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20mA constant current
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:
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. :) --- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/ Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/ +Lasers | Sam's Laser FAQ: http://repairfaq.ece.drexel.edu/sam/lasersam.htm | Mirror Sites: http://repairfaq.ece.drexel.edu/REPAIR/F_mirror.html Note: These links are hopefully temporary until we can sort out the excessive 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 |
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