From: Winfield Hill
Subject: Re: Adjustable current source
Date: 15 Dec 2002 10:42:03 -0800
Organization: Rowland Institute
X-Newsreader: Direct Read News 4.20
> Watson wrote...
>> Billy wrote:
>>> could somebody point me to a circuit of an adjustable current source?
>>> It's gonna be used in a Ni-mh / ni-cad battery charger. It will need
>>> an output of about 3A and it has to be controllable by an 68HC11 either
>>> by d/a or digitally inputs.
>> If you are going to do batt charging, you can do it easily by monitoring
>> both sides of a series resistor. Just monitor both voltages and
>> subtract, use the diff to calculate the current. Your control will be
>> the voltage to the battery. This can be done with a LM350 or similar.
> Here's an s.e.d. posting I made 5 years ago, showing an easy way to make
> adjust current sources from LM317 - LM350 - LM338 adjustable regulators.
> Later I'll modify this circuit into a programmable current source.
> Let me offer the following simple circuit, which I just created and
> haven't tried but "oughta work," as a solution to this problem.
>. | | 0 to 5A current source
>. Vin ----| LM338 |-- 0.25 ohms --+---->
>. |_______| |
>. | cw ,----+
>. | 1k p \_|_
>. ------------> o /_\ LM385-1.2
>. t |
>. ccw '----+
>. I = 1.5 to 3mA sink
> The 1k pot selects a portion of the floating 1.23V reference voltage,
> and tricks the LM317 or LM338 into correspondingly reducing the voltage
> across the 0.25 ohm current-sense resistor. The pot is conventional
> and may be panel mounted. It should be possible to nearly shut off the
> LM338 (a minimum quiescent current will still flow). The current sink,
> I, which powers the floating 1.23V reference, is not critical and may
> be a simple current mirror (sorry to see the TL011 gone!), or even a
> resistor to ground or any available negative voltage, depending upon
> the desired current-source voltage-compliance range. That's it!
> --------------- A blast from the past...
Hmm, I wonder if anyone has ever tried that circuit (I haven't)?
And for my next trick... an adjustable version.
. | | Rs 0 to 5A current source
. Vin ----| LM338 |-- 0.25 ohms ---+---->
. |_______| |
. | R2 250 R2 R3
. | | Iout = Vp -------
. '--------------------+ Rs R1 R4
. ,- R3 1k -, | provided,
. 0 to 5V | __ | Q1 | R2 R3 1.25V
. Vp -- R4 5k --+-+--|- \ | |/ ----- = -----
. | | >--|---| R1 R5 Vee
. -Vee - R5 5k --' ,--|+_/ | |\V
. -5V | | |
. gnd '-----+-- R1 -- gnd
Q1 acts as a 0 to 5mA current sink to generate a 0V to 1.25V offset for
the LM338 adj pin, thereby creating a 5A to 0A output current source. A
simple control formula, Iout = +1A/V, results from the above part values.
The circuit can be modified for operation from a single supply, but this
requires a few more parts including a voltage reference.