RS232 --> ??? --> 0-5VDC/0-700mA

[Marcaroni]

Hello,
This is my first post, please go easy on me. I'm also fairly new to electronics so please forgive me if my terminology is weak.
I've started a little project but i think i may have gotten in a little over my head. I purchased an RS232 controlled potentiometer board. I've already written some software to control it...no problems there. The POTs on the board are rated for 0-5VDC at 1 mA. The POTs range from about 320 to 48k ohms in 255 steps. The problem I'm having is that I was planning on using the board to drive an electronic metering valve rated at 0-5VDC/0-700mA. Obviously I overlooked the fact that the valves draw much more current than the POTs are rated for...like I said, I'm still new to this...I'm learning though. My questions:
Is there a simple "off-the-shelf" solution to get my POT board to drive the valves? I wouldn't mind building some kind of simple circuit if need be, but I don't know if I'm ready for that yet, so I'd prefer something off-the-shelf for now.
Have I gone in the wrong direction altogether?
I've tried to search around the ol' internet but that's proven to be futile since I don't know exactly what I should be searching for.
Sorry if I posted in the wrong section.

Thanks,
Marcaroni

[MrAl]

Hi there,


You basically build a amplifier if you want to build it yourself.

Someone else here is also building almost the same thing BTW.
The member 'khaston' is building four of these amplifiers.
Look in this forum "General" for the post "manage load with digital pot"
and see if that is something you may want to build.

[dyarker]

Off-the-shelf for a job this specific might be hard to find. Luckily, this would be a simple project to start on.

You need current gain (at least 1:700) without voltage gain. That is an emitter follower circuit. See:
http://en.wikipedia.org/wiki/Emitter_follower

Your valve would go in place of Re in figure 1. Transistors that can handle 700mA have current gains of less than 100, so use two transistors as:
http://en.wikipedia.org/wiki/Darlington_transistor

An FET can also be used as source follower, see:
http://en.wikipedia.org/wiki/Common_drain

Your valve would go in place of Rs in the drawing.

If this looks simple enough for you to try (I think it is), I can draw up circuit using bipolar transistors; and someone more knowledgeable in MOSFETs will suggest an FET circuit.

Cheers,

[Marcaroni]

Awesome. Thank you MrAl and dyarker.

[Engineer1138]

Where did you find the RS232 pot? It sounds like a simpler version of something I've been planning to build for a long time.

[Marcaroni]

Where did you find the RS232 pot? It sounds like a simpler version of something I've been planning to build for a long time.

http://www.controlanything.com

their toys are tons of fun. their web site is not. it's some where down the left menu under "digital potentiometers" - usb and rs232 varieties. they're kinda bad with providing specs though...I learned that the hard way.