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OK sounds good. Would it be possible to use a darlington driver to switch a 12V supply to a relay from a 5V input from a CMOS IC? Or am I best relying on a smaller relay driving a bigger one, to drive the motor?
Cheers for all the help. I got five electronics books from the library at weekend but Ive already learned more here :proud:
-- Kev
Since youve got the MOSFETS heres how to connect them to the relay coils but 2 resistors are needed.
Starting at the Relay Coil. Connect one end to Battery +ve the other end to MOSFET drain. Connected MOSFET source to Ground.
Connect one 100k (ish) resistor from MOSFET Gate to MOSFET Source (Ground). This keeps the mosfet off when no voltage on Gate is present.
Connect the other resistor at least 10 times less than other resistor (eg 10k) to the gate and the other end to the input signal 0V = Relay off & 10-15V = relay on (Maybe 5V if logic MOSFET).
You can uses the same circuit for NPN but use lower resistors depending on gain of transistor but 1k & 10k will be typical and will turn on with more than 1V input and Gate = base, Source = emitter, Drain = collector.
When you say Ground thats the same as battery negative i assume - i.e. connect batt -ve to the same ground?
Thanks for that. I might give that a go.
Out of interest - how did you work out those resistor values or is it just an experience thing?
Oh and how do I know if its a logic mosfet? Ill be going through logic gate ICs so if I cant switch the MOSFET on 5V Ill need to use an alternative transistor (either to switch the relay or the MOSFET - see below)
Is it possible to feed a low voltage (5V) to the gate/base from the logic circuit, while passing 12V between source/emitter and drain/collector in an otherwise separate circuit?
Cheers
-- Kev
Ground is the same as 0V- technically speaking there isnt a battery negative, just a battery 0V, unless of course youve got 2 batteries wired up to give a +ve, -ve, and 0V, but thats another story...
The R values are fairly arbitary- but those are fairly standard for voltage driven devices- they are pull down resistors, which stop the ouput device floating, which may turn the MOSFET on, even when you think its off, which obviously isnt particularly desirable. These are reletively high R values, but thats of no consequence in a voltage driven device, in fact its more desirable. Its normally just experience through playing and tinkering and designing such circuits- no real calculations as such.