UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > General Vintage Technology > Components and Circuits

Notices

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.

Closed Thread
 
Thread Tools
Old 4th Aug 2019, 5:10 pm   #1
Jolly 7
Octode
 
Join Date: Feb 2018
Location: Southampton, Hampshire, UK.
Posts: 1,049
Default 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 ?
Jolly 7 is offline  
Old 4th Aug 2019, 5:19 pm   #2
TonyDuell
Dekatron
 
Join Date: Jun 2015
Location: Biggin Hill, London, UK.
Posts: 5,189
Default 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.
TonyDuell is online now  
Old 4th Aug 2019, 7:16 pm   #3
kalee20
Dekatron
 
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,059
Default 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
kalee20 is online now  
Old 5th Aug 2019, 12:49 am   #4
Terry_VK5TM
Nonode
 
Terry_VK5TM's Avatar
 
Join Date: Oct 2010
Location: Tintinara, South Australia, Australia
Posts: 2,324
Default Re: Thermistor in heatshrink on power adaptor ?

And in heatshrink so when it explodes it doesn't spread shrapnel everywhere, supposedly.
__________________
Terry VK5TM
https://www.vk5tm.com/
Terry_VK5TM is offline  
Old 5th Aug 2019, 1:11 am   #5
Jolly 7
Octode
 
Join Date: Feb 2018
Location: Southampton, Hampshire, UK.
Posts: 1,049
Default 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 !
Jolly 7 is offline  
Old 5th Aug 2019, 5:09 am   #6
Terry_VK5TM
Nonode
 
Terry_VK5TM's Avatar
 
Join Date: Oct 2010
Location: Tintinara, South Australia, Australia
Posts: 2,324
Default Re: Thermistor in heatshrink on power adaptor ?

Yes, the stink is not nice but not as bad as Selenium rectifiers.
__________________
Terry VK5TM
https://www.vk5tm.com/
Terry_VK5TM is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 4:38 pm.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.