[Original title: "LM317 Current Regulator Dropping Under Load"]

Hello,

I'm attempting to use a LM317T (TO-220) linear regulator to turn a potentiometer into a 4-20 mA outputter. With my components I calculate I should get an output range from about 3.9 mA to 19 mA (good enough). My schematic currently looks like this:



My question is this. If I put my ammeter directly between the output (essentially the adjust pin) and ground, I get the calculated current. However, if I add a load to it, for example, 400' of 18 AWG wire, I end up with about 2.5 mA less than expected (at least at 100% - the only place I've tested). Why would I see less current? Shouldn't the LM317 just work harder to output the same amount of current given the additional load?

Thanks for you help.

 

Did you put your ammeter in series with the load? Or is it still from the output to ground.

 

In series with the load to ground. See the below updated schematic.

 

What's the value of R_load?

 

It looks like my target resistance will be 233Ω +/- 5Ω in addition to 50 feet of 18 AWG wire (0.3Ω??).

 

You dont have it wired right. Note where the adj terminal goes?

 

You dont have it wired right. Note where the adj terminal goes?

I don't see the difference.

 

a much simpler 4 to 20 ma source is a 10 k 10 turn pot with a 9 volt battery. the turns counting dial will read current output. not exactly 4 to 20 ma, that would take a 10 volt source. we use them here to adjust all kiinds of things.

 

My 4-20mA meter won't even go full scale on a 9V source. I don't see how a 9V battery and pot could work.

 

Make the 82 ohm a 56 ohm or so. Right now 18 ma. drops the 1.2 volts the regulator wants to see from the output to reference.

 

You dont have it wired right. Note where the adj terminal goes?

Don't I have it the same? Connecting after the current setting resistor? The difference I see from your provided schematic is how the wiper is connected.

@alfaclif: Using a 9V battery isn't an option. This would be on an industrial machine that is in service. The intention is not to make a test device.

@ronv: I'll try the 56Ω resistor later when I get a chance. Thanks for the recommendation.

 

What is your source voltage?

 

Source voltage? Incoming to the LM317? 24 VDC. It's labelled on the schematic.

 

Source voltage? Incoming to the LM317? 24 VDC. It's labelled on the schematic.

Sorry, I missed that.

 

What is the ripple voltage or is the source a battery?

 

I don't know what the ripple voltage is. The voltage source will either be an industrial 24 VDC power supply (probably from Puls) or a 10 VDC output from a Mitsubishi VFD.

 

You might also want to add a fixed load of say 360 ohms from the output of the 317 to ground. Most will work with a minimum load of 3.5 ma., but some may want a 10 ma. load as a minimum.

 

I've tested a handful of brands and different ages, they work fine as a one-resistor current regulator down to about 1mA.

For my understanding the minimum requirement of 10mA was on very old parts, and is more to do with being able to run at all possible voltage and transient conditions etc.