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Running a 24v relay from mains voltage?

Discussion in 'Electronic Basics' started by DaveC, Nov 21, 2003.

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  1. DaveC

    DaveC Guest

    I need to power a relay from 120 vac such that its action is delayed for a
    few seconds after power-on. I realize that this means getting DC voltage (via
    diode) and an R-C combination.

    Ideally, a relay with DC-rated coil would be most appropriate, but I've got a
    nice new relay with properly-rated contacts sitting on the shelf with a coil
    rated at 24 vac, 330 ohms, 71 mA.

    How complex would it be to run this relay with mains voltage? Is it as simple
    as a diode followed by a resistor? Would this resistor be 120/0.071/2 = ~500
    ohms and 2.5 watts?

    And a fundamental question: can a relay with an AC coil function properly &
    reliably on DC?

    Since I need to delay the relay's turn-on, I'll be adding a capacitor,
    anyway, whether I use this relay or a true DC-rated type. What should the
    value of the capacitor be to give me a 1 or 2 second delay with the 500 ohm
    resistor (or, if I've miscalculated that value, with the proper value

    Any observations, suggestions, corrections (I'm getting used to these!!) are

  2. DaveC

    DaveC Guest

    It occurred to me that I didn't calculate the *dropping* voltage of the
    resistor, as I should have, and didn't use mains *peak* voltage, as I think I
    also should have:

    ((170/2)-24)/0.071 = ~850 ohms and 5 watts. Since this resistor value is not
    common, I'll have to use a more-common value. These more-common values will
    result in an applied voltage to the relay's coil of:

    750 ohms: 32 vac - too high
    820 ohms: 27 vac - max voltage allowed by relay mfr.
    910 ohms: 20 vac - too low
    1K ohms: 14 vac - way too low

    And yes, John, I learn from all my electronics work, so you can always assume
    it's all for educational benefit :)


  3. Dave:

    Here is a little circuit that will do what you want, delaying the relay from
    latching for a few seconds. Its designed to pump up to 24V with a load of
    330 ohms after about 2.5 seconds. It wastes a couple of watts in heat.

    Don't use this in the bathtub...

  4. Guest

    Lets set aside the other issues, and just address the
    delay time. In rough terms, a resistor in series with
    a capacitor will charge the capacitor to about 63 % of
    full voltage in a time equal to R times C. That figure
    is called the RC time constant. When R is only 330 ohms,
    you'd need a large C to get a few seconds delay - about
    10000 uF to reach 63 percent of full voltage across
    the cap at about 3.3 seconds.

    The rest of this post goes into some light theory and
    ends with a description of a nice circuit you can
    experiment with and possibly use in your amp.

    You could get the same time constant with a much
    bigger resistor, and a much smaller capacitor.
    Make the resistor 33000 ohms, and the cap 100 uf
    (.00010 farads): 33000 * .0001 = 3.3.
    The trouble with that occurs when you connect the
    relay across the cap. All the time the 33000 ohm
    resistor is trying to charge the cap, the 330 ohm
    relay is discharging it. This will keep the cap at
    a very low voltage.

    However, suppose instead of placing the relay coil
    across the cap, we place a very high impedance (many
    times higher than 33K ohms) across it? It won't have
    much of an effect on the time constant - but it will be
    able to "see" the voltage rising on the cap. Now,
    suppose we prevent that high impedance from seeing
    the voltage at all, until it reaches (or exceeds)
    some specific level? Well, we can do that by putting
    a zener diode between the high impedance circuit
    and the cap. Lets select a 9 volt zener, put it
    in series with the 33K ohm/ 100 uf cap junction,
    and stuff the other end of the zener into the base
    of a darlington transistor. Complete the darlington
    (NPN) circuit by connecting the emitter to ground and the
    collector to the relay coil, while the other side of the
    relay coil goes to + 12 volts.

    What happens? When power (12 volts DC) is applied across
    the series RC made of the 33K resistor and the 100 uF cap,
    the cap charges to ~7.5 volts in ~ 3.3 seconds. In
    another second or so, it will reach 9 volts, and the zener
    will conduct. In turn this will cause the darlington to
    conduct and the relay will transfer. If you replaced
    the 33K ohm resistor with a 50K ohm pot, and added a 4.7K
    resistor in series, you would have a nice adjustable
    time delay.

    This is a nice experiment, and the parts are cheap. You need
    to select (or make) a darlington that can easily handle the
    power that the relay coil needs and an appropriate zener and
    relay voltage. For a turn on delay for your amp, precision in
    the timing is not important, so even if the timing varies with
    temperature that should not matter. There are other alternatives
    to the circuit, but for initial experimentation with RC timing,
    it is hard to beat and it could also be a nice fit for your amp.
  5. DaveC

    DaveC Guest

    What kind of caps are these? Electrolytic? Poly? Please specify each so I can
    know what to acquire.

    Thanks for the circuit!
  6. The 4.7uF cap is a big fat non-polar 250V cap. A mylar cap should work fine.
    The voltage rating is marked on the cap, or you specify it when you buy it.
    Don't use anything with a lower voltage rating!

    The 4700uF cap is a big electrolytic cap (you probably can't get anything
    else that big.) The minus on the cap (it'll be marked clearly) goes to the
    ground (which is the bottom line) in the circuit. Since you are going 24V,
    you need something that'll handle a high enough voltage, like a 25V cap. You
    should go 50V just to be safe and to extend the life of the cap.

    You can get both of these, and a sandstone 10 ohm 5W resistor mailorder from

    If you are going to build this, mark HIGH VOLTAGE clearly somewhere on
    inside the case, make sure NOTHING, including the relay, can be touched by
    users, and encase the wires and leads in heatshrink tubing (you can also get
    that at goldmine, I believe.) You could also use a cheapo hot glue gun to
    cover all the wires in goop. That should insulate them. Just make sure the
    goop is actually non-conductive.

