anyone know what kind of transformer would be needed to wire a vex microcontroller to a wall outlet?
i’ve read that its been done in other forums, but want to make sure what the stats are so i don’t fry mah brains.
thanks
Basically, anything that is a regulated DC power adapter in the 6V to 9V range will power the microcontroller. The amperage rating of the adapter will limit how many motors/servos can be run simultaneously. Something that provides 100mA will be fine for programming a controller with nothing attached. An adapter than can provide a full amp will work reasonably well for most applications. An adapter than can provide several amps will be able to handle surge loads much more like a battery.
Avoid unregulated adapters, since they will put out a voltage that is too high under light loads, possibly burning something out. When in doubt, use a volt meter to check the output voltage to make sure it is between 6V and 9VDC.
Cheers,
- Dean
I am also looking to build AC power supply for my VEX bot. Running it off of batteries is not feasible for my application.
2 amps should be plenty of power methinks. I might be on to something with this link:
http://www.zen22142.zen.co.uk/Circuits/Power/9v2a.htm
Then maybe a big laptop brick or something to power it.
Did you ever come up with anything? You have any ideas or links to share?
I haven’t picked one up yet, but I’ve had my eye on these power supplies:
[LIST=1]
*]Not sure if this one is regulated or not, but 7.5VDC @ 1.5A for $4.00 is a good deal.
*]This one is regulated, and about right for a charging station: 8.4VDC @ 0.4A , also $4.00.
*]This is a bare power supply, so you’d need to house it in something. It is a brute, providing 7.5V @ 20A, but could easily handle any surge load that a Vex (or two) could dish out. At $35, it is a great price if you need this much juice. NOTE: This requires 120VAC wiring, so don’t get this unless you understand the necessary safety precautions and are comfortable doing this type of project.
[/LIST]
Cheers,
- Dean
That would work, but you will need to use a nice sized heat sink on the 78S09 regulator. It is going to generate a lot of heat, depending on the current draw and the input voltage.
Lets say you feed it 12V - the regulator has to drop 3V, which it converts into heat. If the robot is drawing 2A, then that is 6W of heat (3V x 2A). That is enough for the tab to get hot enough to burn your finger. If you feed it more than 12V, the heat will obviously be more.
Lots of DC supplies are already regulated - notably laptop bricks tend to be. If you look for the words “regulated” or “switching power supply” in the item descriptions, then you are looking at a power supply that has regulation built-in.
Cheers,
- Dean
That barebones supply looks pretty nice… I do like the idea of a $4 wallwart getting the job done though 
Working with high voltages is not an issue. I worked for a few years on the power grids for AT&T usually dealing with 3-Phase 480v, though I have worked with voltages over 30,000 ( things get really interesting w/ high voltages, even with all the protective gear it’s pretty scary ).
Anyway, thanks for the links, I was having a terrible time finding anything useful ( I think I was searching 7.2V ) I should have thought to check All Electronics 
( I love that place )
Oops - meant to say “about right for a programming station”.
- Dean
I have seen quite a few inexpensive Wall-Warts that are quite un-regulated, so Test, Test, Test, before you install…
I though about developing a Physical Layer Network topology that ran on 3-Phase 480 Volts AC, I wanted to call it Lethal Net. I just couldn’t interest any investors…
Quazar’s just dialed-in…
you mean that’s not how Power Over Ethernet works?
perhaps you have been working with these guys
Well, sort of…
Yikes!!! Too Much Free Time!!!
Do you not have to worry about the amps. Can’t the micro controller only handle so many before is heats up to much, cause it does not have a cooling system. The more amps you pull the hotter the transistors are going to get in the speed controllers.
You won’t have to worry about the amps, because the VEX MicroController has built-in current limiting. Consider that the 7.2v NiCd battery packs can (briefly) put out in excess of 20A if shorted, so the controller can clearly handle high-current power supplies.
You just need to make sure you stay within the 6V-9V nominal supply range.
Cheers,
- Dean
Yea that makes sense.
Also, I probably don’t need too much power anyway, being as I am only powering sensors and servos ( the bot is stationary ).
I would figure you would probably need the higher amperage for driving motors for movement and other heavy lifting applications. I was going with 2A as that was the rating on the RC battery pack I am using.
I would guess if you were needing to really source a lot of current for motors one would be better suited using an external driver / power source for the motors. Eg an ESC for an RC car or similar, or something like the transistor setup I believe you put up plans for.
