Detecting Voltage Drop

I need to detect when the voltage drops in a battery powered circuit. NOT to a certain voltage, but rather when the voltage quickly sags.

I want/need/require to do this using discrete transistors [NO ICs!] and as cheaply as possible.

I was thinking of a resistor and capacitor [in series] across the power - with a PNP transistor wired;

base to V+ side of the resistor
emitter to resistor/capacitor junction

but no luck...............should I 'tie' the collector to ground via a resistor - say 100K?

And get the signal from the collector that way?

The voltage in the circuit is up to 48V - any risk of blowing the transistor?

Thanks!

Ken
 
What sort of an output do you need?

Simplest is a cap and a LED.

Cap from +Vcc to cathode of LED...anode to ground...protection circuitry: a diode in parallel with LED: anode to cathode.

:)

yaaaaaaaaaaaaaa BUT how to detect the voltage drop?

Just need 'some' signal - WHEN the voltage drops.

Ie

zero v - no drop

some V - drop
 
Cap size will determine duration of pulse.

Sorry I used so many diodes...some are for protection.

Close?
 

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I need to detect when the voltage drops in a battery powered circuit. NOT to a certain voltage, but rather when the voltage quickly sags.

I want/need/require to do this using discrete transistors [NO ICs!] and as cheaply as possible.
The easiest, cheapest and most simple solution involves an Op Amp.
An LM358 starts at U$0.22 dropping to U$0.10 bulk.
Care to reconsider?
Please answer:
1) range of voltages to be measured.
2) voltage drop to be detected.
3) knowing the approximate period or duration would help

I was thinking of a resistor and capacitor [in series] across the power - with a PNP transistor wired;

base to V+ side of the resistor
emitter to resistor/capacitor junction

but no luck...............should I 'tie' the collector to ground via a resistor - say 100K?

And get the signal from the collector that way?
This detects instantaneous (here meaning all the time) load current, not exactly the same of what you are asking.
Will trigger at a set total current value , catching only relatively gross ones.
Again, not the same as detecting a drop in supply voltage.

The voltage in the circuit is up to 48V - any risk of blowing the transistor?
Which transistor?:confused:
So far we have selected no one.:shhh:
 
many thanks to all

I get the stupid b*gger of the day award

It does work, as I 1st outlined.................some days, one should NOT do electronics!


The LED limits the voltage across T1 BE, and blocks reverse voltage flow when the voltage drops, keeping the V across the cap.

So when V+ drops, T1 turns on and hence T2 turns on - driving the load. C2 stretches the 'pulse'.

accelator_circuit.png
 
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