Protecting My Propeller from damage

BigFred

Hi,

I want to build a robot that rolls on the ground.

so I am building a circuit that uses a motor controller IC to drive a 6vdc 650mAh Motor
which is on a toy RC car.

Here is it's data sheet for the motor control IC.
http://www.elite-ent.com.hk/upload/090603120608EMD811_&_812.2009.pdf

The idea is to have the propeller control the "switch inputs" on pin 1 and pin 8 of the Motor Ctrl IC.
This will drive the motor in the correct direction based on the pin selected.

Now, I need to make sure I protect the propeller output pins that I use to switch the IC with
so that no current/voltage back flows into the propeller output pins and blows it up!

QUESTION:

How should I do this ?

Can I just use a simple transistor as a switch directly on the pin?
Should I use a Diode Clamp on the output pin ?

This is a hobby for me, I'm not an engineer. I want to use the propeller so I can
program commands in the eeprom for the propeller to execute. These commands would
drive the motors on the RC car.

Thanks for the help,
Fred

Mike Green

It all depends on how careful you want to be. You could use a simple transistor on each input to the IC (an H-bridge ... look at the Wikipedia article on "H-Bridge" for more information). You could use an optoisolator which has a transistor for its output, but looks like a simple LED to the Propeller (look at the Wikipedia article on optoisolator). A 4N25 is the basic LED/transistor optoisolator.

StefanL38

first of all welcome to the propeller-forum.

congratulations to this post! It is well written. You give information about your knowledge level
you provide the datasheet of your hardware you already have thought about solving the problem
and make suggestions.

That's an almost ideal posting.

Ok to come to the technical aspects. If I understand the internal circuitry of the EMD811/812-chip right
it has internal freewheeling diodes that will protect against inductive voltage-spikes.

If you want to be a 1000% sure that nothing ever will happen you could use opto-couplers.
Then the motordriver can be electrical isolated completely from the propeller. This would reuqire two
completely independent powersupplies.

The chip needs an input-current of 6-10mA. So if you insert a darlignton-transistor you could use a 500k-resistor to limit the current flowing from the prop-IO-pin.
The prop-chip is sensible about too high currents flowing through the IO-pins. (maximum 30mA)
The prop-chip has internal clamping diodes that protect against low overvoltages (4V if I remember right but I'm not sure about this maybe the voltage can be higher)
The internal clamping diodes can stand 0,5 mA.

Maybe one of the EE-experts can chime in and tell more about what the prop-IO-pins can stand. And how to protect against this.

To protect the prop-IO-pins against to high currents, when accidentically shortcutting them to +3.3V or GND, I recommend using 470 Ohm-resistors
This will limit the current to 3.3V / 470 ohm = 7 mA. Even if you use pull-up or pull-down-resistors the voltage-drop will be small enough to NOT disturb
the function of the pullup-pulldown-resistors. You can connect the 470 ohm-resistors directly at the prop-io-pins to make sure that everything else
will be connected "behind" them. Only for special purposes like delta-sigma-ADC you have to remove the 470ohm-resistors.

best regards

Stefan