Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

I am on a senior design team at school. We have designed a mechanical system that uses a DeWalt 20V drill motor to drive the system. It is powered by the stock lithium ion batteries. I need to implement a few limit switches. I think 4 will do the trick, two of them will be SPST and two will be SPDT. I ran some tests with an ammeter and found the drill is pulling 17 amps without a load. When loaded, there were a few 100 amp spikes. What limit switches can I use that will handle this sort of power? (I am assuming 20 V * 100 Amps = 2000 Watts) I need them to have a fairly small footprint. Is this even possible to accomplish? Thanks in advance for any help.

I have added a wire schematic, maybe this can help. It was a bit dicey how I measured the current. I pulled the battery out of the drill, and completed the connection with wire and an ammeter to get power to the drill from the battery which was laying on a table. It was crude, but the motor came on, and the ammeter spit out a reading.

Wire Diagram

share|improve this question
How are you measuring current? 100 Amp spikes seem a bit extreme to me. – Scott Seidman Sep 4 '12 at 22:03
Does the drill or its battery pack has a fuse? What current is it rated for? Slow blow fuses are typically used with motors, because they are tolerant to current spikes. Sustained overcurrent will blow them, of course. A switch rated for 20A might be able to work with a 100A spike. Be sure to keep the fuse, though. – Nick Alexeev Sep 4 '12 at 23:35
If the motor is 1/2 ohm, your 100 amp spike would be 50 volts, which probably isn't generated by your 20v battery. I suggest an issue with your current measurements. – Scott Seidman Sep 4 '12 at 23:56
Got it from the last answer -- the spikes are from the motor switching off, not the battery. Don't neglect, the diode, as already advised. – Scott Seidman Sep 5 '12 at 11:00
I edited the post in response to how I measured the current. Again, it was very crude. You are right, the spikes mainly occurred when the trigger was activated, however, when I put a small load on the drill, the current did increase. I didn't see huge spikes, maybe a 15 amp spike. My biggest question is am I going to burn up limit switches with this setup? – Jacob Sep 5 '12 at 20:49

Limit switches don't have to carry all the current, but they can drive a relay that can! I'd go for a normally open relay, with limit switches designed to close the relay when all is good -- this gives a better failure mode (motor off!) if things malfunction. It's probably not as ideal as a direct cut of current to the motor through your switch, but it should get the job done safely.

Think hard about all your failure modes as you go through this exercise!

share|improve this answer
I have considered a relay, I am just trying to minimize the footprint and complexity. If it comes down to it I will opt for a relay, if it is the only option. – Jacob Sep 4 '12 at 22:20
The relay may have a smaller footprint, if it lets you use miniature snap-action switches instead of bulky high-current switches. – markrages Jan 3 '13 at 22:35

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

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