Too high voltage drops across different TIP-based motor drivers?
October 23, 2012
So, I needed to drive a couple of mystery motors I've harvest from a "electric shaving kit". I had a big motor, extracted from the electric razor and a tiny motor extracted from the nose pliers.
big motor -- drive it one way; small motor -- drive it both directions
Case 1: Handling the big motor
UPDATE: The "big motor" crapped out on me. Since yesterday I got some AWG 10 wire (I think) so I used it to replace the weak links (jumper wires I had burned yesterday) and went merrily on to read more current pulling values. So today it started around 5 something amperes and dropped to 4 something. I tried not to run it for long each time, anyhow one of the times the velocity started decreasing and I killed the connection, checked the fuse (4A rated and looks like a brown resistor) it was just warm (I suspect it may be a thermal shutdown fuse), so let it cool a bit and tried it again... the motor spinned a little more very slowly for an instant... and died. Applying direct voltage from a NiCd set of batteries... no movement. All this while using a 3S at ~11.4V 850 mAh 20C LiPo battery.
So for the big motor I just decided to do a TIP120 accordingly to this schematic:
Designed a tiny board board on eagle and homemade it myself, I ended up replacing the 1N4004 by a beefier 1N5408 since I ran out of 1n4004. Last night I've finally tested it, but I was having seemingly considerable voltage drops (comparing battery voltage and voltage across motor leads), more that I think it would be expected for a TIP120. I don't have the exact values here and now but I jolted down some notes on a paper at home. More so, at first my tests led me to believe that this motor would consume ~1.5A "free running"... but while last night I decided to re-measure it while using it plugged to an 2S LiPo... and with a peak at 9A... dropped to 4A for a couple of seconds... then 3.5A and all the while I was expecting it to drop even more and stable at a lower value... the test jumper wires I was using... melted together and went PUFF. So, question is it possible that the current "being ask for" was really the motor, or the bottleneck in cabling also increases current pull?
Case 2: Handling the small motor
Since I needed the motor to run both ways I decided to make a transistor bridge, my initial motor drawed ~600mA during free run, so I was designing a board with 800mA rated "normal" transistors, but I manage to toast that motor and replaced it with a motor with known/reported specs a 4.5V motor (11200 rpm if I recall), the thing was that this new motor drawed ~1.6A free running so taking into consideration the stock of a local electronics store I'm sort of semi-accidently arrived at the TIP122/127 H-bridge combo, I picked the design here: http://www.pocketmagic.net/?p=508
and did it like so:
It runs fine with the exception that again there's a greater voltage drop that it should... from a LiPo at 7.4V ... I measured 3.52 or 3.62V across motor leads... so, am I doing the voltage drop math wrongly, reading specs wrongly or is there some problem on the schematic or might I have a problem with the design/implementation of my self-made board?