How few miliamps do you think I could get away with, having the following hardware:
2 servos ( one a VEX the other is a futaba servo I mounted in a VEX motor module housing )
Sharp analog distance sensor
Proxdot digital proximity sensor
The only thing the servos do is articulate the ‘head’. Hardware setup is ultra simple → I am focusing my efforts on using a computer as the bots brain and the VEX controller as simply a low level hardware interface.
What do you think? Will this 500mah regulated wallwart I have here do the trick?
Servos can actually draw quite a bit of power. One thing they do is jump into action as soon as you start up the controller. I’ve seen cases where the startup current of a few servos is enough to brownout the power supply, causing the microcontroller to reset … which start the process all over again.
Have a look at this thread which covers a similar situation to yours.
I’m not sure what rating you are looking at, but keep in mind that 2 Amp-hours ≠ 2 Amps. 2Ah means that the battery can provide 2A for 1h (or 1A for 2h, etc). The discharge rate for NiCd batteries is generally much greater than their Ah rating. That 2Ah battery might be good for 20A in short bursts.
The microcontroller and sensors will draw negligible power, and they will be fairly steady (which power supplies like). The servos on the other hand will be bursty and quite a significant power draw.
It is really going to come down to how good the power supply is at recovering from changing loads. 2A would be quite safe for your application I think. 500ma might have a problem with the two servos attached. This is probably going to take some experimentation, so you will be better off getting a large-ish supply.
If you are trying to build up an electronics bench, then you might consider investing in a bench supply instead of a wall-wart. I picked up a couple of these on sale once; they are pretty cheaply designed and built, but they do get the job done (they even have a 7.5V setting).
Cheers,
- Dean
That looks like a great power supply. The one I have is 13.8V at 5 amps… I bought it to power my GMRS base station and as a general power supply. An adjustable supply would be nice, but I am trying not to spend too much money on this project so I think I will just be experimenting with some of the stuff I have laying around.
At least I have 2 battery packs so I can swap them out until I find something that works.
Thanks for the tips! I will let you know what I end up using 
That supply would work fine if you use it to feed the 78S09 regulator circuit you posted above. That would be your cheapest option and it should give you plenty of power. Just bolt the regulator to a chunk of metal (even Vex metal will work) to help it radiate heat.
Cheers,
- Dean
So this means I could hook up as many Y cables and motors as I wanted without frying anything?
Also, what about digital outputs? Can the +5v (of the +5, sig., gnd) supply current like the motor/servo ports?
There is no danger of frying anything because the Vex microcontroller is well designed with current limiting devices. That doesn’t mean every possible configuration will work, though - if you draw too much power for too long, the voltage will dip and the system will reset itself.
This probably needs a diagram to properly explain, but here goes:
The “raw” battery power comes into the microcontroller and is immediately run through the power switch and a 4A current limiter. This means that the total current draw of the microcontroller and all attached devices can’t exceed 4A for an extended period. The current limiter is a PVC-type device, which means it will pass more than 4A for a while before it shuts down the flow, so you can draw more than 4A but not continuously.
This current-limited power is run directly to all the motor ports, so the voltage at the motor ports is slightly less than the raw battery voltage, and can draw the full 4A (minus the power needed for the bits described below).
The current-limited power is also run to a 5V voltage regulator that can provide up to 1A. This 5V supply is used to power all the electronics in the microcontroller, as well as provide power to all the other ports (Digital/Analog, Interrupt, Rx1, Rx2).
So, the total power budget for the system is 4A, up to 1A of which can be consumed by 5V devices.
If you exceed the 4A (or 1A) limits, then the current limiter (or 5V regulator) will temporarily shut down. This will cause the CPU to enter reset which will effectively turn off any attached motors. That will cause the current flow to drop to near zero, which will cause the limiter (or regulator) to allow current flow again. The program will restart, and if it again causes too much current draw, the same thing will happen.
Hope this helps,
- Dean
Wow- thanks! Yeah, that cleared it up. I’m also wondering if I could use your circuit for more power. From the diagram, it looks like it completely bypasses the microcontroller (allong with the limiting devices).
Yes, that is exactly right. That circuit provides no current limiting.
You can omit the rectifier from that circuit if you don’t care about being able to operate from a single battery. In other words: with the rectifier present, the 2nd battery is optional - without the rectifier, the 2nd battery is required.
Also note that motors attached via this circuit do not get turned off when the VEX Microcontroller is switched off - you have to actually unplug the 2nd battery when you turn off the microcontroller.
Cheers,
- Dean