backfeeding prop pro dev board

mikea

i made an h bridge with p and n mosfets for a 24 volt motor. i tried the bridge first with 3.3 volts and a bi direction led. when i was in the process of hooking up 24 volts to the dead circuit to try the motor (which wasnt hooked in yet), it lit up the "power on" led on my prop board. the only wires going back to the board are a ground and the 4 gate wires to pins 20-23. the prop was not switched on. i dont want to burn up the prop board. when the board is powered down i assumed there was no path through the i/o pins. does this sound like a normal condition? im having trouble getting a picture here for the h, but its pretty basic and also i have 1000 ohm resistor between the gate and source on each of the 4.-mike

Publison

mikea wrote: »

i made an h bridge with p and n mosfets for a 24 volt motor. i tried the bridge first with 3.3 volts and a bi direction led. when i was in the process of hooking up 24 volts to the dead circuit to try the motor (which wasnt hooked in yet), it lit up the "power on" led on my prop board. the only wires going back to the board are a ground and the 4 gate wires to pins 20-23. the prop was not switched on. i dont want to burn up the prop board. when the board is powered down i assumed there was no path through the i/o pins. does this sound like a normal condition? im having trouble getting a picture here for the h, but its pretty basic and also i have 1000 ohm resistor between the gate and source on each of the 4.-mike

Mike,

I'm pretty sure everyone would love a schematic. It may be me, but I get lost without a road map.

Guys can still ask directions

mikea

Thanks publison. What do guys use to draw the nice schematics posted here? -mike

Duane C. Johnson

Hi mikea;

While many of us have full CAD packages there are good reasons to use nice quick and dirty schematic drawing programs.

The nicest, in my openion, right now is:
Schemeit
This makes nice looking drawings, Low end but works well.
Or
A bit more complicated:
TinyCAD

Duane J

cavelamb

Ages ago (or at least last century) when we were playing with the early circuit board test equipment
I built several circuit board and cable scanners. At that time back to back 8255s were the thing.
(one driving and one shadow input)

One of the stranger things I ran into was that even with DUT (Device Under Test) power shut off,
the 8255s were still completely functional. They were being powered by the address/data buss
connections from the host computer.

That's not all that far from what you are describing.

Designing a circuit that can be partially powered down (safely) is part black art, part witchcraft.
Best advice is don't go there. There are far too many ways to let the magic smoke out!

JLocke

Here's the link that Phil posted to some free online schematic software: http://www.digikey.com/schemeit

cavelamb

Best suggestion is DesignCAD. It's the sleeper of the CAD world.
Easiest to learn and use, but more powerful than a speeding locomotive!
(No, wait. That was Superman. But close.)

Seriously. Best of the best.

Version 22 is the latest greatest. About $99 new. ($69 at Frys - dunno why)
22 is intended for modern systems and Windows 7.

Check Half Priced Books. They usually have 10 to 21 for abut $30 or so.

Mostly I use DC 2000 (version 10) on an 8 year old HP (2.6 Ghz, 2 gig ram).

But I picked up 21 (from 1/2 Price) and use it sometimes for the advanced features.
The video drivers on this box are the limiting factor.

mikea

top fets are p channel which are held low to turn on. lower 2 are n channel held high to turn on. upper left and lower right are grouped as fwd. upper right, lower left as rev. gates are hooked to i/o 20-23. also i left out of the drawing the 24 volt ground is tied into propeller ground. does this circuit appear like it will work? I should probably add the code in here too, but i'm pretty confident that its ok.-mike

Duane C. Johnson

Hi mikea;

OK, it looks like the circuit is not complete.
Are you intending on driving the motors with 48V, (24V - -24V = 48V)?
In my mind 48V implies high power motors.
If so one should look to using power MOSFET drivers that are specifically up to the task.
Take a look at the power MOSFET drivers from International Rectifier, they invented this stuff.
Or
As I suspect, the -24V in the schematic is actually ground.
This then would imply the use of 24V motors.
There is still a problem:
Most MOSFETs have a maximum gate drive voltage of 20V, and most are less than 20V.
Even so, if you drive the gates with 24V signals this can cause the gates to break down.
Again, take a look at the power MOSFET drivers from International Rectifier.
These devices will safely drive the MOSFETs.

Other circuits can be made but are complicated and not as safe as the dedicated drivers.

Duane J

mikea

Thanks Duane J. Yes sir 24 volts total, 2- 12 volt batteries - 24 is ground. i was going to try to drive the gate with the 3.3 from the propeller i/o. There is continuity between the drain and source of the mosfet when no power is on... the meter would read around 1/2 ohm. do i have to set the state of the gates before applying motor circuit power? when i tested the bridge with a 2 way led and 3.3 volts instead of 24 everything worked like i thought, but when i take the led out and start to connect 24 volts sparks start to fly. This is when i saw for a second the "power on" light on my prop board when the switch was still off. -mike

mikea

I just re-checked the between the drain and gate of each. on the left side of the h the upper "p" has continuity and no resistance, same with the one below it. The "p" and "n" on the right side both are open, no continuity. I was wondering if i have to power the gates to set these to fully off before applying motor circuit power?-mike