Voltage Drop Testing

Thread Starter


Joined Nov 11, 2006
I am new to circuit design, but have a solid technical background.
I am looking for best practices, idea's, circuit layout, etc, on a way to build a board which would check voltage drop across three different lines; one 12v DC feed circuit and two DC ground circuits. I need to check each circuit individually and, via LED, indicate to the user if there is excessive voltage drop across any individual circuits.
Recommendations, thoughts, ideas?


Joined Oct 12, 2006
Basic common emitter biased transistor/darlington pair immediately springs to mind. The emitter of the transistor could be connected to your negative probe through a resistor, with an LED sitting between Vcc and the collector. Your positive probe could be driving a potential divider (perhaps with a little trimming pot in there so that you can fine tune it) such that the output only rises above 0.65V when excessive voltage is present, and connect this to the base.

Would this do what you want?


Joined Jun 30, 2006
Would a zener diode + resistor + LED work? The Zener will not conduct unless the voltage is over it's rating, the resistor limits current, the LED should light and work fine if the voltage is over the zener's rating. Maybe zener rating + .7v I forget how that works without testing.

If you want it to latch and hold you could place an SCR in there so that when the zener triggers the SCR keeps the LED on until power is removed.


Joined Dec 2, 2005

If your 'normal' circuit is known, why not consider monitoring the current flowing though it? ...or...
In some applications, (security and fire alarms) the conductors are brought back to the power source via a 'return' pair and the voltage at the 'end of line' is compaired to the source voltage. This could be done with a voltage divider and a couple of comparators (Lm339). This also serves as a way of monitoring the state of the circuit as you can detect both over loading and open circuit faults.