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I'm trying to setup a low cost current injector for testing certain I/O modules. I found a rather simple design for a current source using 7805.

schematic

simulate this circuit – Schematic created using CircuitLab

So I tried making that and changed the resistances to match my requirement. I have a 24 Volt source on the test rack, which powers the I/O modules as well. Since this is a low cost project, I used the same power supply and ended up with this. -->

schematic

simulate this circuit

This circuit worked perfectly when I tested it with a multimeter. It gave 3.6mA to 20.1 mA.

However, while testing with the I/O module, the current through the load (I/O module's internal resistance) never went above 15.6 mA. The I/O module has a floating ground and therefore an isolated current measuring setup. But when I touch the heat sink, the current rose to 20.7mA Thinking this might be due to the capacitance effect from my body, I added a capacitor between the input and GND(what I could find lying around in the workshop was a 35v 100uF). Now I see that the output is in the range of 3.6mA to 20.8mA.

I am not literate enough to calculate the correct capacitance required or evaluate whether this setup is reliable.Please advise.

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  • \$\begingroup\$ Try reading the data sheet for a 7805 - it will tell you that you need two capacitors to generally stabilize it. \$\endgroup\$ – Andy aka Sep 29 '15 at 13:03
  • \$\begingroup\$ The capacitor helped, probably (though not definitively) because the I/O modules picked up noise on the output differently than your multimeter. Other than that, see @i-chodera 's answer. \$\endgroup\$ – Asmyldof Sep 29 '15 at 13:46
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    \$\begingroup\$ An 7805 has a rather large (and varying!) quiecent current. An LM317 is designed to be used in this configuration, and has a much lower (and much more stable!) quiecent current. \$\endgroup\$ – Wouter van Ooijen Sep 29 '15 at 14:51
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    \$\begingroup\$ Most likely it's not happy with the source impedance of your power supply- it is typically stable with just an input capacitor. Try a 100nF capacitor from 7805 'in' to GND within 10-20mm of the 7805. As Wouter says, this is not ideal- the GND current (typically at least several mA) will change at least several percent just from warming up. \$\endgroup\$ – Spehro Pefhany Sep 29 '15 at 14:57
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The 7805 is a generic part, variants are made by lots of manufacturers, you should check the datasheet for the exact part and make you have used. Typically they recommend a 0.1uF capacitor between Out and GND and a 0.33uF capacitor between In and GND. These are required for stability. In practice, you will probably find that any values greater than these will work fine.

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  • \$\begingroup\$ Indeed, at least Fairchild recommends caps, even when used as a constant current source fairchildsemi.com/datasheets/LM/LM7805.pdf (fig 10, p. 19). The LM317 datasheet omits caps when configured as current source. So overall probably even cheaper, never mind better performing in this role as @Wouter van Ooijen noted. \$\endgroup\$ – Fizz Oct 2 '15 at 10:49

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