    One last thing, I'd put a 1MEG 1/4 W resistor in parallel to the 4.7uF cap
    just to be safe. This will keep you from shocking yourself when the circuit
    has been off for 2 hours and you pick it up. If you put the switch right
    before the cap, it could conceivably be charged up to 165V, which would mean
    64mJ of energy stored across the cap, enough to wake you up.

    Bob Monsen
  7. Rather than use that monster 4.7 uF capacitor, it might be cheaper to
    use a small power transformer. About the only place I can remember
    seeing a cap that size is for a fan motor, and it would have to be
    paralleled to get that much capacitance. And it was about the same
    price as a power transformer, maybe $5 or so.

    Among this list, the 7841 looks like it's a good choice; it's cheaper
    than that monster capacitor, and safer because it isolates you from
    the AC line voltage. When it is rectified and filtered, it will be
    about 24VDC. But your relay will take less than 24V if it's DC
    instead of AC.

    @@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@,@@[email protected]@[email protected],@@[email protected]@[email protected]@[email protected]@
    ###Got a Question about ELECTRONICS? Check HERE First:###
    My email address is whitelisted. *All* email sent to it
    goes directly to the trash unless you add NOSPAM in the
    Subject: line with other stuff. alondra101 <at>
    Don't be ripped off by the big book dealers. Go to the URL
    that will give you a choice and save you money(up to half). You'll be glad you did!
    Just when you thought you had all this figured out, the gov't
    changed it:
    @@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@@
  8. Fred Bloggs

    Fred Bloggs Guest

    Where are you getting these numbers? Did you measure 71mA or the 330 DC
    ohms? In either case the AC relay reactance is usually 10x the
    resistance if the relay is of any quality, and therefore all of your
    component planning will not come close. Measure the DC ohms Rdc and the
    total ac-RMS current, Irms. Then total reactance X=24VAC/Irms and
    inductive reactance XL= sqrt(X^2-Rdc^2). Then you solve for series R
    required at 120VAC operation, Rs, as |(Rdc+jXL)/(Rs+Rdc+jXL)|=24/120 etc...
  9. A transformer would also work here. However, you can get two 2.2uF 250 caps
    for a buck at and put them in parallel. That would
    also work (albeit without the isolation)

    Actually, I was thinking this was a DC relay, but the OP clearly stated its
    a 24V AC relay, so... I'm not sure about the delays now. Aren't AC relays
    just DC relays with a diode? If so, the circuit should still work as
    advertised, since he measured DC current/resistance, and the circuit will
    supply DC current. If they are something else, all bets are off.

    Bob Monsen
  10. Da Man

    Da Man Guest

    Many film caps in that range are not that big. I have several 3.3 uF caps
    rated for 250 or 350V, which are only about 1.5" by 0.5" by 1", and I saw
    2.2 uF 250V caps that were about 1" by 1/4" by 1/2".
  11. Da Man

    Da Man Guest

    An AC relay is a DC relay with a shorted turn so that the current induced in
    the shorted turn (remember, this iron core is like a transformer when
    operated on AC), which keeps the magnetic flux around for a while while the
    AC cycle changes direction. The plunger also weighs more in most cases, so
    that it takes more time to accerate ftom the rest of the iron, helping
    prevent the relay from buzzing. When operated from DC, they often turn on at
    much lower voltages.
  12. DaveC

    DaveC Guest

    None of these numbers were measured; they come from the mfr's spec sheet:

    (BTW, I goofed on the coil resistance; it's 303, not 330 ohms.)

    According to Fred (posted later in this thread), an AC coil has one coil

    Bob, how does this change your circuit values, if at all?

  13. Actually, more like a half turn. That's that D shaped piece of copper
    that covers half of the end of the relay's pole.
    @@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@,@@[email protected]@[email protected],@@[email protected]@[email protected]@[email protected]@
    ###Got a Question about ELECTRONICS? Check HERE First:###
    My email address is whitelisted. *All* email sent to it
    goes directly to the trash unless you add NOSPAM in the
    Subject: line with other stuff. alondra101 <at>
    Don't be ripped off by the big book dealers. Go to the URL
    that will give you a choice and save you money(up to half). You'll be glad you did!
    Just when you thought you had all this figured out, the gov't
    changed it:
    @@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@@
  14. According to your OMRON tiff, the thing 'must operate' at 75% of the rated
    voltage of 24V, so thats 18VAC. According to a different poster (Da Man) AC
    relays will operate at even lower values when operated with DC voltage.

    Post the DC voltage that operates the thing, and I'll tell you what values
    to use to get it to operate after 2 seconds.

    Bob Monsen
  15. DaveC

    DaveC Guest

    Since it will work within a range of voltages, shouldn't we aim for one that
    minimizes the size of components? Which would be max voltages, allowing for
    smallest dropping components. (But which, I am reminded, means larger
    components -- ie, working voltages -- for the others.)

  16. If you really want a 2 second pause, you'll have to experiment with the
    values. Since I don't know what voltage the relay will close at, I can't
    choose the values for you.

    If you are just looking for a delay near 2 seconds, then I'd guess (assuming
    it'll close at 15VDC) 3.3uF, 10R, 4700uF, which are standard values. I'd
    also put in a fuse for fire safety in case the 3.3uF cap fails.

    I've updated the web page to reflect this and your new component values.

    Bob Monsen
  17. DaveC

    DaveC Guest

  18. Yes, its the relay coil's resistance. I used it to model the time delay.

    Bob Monsen
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