simple soft start

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well...

...the absolute simplest is to wire something called an NTC thermistor in series with the primary...

You definitely want one, fuses or breakers popping all the time takes the 'shine' off the new uber amp pretty quick... ;-)

An NTC thermistor has a higher resistance cold (about 10ohms), than hot. Once the thing hits about 100c it has a resistance of less than an ohm and interferes little if at all...

Nelson pass uses them a lot and I think he knows what he's up to. The ones most people seem to use are called CL60's, and are one convenient size from a broad selection...

I don't remember the rating, but there are others in the range that have lower cold resistance and higher max current, a couple of these in series would have an appropriate hot and cold rating for a 1kva transformer.

I'm not sure where you'd get them down under, but here digikey has them for a couple of bucks, and eBay sometimes has them for a tenth of that...

HTH

Stuart
 
hi all ive looked on rs components and they have heaps of thermisters but i really need to know the exact values if possible .i was also thinking about using a variable resister that i could set to say 10 ohms on powerup and after about 30seconds turn it to 0 ohms .do you guys have any thoughts on this idea it would seem to be a very simple way of doing it . cheers
 
The one and only
Joined 2001
Paid Member
Class A and very high bias amplifiers work pretty well with the
big Keystone inrush thermistors found in the Digikey catalog, like the
CL60 and CL40 - just pick them by current rating. If your transformer
has two primaries, you can use one thermistor per primary. They can
also be used on secondary windings.

Doing this also has the advantage of stretching out the charge
pulse duration, lowering the noise a bit and the stress on diodes
and capacitors.

If your amp is 1 KVA per channel and Class AB, this is not quite as
good a solution as the small resistance of the hot thermistor still
contributes some limiting of the AC line under high current draw
conditions. For such cases, bypassing them with a relay is a good
alternative.
 
Thermistors

Here's the page from the digikey catalog. Check out the column labelled "inrush current limiters". I, and many others, have used KC006L-ND, which is the same part as some vendors call a CL60.

http://dkc3.digikey.com/PDF/T082/P1891.pdf

IIRC you guys have 240v mains, so using one cl60 at 5A max. would 'limit' you to only 1200va continuous from each transformer...

There are a couple of cons to go with the obvious pros...

1) they are designed to run hot, really hot. Don't want them near anything else that might 'care'

2) If you are in the habit of turning your amp on, then off, then on again quickly you will get a surge, basically because the cl60 will not have had a chance to cool down...

HTH

Stuart
 
slr 5000 said:
hi all im building a daul mono 1000va per side amp and think i may have to use a soft start .id prefer not to but ive heard it will be stressfull for my amp on powerup if i dont .does anyone here know a simple soft start cirtcuit i can make any help would be much appreciated
The biggest problem is probably the mains fuses which may blow now and then. The stress is little for the amp really as long as you have done the design right.
 
thanks for the answers guys its aussie amplifiers latest nxs400 modules and his ps3 power supplys i know my mains fuse can take it as i turn on a 3000 watt electric heater all the time with no probs on the same circuit so i presume the daul 1000va toroids would be ok as well as long as i dont turn the heater on when im using the amp .they are on a 16 amp circuit . i was just worried it may damage the amp .if it wont damage the amp id prefer to not use a softstart
 

taj

diyAudio Member
Joined 2005
>>i know my mains fuse can take it

I think he was referring to the mains fuse inside the amplifier on the primary of the transformer. It's common to pop that fuse regularly due to the copious current that gets sucked through it at start-up.

From what I've read, it's more of an issue with toroidal transformers than EI types.

..Todd
 
slr 5000 said:
thanks for the answers guys its aussie amplifiers latest nxs400 modules and his ps3 power supplys i know my mains fuse can take it as i turn on a 3000 watt electric heater all the time with no probs on the same circuit so i presume the daul 1000va toroids would be ok as well as long as i dont turn the heater on when im using the amp .they are on a 16 amp circuit . i was just worried it may damage the amp .if it wont damage the amp id prefer to not use a softstart
The electric heater will take 10 A peak (if 230 VAC), not more but a toroid will take up to 100 A! That's quite a difference. The max possible peak current is the peak voltage (=325V) divided with the resistance of the primary winding of the transformer. My 2 x 300 VA PS will take 77A, measured with a current probe.... and this without anything connected to the secondary windings. :bigeyes:

The softstart does this:

1 You can have a more suited fuse for the power if you you don't have to take into account the inrush current.

2 The mains switch will have it a bit easier and maybe last longer

3 The rectifier bridge will not be stressed so much (less important normally)

NTC soft start
+ Small
+ Cheap
+ Uncomplicated
- Slow reset (most cool off)
- Can't handle small loads
- Hot :hot:

Electronic softstart:
+ easy to set time
+ easy to set max inrush current
+ fast reset
+ easy to add extra features
- Not small (but not very big either :) )
- Not cheap
- Not uncomplicated
 
Hi,
there are two problems that can be addressed.
Firstly, start up current for a toroid transformer or very large EI transformer.
Secondly, charging the smoothing capacitance after the transformer has started.

These need different solutions.

The first one, namely start up of a toroid can simply be done with an NTC. I prefer a high power resistor. I also relay bypass the resistor and/or the thermistor to ensure they are kept cold, particularly since I have a mains supply that drops out irregularly.
The bypass relay should trigger in about 200 to 500mS.
I did experiment with a 40r 40W resistor + 15r NTC in series. Gives a nice start up characteristic but, it costs more, so I did not implement it.

The second, namely charging of smoothing caps, is rarely implemented.
NTC in the secondary does a great job, but again should be relay bypassed, probably after about 2S to 4S.
 
They usually have their own small DC supply to handle switching the relay. If you had another source of 12V you could probably use that.

I've seen one circuit in Elektor that foregoes the transformer and uses a capacitive dropper circuit to provide the low voltage, but i wouldn't recommend that unless you're fairly experienced.
 
slr 5000 said:
hi all im just wondering why all the soft start circuits seem to have a rectifier in them

I have a large 1Kva transformer for amp I am building.

I have bread boarded the following and it works.

I do not remember the resistor sizes I used, but the 120 volt relay picks at 88 volts AC so I had relay coil votage at about 95 Volts controlled by Resistor2. In case of power line drop I wanted the NTC to be cool for re-switching back on. The cold NTC and Resistor1 will not pick the relay until NTC resistor drops enough.

I did not have scope to check delay but this method uses no rectification and capacitors, just AC relay.

One could also add another resistor in line to transformer primary(+) and make the relay contact point junction after the relay plus
point.

NOTE:
All the periods are in oder to keep the posted message look like the edited one message as all spaces are thrown away

................................Contact
............................___N/O _____
.........................../.....relay..........\
........................../.........................\
Line1__fuse__NTC______Res1_____o_________o Xfmr prim(+)
........................................................\
.........................................................\
........................................................Res2
........................................................../
......................................................AC relay
........................................................\
.........................................................\
Line2___________________________o_____o Xfmr primary(-)
 
slr 5000 said:
hi all im building a daul mono 1000va per side amp and think i may have to use a soft start .id prefer not to but ive heard it will be stressfull for my amp on powerup if i dont .does anyone here know a simple soft start cirtcuit i can make any help would be much appreciated


You could use a relay that is driven from a C that is charged via a resistor. Once the C charged up the relay would come on.
 
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