Host Engineering Forum

General Category => Do-more CPUs and Do-more Designer Software => Topic started by: PLCwannabe on January 29, 2022, 10:20:43 AM

Title: Analog input max voltage
Post by: PLCwannabe on January 29, 2022, 10:20:43 AM: Can 24vdc be applied directly to analog 0-10vdc channel to be used as digital input?
Title: Re: Analog input max voltage
Post by: RBPLC on January 29, 2022, 10:50:39 AM: No. You'll most likely damage the module.
Title: Re: Analog input max voltage
Post by: Controls Guy on January 30, 2022, 02:01:42 PM: Time for a redneck voltage regulator. Either put a resistor about double the input resistance in series, which will give an 8V on-level, or (better regulation), a slightly smaller series resistor and a zener diode in parallel with the input. Set your threshold at 6V maybe.
Title: Re: Analog input max voltage
Post by: BobO on January 30, 2022, 02:58:12 PM: Quote from: Controls Guy on January 30, 2022, 02:01:42 PM
Time for a redneck voltage regulator. Either put a resistor about double the input resistance in series, which will give an 8V on-level, or (better regulation), a slightly smaller series resistor and a zener diode in parallel with the input. Set your threshold at 6V maybe.

In voltage mode the input impedance is very high. I'd personally opt for the zener/resistor combo. I'm from the hills of East Tennessee, but even we have minimum (redneck) design standards. ;)
Title: Re: Analog input max voltage
Post by: Controls Guy on January 30, 2022, 03:18:45 PM: I'm a Lone Star redneck. Howdy!
Title: Re: Analog input max voltage
Post by: PLCwannabe on January 30, 2022, 05:28:23 PM: I did install a 12v Zener in series with the input, but that didn't work, I realize now I'll have to parallel it instead.

Just to give a bit more background on this project, the input is being triggered by a 24vdc PNP proximity switch (that is very hard to get to). When I connect this proximity switch output to a regular 24vdc input, the prox output voltage gets pulled down to about 3 vdc, which isn't enough to trigger the dc input. It's like the proximity switch can't supply enough current to trigger the input, which is why I'm trying to use the higher impedance analog input. I think I've got a faulty prox switch or dc input. I'll get a measurement on the dc input current tommorrow to confirm which part is faulty.
Title: Re: Analog input max voltage
Post by: PLCwannabe on January 30, 2022, 05:34:26 PM: Quote from: BobO on January 30, 2022, 02:58:12 PM
In voltage mode the input impedance is very high. I'd personally opt for the zener/resistor combo. I'm from the hills of East Tennessee, but even we have minimum (redneck) design standards. ;)

I love to use up leftover parts on new projects, so some of my stuff definitely falls into the redneck category.
Title: Re: Analog input max voltage
Post by: Controls Guy on February 01, 2022, 11:03:34 AM: Did you figure out which component wasn't working?

My previous post was based on the assumption you were out of DI's rather than your actual case. You'd probably need to size the resistor & zener differently and with more care for your scenario.
Title: Re: Analog input max voltage
Post by: PLCwannabe on February 01, 2022, 02:03:19 PM: The dc input module uses about 7mA/channel,which sounds about right, so I'm assuming the prox switch just can't supply enough current.
My initial attempt with the 12.5v Zener in series is actually working great. The Zener is dropping 12.5vdc, and the prox switch is also dropping about 3vdc, which leaves about 9vdc at the input. No channel errors. I know this value will fluctuate somewhat with DC power supply voltage variations, but it's close enough.
Title: Re: Analog input max voltage
Post by: Controls Guy on February 02, 2022, 01:34:12 AM: Yeah I was going to say, a series zener should be workable. Not regulating per se, but still workable.