SherryQazi

Greetings,
guys i m making a line following robot weighing abt 12 kg with dimensions of 41x30. for driving the robot i am using power window motors( 2of them) I plan to drive ethem through h bridge but the problem is that my h bridge isnt reliable n heats up and burns up anytime(even after usin heat sink)... ive tried nmos h bridge(P250) as well as bjt h bridge(TIP 120) but its not giving me a reliable h bridge which would run aA motor of abt 6amps at 12 volt dc in bidirection
any schematic or idea??

dknguyen

6A realy isn't that much, but it is too much for most H-bridge ICs. Just make a bigger H-bridge- pick better MOSFETs and drive circuits to turn them on and off quickly.
http://www.modularcircuits.com/h-bridge_secrets1.htm

If you still want to use an IC maybe the two at the bottom on this page will fare better:
http://www.pololu.com/catalog/category/11

As you can see they are claimed to be 30A, but...you will still probably need a heatsink. The current rating manufacturers give for MOSFETs and H-bridges ICs is usually WRONG for practical applications. You have to hunt through the datasheet and get the on-state voltage drops or resistances and use that with your current to calculate power dissipation and then use the thermal resistance to calculate the temperature rise. You will find it's usually at least 5x less than what they say. It has to do with the fact they may or may not be using heatsinks in their measurements and that they may be rating just the temperautre of the silicon and NOT accounting for the fact the silicon is trapped inside a plastic case.

If you look on the product webpage, it seems Polulu has already accounted for all that and seem to say that without a heatsink, the IC won't overheat at >9A or > 14A (depending on the one you choose).

If you build your own H-bridge, do not forget flyback diodes that go in anti-parallel across each power transistor? THese are required to protect the MOSFETs from the voltage spike produced whenenver you disconnect current from flowing through the motor.

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SherryQazi

sorry but i cant use an ic.......i hav to make the h bridge on my own!

Hank Fletcher

Parallel transistors to share the load? I've got a cheap RC car here that does that.

kchriste

Try using a better (rated for more current) nmos than the P250 ( I think you meant P2503 or something). Remember that the start current of a motor is much more (5-10x) than it's full load current, so you must rate your FETs to handle the stall current of the motors or incorporate current limiting in your H-bridge design. Post a diagram of the circuit you tried and maybe we can advise you on better FETs to use.

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ikalogic

I've built two very reliable H-Bridges, they were used on 25Kg robots and 24V motors. They are cheap, using TIP transistors.

Take a look at those pages:
http://www.ikalogic.com/shm_int_h_bridge.php
http://www.ikalogic.com/H_bridge_1.php

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