I noticed tech. questions being answered in this forum. So, I’ll try again. Wholesalers offer CLD’s (current limiting diodes) and that they are great. But I cannot find any reference to them on retail sites. I need help finding and specifying the correct component for my project. Using a 30 vdc supply, I need to keep the current below 10 mA. (2 to 6 mA) would be perfect. I would be willing to go with a lower voltage supply, if I had to. Any input would be greatly appreciated.
plumber
Constant-current diodes seem to be a dead technology. It’s a shame; there have been various times when one would have been perfect for something I was building, but the absurd prices (assuming that I could find the for sale at all) forced the use of some more complicated solution.
Other solutions don’t have to be much more complicated, anyway: an adjustable voltage regulator like the LM317 plus a resistor makes a decent current regulator. You’ll lose a few volts from your maximum power supply voltage, but the same is true of a CC diode as well.
To JasonHarper,
Thank you for your response. You are the first to confirm my suspicions. I am a plumber way in over his head…you provided a part # for the regulator, so how about the resistor? Infinite down to 37 ohms…does that mean anything? Some one suggested a fixed resistor (4200 ohms) in series with a variable resistor (13000 ohms). I almost understand this, but I would have to adjust the pot by hand as the current climbs. Your solution sounds automatic…which is what I am after. I believe the resistor size would be 0.25 watts. But I don’t know what it all means if I use a voltage regulator. Theory and sequence I mean. One more thought, which I don’t understand, is using a transistor and a FET in drain mode with 2 of the FET legs shorted. I was told that is how to “build” a constant current diode. What would you do? Note: A week ago, I didnt know what a milli amp was.
Thanks again, plumber
You can simply use a JFET:
http://www.vishay.com/docs/70596/70596.pdf
That’s how constant current diodes are made.
Leon
Yea, they are not that readily available any more. Internally they are basically a n-ch JFET and a resistor. +V on the Drain, resistor on Source to -V with the gate also to -V (see “Art of Electronics” or a google search).
Another method is a constant source (or sink) using a BJT, two silicon diodes and a resistor. The LM317 constant current configuration also works well.
It just depends on what you really need the constant current for. Do remember that these circuits try to maintain a constant current and vary the voltage to do this. They are not the best for a current limiter with a constant voltage.
Two NPN BJTs and four resistors work very well, also.
Leon
THANK YOU Leon.
It will take me a while to understand how to build this…I knew zip a week ago. I cant get around the math can I?
waltr:
Yea, they are not that readily available any more. Internally they are basically a n-ch JFET and a resistor. +V on the Drain, resistor on Source to -V with the gate also to -V (see “Art of Electronics” or a google search).Another method is a constant source (or sink) using a BJT, two silicon diodes and a resistor. The LM317 constant current configuration also works well.
It just depends on what you really need the constant current for. Do remember that these circuits try to maintain a constant current and vary the voltage to do this. They are not the best for a current limiter with a constant voltage.
Thanks waltr.
What would be the more perfect solution If I wanted to maintain voltage?
Mouser does have a selection of [current-regulator diodes up to 6.5 mA, and they do sell in small quantities.
But for what you described I’d probably just use an LM317 (or LM317L), since I have those lying around already.](http://www.mouser.com/Semiconductors/Discrete-Semiconductors/Current-Regulator-Diodes/)
I have used LM317’s as constant current sources (great for NiCad battery charging) but not as a current limiter.
I’ve been thinking on this. I believe that if the current drawn by the load is less than the constant current set point then the LM317 would drop the minimum voltage (maximum voltage on the output). I would think that the constant current diodes would also do this.
Once the loads current draw exceeds the current set point then the device would produce a larger voltage drop to limit the current to the set point.
The input voltage should be regulated to the voltage expected at the load (say 5V for TTL logic) plus enough extra to account for the constant current devices nominal voltage drop.
If I get a chance I’ll prototype this but you also could.
Thanks waltr and wiml,
Yep. I’m lost. As I get closer to a working solution…I lose it when trying to specify values in the parts search engine provided by online suppliers. I went to mouser and got beat up trying to find the right parts…I’m just not learning fast enough. So whats up with packaging/case ? I’d be using a bread board. So yes, any prototyping help would be great…I swear I’m hooked on this stuff, but for the time I have right now, my brain ain’t going quick enough.
Thanks again, plumber
Look at the second circuit on this page: http://www.cedarlakeinstruments.com/products/led.html
If that will work for you let me know. I had a few wired up for 10mA and may still have some left.
plumber:
Thanks waltr and wiml,Yep. I’m lost. As I get closer to a working solution…I lose it when trying to specify values in the parts search engine provided by online suppliers. I went to mouser and got beat up trying to find the right parts…I’m just not learning fast enough. So whats up with packaging/case ? I’d be using a bread board. So yes, any prototyping help would be great…I swear I’m hooked on this stuff, but for the time I have right now, my brain ain’t going quick enough.
Thanks again, plumber
Plumber,
Look for part that are leaded. That is with wire leads that you can stick into a protoboard. Mouser has a good selection of parts and has links to the data sheets. Near the end of most data sheets are drawings of the packages the part is available in. For resistors, caps and diodes look for the leaded parts, for IC, chips, look for the ones with 0.1 in (2.54mm) lead spacing as most protoboards have holes on a 0.1 in grid.
SMD (surface mount devices) are small and difficult to prototype with until you have gained some experience soldering to small parts.
Take the LM317. It comes in several packages and with different current ratings. The LM317T I found the most useful as its rated for 1.5Amps and is in a TO220 case. Here is Mouse’s page:
http://www.mouser.com/ProductDetail/STM … WmoPQEs%3d
Another LM317LZ is in the TO92 package and is rated for 100mA. This one is a bit smaller size and is good for circuits that don’t need as much current. The -TR in the part number means it comes on a Tape & Reel but Mouser will sell you a quantity of one by cutting one off the reel. Mouser’s page is here: http://www.mouser.com/ProductDetail/STM … WmoPQEs%3d
Look at both of their data sheets for dimensions.
If you have a Radio Shack near you stop in and browse the components they have. Note the devices packages (not the paper and plastic they are wrapped in for store display) to get a better idea of what they look like. They don’t carry nearly what they used to but still have a few parts.
Here is Mouser’s page on current regulator diodes:
http://www.mouser.com/Semiconductors/Di … ?P=1z0z7uz
The DO-35 package is leaded. Check on the first one, 610-1N5294, and then on the data sheet. A drawing is on the second page.
I design electronics instruments for a living, started as a hobby 40+ years ago, and do get confused from time to time as to which is the correct part, part number, etc. So don’t get too discouraged.
If you are unsure of a part post a link to the part you are considering and a description of what you need and I’m sure someone can help.