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ricii

Minimum PLC input voltage!

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Hi, I have a question for you!! I have some scales that have a weight acceptable LED on them (when the weight on the scale is within range, the LED lights up). I have measured the voltage of the LED and it is 2VDC. I would like to use these LED's to give an input to a PLC for further processing. I can't find a PLC that can accept such a low input voltage, and so I have an idea that I could remove the LED and use the 2VDC signal to become an analogue input voltage on a analogue input card on a PLC. Has anyone got any more ideas or options that I could consider? Thanks!! RICII

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It's been a few years since I worked instrumentation, but 2VDC would be enough to forward bias the BE junction of a standard gen purpose transistor. You could then bring 24vdc from your plc to the collector pull up resistor and your plc input to the collector and have the scale drive your plc input. Ant decent circuit board manufacturing outfit could make a board with 4, 8 or 16 of these 'booster transistors" on a board and terminals at the edge to wire to. This convertor card would then mount in your plc cabinet and wire both to the scale and plc. Expect Engineering services on the board to run $1 - $2K and the board to cost $500 - $1K depending on who you contract with.

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depending on which PLC platform you're using (you didn't say) you MIGHT be able to use a TTL (Transistor/Transistor Logic) type input module ... note that there are different types of these (similar to the "sinking" and "sourcing" issues that more people are familiar with) ... do your research before you place your order ... (one size does NOT fit all) ... Edited by Ron Beaufort

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I am open to suggestion on which platform to use, I have considered using one of these, http://uk.rs-online.com/web/search/searchB...p;x=15&y=11 I usually use siemens equipment. Thanks for your really quick replies, Kind regards, RICII

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You could go for a low voltage relay and then make the PLC input whatever you need. Check these at Newark Electronics TYCO ELECTRONICS / POTTER & BRUMFIELD - R10-E1Y1-SS220 - Power Relay PANASONIC EW - TX2SA-LT-1.5V-TH - Signal Relay HTH! Edited by ssommers

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What voltage is present when the LED is removed, as it probably has a series resistor, fed from a higher voltage. If you dont mind digging around you may be able to find this resistor and pick up a voltage from the other end of it.

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Hi Mike, Thanks for your reply, good idea, it might be that I can do this, I'll have a "dig" and see what I can unearth and I'll come back to you Regards, Rich

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