Robot Efficiency & other Robot stuff

Oznog said:

Your schematic shows an external diode in parallel with each PMOS.

This should not be necessary. All PMOS that you get off-the-shelf have an internal body diode inside in this direction already. You can read your spec sheet but usually they are capable of carrying substantial current.

Your schematic lacks a capacitor on the output of the reg. This could cause the reg to oscillate and potentially blow your chip.

You need to put a ceramic cap from Vdd to Vss right at the PIC chip, especially on a power circuit like this. I already had this trouble more than once- nearby power consumers, even though they didn't even draw power out of the reg, were causing the PIC to latch up.

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You need to put a ceramic cap from Vdd to Vss right at the PIC chip,

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Oznog said:

What's the part number for those MOSFETs?

Ones with a threshold of 4V don't turn on very strong with 5V, thus you will only be able to use a fraction of the FET's current rating. Temp variations may make it worse. I had a PWM buck converter working like total crap due to that. Actually, the vgs should have been high enough for the 1 amp current involved, but despite that it wasn't doing even that- I'm not sure why.

The waveform you have suggests just that. The inductance of the coil initially resists the sudden change in current, it appears as a high impedance load so the MOSFET's able to drive vds pretty low. Then the inductor's current increases, you face only the resistance of the stepper coil, and the MOSFET is unable to conduct enough current to keep vds low.
There are ones with thresholds of around 2V which switch much better.

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There are ones with thresholds of around 2V which switch much better

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Oznog said:

IRF510 is not a logic level device, not a good choice. Go to Digikey, search for MOSFET, and select "logic level". Try these from **broken link removed**

IRL520N N-ch MOSFET logic-level gate voltage, 10A, 100V TO-220 $0.67
IRL530N N-ch MOSFET logic-level gate voltage, 17A, 100V TO-220 $0.87
IRL540N N-ch MOSFET logic-level gate voltage, 36A, 100V TO-220 $1.17

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gurgalesci said:

After 3 days of sitting in front of my robot waiting and praying that it would move, it finally did. I spent 72 hours nonstop sitting in front of it. I also have those brake things that someone was mentioning earlier. They don't work very well. Whenever I try to use them, the robot only goes faster and without a brake. Oh well, the brake cost me $5000 to install and buy the brake. As a result from the problem, my robot was badly damaged and I hired a person from Holiday Inn Express to fix it. He did, but during his repair, the Holiday Inn Express Magic wore off and he made the robot look even worse. Now, the robot just spins around in circles signing "Yankee Doodle." I guess its kinda cool around Independence Day.

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