Subject: 20mA constant current
From: "Bianca"
Date: 9/29/2004 8:12 PM Central Daylight Time
Message-id:

For a project I need to have a controlled current of 20mA (tolerance abt 10
%) for connection at the output of an IC that is giving a 1 Hz pulse,
voltage between 5 and 12 VDC, as low-cost and easy as possible...

The next component connected to that output will die if it must eat more
than those 20mA.

Some links or ideas would be helpful.

Hi, Bianca. Your description is a little vague. The way I read it, you've got
a 1Hz logic signal which you want to use to switch a 20 mA constant current
source into a load. The current regulation isn't too important, but it's
necessary that maximum current not exceed 20 mA.

Every method of doing this will require some kind of a minimum voltage drop
across the current regulator, which is a bit of a drawback. The simple
switchable current regulator below has a minimum voltage drop of about 1.7V,
which means that for your minimum input voltage of about 5V, the load will only
see a maximum of 3.3V. If that's OK (view in fixed font or M$ Notepad):

V+
5 to 12V ___ 2N3906
o-o---|___|---- -----o
| 33 ohm v / +
| ---
| |
'---|--|----o
D D |
.-.
| | Switched
1K| | 20 mA output
'-'
1Hz logic |
signal ___ |/
o-----|___|-o-|2N3904
10K | |
.-. |
| | |
10K | |
'-' |
GND | | -
o-----------o---o-------o
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

The above assumes you have a TTL or CMOS logic signal, with logic "1" of 5V to
12VDC, and a logic "0" of 0V to 0.5VDC. All components are available at Radio
Shack or any hobbyist electronics source. Use any silicon diode (1N400X,
1N914, 1N4148 are easily available examples) for D.

Two diodes, two transistors, 4 resistors. Easy and straightforward. If this
doesn't do it for you, you should be a little more descriptive.

Good luck
Chris