Let's Make Robots!

Fried?!...Back to square one...help?

Well I fried my PWM (predicted by ChristheCarpenter) by feeding it about 5 Amps. Alas, back to square one. Would like some definite help. 

I could build another PWM with a higher amp rating, but I noticed that as I put a load on the motor the speed was not constant. If would speed up, then bog down, then speed up. I'm not sure if this was due to a insufficient PWM, not enough Amps from the 1.5 A power supply, or lack of a true motor controller.

Hence I am prepared to start over from square one. I do not currently have a microcontroller of any kind. I know I need to buy one in the future because I would like to control my bots speed via laptop in realtime. 

Thus, I am asking for help and advice. What do I need? Please remember that my bot needs to push a 3lb box around the floor, with good speed (16 in/s).

I thought about using something like this: http://www.seattlerobotics.org/encoder/200001/simplemotor.htm

however the motor I have pulls SERIOUS amps and I think it would burn out the IC.

Help? 

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I fixed my PWM, added a MOSFET with a higher amp rating, put a 4700µf capacitor on the 12VDC power source line followed by a 5 amp fast blow fuse, to smooth out the power from the transformer, which also burned out i think. I think I'm going to switch to a 12V 3 amp mountable transformer.

maybe that will be enough...after that its a 5 amp brick, and so on and so on.

still want to go with a digital PWM and I discovered a little post on the end of the motor that rotates, maybe I can thead it, add a connector post, an extension and then add a rotary encoder to it.

Overbuild it.

It gets depressing when you keep frying stuff.  I bought a simple-H for my motors, but they are under-built.  The PWM generator I built has been very satisfactory, however, when I accidently stopped the motors, the voltage spike from the coil energy was too much annd POOF . A fried simple-H.  I have built several H-bridges before.  

The MOSFET bridges I built were the most capable of handling large amounts of current.  If fact when my robot stalled it vaporized the wires before toasting the FETS.  

Now I'm going to make an H-bridge with some big power transistors I got on the cheap from ebay!

The simple-H bridges would probably be sufficient for your purposes - Chris Baron @ robotpower.com does great work, but of course, its more expensive.

FETs bridges can be more complicated because of the need for the highside FETs to be activated with a higher voltage (boostrap)

PNP NPN combo H-bridges can use the minimal amount of components - nice for people who aren't solder masters (like myself)

Links: 
My latest speed control tragedy
Pricey simple H-bridge
MOSFET bridge with burned wires

Good Luck,
GroG

What rating does the motors have? Volts, stall current, etc.

Even just a motor brand and number might help to look up specs.

Current Motor is a 12VDC Skil Drill (model number: 2467-02) motor, not sure of its stall current, probably about 30A. Nominal is 0.5A. Ofcourse it uses 20 gauge wire inside the drill, so it cant draw anymore than 8.5 Amps or so.

I will change motors if necessary, otherwise I will need to power my bot with a PC power supply, since I want continuous use of longer than 30 to 60 minutes 

I'm assuming I will need an HBridge controlled my the Picaxe? 

Pololu has some good devices for the money. Their Dual VNH2SP30 motor driver carrier MD03A works very well for some window motors I'm using, and is rated for 12 volt motors, up to 14 A continuous, with a 30 A peak. It should just be able to handle both drll motors in one package for $60. Probably be good to have a fan over the device, and monitor current, take it easy driving. This one would require PWM generated by the PICAxe.  Pololu makes a TReX dual motor controller DMC01 that also takes servo pulses, serial, or analog voltage commands using the same driver board above for $100. Another h-bridge I've liked in the mid-range is the Robot-Power Scorpion XL for $120 for 2 motors, 12.5 A, 45 A peak, up to 28 volt for RC servo pulse control or a pair of Simple-H for PWM control. There is a Scorpion XXL for higher current RC control too. Almost forgot, Dimension Engineerings Sabertooth Dual 25A motor driver 6-24 v 25 A, 50 A peak for 2 motors, $125.

The Devantech single 20 A is a good board, but these perform as well or better for cheaper. Unless you must have I2C.

I personally prefer generating my own PWM, for motor control PID algorithms, just depends on your preferences.