo's Avatar
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    Oh I trust RG for sure, but he did say it would be interesting to find out. A diode would indeed be cheaper than a magnet.

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    "Thermionic Apocalypse" -JT nickb's Avatar
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    Quote Originally Posted by Enzo View Post
    Oh I trust RG for sure, but he did say it would be interesting to find out. A diode would indeed be cheaper than a magnet.
    I thought RG's suggestion was a jolly good. It just so happens that I pulled a bad FRT5 polarized 24V relay today. So...I hooked it up to a bench supply set to 24V and 20mA. Relay clicks in and pulls 10mA one way round. When the other way round it fails to operate but still pulls 10mA with a 24V drop. No diode in this specimen.

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    Old Timer Tom Phillips's Avatar
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    I'm thinking that the relay design is such that a DC voltage applied to the coil in One Direction pulls in the contacts but if the DC voltage is reversed then it repels the contacts. The current draw and voltage drop would be the same in either case.

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    Quote Originally Posted by Enzo View Post
    Do this experiment:
    take a plain old 1N4007 diode, and clip a 410 ohm resistor in parallel with it. Oh 390 ohms is a close standard value. Now try measuring the diode. I think you will find the low resistance obscures the diode function.


    In my days working on pinball machines, we drove the solenoids - the coils that worked the moving parts to slap the ball - with DC voltage and transistors. Those solenoid coils would destroy a transistor in an instant, were it not for a reverse diode across each coil. COnsidering a coil might be 550 turns of #24 wire, and so very low resistance, I had to convince them they would never detect a shorted diode with an ohm meter.
    That explains the resistance measurement with a DVM. I don't see how that explains the voltage drop of 9.15V. If there were a diode in the relay and it were forward biased, you'd read a diode junction (about .7V) instead of 9.15V. I don't see how it could be as simple as a snubber diode in this instance.

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    Old Timer J M Fahey's Avatar
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    The "added magnet" may be built in and cost nothing.

    Armature "should" be made of soft low carbon steel, think SAE1008 or its modern incarnations ... which nowadays are harder to find, recently one of our friends in the Pickup section was worried about unobtanium 1008 and 1012 rods for his pickups.

    Higher carbon steel is abbundant and cheap because of it´s mechanical properties so everybody carries it and low carbon (magnetically good) can be easily ordered ... 500 Tons minimum batch and probably a couple Months delay.

    So some bright bulb at Omron goes with the higher carbon type (which retains some magnetism) and turns a problem into a feature.

    Just to make it consistent they pulse each armature (or a tray full of them) in a speaker type magnetizer ... FWIW even I could precharge , say, from 40 to 120 of them on a single "kick" which proves it´s not exactly NASA technology.

    Such a relay would test symmetrical to a multimeter but not to applied current.

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    Lifetime Member Enzo's Avatar
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    Oddly enough it reads the same in both directions. Resistance as well as semi. 410ohms/517mv.
    This is what I saw, I didn't see 9v. The 517mv.

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    Lowell,

    Did you noticed that the schematic in the datasheet says: Bottom View? This is a standard and I would be very surprised if suddenly one company would manufacture relays with reversed polarity. No one would use such relays.
    Other datasheets of similar relays have a note: VIEWED TOWARD TERMINALS (which is the same as BOTTOM VIEW).

    Mark

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    Supporting Member The Dude's Avatar
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    Quote Originally Posted by Enzo View Post
    This is what I saw, I didn't see 9v. The 517mv.
    Gotcha. I was going from the first post in the thread: 9.15V dropped with the relay coil polarity reversed.

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