simple 150uA constant current supply

[Dave]

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

 

[Martin Griffith]

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

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


martin

 

[[email protected]]

I would like to find the simplest way to provide a constant current of
around 150uA (+/-20%) at around 2V.

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 = (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

 

[Dave]

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

 

[[email protected]]

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 = (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

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

 

[Joerg]

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.

 

[Joerg]

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

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

http://www.national.com/ds/LM/LM134.pdf

 

[Joerg]

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

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

http://www.onsemi.com/pub_link/Collateral/TLV431A-D.PDF

 

[Joerg]

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.

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.

 

[Paul E. Schoen]

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

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

=========================================================================

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

 

[Spehro Pefhany]

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

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



Best regards,
Spehro Pefhany

 

[Phil Allison]

"Dave the Dickhead "

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.

** Look up " fet current source ".

One JFET plus one resistor.

Dickhead.



...... Phil

 

[Phil Allison]

"John Larkin"
"Phil Allison"

Look up "typical Idss variation for production jfets."

** Already very familiar with that - you posturing asshole.

The OP merely needs to chose his FET and resistor ( or trim pot) to get
150uA.

Amateur.

** Autistic prick.



...... Phil

 

[Phil Allison]

"Jamie the Lying Fuckwit Asshole "



** Drop dead - pisshead.





..... Phil

 

[Don Klipstein]

"John Larkin"
"Phil Allison"

** Already very familiar with that - you posturing asshole.

The OP merely needs to chose his FET and resistor ( or trim pot) to get
150uA.


** Autistic prick.

You need to advise someone towards a design that is likely to need a
trim pot?

- Don Klipstein ([email protected])

 

[Jamie]

Look up "typical Idss variation for production jfets."

Amateur.

John

Now now children!


http://webpages.charter.net/jamie_5"

 

[Phil Allison]

"Don Klipstein"

You need to advise someone towards a design that is likely to need a
trim pot?

** Why the heck not?

The half witted OP is making a one-off - he ain't no designer.

Very wrong of YOU and other "experts " here always * assume * mass
production is envisaged.




...... Phil

 

[Jamie]

"Jamie the Lying Fuckwit Asshole "



** Drop dead - pisshead.





.... Phil

Phil, please stop licking the bus windows and go play with
the barn yarn animals.

 

[Bill Sloman]

There are microscopic nanopower opamps from Linear and others. Since all
you want is nominal +/-20% , and this is the same as Vbatt ranging from
2-3V, then something simple that does not require much battery current
would be like so, total bias around 16ua, compliance should be to within a
few hundred millivolts of the rail, and you could do even better with a
PFET:

A simple asymmetric current mirror built with a dual PNP transistor - as
drawn below - or an NPN part would have much the same accuracy (depending on
the Vbe matching between the two transistors in the dual) would probably be
cheaper and would use fewer parts, and is unlikely to oscillate.

Farnell stock four NPN and five PNP duals from NXP (was Philips) as well as
five current mirrors - three NPN and two PNP three of them from NXP and and
two from Infineon (was Siemens).
You'd need to look at the data sheets to work out which would suit you best

View in a fixed-width font such as Courier.

BATT---------------+-----------.
. | |
. | |
. [3K9] |
. | |
. | |
. / /
. |< |<
. | |
. +---------+-----------|
. | |\ |\
. | | |
. | | |
. | | Iout
. | |
. [166k] |
. | |
. | |
. COM---+-----------+-------

The 10:1 asymmetry is a bit on the high side - your spare 30uA would
accomodate 5:1 - 80k and 1k4

 

[Dave]

Lots of interesting stuff here thanks.
Just to clarify I have a 2 x AAA cell supply and want to drive an LED at
150uA which will be around 2V.
The AA supply will obviously not be completely exhausted by the time there
is insufficient voltage to light the LED but the intensity needs to be
reasonably constant until then.
By the way, why do trolls despise themselves so much as indicated by the
intense anger?
Dave