inrush current

Hi Aandy,
which is old hat and out of date?

Doktor,
Relay bypassed power resistors on the mains primary side of the transformer give good soft start characteristics. 300mS delay is appropriate

Relay bypassed Thermistor on the secondary side to slow charge the smoothing caps give the amp/caps an easier time. 2S to 10S delay is appropriate.

Thermistors on their own have too many downsides for my audio type uses.

One of the Thermistor's biggest failings is poor performance with repeated re-starts due to mains supply failure.
 

anatech

Administrator
Paid Member
2004-06-06 8:31 pm
Georgetown, On
Hi Aandy,
old hat and unreliable and out of date
I'm sorry Aandy, but I completely disagree with you on all counts. I am rather stunned by your answer really.

There are more problems with your NTC implementation by far. It remains in series with your AC line and will vary it's resistance depending on current flow. A relay to short that mess out is a good idea, by what is wrong with a properly sized resistor instead? I'm sure that may have been where you experienced your earlier problems.

When done properly, a resistor shorted out as Andrew has described will not create hot parts. There is an added plus in that the relay can be made to drop out under fault conditions, causing the resistor to overheat and go open. Many other things hot and smoking at this point so we might as well limit the damage. Keeps the fuse jockeys from destroying the gear. :devilr: Seen more than my share of those clowns.

-Chris

Edit: The size of the amplifier has no bearing on the method. You could do this on a 2 wpc amplifier (but why would you?).
 
I have often wondered this - what is the downside of using a triac to short the resistor/thermistor out?

I think if anything, the thermistor is a cheaper option than a good size power resistor. I have a 68R aluminium clad one good for 50W that I plan to put to use for starting a 500VA toroid. Having had a quick look at suppliers, a 22R NTC is a lot cheaper and would work just as well.
 
I'm with the others that the relay shorted option is the best. Thermistors are not suited to class-b amps because of the non-linear current load. Also the switch on/off issue.

Inrush current comes mainly from the magnetising current of big toroids. Therefore the only sensible place to put a limiter is in the primary.

Not sure what the issue with triacs is other than the (more complex) control circuitry necessary.
 
Also me I believe that the thermistor it is a good solution, mainly because it does not seize space. What has importance I believe, it is the careful calculation of the activation time of relay that shorts the thermistor. Of course we talk for a thermistor placed on the mains side of xformer. If this time is bigger than thermistor needs to zeroing almost his resistance then the more current will pass through the way that presents the smaller resistance. It should not we forget that also the relay contacts (or contact; in this place for certainty must be used a standard type SPST NO with 30A contact) have certain be it so negligible resistance and I am not sure that thermistor it cannot reach also in so much low resistance. From my experience I have to say the following: if you use a relay with coil for example 12Vdc (of good quality such as NAIS) supply it with 14 to 15 Vdc. If the relay it is of good quality there is no danger to the coil (I have tried this for 2 years in real amplifiers) and simultaneously it pulls with bigger force his contacts minimizing thus furthermore their resistance.
If you are reserved yet for the thermistor use and you don’t trust that I wrote, tell me this and I will post 2 circuits that use exactly Crest Audio and Amcron-Crown.
Fotios
 

anatech

Administrator
Paid Member
2004-06-06 8:31 pm
Georgetown, On
Hi Andy,
how many amplifiers have you built?
Too many to count. Also power supplies and other assorted items. Why?

glad to here some one at last knows where the inrush comes from!
This is stunningly obvious. Why did you even pose the question? I agree completely with richie00boy on this issue and can't see anything else to add to his comments.

For the record, I prefer EI transformers. I just use taller casework.

in addition to the hi inrush 3-500amps on >1kw
The resistor in series exists specifically to control and reduce this inrush current. For the most part, the relay contacts (or triac) only needs to carry the normal operating current. The reason you shorted your triacs out to begin with was your failure to include the anti-surge circuit.

My comment about smaller amplifiers was only to illustrate that an inrush surge occurs with all designs using a transformer rectifier filter type supply. The smaller amplifiers simply have a lower peak and steady state current requirement.

-Chris
 

anatech

Administrator
Paid Member
2004-06-06 8:31 pm
Georgetown, On
Hi Andy,
They don't saturate as easily as toroid types and are generally less expensive than toroids also. A wide band device connected to the mains is also not something I view as a benefit either. It's also easier to remove heat from the core.

If I had to design a narrow amplifier to go into a rack, my choice would then be a toroid transformer. It's hard to get EI cores inexpensively that fit into 1U chassis spaces.

Why do you use toroid types?

-Chris
 
Hi Chris
i have used over the years both types.
there overload behaviour is very different,and for use in hi power amplifiers,saturation is no problem and can have advantages.
you can get many more times the average power out of a toridal as you can an EI,
heat removal from the core (iron losses)this has a lot to do with core material,and i have never had heat problems with them.
if i was to build a 3kw pa amp with a EI tx it would be around 50kgs for the transformer alone.
so to use EI in pro audio has no chance.
for the powers i build amps at EI's cost around 2-3times the price of a toridal.
iron and steel!
also if a toridal is made correctly,it will have much less stray flux.
my exspearience has been that most toridal makers donot know how to build them correctly.

i have tested a lot of trash made toridals.

the main problem with them is the much higher inrush than EI's
andy
 

anatech

Administrator
Paid Member
2004-06-06 8:31 pm
Georgetown, On
Hi Andy,
the main problem with them is the much higher inrush than EI's
True.

Have you tried high frequency switch mode supplies for the larger amplifiers? Similar to some of the Carver ideas. A current type transformer (he calls them Mag coils) is even more efficient than a toroid.

When you reach really high power, the rules change. For home use, keeping line trash out of my supplies is more important, and since I do not make my own transformers I rely on what is available to me. That means an EI normally gets the nod.

-Chris
 
Hi Chris
the carver amp uses a c core tx in near saturation mode,
this is not a high frequency power supply,but uses a triac as a light dimmer!
sorry i have fixed them.
i am working on hipower smps at the moment for my amplifiers.
and have built them for my laser power suppliyes up to 12kw.
however these are water cooled due to there hipower.
and three phase baced on double half bridge.

it is quite likely you have had junk toridals, there are a lot about.
these quality problems resaulted in me putting in my old winding machines.
i have also managed a winding dept for EI txs 18years ago.
so if you came to the UK you could build your own at my place.
problem is copper and iron cost money and the supplyers want to cut corners!
andy