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Too high voltage drops across different TIP-based motor drivers?

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.

My needs:

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?

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Ok, so I'm trying to learn the meaning of the Vce(sat) and how to/what can be read from its values on datasheets.

As you said, "The Vce(sat) voltage is 2V with a 3A load", this would mean that if my motor was drawing 3A of current... I would expect a 2V drop across the TIP122... but since in the H-bridge the current would have to travel through a TIP122/127 matching pair... a 4V would be expected? (well, at least last night my smaller motor was drawing around 2.1A through my TIP127/122 h-bridge and from an ~8V battery I was getting around ~4V ... I would further guess that in the physical world things aren't are perfect as in ideal datasheet numbers)

 

Also regarding the BD681/680 recommendation given that the Vce(sat) for Ic = 1.5A, Ib = 30mA is 2.5V wouldn't this make it a worse choice? bigger drop at a lower current?

In summary and for practical purposes I would want to find the NPN/PNP pair with the lowest Vce(sat) I could find, provided they met the other necessary requirements (Ib, Ic, etc) right? (or oh so very wrong?)

In the meantime I'm considering going with a simple relay driver setup. At the moment seems like the best (quickest & available) solution regarding voltage drops.

Your problem is that those transistors are designed for high voltage applications where a 2-4V drop is not a big deal. If you look at the datasheet for the TIP122, The Vce(sat) voltage is 2V with a 3A load and 4V with a 5A load.

This means that with a load of 3A, about 6W of power is wasted as heat!!

For more than 1A, BJT transistors are not very efficient. You tend to need big heatsinks. For the small motor, a SN754410D would be a better choice. Just bridge the outputs.