Limit voltage

[mmainland]

Hello,
I am new to this forum and need some help. We are measuring resistance with a Dataforth signal conditioner, 0 to 100 ohm input = 0 to 10vdc output. The problem is on the signal conditioner when there is no input resistance on the Dataforth sig cond. the outputs are saturated and actually put out 12.5vdc. The output of the sig cond. is going into a analog input card on a PLC. The card in the PLC is a 0-5vdc input card. We will only be reading about 4.87 ohms on the input of the dataforth card so the output will allways be below 5vdc except when the input to the dataforth card is not hooked up. Is there a way I can limit the voltage that my PLC analog input card see's? would a zener diode work in this application? I just need to make sure my analog input card never see's 5vdc or more. Any help would appreceiated.

Thanks,
Mark

 

[Reloadron]

What is the model of the signal conditioner? You likely could use a zener diode to serve as a clamp at the PLC input.

Ron

 

[mmainland]

Hi Ron,
It's a Dataforth DSCA36-01. When I talked to Dataforth I found out the outputs on the sig cond saturate to 12.5vdc when there is no input. It took out the analog input card on the PLC, which was a 0-5vdc. Could you explain to me how I could use wire in a clampling (zener?) diode to limit the voltage going into the analog input card?

Mark

 

[Reloadron]

OK, typically your device outputs based on a 100 Ohm pot input (or 0 to 100 Ohm resistor input). The device outputs normal 0 to 10 Volts proportional to resistance. Since your measured resistance never would exceed enough for the device to output > 5 volts there was no concern but with open input the device can drive over 12 volts out and cook your input device. The current maximum out in the voltage output mode is limited to 8 mA. You could configure a voltage divider on the output of the Danforth but that would likely screw up your scaling of whatever you are doing and require a code change.

This is from the manual of your device:

Output Range: See Ordering Information
Load Resistance: (IOUT) 600W max
Current Limit 8mA: (VOUT), 30mA(IOUT)
Output Protection: Short to Ground Continuous

Since the device has output protection and output current is limited to 8 mA I would just find a 5 Volt zener and place it across the danforth output. Cathode to + (POS.) out and Anode to - (NEG.) out and let the Zener shunt it. If the output goes above 5 volts the Zener will protect the next device inline. Simple and effective. You can check with danforth but I am sure they will likely agree. Any 5 volt 1/4 watt or greater zener should do fine.

Ron

Ron

 

[mmainland]

Thank you!
Mark