# MOSFET current source / current limiter request for review

I need a small (somewhat) voltage independent current (about 10 mA), with a voltage range of 3-30V. I came up with the following circuit.

simulate this circuit – Schematic created using CircuitLab

I drew the resistor U/I characteristic in the Id-Vgs characteristic of the MOSFET:

The idea is the circuit will quiesce at the intersection of both characteristics.

The MOSFET is a depletion mode N channel type. Circuitlab doesn't seem to have distinctive symbols for enhancement and depletion mode.

I choose a type that can handle over 1W of power to keep well away from SOA limits.

I am aware that a similar solution exists for JFET's, but I could not find a JFET type that can handle the voltage requirement. Also I know similar devices with JFET's exist as discrete components (constant current diodes), but these are rather expensive (\$5 or more. The aim is to both keep component count and price low. A Widlar source is not an option, because it needs a reference current, which is not available.

Current constancy is less of an issue, it is meant to limit the current of an unknown voltage, supplied to a low resistance (20-50 ohm), 10 - 20 mA is fine. Neither are response times very important.

I don't feel really comfortable working at the lower end of the characteristic. Does anyone see a problem with this setup, something I overlooked, thermal stability issues, and maybe points for improvement? Maybe I am too much reliant on constancy of performance of individual devices?

• RdsOn of Nch shown is <30 to 60 Ohms is too high for circuit shown with 80 Ohms current sensing for accuracy. It's Idss=20mA. but for rough approximation, ok. Using a FET with Idss=10mA is better with R1=0 or a BJT current limiter Oct 6 '16 at 15:03
• The LM317 and a resistor will make a nice ~10mA current source. Oct 6 '16 at 15:22
• Like George is indicating LM 317 TO92 can do the trick. Costs amost nothing. Oct 6 '16 at 15:29
• @GeorgeHerold 10 mA is right at the bottom of the range for an LM317. I looked for something where 10 mA is within the operating range for a device and found the LT3092, which is handily available in the same package as a BSP135. Could that be a better option? Oct 6 '16 at 18:38
• @mkeith I think the OP is stating that he doesn't have such rails indirectly by writing, "A Widlar source is not an option, because it needs a reference current, which is not available." It there was a rail, then there would be a way to make a reference current.
– jonk
Oct 7 '16 at 8:59

The aim is to both keep component count and price low

Given your other limitations, it's hard to beat just a depletion mode MOSFET and a resistor. (I see where you are coming from, setting aside the JFETs.) I gather you are thinking of this:

simulate this circuit – Schematic created using CircuitLab

I can't tell from your writing just how repeatable you need to make this, though. The value of $V_{GS\left(off\right)}$ isn't all that precise. So you may need a potentiometer in there. Which ups the price, physical size, etc. Or you could just select parts, I suppose, and work it that way.

For example, the Supertex DN3525 specs $V_{GS\left(off\right)}$ anywhere from $-1.5\:\textrm{V}$ to $-3.5\:\textrm{V}$, with $V_{DS} = 15\:\textrm{V}$ and $I_D = 1.0\:\textrm{mA}$. Will that be repeatable enough without either selecting parts or selecting the resistor value?

I am aware that a similar solution exists for JFET's, but I could not find a JFET type that can handle the voltage requirement.

You could use this topology to cover that concern:

simulate this circuit

However, with the devices shown you may need more headroom than just $3\:\textrm{V}$.

I know the load line approach would seem to get you there. But isn't the variability pretty significant? Or do you think it's good enough?

A bipolar transistor current source can be used for your application. This one still regulates (barely) with Vin2=3v, but wastes nearly 3mA when Vin2=30v:

simulate this circuit – Schematic created using CircuitLab

This is a low-side current regulator using generic NPN silicon transistors. R3 determines the current-regulation setpoint. About 10mA flowing through R3 generates enough voltage to turn on Q3, which begins to current-regulate. This circuit has a voltage-burden of a bit more than a volt, better than a MOSfet. Q2 dissipates some heat. A generic 2N3904 will get warm with 10mA, a bit too-warm with 20mA. For more than 10mA, substitute for Q2 something beefier, like MJE181 that can dissipate heat more easily.