50ma Constant current source

[technogeek]

I found this schematic here: **broken link removed**

My question is, what happens if the output is accidentally shorted?

It appears that it would still try to put out 50ma, through the 56 ohm resistor. This would be a drop of 2.8V, leaving 169-2.8=166V across the LM317.

The maximum of the device is 37V, so I think it would likely turn into smoke, no?

And if so, are there other devices that would work, and have short circuit protection built in?

*also* - would the power dissipation be the voltage drop of the device * the current output? i.e. in the short circuit case, 166V*0.05=8w?

 

[Sceadwian]

Did you notice that it says low voltage AC input? Some people think 120 volts is low voltage but not I, and certainly not an LM317. You can't feed that circuit with AC voltage over about 25 volts RMS.

 

[technogeek]

True, that would be the safe answer....

 

[Sceadwian]

As long as you stay within the regulators voltage range a short isn't going to hurt anything.

 

[technogeek]

Have you heard of a "current limiting diode"? Do they still make these, or have they gone the way of the vacuum tube?

Seems like that would eliminate most of the parts in the above schematic...... I'm all about simplicity.

 

[kchriste]

You can use a JFET as a current limiter by tying the gate to the source terminal.
That's a pretty simple low voltage solution.

 

[Hero999]

Did you notice that it says low voltage AC input? Some people think 120 volts is low voltage but not I.

Lol, anything below 1kV is low voltage as far as I'm concerned.

I think it means safe extra low voltage, <25VAC.