Let's Make Robots!

I need to build a BIG motor controller

Simple question,

I need to run a motor drawing between 7 and 10 amps at about 18 volts. (I think a similar question has come up reciently)

I am looking a these which is a big brother to the L293D. It says it will handle 2 amps per channel.

Question: Is it as simple as ganging up in 5 of these units in paralell to get my 10 amps? 


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Here's a schematic and a PCB layout. Uses DPDT relays to control direction. Will handle 10A at 40V. Speed control is by PWM. I've added an attachment with is the Eagle CAD schematic and layout.

Simple answer, No. It won't work.

Long answer. The L298 consists of bipolar junction transistors BJTs as the load carrying devices that have a negative temperature coefficient of resistance when going through current overload. One transistor gets hotter as more current is passes, dropping it's resistance which allows more current to pass, getting hotter, etc until failure. Essentially one L298 will attempt to carry as much current as possible, til it melts, then the next, so on, down the line.

Field Effect transistor FET devices like the LMD18200, have a positive temperature coefficient, in which they have a growing resistance as they get hotter. So one FET device will get hot, causing more resistance, which will push current to other parllel FET devices, allowing the first to cool, and allowing a sort of load balancing.

Or you could buy a couple BTS7960B half h-bridges that carry 20+ A  up to 24 volts. (bottom of page, need 2 for full h-bridge) These are surface mount, but not a small package.

I never thought about that. I just realised that's what the OSMC is: a load of FETs in parallel. I didn't realise that was an option. Nice one.

Wut?  We've been through this before, ya muppet :P

 Actually, I mean no disrespect..  I think your design is brilliant!  I can't tell you the amount of pain you could suffer, designing voltage doublers circuits (or kludging additional batteries in series) or  looking for rare, expensive, and inefficient P channels FETs ..  The relay FET combo is a GREAT design!  It utilizes the best from both worlds, simple current switching with solid state PWM.
Kudos!  And of course since emulation is the highest form of complement, I would like to compliment you and use your design :)

However, as my reply at the end states,  I would use some flyback diodes in the design.  In my setup, it means the difference between smoke and no smoke.  Another scary thing is when a FET burns from a voltage spike it will burn Open.  This means your bot will go 100% full blast until it hits something which can stop it - the motors will stall - then more things will smoke.

Here is my bread board testing FETs and diodes since my last simple-H lit up. (see here) - the terminal wires were ganged up for more current capacity.


I'll still probably end up using my 555 PWM and DIO for speed control, which probably won't be relevant to Chris, because he'll just use the PWM on the PicAxe.  Is that your plan Chris?

Good Luck,


When you say "burn open" you mean "burn closed" don't you? As in the switch closes, causing your bot to go flat out?

Left, right?

Yes, I meant to say it will fry in a position that will be open throttle... (admittedly, not the best choice of words)

but most definately fried closed..

Great explaination of the resistance/ heat thing --good stuff.

I went over to the "fried/ square one" post and found some links to some ready-made controllers... If money were no object, this guy seems the most promising... However, money IS an object so I am liking the BOA approch more and more.  I would hesatate, but I re-watched the video of BOA driving his plywood plank around via remote and his little homeade controller seemed to be working fine. -I like the PWM aspect as well, as the motors I am driving are wicked fast. I plan to start going through BOA's controller tonight and getting a parts list together. 

One question though: are there any FET's available bigger than what you are using? 10amps will probably work for me as there will be alot of gearing down and the motors will not see much load but on the other hand, they are showing 7.2 amps just spinning in mid air... I might be crossing the 10 amp threashold from time to time...

So here's what I got (based on your schematic)schematic_1_002.jpg


Do I have the Reverse on/off and PWM inputs right?

Is the 2N4401 (Radio Shack) NPN Transistor the right one to use?

What's up with the 330 and 470k resistors on the bottom -do I need them?

It's a 12v relay, right?

Is there a bigger FET to replace the IRF740 to allow for greater amps?


OK, that's should be it... Gimme what you got!

Nope. You really do need the transistor drive to pull the FET gate all the way up to your drive voltage. These are still something of a black art to me, so I won't advise you, but I learned that driving the gate with only 5V from the PIC caused only 5V to appear at the source regarless of drain voltage.

Also, be aware that the transistor will invert the PWM, so 100% = off and 0%= full on. AND it does not fail safe. If the MCU fails or the FET lets smoke out, the motors will keep running. (Consider an additional SPST relay at the output so if you lose one of the above, the MCU still has full authority to switch the motor off.)

Pay attention to the recent posts on flyback diodes. My design SHOULD have them and there's a distinct possibility that I've just been getting lucky.

I'll tell you something: motors that are drawing 7A at idle seem likely to pull a LOT more under load. Consider measuring the coil resistance and calculating the stall current. It seems you don't need bigger FETs. Neighbouring posts seem to indicate that you can just bung a load of them in parallel. It looks like that's what's going on here.

I can't comment on the suitability of the transistors, but I'm pretty sure basically any old NPN transistor will do so long as it can sink the current for the coil i nthe relay. Do the V=IR thing for the relay coil to find your current.

You're nearly there, though!


I found a book once on the "black art" and it explained it this way:

1. not happy unless fully on or fully off.  (unhappy = heat)
2. you need a .7 voltage boost on the Gate relative to the Drain to be fully open (Happy!)
3. this is why N channel FETs are easy to deal with on the bottom end of a motor - because the voltage drop after driving the motor is much more than .7 volts - so if the gate is being driven by a rail it turns on ALL the way!

A picture's worth a thousand words, so here