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Old 7th April 2013, 11:40 PM   #1
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Default 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
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Old 8th April 2013, 12:44 AM   #2
DUG is offline DUG  Canada
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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.

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Old 8th April 2013, 12:46 AM   #3
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Hi,
I think what you are looking can do it with a voltage comparator op amp. You set the limit and if the voltage fall below it will set and output. You can use an led to indicate when you have a voltage down. Do a Google "voltage comparator op amp" and you will find a lots of web side explaining it.
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Old 8th April 2013, 01:11 AM   #4
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Quote:
Originally Posted by DUG View Post
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
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Old 8th April 2013, 01:14 AM   #5
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Quote:
Originally Posted by tauro0221 View Post
Hi,
I think what you are looking can do it with a voltage comparator op amp.
Yes, BUT the requirment is NO chips, NO op-amps, NO ICs

only discrete solid-state - transistors
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Old 8th April 2013, 01:43 AM   #6
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What is the output?

Open collector?
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Old 8th April 2013, 01:54 AM   #7
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I tried open collector but got nothing...............

I'll try connecting the collector via 100k resistor to ground, and see if I get something from the collector.................
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Old 8th April 2013, 02:25 AM   #8
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Cap size will determine duration of pulse.

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

Close?
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File Type: jpg drop-det.JPG (14.7 KB, 46 views)
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Old 8th April 2013, 03:08 AM   #9
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Quote:
Originally Posted by KMossman View Post
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

Quote:
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.

Quote:
The voltage in the circuit is up to 48V - any risk of blowing the transistor?
Which transistor?
So far we have selected no one.
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Old 8th April 2013, 03:29 AM   #10
DUG is offline DUG  Canada
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diodes 1N4002
transistors MPSA06 and MPSA56
Resistors 100K
Caps: make the one on the right 10 times as large a value as the one on the left

You can also use one less diode...see new jpg

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