Transistor Confusion and other Electronic Gibberish

All,

Ive been designing a simple speed controller (well, kind of) based on logic gates, transistors and relays to allow F, B, FL, FR, BL, BR and Spin L and R all at full speed only. Im by no means an electronics whizz so Id appreciate some guidance.

Firstly - can I use 74 series CMOS logic gate ICs on 12V? If so, do I need to put resistors in series with them or anything in order to increase/reduce the current? My design is entirely based on this premise and if this is not possible then Im going to need a rethink, or a separate level of relay switching a relay switching a motor.

Second : I bought a couple of MOSFETs and some automotive relays at weekend. When I connected battery to MOSFET (drain I think) to Relay (via Source of MOSFET) to battery, the relay activated whether the gate had a current or not. If I connected the gate the relay stayed on even when the drain was disconnected, but if I just connected gate and source, the relay did not activate. Are my FETs fried, am I using them wrong, or do I need some extra resistance or something to prevent the FETs being triggered?

Thatll do for now. Im sure any responses will throw up further questions.

Cheers

-- Kev

Incidentally Ive put this in the wrong forum. I meant to put it in featherweights, but Im not sure where it best fits to be honest :proud:

-- Kev

You cant drive the 74xx series at 12v. They are 5v only. You can drive 4xxx series at 12v, but I dont think you can do that with all of them.
Read the datasheets! There are tons of them on the net.
If you dont have a part number of the fet I cant help you with that, although odds are the pins are gate-drain-source. It sounds like it is still OK. Did you know they are sensitive to static electricity? So handle them carefully.

The pins are gate-drain-source, but if I connect the load across drain and source the relay shouldnt activate until the gate is connected should it?

Thanks for the IC info. More plans needed I think. Might have to incorporate a 5V relay to activate the 12V relay - unless anyone can think of a better way?

-- Kev

Oh and the part number of the FET was (I think) BUZ10. Or at least thats how the maplin catalogue described it.

-- Kev

Unlike a normal transistor, FETs are turned on or off by the voltage of the gate terminal relative to the source, rather than a current flowing into the gate.

The gate has a small capacitance so it will stay charged to a high voltage after it is disconnected, so it may be that you need to connect the gate to the source in order to turn it off. In fact you can often turn them on or off by touching the gate terminal with one hand and touching the battery postive or negative terminal with the other hand. They are very static sensitive though, as Stefan says.

So if I connected it backwards by mistake (i.e. batt + to source), the gate and source may both be positive and the thing might turn on? Or am I misunderstanding there?

Would I be more successful using a high power standard (bipolar) transistor? (as I tried the same experiment with some unspecified transistor and the same thing happened) - would it even need to be high power?

Cheers

-- Stupid Kev

Most MOSFETs have an internal diode (or behave as though they do) so current will flow form source to drain regardless of the state of the gate.

You dont need a super high-power transistor - normal car relays have a coil resistance of 85 ohms, which is about 150 mA at 12V. Id use a transistor with at least twice that current capacity as its an inductive load. Personally I would use a Darlington array IC - search for ULN2803 on Maplin.co.uk. This will probably be cheaper than building it out of discrete components as well.

Also, if youre using a MOSFET or NPN transistor, connect the source or emitter to ground and the load between the battery positive and drain/collector.

Ive seen darlington arrays and wondered what they did - so are they basically a complex transistor? Or a transistor with built in safety stuff?

Sorry for the silly questions but Im going from first principles here :proud:

A darlington driver is basically two transitors connected in sequence. One small transistor drives the base of the bigger transistor. This provides a lot of amplification so you can control large loads from a CMOS level output. They dont provide any extra safety as such.

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.