# What's the best way to kick 12 > 24vdc for PLC input?

S

#### Scott J

Hello,
Issue: 24vdc input PLC card trying to monitor a 12vdc signal. OK, I know there are a number of ways to change this voltage, and I have tried a couple, some not so reliable. I hope to learn from all of you, "what is the best way? What methods do you guys use? What is the most reliable? Do you use ready made converters?
I thank you ahead of time for any response.
Good Day,
Scott J

J

#### Jake Thompson

Hello Scott,
If this is an on/off signal like from a push button or limit switch, the easiest way to go from 12vdc to 24vdc is to use a simple SPDT relay with a 12vdc coil. Put the 12vdc signal to the coil then add 24vdc into the common of the relay and your input to the normally open side of the relay. Hope this helps!

T

#### Todd

If this is for a digital input, check out the relays' requirements for turn-on. Most solid-state and electro-mechanical relays work at 12V. If it doesn't turn-on at 12V, then I would just tie an extra 12V to that line from a power supply, or some other source. Just make sure that the two wires share the same common.

If this is for an analog input, our software allows us to calibrate the input signal. So 0-12V actually 'looks' like 0-24V. The only problem is that you lose resolution this way.

Todd

M

#### Mike Trombley

Scott:
Here is something to think about. Most input cards are optically isolated inputs, a series circuit of a light emitting diode and a resistor, the diode only being 5vdc. Perhaps changing your voltage is not the answer but reducing the need for the 24vdc is. If you series two resistors they add together. If you parallel them they reduce. Here is the formula:
R1xR2/R1+R2
Measure your resistance and parallel a resistor across the input and common. Its worth a try.

M

#### Mike Trombley

Scott:
Here is something to think about. Most input cards are optically isolated
inputs, a series circuit of a light emitting diode and a resistor, the
diode only being 5vdc. Perhaps changing your voltage is not the answer
but reducing the need for the 24vdc is. If you series two resistors they
add together. If you parallel them they reduce. Here is the formula:
R1xR2/R1+R2 Measure your resistance and parallel a resistor across the
input and common. Its worth a try.

L

#### List Manager

I would hav e put a relay with a 12Vdc coil and energize that
from the signal I want to monitor and close the 24Vdc signal
most certanly availiable to the input card.

/Johan Bengtsson

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J

#### Johan Bengtsson

That won't help, since the resistor you are thinking about is
located inside the card. If the input is happy with 12Vdc it
works just fine without the extra resitor, if it is not then
the extra resisitor will not increase the current thru the LED
anyway, just the current drawn from the signal.

By the way, a LED does typically need a voltage somewhere in
the ballpark of 1.5-2.5V depending on wavelenght, some blue
and green/blue ones have higher voltage but I doubt they are
located at that place. Measuring resistance will not give you
the resistance of the resistor connected in series with the
LED, sorry.

If this is going to work the extra ressitor have to be parallel
to the series resistor and NOT the LED, and that means opening
the case (and reading the resistor value is probably easier
than measuring)

/Johan Bengtsson

----------------------------------------
P&L, Innovation in training
Box 252, S-281 23 H{ssleholm SWEDEN
Tel: +46 451 49 460, Fax: +46 451 89 833
E-mail: [email protected]
Internet: http://www.pol.se/
----------------------------------------

J

#### Jeffrey Eggenberger

Use a relay that accepts 12V, and use contacts from that to control the 24
volts to the next input.

There is no such thing as a natural-born pilot. Chuck Yeager

Jeff Eggenberger

C

#### Curt Wuollet

For analog:
I would set up an opamp with a gain of 2. Less than a dollars worth of components and it's a precision solution. You would be best served
by a single supply opamp with "rail to rail" inputs and outputs.

For digital:
A pair of FET switches and a few resistors do well.

Regards

cww

H

#### Hakan Ozevin

The most reliable method is to use 12 V DC input modules of the PLC of course. But, if you already have the PLC with 24 V DC inputs, you can make a very simple and very reliable transistor circuit. You can use a transistor as a switch in cutoff and saturation regions as you know. You only need the simplest transistor and two resistors.

Hakan Ozevin

H

#### Hakan Ozevin

The most reliable method is to use 12 V DC input modules of the PLC of course. But, if you already have the PLC with 24 V DC inputs, you can make a very simple and very reliable transistor circuit. You can use a transistor as a switch in cutoff and saturation regions as you know. You only need the simplest transistor and two resistors.

Hakan Ozevin