current sense led circuit

[Flump]

can anyone suggest a way of making a led light up when current drawn by a device drops below 300mA ?

it is for a ocxo, when the heaters on it draws 400mA but when its up to temp it settles to about 250mA
i would like visual confirmation of that.

an example circuit to look at would be great if anyone has something like it.

[V8Snail]

Do you have a link to the model of OCXO you are using?

[Flump]

yep its one of these http://www.sungwhatech.com/product/pdf/aging/8663.pdf

[TerminalJack505]

Here's one way to do it.  Assuming that you can put a resistor between the heater and ground, that is.

[V8Snail]

Here's one way to do it.  Assuming that you can put a resistor between the heater and ground, that is.

This is along the lines I was thinking, except instead having just the heater monitored, which looks to be impractical with this package, connect pin 4 and GND of the 8663 package between VCC and R1 of this circuit.

Bear in mind, the unit he referred to is 12V not 5V, but this is a good way to ensure the voltage drop across the current sense resistor stays well within the required 5% of 12V VCC.

[TerminalJack505]

Here's one way to do it.  Assuming that you can put a resistor between the heater and ground, that is.

This is along the lines I was thinking, except instead having just the heater monitored, which looks to be impractical with this package, connect pin 4 and GND of the 8663 package between VCC and R1 of this circuit.

Bear in mind, the unit he referred to is 12V not 5V, but this is a good way to ensure the voltage drop across the current sense resistor stays well within the required 5% of 12V VCC.

Whoops.  I didn't look at the OCX's datasheet before drawing the schematic.

It might be better to monitor the current from the high-side.  This way the sense resistor doesn't affect Vref.  That would be a little trickier.

[TerminalJack505]

Here are a couple of circuits that should work for sensing the current from the high-side. 

One uses an INA138 high-side current monitor IC. 

The other uses more jellybean parts.  The P-channel MOSFET shown in the circuit can be pretty much any part.

[rx8pilot]

I think there may be some of the INA current sense amplifiers with fault outputs. Could have a lower part count. Set it to reverse indicate anything over 300ma or whatever threshold you need.

[Flump]

thank you all for the advise