Let's Make Robots!

Tracking fuse blowing short.

Last night while testing my code,  I blew a fuse.  I went through and checked continuity with all the connections I could think of.  Not finding anything I went through and changed out all the wires/solder connections fuses and battery connections to my switches. (everything between the cpu/motor controller to the battery except the switches.) Right now everything is bare minimum of connections and it will still blow the fuse.  I only have the arduino 2560 with v2 sensor shield(corrected the pin to usb short/bug) .  Hi amp dual h-bridge (no name import)  as well as 2 switches. All connections are soldered including battery wire/crimp connectors

 I may be wrong,  but it looks like it might be related to the order I turn it off in.  It might be blowing when I turn off the arduino before the H-bridge. It blew last night, not sure which order.  I put in a new fuse and turned it on = cpu/motor and off = motor/cpu. Thought about the order and tested it by turning it off opposite cpu/motor control and the fuse blew immediately. 

 The whole thing is wired as common ground from battery splits to arduino ground and motor power ground.  A single positive lead splits off to both switches.  The second terminal of the 1st switch leads to the arduino's regulator plug.  Second terminal on switch #2 goes to the positive terminal of the motor controllers motor power in.  (motor control log + pos and - neg comes from an arduino 5v source.). Fuse is inline from batter's pos. 

It seems to work fine when programming with power via USB port (arduino regulator unplugged to prevent any possible backfeed,  no motor power or battery connected). Doesn't seem to blow the few times I have powered the arduino on and off using the battery.  

I am pretty sure it blows the fuse while the power is off since I had a led voltage meter connected to the + switch split and it was the first thing I was turning on during the first fuse blows.  Wouldn't come on.  Still getting the blown fuse when powering off with bare minimum electronics connected. 

The only other possible thing I  could see affecting it might be the actual switches.  They were salvaged off some old electronics,  so I am going to look for replacements today. 

Is it possible the cheap motor driver may be backfeeding causing the simulated short or more likely it's a bad switch?  Could it be fast blow fuses connected to the switches?  Bad switches?  (trying to think of everything here).  Anything I can do to isolate the area of the problem (or prevent it/damage?) 

 Switches will be swapped out for new.   I really would like to fix it without continued risk to my equipment. 

Any ideas? 

Ed

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Without seeing the schematic for your motor driver I would agree with Duane.

As a general rule of thumb your motor driver should not have motor power unless it has logic power. If you use a seperate battery to power the motors then make sure this is turned off first or at the same time as the logic power. For example use a DPDT switch that switches the motor batteries and logic batteries at the same time.

I've seen recommendations of adding pull-down resistors on motor control input lines. I think there can be a problem if the control lines floating while the uC is off or hasn't established control of the I/O pins yet.

What is the rating of the fuse you are using? I'm guessing it's just under a stall current spike going to the motors on power up.

That was one of my first thoughts,  it's  now using 2 new motors.  Radioshack 9v,  115mA max each. The fuse is a 4amp fast blow 5x20mm glass fuse.  The first fuse to blow was a 5amp fast.  Motors were not moving the last time it blew.  (I didn't know the rating of the larger motors that blew the first 5 amp,  but they are also mini hobby motors,  can't imagine them pulling a couple amps.  I guess they are <0.5 amp max,  but don't know. ) 

I've now replaced the 2 switches as well. Everything has been replaced except the arduino and motor driver themselves.   I  hope it was something I  corrected.  Maybe a poorly soldered/insulted joint or the suspected switch. 

I agree with the things that have already been said.  Often, motors can take a LOT more current than you would think.  Take a look here http://www.radioshack.com/product/index.jsp?productId=2102827&znt_campaign=Category_CMS&znt_source=CAT&znt_medium=RSCOM&znt_content=CT2032230

down near the bottom under "crank things up" at the current draw of this small motor.  Those ratings are "under load" not max.