Digital/Analog Conversion

Hello,

I have a project where I am going to create a 2-10V analog signal with a DL05 PLC using 4 relay outputs with resistors. This is going to be used to control an 8 stage step controller that accepts a 2-10V signal. 2-3V commands 1 stage, 3-4V commands 2 stages, etc. I am using bits of a modbus word to control each of the relay outputs, so output 1 only would be a word of 1, output 2 would be a word of 2, output 3 would be a word of 4, and output 4 would be a word of 8. Combinations of the relay outputs are also used. The 0VDC side has a 1000ohm resistor wired in series with the relay output commons. The control signal comes off of both sides of this resistor. The other side of the relay outputs have a 2320ohm, 4220ohm, 6490ohm, and a 8250ohm resistor each wired to a different output with the other side of each resistor tied to +24VDC. This way, with no relay outputs active, the control signal is 0VDC. This seems to work fine for me to activate the stage outputs. My question is, has anyone else ever done anything like this? It seems to be a decent way to get low resolution control but figuring out the proper resistance values is the hard part. Does anyone have any D/A schematics that they have used to do this? The resolution can only get better with more outputs and perfectly chosen resistor values.

Have you looked at using one output with PWM?

kittydog42 said:

My question is, has anyone else ever done anything like this?

Click to expand...

Yep, poor mans D/A converter and you're right, more outputs gives you more resolution.

You'll have to be carefull of the input impedence of the circuit you'll be driving. Too low and it will effect the resistor voltage divider network that you've created and it will change the voltage values.

I guess it depends on your definition of decent. The Automation Direct DLO5 analog card is so dirt cheap that it hardly seems like your method is worth all of that work and time. Sometimes wasting a day on a contraption is like stepping over dollars to pick up a dime. Put the money you would spend on the relays towards an analog card.