Variable Current Supply

You don't mention the 'burden' of the instrument you're trying to drive. What will be the resistance of the load you will be driving?

daestrom

Depends on what range of output voltage you have to have.

If you can afford to lose 5 or so volts, use a 5 volt three pin regulator, together with a variable resistor in series with its output pin. Now connect the "common" pin of the regulator to the other terminal of the resistor. Use that as the output pin. The other output pin is the battery negative.

The regulator will limit the voltage across the variable resistor at 5v. The output current will be limited to 5v divided by the resistance value.

Thus by selecting the right value of resistor, you can set the maximum output current to 4-20mA, as necessary.

Depending on whether you are fussed about noise. But it will probably work fine without.

I use my Fluke 787 or my Fluke 924 or any process control meter.

Thanks, I was going to approch it this way, just wanted to get a second opinion before I went ahead and built it.

YVW. But bear in mind that you are going to lose lots of volts in both the regulator and the resistor. It may not produce the range of voltages that you need, if the load is not very low impedance.

Another simple solution is a single (or Darlington) transistor with its base held at constant voltage by a resistor/zener chain and a variable emitter resistor. The collector will drive a much wider range of loads at constant current.

------- 62.5 ohms

+9 -----in| LM317 |out---[R]---+ ------- | Adj / | \ +--------------->/ 250 ohms | \ | +-------------------------> 4-20 mA out

Gnd ------------------------------------>

This gives you a higher voltage output than a 5 volt regulator and is continuously adjustable from 4 mA to 20 mA.

Ed

I now need to change my design so I can adjust the output between

4-8mA, any ideas?

Simply increase the value of the series resistor.

These circuits actually sense the voltage across the series element and hold that constant.

Thus, if you double the value of the series resistor, you half the current needed.

If a 5v regulator is used, the series resistor would need to be 5/4 kohms for 4mA and 5/8 k ohms for 8 mA.

A fixed resistor of 500 ohms, in series with a variable resistor of 1k ohm, would give an adjustable range of about 3mA to about 10mA..

------- 156 ohms

+9 -----in| LM317 |out---[R]---+----------+ ------- | | Adj / | | \ 440 ohms +--------------->/ 250 | | \ ohms | | | +---------------------------+-----> 4-8 mA out

You can put 2 220 ohm resistors in series for the 440 ohm resistor.

Ed

Doesn't the LM317 have some sort of minimum current spec for reliable operation? Or, is that only when used as a voltage regulator? I thought it was around 10mA.

-Bill

Have a look at the datasheet, page 6

It will work starting at a little less than 1.5 mA. It needs more current as delta V (Vin-Vout) increases and as temperasture increases, but still needs less than 4 mA, worst case.

Ed Ed

it is about 4-5 ma under normal temp conditions and you need that Iadj current whether using it as a floating current regulator or a voltage regulator.

-bob