AC Adapter

I was wondering if any one has ever tried to hook an AC adapter up to the Vex microcontroller. I want to make a robot that runs a long time and this would probably drain the battery. Also, when programming I am constantly leaving the battery on and this wastes power. I am going to try to hook an ac adapter up to a tamiya connector. Has any one tried this before, or have any tips?!**+****=**



I use a bench power supply for this, which is essentially the same thing. I just set it to 7.2V, and it has a digital readout for current, so I can see how close I am to the 4A limit.

There is no reason you can’t use a wall-wart the same way. Just make sure to use one with built-in regulation, and that the voltage is in the allowed range (between 6V and 9V). You’ll also want to make sure it can supply enough current for what you need.

If you are just using it to power the controller during programming, 500mA should be enough (something like this). If you actually want to be able to power motors and such, then you should try to find one that can supply the full 4A (something like this)

The current demands of a robot are highly variable. When at rest, it may be only a fraction of an Amp, but can spike to well over 4A when all the motors are going. The 4A limiter in the controller isn’t instant - I’ve pulled over 10A through it for a few seconds. If your supply can’t provide enough current during these spikes, then the voltage will “sag” and the controller will reset. That is discussed over on this thread. The workaround is to wire a large-ish capacitor in parallel to the power wires near the VEX controller. This will serve kind of like a battery, charging while the current demands are low, and helping out when they are high.

Actually, a plug-in power supply would be an outstanding product for IFI to add the the VEX lineup.


  • Dean

see my thread

My bad, I thought I searched before starting a new thread. And Quazar tose 2 look good but I am sick up ordering stuff offline. Do you know any ways to make an ac adapter cheaply, and with parts from Radio Shack? Maybe one of those cool pictures!! But if you don’t want to, I understand.:slight_smile:

Making a regulated linear supply is pretty easy, but Radio Shack doesn’t have quite the right parts to make one that can go up to 4A. I could put a diagram together easily enough, but you’d still have to order some of the parts on-line. Truth is, you can usually get some good deals on surplus transformers and other parts that make a nice brute of a power supply for cheap. But it is hard to put a project page together for surplus parts, since everybody will wind up using different parts.

Actually, the power supply for the VEX Battery Charger would do OK if the voltage were regulated down a bit. Perhaps I’ll put a project page together showing how to do that…

In the mean time: If you just want to program the VEX, and aren’t going to be pulling a lot of power, then something like this would do (set it to 7.5V). Rather than cut off the end to wire up the battery connector, you could get this and wire up the battery connector as another optional tip.


  • Dean

I’m just gonna throw this out there because I have one lying around, but I bet you could pull the power supply out of an old computer (I found one that had a built in switch) and modify it to power a vex bot. They output 5 and 10 volts, so it would be reasonable to hook up to the 10 and reduce it to between 5 and 9 volts. you could even use a potentiometer and be able to vary your output. I get paid today, so I’ll get some stuff do try this.

One problem you are likely to run into with this approach is that many computer supplies have a required minimum power draw - they won’t start up if there isn’t at least a minimum load on some of the outputs. The details vary with different supplies, of course, so YMMV.

As for using a potentiometer to reduce the voltage, you will quickly fry your pot. Lets say you want to drop the 12V (they are typically 12V, not 10V) to 9V. That is a voltage drop of 3V. If the VEX draws 4A, that is 12 Watts of power being dissipated by the pot (4A x 3V = 12W). Most pots are rated an an eighth or a quarter of a watt.

To make that work, you’ll want to use a voltage regulator like the 7808 to drop the voltage down to 8V, but that is limited to about an amp. If you want to drive the full 4 amps, then you should look at the LM338 - it’ll drive 5 amps and only requires two external resistors to get it going (one of which can be a pot to make the output voltage adjustable).


  • Dean

Thanks for pointing this out before I toasted something. I’ll take a look at those two.

I was just looking around on the internet and I saw this:

I think this could work as an ac adpater! On the website it says it is ment to charge a 7.2v battery, just like VEX. Can some one back me up, or does anyone see any problems with this product?



You would need to change the connector type on the end of the cord to match it with the microcontroller port, but I bet it would work They don’t give the Max current output for it on the website, but it’s probably enough to drive Vex motors. I didn’t realize that the iRobot Looj used a standardized battery though, that’s interesting.

I built an 18A 8.5V supply. Runs 2 fully loaded bots and 4 remote controls nicely so its good for training and practice. :slight_smile:

I had a similar issue with a wall mounted display in my classroom that used a sonar, buttons, speaker, and a 269 motor. I tried hooking it to a 9 V wall wart (actually putting out 9.3 V with no current). Upon turning on, the motor would instantly start at full power, and the cortex would then reset itself. I changed the code to delay the motor working for a few seconds after startup, and only allowed the motor to run at a max of 60 instead of 127. Now it seems to consistently running off the 9-ish V wall wart, and i’m guessing any voltage sags on motor startup are not enough to reset the cortex.

However, if I was using a robot with multiple motors that needed to run at high power off a wall wart, I bet i would be facing the same issue with cortex reset. I agree that a DC power supply that plugs into the wall for a VEX cortex would be a nice product to have available for us.