Electrical – Why did I burn the L9110s H bridge

atxh-bridgeheatmotor

I have recently bought a L9110s H bridge (pictured below), and while testing I accidentally burned it up. I'd like to know why, to avoid making the same mistake and if maybe I should try to get a refund.

Datasheet:
+=========+====================+=======+===========+=======+=========+
| Symbol  |     Parameters     |       |   Range   |       |  Units  |
+=========+====================+=======+===========+=======+=========+
|         |                    |  Min  |  Typical  |  Max  |         |
+---------+--------------------+-------+-----------+-------+---------+
| VCC     | Supply Voltage     | 2.5   | 6         | 12    | V       |
+---------+--------------------+-------+-----------+-------+---------+
| Idd     | Quiescent Current  | -     | 0         | 2     | uA      |
+---------+--------------------+-------+-----------+-------+---------+
| Iin     | Operating current  | 200   | 350       | 500   | uA      |
+---------+--------------------+-------+-----------+-------+---------+
| IC      | Continuous         | 750   | 800       | 850   | mA      |
+---------+--------------------+-------+-----------+-------+---------+
| IMax    | Current peak       | -     | 1500      | 2000  | mA      |
+---------+--------------------+-------+-----------+-------+---------+

My setup:

  • Motors: Generic motors from an old RC car. I can't see any number on them so I don't have much info. They came with a 100nF ceramic capacitor soldered on both ends. I have tried to connect them manually to the 5V and GND terminals of the power supply, and they worked just fine did not show any problems.
  • Power: ATX PC Power Supply (5A/3A current), with VCC connected to the 5V entry.
  • Situation: I connected the VCC and GND to a breadboard with 5V and GND from the power supply. When I connected A-IB to the 5V (to power a motor up), I heard a crackling noise, the two transistors started heating and emitting magic smoke, and they got so hot they actually fell from the chip. I screwed up real bad.

Schematic:

schematic

simulate this circuit – Schematic created using CircuitLab

There are two things I'd like to know:

  1. What was my mistake and
  2. Was my L9110s bridge defectuous? I know it was probably my fault, but I think it's weird that the soldering was so weak the transistors fell out when it overheat.

Best Answer

Unconnected inputs on CMOS devices are bad; they cause the internal transistors to be neither on nor off. It's possible that this caused the top and bottom of the H-bridge to be on at the same time, directly shorting through the device. This would explain why both chips failed when you only provided one input to one chip. You need to tie the unused inputs off to GND.

It's also possible that the motor draws more than 750ma - you should connect it up to the power supply via a multimeter in Amps mode to check this.

The solder melts at about 300C. Presumably the chips simply got this hot. I know it's cheap hardware but I very much doubt it was any kind of manufacturing fault.

It might be better to start with a current-limited power supply: e.g. 500ma USB charger. Run it for a few seconds and see if anything heats up unacceptably. It should at least take longer to self-destruct at reduced power.

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