Mosfet for current boost.

[sureshot]

Having used BJT's for boosting regulator current, i was curious as to whether a mosfet could be used instead. The diagram schematic is all i can find on the subject. RX is said to be 10 ohms 10 watts for the gate resistor. Can anyone tell me if this circuit would function. I'm aware the LM317 built in protection might be compromised. Just curious to know if it would work.
Thanks for reading, any advice appreciated.

[digsys]

FETs can be ideal for Linear Regulators, but definitely not that one, and not that circuit.
First - the FET MUST be rated for linear mode operation, ie the SOA MUST include DC. That one is only specified to 10mS
Second - in that circuit, you will need up to 4V LOSS across Rx to put it in linear mode. With ~3V+ loss on the LM317, that's a lot !
Third - that circuit won't work :-)
If others don't chime in, I'll post a few circuits that I've used, or google Linear FET regulator. It is quite common

[JourneymanWizard]

You don't have to use a FET designed for linear operation (though it does help).  SOA at/below 10mS is usually considered "DC" enough.

For "triode" operation (the ohmic or linear region of a FET), you simply calculate the FET power dissipation based on the current through the FET times the voltage drop across the FET.  That is how much power the FET has to dissipate.  Usually the FET has a power rating listed somewhere (5W, 225mW, etc), especially if it is a power package like a TO-220, D2Pak, etc.  Read the fine print - it usually says that the power dissipation requires a certain amount of metal surface (10^2 mm 2oz Cu) to help spread the heat.

Note that in this circuit, the turn-on voltage required for the FET will be more than a BJT - Vgs may be 4 or more volts instead of 0.8-ish volts.  This will affect your headroom (as digsys said), and this value isn't as stable as the Vbe of a BJT

[Zero999]

Why do you want to use a MOSFET, rather than a BJT?

The only advantage I can think of is, generally MOSFETs have a wider safe operating area, than BJTs, but this isn't a factor, unless it needs to drop a large voltage, say >40V.

As mentioned above, the main issue is a MOSFET has higher threshold voltage, than a BJT, so it will have a much higher drop-out voltage, than if this circuit were made with a BJT.

Yes, this circuit does compromise the LM317's over-current protection, but there are ways to fix that. How much current are you planning to draw from the output?

[sureshot]

Thanks for your replys. About 3 to 5 Amps maximum for a single transistor fet.
Using for no other reason than to try it, see how well it works. So if I have an unregulated voltage of around 26 Volts, would it not work with a 10 ohm 10 watt resistor ?
I would like to keep the LM317 variable with voltage, if that's possible.

[Audioguru]

If the Mosfet needs a gate-source voltage of 5V to barely turn on then the 10 ohms resistor current and the current in the Mosfet is only 5V/10 ohms= 0.5A that an LM317 can do by itself if the input to output voltage is not too high. If you use a 1 ohm resistor then it will have about 5V across it and a current of about 5A.

[sureshot]

So would that be 5 Amps available from the mosfet, or 5 Amps the 1 ohm resistor is running at ? I think it means 5 Amps current from the mosfet, but what value in watts would a 1 ohm resistor need to be, possibly more than 10 watts ? I'm not sure. Would a fixed value voltage regulator like the 7812 and a mosfet be viable to do ? Again input voltage is about 26 Volts DC after rectifying and filtering. Thank you for the replys.

[sureshot]

I can put circuits together relatively easily, but my maths sucks. It's almost like number dyslexia for me, not very helpful in the hobby of electronics. I'm looking at the 1 ohm resistor again, now I think it means the lm317 would be doing 5 Amps, and that's not possible for that regulator. Please correct me if that's wrong. I'd really like to try a mosfet as a series pass current booster for a linear regulator IC.