Signal conversion

I have, coincidentally, two issues dealing with signal conversions (sorta). I have a customer who bought some equipment without consulting anyone so here I am (again) dealing with a 'make it work' type scenario.

1) I know to convert a 4-20mA signal to 1-5Vdc, I use a 250 ohm resistor. How do I do the do the opposite (1-5Vdc to 4-20mA)? Or can I? This is a Mitsubishi FX1N with an FX1N-2AD-BD (Mitsu guys know what this is ...)a $40-$60 add-on analog board that can be used for 12-bit analog signals, either 4-20mA or 0-10Vdc) board. Really neat and easy to use if the customer talks to you about the interface device first. I could factor down the 0-10Vdc, I guess, but I would like to use the full 12-bit resolution of the board.

2) I have another PLC with a 0-10Vdc input for an analog signal. I'd like to connect a potentiometer to it but only have 24Vdc in the system. Ideas? Here I'm using an Idec MicroSmart slim line unit (if anyone's familiar with it) that has a 0-10Vdc, 8-bit analog input built in (fixed as a voltage input).

Thanks for whatever feedback I can get.

... rab

Try this link for some off-the-shelf signal conditioners. http://www.api-usa.com/
Answer to question 2.
To get 0-10V with a pot connected to 24V. Connect low end of pot to 0V, connect high end of pot in series with a fixed value resistor, connect other end of fixed resistor to 24V. 0-10V derived between 0V and wiper arm of pot. The values are, for example, 1000 ohms for the pot and 1400 ohms for the fixed value resistor to equal a total of 2400 ohms. CAUTION... don't exceed wattage ratings for the devices. Double or triple ohm values for pot and fixed resistors to keep wattage down. Maintain the ratio of pot ohms to fixed resistor ohms for a 0-10V adjustment range.

There are lots of sources for 1-5 VDC to 4-20 mA signal conditioners, and that is what you will probably need. Most vlotage sources have fairly low current capabilities, and getting 4-20 mA directly using resistors etc. isn't usually going to work. In addition to API try M-Systems and Action Instruments.

For the pot look at this link for some infor:

http://www.plctalk.net/qanda/showthread.php?s=&threadid=2361&highlight=potentiometer

This one and it's links can also help.

http://www.plctalk.net/qanda/showthread.php?s=&threadid=1152&highlight=potentiometer

Thanks ...

I expected that the 1-5V to 4-20mA wasn't an easy fix like the reverse. I think that I understand that the solution to the potentiometer is a voltage divider circuit of sorts.

My background is as a machine designer who fell into electrical (industrial) controls (relay logic then PLCs) about 15 years ago. When it comes to electronics (resistors, capacitors and the like) I have only enough background to have a rough understanding but not to create more than the most simple circuit. I still struggle with the pull-up, pull-down concept when faced with a NPN-PNP concept (return here to review the various discussions on this when I have to implement). It took me a year and a half to understand how a transistor could be used as a switch and how to size the base resistor.