[chbot] Circuit help please, low voltage cutout

Chris Hellyar chchrobotics@lists.linuxnut.co.nz
Mon, 23 Apr 2007 09:12:02 +1200

ZD1 sets a stable voltage reference of about 5.3-5.5V (depends on
characteristics of the zener) on the '-' input to the opamp...

This is compared to the voltage set by the 22k and 20K pot using the
opamp as a comparator, which goes down as the cell does...  When the
voltage cross the resistor/pot is less than the reference set by ZD1 the
opamp puts it's output low, which turns off Q1, making the reference at
the - input go to the + rail from the battery, latching the circuit.

Pushing S1 turns on Q1, which forces the ZD1 reference voltage to
5.3-5.5V, so assuming that the battery voltage is high enough to bring
the + input higher than that the opamp output will turn on, turning on
the load and Q1, latching the reference to the lower voltage again.

You could replace the mosfet with a conventional transistor, although
you'd need a buffer of some sort after the op-amp, and you'd get a .6V
drop across the transitor..  Use an IRFZ44 or similar, they have a
22mOhm (milli not mega) on resistance, so you'll get minimal drop, and
they only cost a couple of bucks...  Or I can post you one for nothing,
I wound up with a few of them in error. :-).

Cheers, Me.

On Mon, 2007-04-23 at 00:53 +1200, Carl Ranson wrote:
> Ive been looking at this circuit for my bot's battery protector circuit:
> http://www.siliconchip.com.au/cms/A_104378/article.html
> There  are a couple of things I don't understand about it:
> a. Whats the point of ZD1? How does its value effect the cutout voltage 
> which I thought was determined by the voltage divider involving VR1?
> b. Can Q2 be replaced by a normal transistor and a relay? is the choice 
> of a mosfet because it will use less power than a relay?
> I know it was mooted that i could do it by using a voltage divider to 
> one of the pics ADC pins etc, but i want it to cover all of the 
> circuitry on the bot (even if the controller is disconnected).
> I will probably still monitor the voltage into the controller to 
> determine bot behavior but thats a ways off yet.
> Thanks all,
> CR
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