I would like to find the simplest way to provide a constant current of around 150uA (+/-20%) at around 2V. This needs to be discrete components rather than IC and should not involve pulse width modulation. The simplest form I thought of was a 3v voltage reg with a 20K resistor between it's 'reference ground' and the output but this seems clumsy. Anyone ? Thanks Dave
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
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.
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.
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).
SOT-23-5 LMV431, an inexpensive SMT BJT and a couple of tiny resistors.
Best regards, Spehro Pefhany
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If you\'re not allowed to use integrated circuits, then why don\'t you
post the schematic for the 3V regulator you\'ve come up with, as well
as with what\'s feeding it and what the load looks like?
JF
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