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.

On Thu, 30 Sep 2004 09:12:34 +0800, "Bianca" wrote:

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.

Grab the data sheet for the LM317 family. Constant current source with one
resistor. For 20mA I'd use the 317LZ (TO-92).

"Bianca" wrote in message
...
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.





Have you bothered to type something like Constant Current Source into
Google?

"Bianca" wrote in message
...
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.





Simple. Put the device needing constant current drawn through it
between a power source and the input to a fixed voltage regulator IC,
such as a 7805 or whatever. Put a resistor that will draw the desired
current on the regulated output of the IC to earth. As the IC itself
draws little or no current (microamps) the current drawn from the
power source through your device will thus be the current through the
resistor.

At 20mA one of the TO-92 encapsulated types will do the job admirably.


--
Woody

Bianca wrote:
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.

Do you actually need a constant current device? Or do you just need
some way to ensure that the current cannot exceed 20 mA?

Your description sounds like the latter, in which case the answer is
probably as simple as a resitor.

Bill

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

(CFoley1064) writes:

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.

Then again, requirements were known better, 1 or 2 resistors might be
sufficient.

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Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/
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traffic on Repairfaq.org.

Important: Anything sent to the email address in the message header is ignored.
To contact me, please use the feedback form on the S.E.R FAQ Web sites.

James Sweet schreef in berichtnieuws
d1M6d.9728$me5.4783@trnddc06...

"Bianca" wrote in message
...
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.





Have you bothered to type something like Constant Current Source into
Google?



Yupz.
I always try my fiance first. ;o)
All too complicated or not working.

Bill Jeffrey schreef in berichtnieuws
...
Bianca wrote:
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.

Do you actually need a constant current device? Or do you just need
some way to ensure that the current cannot exceed 20 mA?


More special: not above 20mA, not below 18mA...

Your description sounds like the latter, in which case the answer is
probably as simple as a resitor.


To variable, it will damage the thing that is connected.


Bill

Do you actually need a constant current device? Or do you just need
some way to ensure that the current cannot exceed 20 mA?


More special: not above 20mA, not below 18mA.

Bianca-

If you look at the characteristic curves of an FET, you will see lines that
maintain a nearly constant current over a wide voltage range. One of those
lines corresponds to the case of zero volts between gate and source.

In other words, an FET with gate tied to its source, becomes a constant-current
diode. You just have to try several to find one that meets your requirement.
(In fact, such a device is available with the leads tied together internally.)

I don't know how well the current remains constant over temperature, but this
approach is something to consider.

Fred

Kibo informs me that "Bianca" stated that:

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.

Couldn't be easier:

,-------.
1 | LM317 | 3
+Vin O-----o o----------o-----O +Vout
'---+---' |
o 2 |
| |
`--[56R]-[6R8]-'

-Vin O------------------------------O -Vout

As mentioned by others in this thread, the little TO-92 package version
of the LM-317 will be fine for 20mA.

--
W
. | ,. w , "Some people are alive only because
\|/ \|/ it is illegal to kill them." Perna condita delenda est
---^----^---------------------------------------------------------------

Fred McKenzie wrote:

Bianca-

If you look at the characteristic curves of an FET, you will see lines that
maintain a nearly constant current over a wide voltage range. One of those
lines corresponds to the case of zero volts between gate and source.

In other words, an FET with gate tied to its source, becomes a constant-current
diode. You just have to try several to find one that meets your requirement.
(In fact, such a device is available with the leads tied together internally.)

Fred -

You are exactly right. And if you put a resistor in the source lead
(between the source and the connection to the gate), you can change the
value of the constant current. Start with FET that delivers a bit too
much current. Connect a variable resistor, and dial the current down to
where you want it. Then replace the variable resistor with a fixed one.
Easy!

Bill