Super-Simple Power MOSFET Linear Current Regulator

[lctorana]

Question -

could you use a power transistor instead of a MOSFET for Q1?

 

[OddOne]

Question -

could you use a power transistor instead of a MOSFET for Q1?

Yes, and jtr1962's circuit shows precisely that, only his example circuit is driving a string of low-power LEDs. Change the value for R1 in his diagram and use a MJE3055T or TIP3x/4x series for Q2 and you'll get the same thing with a bit less efficiency.

 

[OddOne]

The thing with shunt (linear) regulators is that they are not very efficient. Take the example of a 12VDC supply driving a single LED at 0.750A. The power going to the LED is about 2.70W (3.6V x 0.750A). The total power is 9W (12V x 0.75) giving an efficiency of only 30% - 70% of the power is lost as heat (6.3W) which has to be got rid of. The efficiency will go DOWN even further if the supply voltage is increased (it will go UP with lower supply voltages).

A switching buck regulator will typically be 80-90% efficient - lets use 85% as an example. Using the same 12V supply, the buck regulator will draw 265mA instead of 750mA - so for a given battery size, the buck regulator will last 2.83x longer (750/265) than the linear regulator. Also, the amount of power/heat that needs to be got rid off is 0.476W compared to 6.3W - less than 8% of the linear regulator.

A buck regulator is not terribly difficult to put together (a boost, buck/boost or SEPIC is a bit more complex). However, if efficiency is not a concern, then the simplicity of a linear regulator may be a viable solution.

Naturally I would have to agree - switchers are generally the best way to go. That said, a decent linear can be reasonably efficient (read: "efficient enough") if used properly, i.e. used to step down a supply that's only slightly higher voltage than the load wants. In my circuit example, driving one LED @750mA from 12VDC would be wasteful (12VDC - 3.4VDC - 0.6VDC = 8VDC, or 6W wasted through Q1), but nowhere near as bad when running three LEDs @ 750mA (12VDC - 10.2VDC - 0.6VDC = 1.2VDC, or 0.9W wasted through Q1). At load-voltage-close-to-supply-voltage operating conditions, the question becomes one of whether the efficiency difference is enough to justify the complexity and expense to go with a switcher.

 

[invent]

Hello,

I built the above circuit, current regulator with a few modifications. I used the base emitter junction of a transistor for the diode from the collector of the same type 2n3904 transistor, to the source of the mosfet. My current sense resistor is .75 ohms and I used two diodes across the matching voltage resistors above and in the transistors emitter's path to allow me to set voltage across the emitter's resistor to .375 volts. I regulated the voltage to the circuit to 6.5 volts. The voltage to the drain through the load is ~30 volts, with the ground tied to the 6.5 volt ground. I am not using a logic gate mosfet, I am using an irf510 but with the above changes I can get the Mosfet to turn full on or full off. I initially set the unit up to .5 amps. Most of the voltage is across my load (series Leeds). The problem I have is that the current changes drastically over time, related to the temperature of the mosfet.
1. I am dropping about 27 volts across my load.
2. This leaves about three volts across the current sense resistor and mosfet.
3. Ignoring the resistor, the mosfet sees about 1.5 watts
4. I can touch the heat sink and it does get warm, but not that I can not touch it.
5. My current continues to drop the warmer the mosfet gets.
6. If I blow across the mosfet the current will start to rise toward the original setting
7. If I leave the unit on in about 20 minutes (72 degrees room temp) I drop about .060 mili amps.
What changes did I make that could be causing these results?
I am setting the sense resistor to .375 volts to get my .5 amps through the load. Thanks for any help.
[email protected]
Dean

 

[invent]

Hello,

I forgot to reference the location that lists my version of the above circuit, before I altered it, see the link below. I hope this helps someone understand and offer help with my problem.

Thanks,


http://radiolocation.tripod.com/LEDdimmer/LEDlampDimmer.html

Dean