# using 12 volt battery with a 7.2 volt drill

Discussion in 'Electronic Basics' started by Mike, Aug 3, 2006.

1. ### MikeGuest

Hello
Can I use a 12 volt battery with a 7.2 volt drill. these are for basic
robotics we are trying to get into. Also would a 12 volt battery power
a 14.4 or 18 volt drill.
Thanks
Mike

2. ### EeyoreGuest

WTF makes you think you *could* ?

Graham

3. ### jasenGuest

with PWM limiting the current that should be OK. without it may not last long.
can't get any 12V drills cheaply ?

14.4 would probably be OK, 18 probably not.

Bye.
Jasen

4. ### Tom BiasiGuest

Yes you can but what would be the results?
DC motors of this type are quite tolerant of overvoltage and would spin a
little faster and get a little hotter, (more than just a little under load).
Providing less voltage will reduce the RPMs and torque.
Since you are beginning robotics, I would like to suggest that you get used
to following specifications.
Regards,
Tom

5. ### ChrisGuest

Hi, Mike. I guess you could use a 12V battery for a 7.2V motor, if you
drop the extra volts. The simplest way to do this would be to get a
string of five 1N5402 diodes, and put them in series between the
battery and the motor like this (view in fixed font or M\$ Notepad):

|
| 5 X 1N5402
|
| .-->|-->|-->|-->|-->|--.
| | |
| +| / \
| --- ( M )
| 12V - \_/
| | |
| | |
| '----------------------'
|
|
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

This is OK for a small motor that's drawing less than 3 amps full load.
Of course, you'd be wasting 5/12ths of your battery power as useless
heat. And the diodes aren't going to regulate the motor voltage all
that well.

Speed and power of DC motors is dependent on input voltage. So, your
14.4V and 18V motors will run, although the 18V one will be slow and
weak.

Better to think 'er over, and get the right parts for the job. There
are plenty of DC motors available for robotics that are made for 12V
batteries.

Good luck
Chris

6. ### Roger DewhurstGuest

Drop the voltage with a bunch of heavy diodes in series. I know of someone
who operates 7 volt drills off a car battery. He uses the drills in
connection with a mobile logging operation.

R

7. ### Rich GriseGuest

If it's for a robot, you're going to be using PWM control anyway, right?

Just make sure that your maximum duty cycle <= (7.2/12).

Is it really a 12V battery, or is a lead-acid? In that case a nominally
12V battery usually has 13.2 volts or so open-circuit.

Good Luck!
Rich

8. ### Mark ClementsGuest

Hi,

could you possibly explain why you prefer diodes here to using a voltage
divider? I've googled a bit and I think I understand that it's because
the load is significant and with a voltage divider the voltage drop is
proportional to current

ie increase the load => increased current => increased voltage drop
=> thereby limiting the voltage available for the load

Is my understanding along the right lines?

thanks,

Mark

9. ### jasenGuest

could you possibly explain why you prefer diodes here to using a voltage
yes.

silicon diodes will drop about 0.6 to 0.8 V each over a pretty wide
range of load currents and temperatures, resistors will give a drop
that's proportional to the current flowing through them.

10. ### Alan BGuest

Yes, your reasoning is correct. The diode scheme - let's say the logger
uses seven diodes to run a 3A 7V drill from a 12V battery - will be wasting
~2W per diode. So why not use a voltage divider with a minimum 14W source
resistor? The problem is that unless your voltage divider itself draws at
least 10x the current as the parallel load, the varying load will
significantly vary the voltage across the motor. In the case of a 3A load,
you've got to have a divider that draws at least 30A. Wowsers! Since
diodes are non-linear, they will drop the same voltage regardless of
current, so you simply get diodes hefty enough to handle the maximum load.

Note that I said the diodes will waste ~2W per diode: energy used that is
not doing useful work. That's one answer for the OP's question - be
cautious when mis-matching source and load, as apart from any other
concerns, you are wasting energy.

11. ### Mark ClementsGuest

OK - understood. Thank-you to you and to Alan B for your responses.

regards,

Mark