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rparigoris
Joined: 24 Nov 2009
Posts: 791
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 Posted: Tue Mar 30, 2021 7:06 pm Post subject: Common ground on LM335 doesn't work but common power does? |
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Hi Group Have some Gremlins visiting at hangar and wondering why they are now playing nice. We have a Ilec SC7 variometer. It has a momentary switch that will measure temperature. The temperature sender is a ST LM335 temperature sender. It I think uses Zener technology. The Ilec provides 3 volts to the sender. The LM335 has 3 leads, power, ground and you can add a trimmer pot. We are just using 2 legs like the factory sender is using. Now for our problem, the factory sender wired direct works perfect. Perhaps 1 or 2 degree changes. We decided we want 6 channels. Purchased a nice rotary switch from McMaster good for 2 amps and 30 volts DC. We bread boarded 6 senders with a common ground and switched power. Channels would go bonkers from time to time. It would act like the sender was shorted which would have temp fall way down and it would climb back up, then do it again. We were first using a very precise power supply that we can vary voltage. We went from 12 to 14.5 volts, same problem. We had grounds at power supply. Then we switched grounds to the ground at Ilec. No joy. We tried a battery, no joy. We bypassed the switch completely, no joy, it's not the switch. We had things on a bread board, made soldered connections, no joy? Then we decided to share power and switch grounds. All is well? Anyone have any ideas why switching power to senders and using common ground would give problems but sharing power and switching grounds make things OK? BTW we have a handful of senders, we tried many, when grounds were shared random flakiness. Thx. Ron P. P=Perplexed!
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user9253
Joined: 28 Mar 2008
Posts: 1920
Location: Riley TWP Michigan
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| Posted: Wed Mar 31, 2021 6:11 am Post subject: Re: Common ground on LM335 doesn't work but common power doe |
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You wrote, "fall way down and it would climb back up, then do it again".
That makes me think of a capacitor charging and discharging.
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