Need help to use a transistor or a triac as a switch

MAJTAJMAJTAJ Posts: 3
edited 2019-09-11 - 08:16:27 in General Discussion
Hello everyone,

I have a digital programmable timer with signal output of 1.2V DC. It runs on rechargeable battery of 1.2V. I need to use this small output to run a small 6V relay. To my general knowledge, I need at least 3 volts to trigger a small transistor. Is there a transistor that can be triggered by 1.2V signal? or can I use a photoisolator triac to drive the relay with the small 1.2V signal? Thanks.

Comments

  • Since you say your battery is rechargeable, are you sure it really puts out 1.5V? Have you measured it? NiCads and NiMH batteries typically provide only 1.2V when fully charged.

    -Phil
    “Perfection is achieved not when there is nothing more to add, but when there is nothing left to take away. -Antoine de Saint-Exupery
  • tonyp12tonyp12 Posts: 1,949
    edited 2019-09-10 - 23:09:08
    "digital programmable timer" and that one is a of the shelf item with a data sheet?
    If you could make it provide a square wave going to mosfet gate, charge-pumping the 1.2V x3 probably only way.

    small relay, where are you getting its drive voltage, works on 1.2-1.5V?

    Start from scratch and use the 115V AC you have nearby
    https://www.mouser.com/datasheet/2/308/NCP786L-D-972808.pdf
    half/full rectified 115/220VAC to 5V DC at 5mA LDO

    Or 3.3vdc
    https://www.mouser.com/ProductDetail/ON-Semiconductor/NCP785AH33T1G?qs=sGAEpiMZZMuTlc58v6LktvfrgJyT5U5KtZx97xKWlSs=

    2.2uF HV cap on input.
    https://www.mouser.com/ProductDetail/EPCOS-TDK/B32923C3225M?qs=sGAEpiMZZMsh%2B1woXyUXj2wKL59MZ/VjZOo0QupFjOc=
    https://www.mouser.com/ProductDetail/Nichicon/UVR2W2R2MPD?qs=sGAEpiMZZMvwFf0viD3Y3bKr/78AuBoo/%2B9btcJXPUE=
  • I am assuming that the trigger signal is the only low voltage component and that what he is triggering is separately powered. What about sensing this with an analog input of a small microcontroller and have it drive the power circuit?
    Failure is not an option...it's bundled with the software.
  • Failure is not an option...it's bundled with the software.
  • Peter JakackiPeter Jakacki Posts: 8,641
    edited 2019-09-10 - 22:55:09
    A BJT such as a common NPN is driven by current, not voltage directly, but have a voltage threshold of 0.6V so they are very easy to use at low voltages but that is why you need a current limit resistor feeding the base.
    However even if you had this your desire to drive an AC from such a minuscule voltage source pretty much rules out relays at that low voltage. You would be better to drive the gate of a TRIAC directly, but now I can't give you any details because you want to work with AC and life and limb are involved here.

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  • I asked my question about the battery, thinking that an optoisolated triac driver (e.g. MOC3010) might be the answer. But there's a big difference between 1.2V and 1.5V as regards the forward voltage of the IRED in those devices.

    -Phil
    “Perfection is achieved not when there is nothing more to add, but when there is nothing left to take away. -Antoine de Saint-Exupery
  • Thank you for all your replies and advice.
    The battery voltage is 1.28V, it is a small Ni-Cad.
    I want to drive a small 6V dc relay, and that will control the AC load. So the AC is isolated from the DC components.
    The Optoisolated triac driver suggestion is great, but I'v never used one before. Would it run on 1.28V signal? and can it trigger/drive a 6V relay ? Please advise?
    Kind Regards
  • Peter JakackiPeter Jakacki Posts: 8,641
    edited 2019-09-11 - 04:31:12
    Your voltage is too low and while it could drive the IR LED of an opto, it is very marginal and LED current would vary greatly depending upon whether your battery was 1.2V or 1.4V etc.
    An NPN will definitely work but is of no use as you only have 1.2V to switch anyway and I'm not aware of any relay that runs at that low a voltage. You would need a TRIAC which you can turn on using the NPN from gate to MT1 for instance which would then be connected to the battery common. If all this was absolutely isolated from anyone accessing the battery while the thing was connected to AC then it should be safe, otherwise, as I say, death death death.

