Power output on I/O port
 

I'm trying to mess around with the controller, and what I'm trying to reach is to provide >7 volt out of either an I/O or motor port on Channel 6.

I've noticed that the three ports are:
- Common gound
- V++
- Control

The control signal is minimal power, and the V++ plug is constent.

I was thinking of some sort of tri-state buffer, or relay so that the control signal will open up the V++...

Anyone have any ideas for me?

Re: Power output on I/O port
 

Your are going to need to be more specific in what you are trying to do. All outputs on the RC are tied back to the +5 volt internally regulated power rail. Although the 5 volts is unused in the speed controllers, it is used in the servos. If you are using the RC for non competition controls there is a variety of things you could do, but... For competition, you may not put any custom circuits between the RC and motors, solenoids, etc. that will affect the devices other than the parts supplied, i.e. speed contollers, spike relays, servos, and team LEDs.

Re: Power output on I/O port
 

Luckly, this it for a non-competition project I'm playing around with.

It appears that the constent power on the Servo outputs is ~7.2v. What I would like to do is be able to send either no power, or full 7.2v power based on either a bummper switch or Channel 6 on controller.
I'm fine with the programming side of things, but I can't figure out how to get the full 7.2v based on any programmatic logic.

A spike relay sounds like it might be the direction I'm thinking. Would it use the control plug of one of the I/O or servo ports to send the full power rail through? If thats the case, would I be better off using an exterval 9v battery as my provided power so I don't fry the RC?

Thanks

Re: Power output on I/O port
 

This is a real issue for many people, remember that the servo does not work on supplied voltage, it requires PWM encoded direction and speed info. The power is supplied so that the the electronics inside the servo and the motor have a working power supply when the controller generates a PWM signal. Changing the voltage to a servo will not get it to start or run at a variable speed and may in fact damage it. If you read further into the servo spec sheet you will see that they will run over a specified voltage range with 7.2 volts as the nominal. This voltage was determined ( a long time ago) by two factors. Six 1.2 volt NiCad batteries add up to 7.2 volts (a six pack of AA Nicad Cells is a nice rectangular package and is easily available) and three terminal voltage regulators (of which one each is inside of every servo) run happy with a 7.2 volt input. Now a close look at the servos supplied in the kit are of the 4.8 volt type ( I think and will check) which run just fine on 5 volts.

Re: Power output on I/O port
 

Sorry, I may have been mis-leading in my previous post.
I'm not planning on using a servo for this project, I'm actually trying to get a magnetic solenoid to extend by providing it with > 7v power.

The solenoid only has 2 wires, ground and Vcc... when >7v (7-9v capable) is supplied, the soleniod will extend, when its removed, the soleniod retracks.

My thought was to use some sort of 3-state buffer that when the PWM is set high, it will relay the Vcc through and provide the soleniod with 7.2v power.

Sorry for the lack of prior detail

Re: Power output on I/O port
 

It sounds like a spike is just what you need with the spike your code will look like this when it is connected to the relay outputs:

relay3_fwd = p3_sw_trig;
relay3_rev = 0;//So it cannot recieve reverse polarity possibly damaging something

I am pretty sure that you want to set that to 0 you may want to ask a more FRC aquianted programmer than myself

Re: Power output on I/O port
 

I was under the impression (from stuff I've read and experience) that the power line on the PWM outputs is 7.2V, direct from the backup battery. I thought only the digital IO and analog ports had 5V.

You should be able to accomplish this using a digital IO port to drive a BJT (transistor) that switches power from the 7.2V line avilable from the PWM outputs. Google "BJT switching" or something similar for more info.

Re: Power output on I/O port
 

The power pin on the IO ports or PWM port is designed to power a servo, not to drive motors or solenoids. If you load it too much you might damage the robot controller.

I strongly suggest you refer to the robot controller manual, and start with a relay spike to tinker with your solenoid. If you want to make custom circuits to power devices off the IO ports, you really need to understand what you are doing, or you could wipe out the RC.

Re: Power output on I/O port
 

It sounds like the Spike Relay is exactly what I'm looking for...

Where can i pick a few of these modules up?

Re: Power output on I/O port
 

Are you talking about the Spike Relay modules from Innovation First?
Ouch, $30 a pop...

I'm just messing around with the little VEX RC... I'm sure there is a way to pull this off cheaper then buying 4 * $30 modules...

Thats why i was thinking along the lines of a simple tri-state... and other ideas other then the Spike Modules?

Re: Power output on I/O port
 

A relay is the way to go here. There is no control of the power pins on the RC. and the solenoid would draw current that could exceed the capabilities of an RC output. A simple tristate interface would not handle the current from the solenoid but a high current hex driver might. You need the current specs from the solenoid to determine the best course of action. Don't forget that the solenoid will generate a rather large voltage spike when the power is removed and the device driver needs to be protected from that abuse.
Yes the PWM connections on the RC are tied to +7.2 volt backup battery (in 2004 and later) and as such power is not available without a battery connected.

Re: Power output on I/O port
 

There are 2 problems with a solenoid. In rush current and the spike when power is removed from the field collapse. To handle these loads you can use a relay, motor-solenoid driver chip or a logic level FET. You can't use the 5 volt control output of the vex controller to power a relay directly. You need to use a transistor to power it. A PNP transistor like a pn2222a would do. If you go to the parallax basic stamp web site there is a tutorial called stamps in class that goes over choosing and wiring a transistor to power relays. I believe it is the 2nd chapter. There is also this link-
http://kronosrobotics.com/Anotes/Relays.pdf

Re: Power output on I/O port
 

Thank you Gdeaver... thats exactly what I needed to hear!!!

The provided link you sent gave me a great idea, and I think this is what I'm going go with:

I'm going to use the pn2222a transistor and a Solid State Relay (Radio Shack Part# 275-310)