relay coils and transistors - relay polarity

ArchiverArchiver Posts: 46,084
I'd like to mention that whereas relays are not
commonly polarized there is a class of relays built
which _do_ respond differently to polarity across
their coils.

These are usually latching relays - they stay in the
last state they were driven to rather than "dropping out"
when voltage is removed from their coils.

Latching relays may have a permanent magnet inside
which can hold them in either position and the
electromagnetic coil counters or aids the permanent
magnet field to switch them.

The big advantage is that the relay stays in the
same state on loss of power and only require a power
pulse to activate rather than steady current.
[noparse][[/noparse]there are also mechanically operated latching types
which have no permanent magnet inside].

Note that there are two coil latching types which
do not require polarity reversal but stay in the
state last activated by one or the other coil.

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  • 7 Comments sorted by Date Added Votes
  • Polarized Relays:

    Just to re-visit an important subject, and something that caught me up because I didn't know it existed. I was trying to use up some salvaged relays, and create a dual relay board for a project. Without a datasheet and no schematic printed on the relay itself, I had a 50/50 chance on getting it right the first time. Well I got it half right anyway, re-work in progress.

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  • Always worth posting a request for information. Surprising how often a forum member has the info to help out.
    In science there is no authority. There is only experiment.
    Life is unpredictable. Eat dessert first.
  • YanomaniYanomani Posts: 470
    edited October 2016 Vote Up0Vote Down
    Hi MikeDYur

    As per a newer datasheet I'd found at Jameco's web site ( ), the salvaged relays you have at hand are from the Single Side Stable type.

    Looking at your excellent perfboard work (in fact, very nice one) and provided my someway old glasses didn't cheated me, in the top view picture, the rightmost relay freewheeling diode should be reversed.

    Sixty years old eyes (even with glasses) couldn't discriminate more than this, then is up to you to fix the switching transistor circuit accordingly.

    Also, the S in the relay spec seems to mean that it pertains to the 200 mW nominal operating power, so, about 40 mA should be flowing thru the coil, @5 Vdc operation.

    From the datasheet, a S type relay @ 5Vdc will have a pick-up voltage of 3.5V DC (max.), then, if you are operating it with a 5 Vdc power source, the series resistor value should be lowered, from the actual 220 R to something in the range of 27.5 - 32.5 R, assuming a 0.2 - 0.4 VCE drop, in the saturated transistor switch.

    Hope it helps a bit.


    P.S. Sure can be my poor eyes, but, looking at the solder side, it now appears that the 220R resistor is in series with the freewheeling diode.
    In my opinion, it should be placed in series with the transistor collector, assuming the emmiter is at the ground potential. The diode should be left alone, in parallel with the coil.

    P.S. 2 - Perhaps a better approach...

    R1 is in the 27.5 - 32.5 Ohm range, in order to limit the freewheeling diode discharge current thru the 5 Vdc supply circuit, which in turn should have its regulator output protected against sudden overvoltage spikes generated by the coil(s) discharge diode(s).

    (only half the actual number of DPDT contacts available into each relay shown!)


  • There are also some excellent application notes from TE Connectivity (formerly Tyco) about the subject of properly dealing with relay coils transient supression circuits and its effects on reliability and operational life of relays.

    Just Google "TE Connectivity relay application notes" that leads to

  • Henrique, Thank you for your help on this.

    I had checked the circuit last night and checked out OK. So I had researched the relays before my first post. It was just a matter of swapping the connections on relay number two, all works fine now. The 220 Ohm current limmiting resistor's on the LEDs is a little much for 5v, but I have a lot of them and may lengthen the life of the lamps.

    Once again thanks for the help, and I appreciate the kind words.

    BTW: I do know how that eyeglasses thing works, can't see nothing without them, and I can't see to much with them. Wouldn't you know the wife and I had eye doctor appointments yesterday, she went and I didn't, not too smart of me.

    Mike Y.

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  • YanomaniYanomani Posts: 470
    edited October 2016 Vote Up0Vote Down
    Myke Y.

    My bad! The leds, I should have seen the leds and their associated resistors.

    My wife and I, we live almost next door to a hospital; less than 100m to be true.

    Many young doctors, nurses and so on... Lots of opportunities to have an appointment with one of them, and do some eyes examination, but me too, not too smart...

    Also a nice video clip, very instructive! The clicks did resemble some Flamenco dancer, her shoes tapping and stomping...

    A solid-state relay will never produce that sound! The future generations will lack both the audible and tactile feedback.

  • Once upon a time I made relay control for my 70-cm antenna. I wanted polarisation switch etc features, so. I started to search for high frequency relays. I found Omron G6Z-1P 9V DC. They are standard relays (meaning: no polarized). I was surprised when apply 9 volts. They work only when DC applied in correct polarity. Similar case with small signal (don't remember brand) 5 Volt relays from old radio relay link module. Why? Answer is simple: (proudly) made in Japan. They are making polarised relays and by removing some mechanical parts relay become standard (momentary) relay. Didn't figured out why some still need correct polarity. Recommendation: If you are using high quality brand relays, check polarity before making PCB.
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