N Chan Mosfet as current source

[Freddybaby]

I need a current source with up too 350V compliance. I have a bunch of IRF710 (400V, 2A). I only need about 10mA.

My actual application is running an LED from rectified mains (20-240VAC) The input voltage will vary so just a resistor will not work.

Anyone have a simple schem or is this not a great approach.

 

[keith1200rs]

Bear in mind rectified mains will peak at around 340V.

I did draw out a circuit based on a conventional opamp based constant current source that would work using the IRF710, but I think it is a bit risky so I haven't posted it.

The high capacitance of the transistor will give a high surge current when you apply power and could blow up the LED. A current limiting resistor would help but if you want to work down at 20V your scope for that is limited. A resistor to limit the surge current to say 20mA at 20V could be around 0.4A at 340V.

There is a risk of kickback though the MOSFET drain-gate capacitance blowing up any driving circuitry. A limiting resistor is a more workable solution there.

Bear in mind that dropping nearly 340V even at 10mA will dissipate significant power in the transistor (3.4W).

I think a switching regulator approach would be better. There may be devices around which will do that but I haven't used any. Hopefully some else can suggest one (or a circuit configuration with the IRF710 which doesn't kill you). Maybe a more crude, all discrete circuit which uses a simple zener reference and source resistor would be better, but I would still be worried about the power dissipation and surge current.

Keith.

 

[Fom]

Resistor+Zener diode. ZD is connected to gate of IRF710. Between Source and GND another resistor to limit current by 10mA.

 

[keith1200rs]

Resistor+Zener diode. ZD is connected to gate of IRF710. Between Source and GND another resistor to limit current by 10mA.

You need to be careful of the switch on surge current due to the MOSFET capacitance. A capacitor across the LED might solve that. You will still have to be careful about the power dissipation in the various components. The gate resistor has to turn on the zener at 20V and also at 350V. The precision of the current will be poor due to the Vt variation, but maybe that doesn't matter in this application.

Keith.

 

[Freddybaby]

I think I'll air on the side of caution and go with a resistor. This will slow down the response of my sensor at the lower voltages (Led is an optocoupler) but I'll have to live with it.. Fortunately this particular application may be limited to a single input voltage but I wanted the possibility of working with wide input voltages.

Thanks again for the comments and practical advice.

F

p.s.

I did find some current regulating diodes but they are cost prohibitive ($ 4.75 a piece and I would need 4 or 5) and I have never worked with these devices either.

 

[erikl]

.. Fortunately this particular application may be limited to a single input voltage
... current regulating diodes ... are cost prohibitive

At a limited input voltage range - say 200..240V~ - you could use a capacitive current limiting with a ≥ 400V rated X- or Y-capacitor of about 0.15µF + bridge + surge-limiting protection resistor. This would avoid the power dissipation of >2W.

 

[pawanbahl]

At a limited input voltage range - say 200..240V~ - you could use a capacitive current limiting with a ≥ 400V rated X- or Y-capacitor of about 0.15µF + bridge + surge-limiting protection resistor. This would avoid the power dissipation of >2W.

If all you are doing is driving LED of an optocupler, i suggest you use the LR645 of Supertex. its a 400v device. add a resistance to its output to precisely set the LED current directly from mains.