    BTW, with what would you operate a 6V relay with when you only have 1.2V or AC?
    Please note fellow forumistas - we are treading on litigious and dangerous ground going any further with this at this very basic hobbyist level. Even if litigation or any law was not involved, can we live with the knowledge that someone was killed because our advice was acted upon, even if it was done totally incorrectly?


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  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 22,440
    edited 2019-09-12 - 01:07:55
    MAJTAJ,

    Would your timer work on 3V? If so, you might consider using a lithium primary cell (e.g. CR123) instead of a 1.2V rechargeable cell. The lithiums are not rechargeable, but they do provide long life. A 3V output would give you a lot more options.

    -Phil
    “Perfection is achieved not when there is nothing more to add, but when there is nothing left to take away. -Antoine de Saint-Exupery
  • we are treading on litigious and dangerous ground going any further with this at this very basic hobbyist level. Even if litigation or any law was not involved, can we live with the knowledge that someone was killed because our advice was acted upon, even if it was done totally incorrectly?

    Lots of "qualified" idiots out there Peter.

    I was working with an Italian machinery manufacturer, some years ago. 3-phase supply, transformed and rectified to various voltages. The 5v distribution rail was heavily guarded...nothing else. When I questioned this, the head of the electrical department looked at me with a "don't you know anything?" look. "The 5v rail can supply up to 200A...it's current that kills"

    I just walked away.

    Failure is not an option...it's bundled with the software.
  • MAJTAJ wrote: »
    Thank you for all your replies and advice.
    The battery voltage is 1.28V, it is a small Ni-Cad.
    I want to drive a small 6V dc relay, and that will control the AC load. So the AC is isolated from the DC components.
    The Optoisolated triac driver suggestion is great, but I'v never used one before. Would it run on 1.28V signal? and can it trigger/drive a 6V relay ? Please advise?
    Kind Regards

    While the 1.2V signal will be enough to turn a BJT transistor on it is not enough to turn on the 6V relay. That will require a 6V power supply of some kind.
    In science there is no authority. There is only experiment.
    Life is unpredictable. Eat dessert first.
  • What about using the Parallax relay board 27115.

    It uses it's own 5v supply to turn the unit on and off with just 3.3v.

    Mike
  • iseries wrote: »
    What about using the Parallax relay board 27115.

    It uses it's own 5v supply to turn the unit on and off with just 3.3v.

    Mike

    Excellent choice, but it would still need a 5V supply for the relay board, and probably a lower value base resistor for the 1.2V control signal to turn the transistor on to activate the relay.
    In science there is no authority. There is only experiment.
    Life is unpredictable. Eat dessert first.
  • Hello Everyone,

    I tried one simple NPN circuit that was suggested earlier, and it worked with the 1.2V trigger signal from the timer. I used 2N3904.
    Here is the circuit:

    Thanks a lot for all your help.
    873 x 304 - 6K
  • I'd hardly think that introducing another power supply was the "solution". The >600mV turns on the base of the NPN via a resistor and the collector drives the TRIAC gate directly shorting it to MT1 where the emitter is connected. That will work but of course the timer is now live and what's more, it has a rechargeable battery and that charging it in circuit or even accessing the battery may be lethal.

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  • I'd hardly think that introducing another power supply was the "solution". The >600mV turns on the base of the NPN via a resistor and the collector drives the TRIAC gate directly shorting it to MT1 where the emitter is connected. That will work but of course the timer is now live and what's more, it has a rechargeable battery and that charging it in circuit or even accessing the battery may be lethal.

    What TRIAC? The circuit MAJTAJ posted is the typical circuit seen on almost all the relay boards being sold these days. As long as the power supply for V1 in that circuit is is isolated from the AC line voltage as most wall warts and modular PS boards are it should be quite safe since the relay contacts where the mains AC lines in and out are connected are isolated from the rest of the circuit.
    In science there is no authority. There is only experiment.
    Life is unpredictable. Eat dessert first.
  • The subject line indicates a transistor OR a triac. By using both you can do this without using any external DC supplies.

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