How can you make a mosfet switch at a voltage of your choice?
Typically I'm looking to switch on a relay or very small resistive load at 2.8volts
My rail will be 12vdc the gate source is related to the rail supply but I do need resolution of .1volt. Some sort of potentiometer will be needed to adjust the gate threshold as 2.8 might not be the voltage I need transition at

 

The gate voltage needed to turn on a mosfet fully is ~10V, or ~3-4V for a 'logic-level' type. So for a turn-on voltage of 2.8V you will need a comparator/opamp preceding the mosfet.
Edit: Can you use a mosfet instead of the relay to switch your load?

 

The gate voltage needed to turn on a mosfet fully is ~10V, or ~3-4V for a 'logic-level' type. So for a turn-on voltage of 2.8V you will need a comparator/opamp preceding the mosfet.
Edit: Can you use a mosfet instead of the relay to switch your load?

you say a mosfet is not what i need for my aim but then say can i use a mosfet?

 

There are Mosfets with even lower threshold voltages, I used some that turn on at 1V. But these thresholds are not adjustable as was mentioned.

As was mentioned, you need some kind of comparator before that. Depending on load current all you need would be that comparator, not even a Mosfet.

 

You can drive your mosfet with this circuit fragment.
1v will drive the output to 4.3 volts.

Should work well to drive a mosfet at lower frequencies.

Cheers.

 

200ma.
Some resistors will surely be needed to get my transition right at 2.8

 

You didn't say what the adjustable range is. Have a look here too: http://www.allaboutcircuits.com/vol_6/chpt_6/2.html

The output of the comparator can directly be connected to a Mosfet gate. use a open-drain comparator with a minimum output voltage spec of 15V. Then drive a common Mosfet with it.

 

The signal range is 0-5v

 

Just to make sure: The adjustment can be 0.1V or 4.9V or anything in between, correct?

The input signal to be compared can be anything between 0 and 5V?

 

Just to make sure: The adjustment can be 0.1V or 4.9V or anything in between, correct?

The input signal to be compared can be anything between 0 and 5V?

Don't understand the first question but yes the signal could be 0-5vdc and I will want an output when 2.8v is reached

 

Trying to boil this down, mosfets have miserable input specifications for selecting a certain voltage. You absolutely need an adjustable switching circuit. I drew one. When you find out how much current you need switched on, you can choose a comparator chip to do the job or have a comparator switch a mosfet.

 

200ma.....

 

I know that's a comparator symbol but what would the pin number connections be

 

Depends on the comparator. As I mentioned there are thousands. Do you have any or where do you buy them?

 

EBay or maplins uk

 

That makes a difference.
You could go to the top of the page and click on, "User CP" then enter the country you live in. That helps us find parts.

Maplin is weak! Their search engine wants the weight, depth, and width of a mosfet!

Can somebody find me a logic level mosfet for this job at Maplins?

 

You're spending a lot of time looking into mosfets, but it seems like a programmable ujt would fit your needs much better?

 

I'm a beginner in electronics so correct me if I'm wrong, but it seems that to my understanding a PUT would work easier.

 

I'm a beginner in electronics so correct me if I'm wrong, but it seems that to my understanding a PUT would work easier.

I've no experience so open to ideas

 

You're spending a lot of time looking into mosfets, but it seems like a programmable ujt would fit your needs much better?

I'm a beginner in electronics so correct me if I'm wrong, but it seems that to my understanding a PUT would work easier.

No, UJT or a PUT will not do this very easily. Please don't confuse the issue unless you actually have a circuit in mind and are willing to show it. The OP is getting good help from people who know what they are doing.

Comparators are cheap and reliable, very tested.