simple 150uA constant current supply

Sounds like a homework assignment, but I'd start off with a 150V rail and a 1Meg resistor

martin

Connect a voltage supply (Vin) to the emitter of an NPN transistor through a resistor (R). Connect a reference voltage (Vref) between the base and ground. Let Vbe be the base-emitter diode drop. Then Iout =3D (Vin-Vref - Vbe)/R. The higher Vin is, the less sensitive to Vbe the output current will be. You should use a transistor that has a reasonably high beta at 150 uA. The alternative is to use a Darlington transistor. The disadvantage is that now you have to base- emitter drops to deal with. Regards, Jon

Thanks Martin but, it's not homework, I don't have any space for an IC and this is battery powered. I can only spare about 30uA to 'generate' this supply. Thanks Dave

find the simplest way to provide a constant current of

Oops! I meant a PNP transistor for a positive output current (Conventional current flow out of the collector)..

Uhm, so you contemplated a regulator chip but don't want an IC. Then you said you have 30uA to generate 150uA but do not want PWM. That's all quite confusing. Maybe you should share some more data.

How constant versus terminal voltage? What supply voltage is available?

This is always fun:

V+-----------------+--------------+ | | | | | R2 red led a | k | | e +-------------b pnp | c | | R1 | | | | | | out gnd

John

I've used the LM334, only two parts but a bit pricey and it doesn't come any smaller than SO8:

The TLV431 also comes in SC70-6 and TSOP-5:

Oh, a PTAT.

The world really needs a simple, low saturation voltage, resistor-programmable current source gadget in a tiny package. I keep making these from opamps and transistors. Whatta nuisance.

John

These saturate between 0.7V and 1.1V, depending on current. Nice parts but over 30c a pop. I am afraid it won't get much use in designs at that price and then we won't see smaller packages.

Maybe Jim can whip one out and have Lansdale sell it. With s.e.d. discount, of course.

A very simple current regulator using a 2N3906 PNP, two 1N4148 diodes, and two resistors will work on a 3 VDC supply, but it draws 500 uA from the supply to supply the 150 uA to the load. Here's the LTSpice circuit. If your 3 volt supply is well regulated, there might be a more efficient way to do it. But there's no way to get 150 uA from a supply current of 30 uA without some sort of switching regulator (or maybe magic). And what is the maximum load resistance? 2 volts at 150 uA is 13.3k. The following circuit will just barely do it, up to about 16k (2.44V).

Paul

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Version 4 SHEET 1 880 680 WIRE -48 128 -112 128 WIRE 80 128 32 128 WIRE 256 128 176 128 WIRE 256 208 256 128 WIRE -112 224 -112 128 WIRE -80 224 -112 224 WIRE 0 224 -16 224 WIRE 128 224 128 192 WIRE 128 224 64 224 WIRE -112 256 -112 224 WIRE 128 256 128 224 WIRE -112 368 -112 336 WIRE 128 368 128 336 WIRE 128 368 -112 368 WIRE 256 368 256 288 WIRE 256 368 128 368 WIRE -112 400 -112 368 FLAG -112 400 0 SYMBOL voltage -112 240 R0 WINDOW 0 -50 11 Left 0 WINDOW 3 27 94 Left 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value 3 SYMBOL pnp 176 192 M270 SYMATTR InstName Q1 SYMATTR Value 2N3906 SYMBOL diode 0 240 R270 WINDOW 0 32 32 VTop 0 WINDOW 3 -2 52 VBottom 0 SYMATTR InstName D1 SYMATTR Value 1N4148 SYMBOL diode -80 240 R270 WINDOW 0 32 32 VTop 0 WINDOW 3 0 32 VBottom 0 SYMATTR InstName D2 SYMATTR Value 1N4148 SYMBOL res 48 112 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R1 SYMATTR Value 3.0k SYMBOL res 112 240 R0 SYMATTR InstName R2 SYMATTR Value 5.6k SYMBOL res 240 192 R0 SYMATTR InstName R3 SYMATTR Value 10 TEXT -66 418 Left 0 !.tran .1 startup

SOT-23-5 LMV431, an inexpensive SMT BJT and a couple of tiny resistors.

Best regards, Spehro Pefhany

I think this article might give you some starting point:

CCLM 0130 from central semi. -------o|---------

Bob

Does "at around 2V" mean your available supply is 2V or that the maximum voltage developed across the load by the 150uA is 2V, or both?

Bob

Bob

An IC is actually the simplest and smallest way to build this National LM134 but it does require at least 1vdc to operate.

Bob