Need help to use a transistor or a triac as a switch
MAJTAJ
Posts: 3
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.
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
-Phil
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=
Maybe this?
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.
-Phil
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
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?
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
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.
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.
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.
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.
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.