Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Parts Where to get, and how to make the best bits. PCB's, caps, transformers, etc.

Thermistor vs Soft Start
Thermistor vs Soft Start
Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Thread Tools Search this Thread
Old 19th September 2005, 03:48 PM   #1
argofanatic is offline argofanatic  Canada
diyAudio Member
Join Date: Jul 2005
Location: Quebec, Canada
Default Thermistor vs Soft Start


I'm building several amps using 500VA Toroids. I'm designing PCB's with soft start circuits and was wondering about thermistors. Can I eliminate the soft start by using a thermistor or should I use a thermistor before the soft start ?

Any enlightenment would be appreciated.
  Reply With Quote
Old 20th September 2005, 04:12 AM   #2
BrianL is offline BrianL  United States
diyAudio Member
Join Date: Mar 2002
Location: USA
Thermistor vs Soft Start
One can use NEGATIVE temperature coefficient (NTC)
thermistors to help "soft start" a power supply. At initial
turn-on their resistance is high and then drops as they
heat up. Thus they limit corrent more at the very start,
and have lower resistance when warmed up.

The trick is picking one with the correct characteristics for your
application, i.e., enough current limit at turn-on, minimal
resistance when warmed up and a reasonable time

But it's the simplest "soft start" you can do in terms of component
  Reply With Quote
Old 20th September 2005, 04:40 AM   #3
Loial is offline Loial  Sweden
diyAudio Member
Join Date: Nov 2004
Location: Gävle
Take a look at http://www.rti-corp.com/Electronics/surggard.htm
They have a small selection guide on that page to.
  Reply With Quote
Old 21st September 2005, 02:45 PM   #4
macboy is offline macboy  Canada
diyAudio Member
Join Date: Oct 2003
Location: Ottawa, Canada
I have built soft-starts for my home theatre receivers using the circuit below. This is a modified version of one of the soft start circuits found on Rod Elliot's ESP pages. I modified it to use the existing 12V standby power supply and logic-level trigger signal in the receiver. I use a single ordinary 25 ohm, 10 W ceramic coffin resistor, which doesn't even get warm in use. I used R1=47K in my circuits, giving approx 0.8 second start time.

This is a more complicated circuit than many others, but it has important features great for toroids... it will dropout very quickly on a power blip, and the soft-start time is just as long following that as any other instance. An NTC thermistor, or the simple soft-starts that use a slowly charging capacitor on the secondary to power a relay, will not have either of those properties. That means that if your power "hiccups", your toroid will thud and draw a surge current just as if the soft-start wasn't there. The active circuit will however soft-start the toroid in this case, just as always.
Attached Images
File Type: gif softstart.gif (10.1 KB, 1239 views)
  Reply With Quote
Old 21st September 2005, 03:13 PM   #5
lgreen is offline lgreen  United States
diyAudio Member
lgreen's Avatar
Join Date: May 2003
Location: San Diego, USA
Like macboy I built a softstart using ESP project 39, figure 2 to power my krell clone that has two large 700 VA xformers. Like you are thinking I used a thermister and a delay board.

People will tell you that you could just put a CL-60 thermister in line with the primary of your transformer and it would work great for years to come. They will say that Pass Labs does it this way. So it will work but note that it will not softstart if the amp has been on for a long time (thermister is hot) and then turned off and on again rapidly. A rare case to be sure but it does happen.

On the other hand a delay circ will always work, switching out either a resistor or thermister after a certain delay will keep that element cool and ready for another softstart.
thread 1
Here are some links to the way I did it-
My soft start circ.
My startup wiring diagram
  Reply With Quote
Old 21st September 2005, 03:47 PM   #6
richie00boy is offline richie00boy  United Kingdom
Did it Himself
diyAudio Member
richie00boy's Avatar
Join Date: Nov 2003
Location: Gloucestershire, England, UK
Thermistor vs Soft Start
Thermistor soft starts are fine for constant loads like class-a amps, but the highly dynamic current draw nature of class-b amps precludes their effectiveness. They will limit the inrush but also spoil your dynamic headroom and burst power availability, and also increase distortion.
www.readresearch.co.uk my website for UK diy audio people - designs, PCBs, modules and more.
  Reply With Quote
Old 21st September 2005, 04:34 PM   #7
argofanatic is offline argofanatic  Canada
diyAudio Member
Join Date: Jul 2005
Location: Quebec, Canada
Thanks for the replies.

I'm building some active speakers with a variation of MaxHawk's soft switch. This will avoid the inrush upon "hiccup" because the amps will remain off. I think I'll take the simple route and put in an NTC thermistor. By the time I get up to turn that puppy back on, the thermistor should have cooled and be ready to work again.

  Reply With Quote
Old 21st September 2005, 06:14 PM   #8
95Honda is offline 95Honda  Germany
diyAudio Member
Join Date: Feb 2004
Location: Germany
The thermister worked great on my Aksa 100s. I had twin 400va toroids and went with a 10 amp, 1.8ohm piece, but I would have to double check to be sure. I bought several different types and figured out wich one took about 1-2 seconds for the voltage rails to come up. It's been over a year and I haven't had any problems. The thermister doesn't get hot either. Negligable voltage drop across it during normal operation also.

  Reply With Quote
Old 22nd September 2005, 12:44 AM   #9
wes-ninja250 is offline wes-ninja250  Canada
diyAudio Member
wes-ninja250's Avatar
Join Date: Feb 2005
Location: Inverary, ON
Hey, this may be an insane idea, but I thought I'd throw it out there anyway.

Comments? The goal is to have "hard wires" once the soft start procedure is complete (enough). And to have a circuit simple enough for me to understand!

Immediate flaw -- there will be a very small period of time (during the switchover) where there is no current supplied to the transformer. Hopefully capacitors down the line would absorb that. Or maybe use a "make before break" relay.

Obviously values for the components are somewhat critical.

BTW, I got the idea from my motorcycle's junction box... the headlight-on-only-after-start circuit.

Attached Images
File Type: gif soft-start.gif (7.5 KB, 616 views)
Do daemons dream of electric sleep()?
  Reply With Quote
Old 22nd September 2005, 02:07 AM   #10
anatech is offline anatech  Canada
diyAudio Moderator
anatech's Avatar
Join Date: Jun 2004
Location: Georgetown, On
Thermistor vs Soft Start
Hi Wes,
Normally in a soft start circuit, the contacts only short out the thermistor or resistor. The AC is applied to the resistive element all the time with the contacts across the resistive element. No interrupton of current then. We would also use a fixed resistor, not a thermistor. That's entirely up to you.

  Reply With Quote


Thermistor vs Soft StartHide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Soft Start Necessary? dtaylo3 Class D 2 26th December 2008 10:05 AM
B+ soft start Gluca Tubes / Valves 2 3rd September 2007 02:04 PM
Need a soft start? maxpou Solid State 19 18th April 2007 10:23 PM
Soft start & Soft Switch circuit: can anybody help? m.parigi Solid State 95 22nd August 2005 04:32 PM

New To Site? Need Help?

All times are GMT. The time now is 02:09 AM.

Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 15.79%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Copyright ©1999-2018 diyAudio