How To energise my 24V Relay sensor's with sbRIO 9642 3.3V DIO

[sathya20]

Hi every one. This is first time i've started to work on sbRIO 9642 cards which has nearly 110 3.3V DIO and 32 AI & 4 AO.I do request some one to suggest me that what kind relay card that will be appropriate for me to use the DIO to configure to my relay sensor that used to energies only on 24V. The Industrial standard sensors are about to work only on 24VDC but how to make a control over my digital input with 3.3V.

[Buechsenschuetz]

It is not quite clear to me what you want to do. Do you

- want to drive a 24V relay with one of your 3.3V DIO lines or

- want to interface one of your 3.3V DIO input lines to a system working with 24V signal level?

[sathya20]

Hi,

Thanks for your reply, Exactly the second point of yours. Want to interface with one of 3.3V DIO lines to a working with 24V signal level Sensor?

How it's possible to do, what type of hardware is required to compete between the sensor and sbRIO card.

[Buechsenschuetz]

It seems as if the 3.3V logic has not reached the control systems manufacturers yet ... all I could find is a four-channel level converter converting 24V level signals to TTL (5V) levels. If your board will continuously withstand a 5V overdrive of the 3.3V inputs you could use such a device. I am located in Germany and just have a german source:

http://www.leg-gmbh.de/en/SU2-en.pdf

I also found a manufacturer for a 16channel converter board (again, to 5V TTL level only):

http://shop.trenz-electronic.de/catalog/product_info.php?cPath=25&products_id=39&language=en

[crossrulz]

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@Buechsenschuetz wrote:

If your board will continuously withstand a 5V overdrive of the 3.3V inputs you could use such a device.

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The spec for the sbRIO-9642 says it will accept 5V.  I would add a series resistor to lower the voltage just a little bit to make sure.  But the 24V to 5V device sounds like a good solution for you.

[Buechsenschuetz]

Adding a series resistor in a TTL signal line usually is not a very good idea. The TTL specs say that in low or "0" state an input line has to sink a certain amount of current. If the resistance of the input line is too high, this current flowing out of the input in reverse direction may cause a voltage drop and thus increase the low level of the signal higher than expected by the input (usually everything below 0.8V is detected as a low or "0" state, but you never can be sure what happens if the level of the signal line is between this level and the level for the high state).