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argofanatic 19th September 2005 02:48 PM

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.

BrianL 20th September 2005 03:12 AM

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

Loial 20th September 2005 03:40 AM

Take a look at
They have a small selection guide on that page to.

macboy 21st September 2005 01:45 PM

2 Attachment(s)
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.

lgreen 21st September 2005 02:13 PM

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

richie00boy 21st September 2005 02:47 PM

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.

argofanatic 21st September 2005 03:34 PM

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.


95Honda 21st September 2005 05:14 PM

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.


wes-ninja250 21st September 2005 11:44 PM

2 Attachment(s)
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.


anatech 22nd September 2005 01:07 AM

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.


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