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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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4th Aug 2019, 5:10 pm | #1 |
Octode
Join Date: Feb 2018
Location: Southampton, Hampshire, UK.
Posts: 1,049
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Thermistor in heatshrink on power adaptor ?
Recently I've been dismantling an old 240V to 12V external SMPS power adaptor for parts. There seems to be a thermistor of some sort that's completely covered in heatshrink tubing. What purpose does the thermistor serve and why wrap it in heatshrink ?
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4th Aug 2019, 5:19 pm | #2 |
Dekatron
Join Date: Jun 2015
Location: Biggin Hill, London, UK.
Posts: 5,189
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Re: Thermistor in heatshrink on power adaptor ?
Possibly to limit the mains inrush current. When you connect an SMPSU to the mains you charge a capacitor from the rectified mains which can take a considerable current initially. The thermistor, in series with the mains to the rectifier starts off at high resistance (limiting the current), then the resistance drops as it warms up (due to the current flowing through it) and it has little further effect on the circuit.
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4th Aug 2019, 7:16 pm | #3 |
Dekatron
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,059
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Re: Thermistor in heatshrink on power adaptor ?
Almost certainly inrush-current limiting, as Tony says. It's a cheap and cheerful approach, and works well. It has its limitations, if the unit is switched off and on again a few seconds later, the reservoir capacitor may be discharged but the thermistor still hot and low-resistance, so the switch-on surge will be large.
Last edited by kalee20; 4th Aug 2019 at 7:18 pm. Reason: Typo |
5th Aug 2019, 12:49 am | #4 |
Nonode
Join Date: Oct 2010
Location: Tintinara, South Australia, Australia
Posts: 2,324
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Re: Thermistor in heatshrink on power adaptor ?
And in heatshrink so when it explodes it doesn't spread shrapnel everywhere, supposedly.
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5th Aug 2019, 1:11 am | #5 |
Octode
Join Date: Feb 2018
Location: Southampton, Hampshire, UK.
Posts: 1,049
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Re: Thermistor in heatshrink on power adaptor ?
Thanks for the replies explaining how a thermistor works and it's disadvantages. Terry, thanks for explaining why a heatshrink tube should be used. Here's what I found elsewhere too:
"Take precautions with these inrush current limiters, especially if you're experimenting. They explode suddenly like firecrackers throwing pieces around, and let out a terrible stink and smoke..." Scary stuff ! |
5th Aug 2019, 5:09 am | #6 |
Nonode
Join Date: Oct 2010
Location: Tintinara, South Australia, Australia
Posts: 2,324
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Re: Thermistor in heatshrink on power adaptor ?
Yes, the stink is not nice but not as bad as Selenium rectifiers.
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