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cheap electronic relay circuit

Discussion in 'Electronic Design' started by CampinGazz, Jan 27, 2004.

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  1. CampinGazz

    CampinGazz Guest

    i need to build a number of circuits that can replace relays,

    the voltage will be 12 volts, the load to be switched is a max of 1.5 amps,
    the trigger viltage will be 12 volts, and a max current of 50Ma.

    Basicaly i have a 8 way light switch (tough panel) that has latching ouputs,
    that shoud power relays to switch the lights on and off, but the system
    needs to be economical on the power, so having 8 relays drawing power must
    be avoided,

    it's all Direct current BTW.

    i need something simple to build, that can replace a relay, i knwo you can
    buy solid state relays, but they are too expensive for this project,

    was told about SCR's, but they latch and need the power removing from the
    load to turn off, i need something that will allow power to flow between 2
    points when voltage is applied to another point, and interupt the power flow
    when the voltage is removed from that point.. just like a relay, but
    electronic.. mosfet circuit??
  2. Rolavine

    Rolavine Guest

    ubject: cheap electronic relay circuit
    If you can stand the drop why not use PNP darlington transistor like a TIP125.
    Connect the emitter to the +12, and the collector to your load to ground. With
    an open to the base they are off, pull the base low via a resistor to about a
    volt below the 12, and it turns on. I think this sounds like what you need.
    TIP125's can be had in quantity for about 50 cents each.

  3. Yup, you need the Mosfet's !!
  4. You can use latching relays. They need current only when changing their

    If you don't like that, use power mosfets. Just apply the 'trigger viltage'
    to the gate. Most mosfets have three pins, a gate, a source, and a drain.

    An N-channel MOSFET whose source pin is 0V will turn on (its resistance will
    go to near 0) when the gate goes from 0 to 12V. You connect your load
    between 12V and the drain pin. The gate doesn't need any current, just the
    voltage, so its economical on power.

    1.5A is not alot of power for a mosfet, but it might get hot, so a heatsink
    would be required.

    In the US, you can get them at Radio Shack, or online at places like or

    Bob Monsen
  5. Rolavine

    Rolavine Guest

    Subject: Re: cheap electronic relay circuit
    If you want to use an N channel logic level mosfet you will need about 17 volts
    to apply to the gate to turn them on. This is because your switching the 'high
    side'. Using a P channel mostet makes more sense for you.

    Since you seem unsure of how to design this, you might want to look at "High
    Side" Driver ICs, this will make life easier for you. These usually have some
    protection built in which is a nice feature. On a switched power circuit,
    unless the signal is isolated to the same circuit board I always put protection
    on it (well unless it's got to be cheaper) I just designed some International
    Rectifier high sides into some aircraft electronics, and they worked great.
    Digi Key has them.



    Good luck.
  6. Keep in mind that with most of these you can't have the "relay"
    continously "on" because of the way the bootstrap supply works.

    Best regards,
    Spehro Pefhany
  7. Fred Bloggs

    Fred Bloggs Guest

    If you are switching 1.5A loads, then it makes no sense to skimp over
    20mA of relay drive- that is 0,020 Amps :

    Please view in a fixed-width font such as Courier.

    | |
    | / |
    +--|----o o-----|---------> to load
    | | |
    12VDC>--+--|------+ |
    | | | |
    | | )|| |
    | | )|| |
    | | )|| |
    | | )|| |
    | | | |
    | ---------------
    | |
    | |
    | |
    | 1N4001 |
    3.3K |/
    touch panel>--/\/\--| 2N4401
    output |\
  8. I read in that Fred Bloggs <>
    You could tell him how to reduce it, even so, with an RC parallel
    network in series with the relay.
  9. CampinGazz

    CampinGazz Guest

    OK, i'm lost now,

    i guess i was assuming i'd use the standard automotive type relays, which
    pull 180 Ma for the coil,
    the largest load will be 1.2 amps for a standard 12 volt fluorescent tube,
    smallest 50ma or so for a small led array, the rest 700ma for the ccfl's,

    i'd also have a few other items i want to switch in the van where i could do
    with loosing a relay to a solid state circuit,

    but the only voltage i'll have onboard will be 12 volts (14.8 when the
    charger is on line.. but most of the time it'll be 12.8 volts or lower as
    the battery gets drained)

    simple is what i'm after idealy, i'd found a simple circuit that just uses a
    npn bipolar transistor and a 47k resistor to switch the negative side of a
    load on when +12 volts is applied to one of the transistors legs through the
    this particular circuit is to switch a relay on.. but is done to present a
    very low drain current to the power source turning the relay on.. in this
    case a car stereo's power antenna output, that must be no more than 500ma,
    apparantly it presents a 0.005 amp draw to the switch source,

    that circuit handles 400ma, so i'd need something beefier for my circuit..
    but i'd idealy like to switch the positive side rather than the negative
    side of the circuits, is the page with that circuit on, half way
    down.. titled "EXCESS JUNK"
    that's where i got the origional idea from,

    i was hoping to do something similar with a larger transistor and maybe a
    few more components to switch the pos side,

    on boats they have solid state split charge units.. sold as zero volt drop
    diodes.. basicaly they need to connect 2 batteries together only when the
    engine is running, the old way was to use a skottky diode or 2, and split
    the alternators output, but the resulting volt drops affected the charge
    but now they use a few mosfets back to back in the zero volt drop units, and
    they are switched when the engine is started, basicaly a high power solid
    state relay, but i'd imagine there's more to the circuit than a few mosfets
    in a heatsink case.
  10. Why does he need high side? Seems simpler with N-MOSFETs, given his
    activation signal (high = light on). He is just turning on lights. They can
    live between the mosfets and the 12V supply, right?

    In order to do high side, he'll need a way to invert the signal (ie, your
    high side driver IC)

    Bob Monsen
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