On Wed, Aug 05, 2009 at 07:17:47AM -0400, Chris Loper wrote:

> Reposted, this time with the schematic. Schematic also here:
> www.chrisloper.com/low_side.png

> 
> A few weeks back, I asked for advice on building an 
> H-bridge for an electric go-cart (24V 500W motor).
> Heeding the sage advice I received, I have instead built this 
> PWM'ed lowside driver using a 16F88 and an IRF540.
>    low_side.png
> Running on my desk, with no load on the motor (drawing ~1A), it runs great!

> 
> I'm sure that there are all kinds of problems with it;
> I've never done anything like this before and
> I would appreciate any suggestions.

If you have some time and patience, there is a long standing thread for an
open system EV motor controller running on EcoModder.com here:

http://ecomodder.com/forum/showthread.php/paul-sabrinas-cheap-144v-motor-controller-6404.html

If you read all 215 pages of the thread, you can become a electric motor
controller guru.

> 
> I would like to test with a load, but haven't figured out 
> a way to do that yet. (The cart is not ready yet.)
> Anybody know how to put load on a 500W motor sitting on a desk?

Actually pretty simple. Max load for a motor is at stall. So all you really
need to do is solidly mount the motor then lock the shaft in a vice for
example.

> 
> I know that driving the gate at only 5V is not good, 

Nope.

> and I plan to replace the IR540 with a logic level FET.

Bad idea. You need a true totem pole MOSFET driver. Check the thread for
examples.

> I did a search on Digikey and found this one:
>    DIGIKEY IPP0165N03L G   30V   50A   Rdson=.0065   $1.03

0.0065 is about 8 times too high for a good motor controller. It'll
generate a ton of heat that you'll have to dissapate at load.

And 50A really isn't 50A. That's 50A at max heat dissapation for a limited
pulse. You'll want to derate that to about 20A for continuous load. So
you're going to need more than one MOSFET.

>    http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=IPP065N03LGIN-ND
> Datasheet:
> http://www.infineon.com/dgdl/IPP065N03L_rev1.02.pdf?folderId=db3a30431441fb5d01148c401f250e27&fileId=db3a30431441fb5d011492371ebc0fe2
> One US dollar!  It seems almost too good to be true. 
> Are there other parameters that I need to check?

Decent price. Buy a double handful of whatever FET you end up using. Again
the thread on Ecomodder talks about MOSFET parameters.

I'm snipping the rest. Go sit down and take a read of the thread. Every
aspect of controller design and programming have been discussed over the
last 10 months.

Good Luck.

BAJ
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