Temperature Sensor Switching Off

Electronics related to CNC

Temperature Sensor Switching Off

Postby beentheunseen » Sat Oct 20, 2012 1:43 pm

Hi All,

I have this really annoying problem with my laser cutter build - I have a cheap pc temperature sensor running off the 5v supply in in my machine, to allow me to monitor the laser tube temperature. Unfortunately it seems to be a bit sensitive to spikes in the 5v supply, as whenever any of the relays or the laser powered up it would bail on me and just switch off. I managed to fix this problem on the relays by adding flyback diodes across their coils. However, it still happens when the laser is firing. I also tried adding a capacitor in parallel with the temperature sensor in case the voltage was briefly dropping...

Anybody else had a problem like this? Any tips for how to clean up the supply?

Thanks!
Last edited by beentheunseen on Sat Oct 20, 2012 5:24 pm, edited 1 time in total.
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Re: Temperature Sensor Switching Off

Postby bdring » Sat Oct 20, 2012 2:12 pm

The diodes on the relays should always be used and a cap can help a little too in some cases.

You are not giving us much to go on like what drives the relays, what type of sensor it is, how the sensor attaches to the rest of the system or how long the cables are.

You want to use good wiring techniques, like twisting pairs of wires that go to relays and sensors or using shielding wires. You want the shortest lengths possible and separate the sensor from other wires as much as possible.

If the sensor is plugged right into some logic is only takes a tiny noise spike to cause a problem. Temperature is a slow changing thing. You could put a pretty slow R/C filter on the sensor. There are lot of RC filter calculators on the web.
Bart
"If you didn't build it, you will never own it."
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Re: Temperature Sensor Switching Off

Postby beentheunseen » Sat Oct 20, 2012 5:23 pm

The relays are elsewhere in the 5v circuit, and unrelated to the temperature sensor. They're not causing a problem anymore now they have flyback diodes.

This is the sensor: http://www.ebay.co.uk/itm/190582664185? ... 1497.l2649

Sorry, I should have mentioned that it's a self contained LCD screen and sensor. It's just a crappy one designed for a pc, with a 5v power feed, then a thermistor on the end of a long wire. It doesn't feed into the system at any point, just gives a visual readout.
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