loss of power from h-bridge

Other Hardware Motors, Mechanics, Power and CNC
1

I'm still fairly new to electronics and I may be doing something wrong, but are we expected to lose some power from an h-bridge?

I say this because if I connect my motor directly to a battery, it goes fairly fast. But if I do it through the h-bridge (and I make sure the voltage is being sent to the h-bridge pin that powers the motor), and I set the h-bridge enable at 100%, I can't get it to rev as fast as it did when I did it directly. It is noticeably slower.

Is this expected? Am I perhaps doing something wrong?

Thanks!

2

Which h-bridge?

3

Hello,
The H-Bridge is essentially two switches in between your motor and the power supply.

So there is always SOME loss.

A typical L298N H-Bridge used Bipolar power transistors internally and so the voltage drop can easily be 2 volts. If you are running 5V motors from a 5V supply that will be very noticeable. But they are inexpensive.

A high-end Power-FET based H-Bridge will have a lot less voltage drop.

Just what are you running??

Take a look at some Motor Driver How-To here: http://arduino-info.wikispaces.com/MotorDrivers
Some overall Motor Info here: http://arduino-info.wikispaces.com/DC-Motors

4

Thanks for the info. It feels like I'm getting way too much motor loss.

My h-bridge is just a standard SN754410. Thoughts? :slight_smile:

5

You don't say how large or how much current your motor draws. You might try a TLE5206-2 h-bridge which is rated for 5a. DigiKey shows them in stock.

6

That H-bridge has Darlington output stages so it will lose about 2 to 3V - pretty poor if you are using a 6V motor (I'm guessing you are). A MOSFET based H-bridge is much more suitable for low voltage motors.

Unfortunately there don't seem to be any good cheap MOSFET H-bridges in DIP packages, and a lot of people go with the L293, L298, SN754410 which are cheap and cheerful, but lose lots of voltage (and thus get hot at high currents). That TLE5206-2 sounds good (Farnell don't stock it alas...)

Some of the high performance MOSFET bridges are available as motor controller modules (Pololu is a good source of these BTW).

(A Darlington stage is a pair of transistors with high gain but minimum saturation voltage of about 1 to 1.5V)

7

actually the main motors I am using this on is a 1.5v to 3v motor.

The weird thing is despite if I connect a 9v battery or 4 AA batteries (6v), I still can't get it anywhere as close to the same speed if I was to connect it directly to the batterie, without the h-bridge. Is this expected?

8

Is this expected?

Check the data sheet for the h-bridge chip you are using for the voltage drop across it and its internal resistance. Test the h-bridge making the input control pin high by direct connecion without using the arduino.

9

I still can't get it anywhere as close to the same speed if I was to connect it directly to the batterie, without the h-bridge. Is this expected?

Yes it is expected because as has been said before you loose some volts, so it will always loose power compared with the raw voltage.

The fact you have a 3V motor means you are over driving it with a bridge and even more over driving it with no bridge.