110 Vac Prox

sprek · Jan 8, 2014

I am having a problem with a 110 vac capactitive prox leaking enough voltage to always light an indicator. This started when the original bulb type was switched with an led. The led doesnt use enough load to pull the AC down to near zero. The output of the prox goes to the indicator and an input to a ML1500 PLC.

Does anyone have an idea what I can do to pull the voltage down?

bguinn · Jan 8, 2014

You could use a relay.

bernie_carlton · Jan 8, 2014

Or a resistor in parallel with the LED but be sure to get the wattage rating right. It could get HOT. Maybe the relay is the more practical solution.

sprek · Jan 8, 2014

I tried a relay it activates the relay when its covered but wont switch back stays on all the time.

I measured the voltage on the way its normally hooked up and it is 13 vac with the prox uncovered with a relay inplace it reads 26 vac uncovered

bernie_carlton · Jan 8, 2014

Could you post your wiring diagram with and without the relay?

Nethop · Jan 8, 2014

Resistor Sizing

bernie_carlton said:
Or a resistor in parallel with the LED but be sure to get the wattage rating right. It could get HOT. Maybe the relay is the more practical solution.

A bit of a tangent from the OP but... In a recent temperature sensor installation where I had asked for (and marked up the wiring diagram for the electrician) the 4-20mA signal from the sensor, he wired the discrete instead which was 24VDC. So at the strip where I was to convert the signal with a 1/2 watt 250 ohm resistor from 4-20mA to 1-5V, things got rather hot!

I guess the dissipation on the 500mW resistor was ~2.3W instead of the 100mW as anticipated.

Dan.

sprek · Jan 8, 2014

I was thinking a resister is what I need to do this wasnt sure on the size.

I did try a 2w 400 ohm and when the smoke came out for some reason it didn't work anymore

anyway here is the print

http://www.sprektech.com/pdf/wiring.pdf

kamenges · Jan 9, 2014

That'll happen.

P = E^2/R = 110^2/400 = 30.25 watts. Looks like smoke to me.

The default burden resistor I usually fall back on is a 5K 5W resistor. 4.7K is probably easier to find. At that resistance the power through the resistor is 2.5W so you have alot of power margin. That means the resistor won't get so hot. That resistance will also keep the burden current under 20mA when the sensor is on, which is a pretty reasonable number.

Keith

sprek · Jan 13, 2014

Wow I was making that more complicated that it needed to be. Never thought of letting the ohms law do the work for me.

I was looking at the manual for the prox it says control from 10 to 200 ma so I figured the resister should absorb about 26ma. should do the trick

I had to order the resistor I will post again when it is installed

110 Vac Prox

sprek

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