0
\$\begingroup\$

I have a motor that came with a 9 volt, 1.0 amp AC to DC wall converter to power it and the text next to the power connector on the motor seems to indicate 9 volts @ 1 amp is the preferred input. I wanted to make a battery pack for it so I have 6 1.5v 'D' alkaline batteries in series with a barrel jack to connect to the motor.

As is, the battery pack runs straight to the barrel jack. Should there be a current limiting component on my battery pack such as a resistor or capacitor?

\$\endgroup\$
1
\$\begingroup\$

1.5 volts per cell suggests that your batteries are alkaline chemistry. You're just discharging a battery through a motor. So, you probably don't need a current limiter.

I suggest adding a fuse between the battery pack and the motor. If the motor fails and becomes a short circuit, the fuse will blow and cut off the battery current. Motors usually work better with slow blow fuses (as opposed to fast blow fuses).

As an experiment, connect the ampere meter between the battery and the motor. See how much current actually flows.

\$\endgroup\$
  • \$\begingroup\$ Thanks for the confirmation. I have an old voltmeter that says it can measure up to 10 amps (which should be enough cushion) but it didn't respond when I hooked it up to the battery pack. I checked the fuse and it wasn't blown, so I'm not sure what's wrong with it. The volts measured a little high, about 9.5, but that should go down once it is actually hooked up. (right?) \$\endgroup\$ – tehDorf May 18 '12 at 0:25
  • \$\begingroup\$ My NiMHs are 1.2V. \$\endgroup\$ – stevenvh May 18 '12 at 4:09
  • 1
    \$\begingroup\$ Is your multimeter the type with a different probe socket for measuring current? Also, did you really just put it into current mode and short it across the battery pack? You should wire it in series with the motor to measure actual current draw. \$\endgroup\$ – pjc50 May 18 '12 at 9:44
  • \$\begingroup\$ @pjc50 - Yes and yes. It has a separate plug for the probe for measuring 10 amp currents, which I used. I also just hooked the probe up to the +/- terminals of my battery back without the motor connected. It looks like that was my problem? \$\endgroup\$ – tehDorf May 18 '12 at 20:43
1
\$\begingroup\$

A 9V motor will "look after itself" when operated from 9V in normal operation. If stalled it may draw excessive current. A slow blow fuse in the motor circuit will minimise the risk of damage under overload conditions.

If you have a "proper" plug/socket on the battery connection it would be common to have no protection in the battery pack. shorting 6 x D cells in series can cause a fire so a slow-blow fuse in the pack itself would not be a bad idea but in a one only situation it's up to your personal risk aversion and carelessness factors whether you use one. If this was for a high volume consumer application then a fuse would probably be required to avoid potential liability issues.

BUT

You have a potential technical problem with your proposed solution. 6 x D cells will produce about 9 volt when new (Alkaline, Zinc, "Heavy dury", ...) or about 8 Volt for NimH or NiCd. BUT as they discharge the voltage will drop, rapidly at firs and then more slowly, to about 1 Volt per cell when nearly fully discharged. A non rechargeable cell will spend a lot of its life in the 1.1 to 1.3 Volt range or say 6.6V to 8V output. Your motor may work PK at these lower voltage or may work badly or not at all depending on the application. You need to try the motor at lowest battery voltage (about 6 Volts or even less loaded) and see if it works acceptably.

\$\endgroup\$
  • \$\begingroup\$ Thanks for the note about the voltage drop. I tried it with just 4 fresh batteries (~6.4 volts) and it was ok. \$\endgroup\$ – tehDorf May 18 '12 at 20:46

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.