From: Ron 
Subject: Re: [drakelist] AC-4 Cap values


Ron  made an utterence to the drakelist gang
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These NTC Thermistors are often sold as "Inrush Current Limiters"
 They are 
made by Keystone Electronics, come in a variety of values, and (I think) 
can still be bought from Digikey.  I bought a selection of values from them 
years ago when I built a 500W 6550 based audio amp.  It had 8000 MF of 
capacitance in the PS, and would blow a 25 amp fuse every other time you 
turned it on.  The inrush limiter solved that problem perfectly: 12 years 
later I still haven't popped a fuse.



At 06:49 PM 3/3/2005, Carl Strode wrote:

>Carl Strode  made an utterence to the drakelist gang
>----------------------------------------------------------------------
>What I may do is to install a NTC (Negative Temperature Coefficient) high 
>current thermistor in series with the incoming AC line. These are really 
>cool devices. At room temperature (25C/ 77F) they exhibit a high 
>resistance, as they self heat, the resistance drops. The one I will try 
>starts out at 47 ohms, then slowly drops to less than one ohm in less than 
>a second. Steady state current rating is 4 amps.
>
>Perfect for a surge limiter.
>
>Gerry wrote:
>
>>Unfortunately, inserting series resistors in the B+ leads will, depending 
>>on the value, adversely affect regulation. It may work fine for static 
>>loads such as the +250 supply but is not advisable for HV supplies with 
>>dynamic loads. Years ago I tried a home brew power supply constructed 
>>with mil surplus components. What I found is that choke input supplies 
>>need a critical value resistive load to maintain the voltage below 
>>allowable limits. With no load on the +650 volts, it went almost to the 
>>rating of the filter caps which I believe was 800 volts. Trying another 
>>approach with a capacitive input filter and a choke meant I had to find 
>>another transformer. When I did, the 50 Ohms resistance of the choke 
>>became somewhat of a problem which all made sense later. At 450 mils, 
>>that would be a 22.5 volt drop on top of whatever the drop was in the 
>>first place. The thump comes from the transformer and is loudest when 
>>power is switched on at the peak of the primary AC cycle. Sometimes you 
>>can hit it just right. When I built my Heathkit SB-1000 linear, there was 
>>an alignment procedure which called for removing the screws and sliding 
>>the case back about an inch or so. This was done per the assembly 
>>instructions to align the input coils. When I turned on the amp for the 
>>first time, there was a loud KABAM! After changing my underwear, it 
>>became obvious what happened: there was a surge at turn on and the steel 
>>cover acted like a sounding board. Nothing bad happened to the amp, it 
>>was just one of those things that makes you tremble whenever you reach 
>>for the ON switch. This brings up another point about the need for 
>>slow-start on such devices. I don't use the linear often but it is on my 
>>to-do list.
>>
>>-----Original Message-----
>>*From:* owner-drakelist{at}www.zerobeat.net 
>>[mailto:owner-drakelist{at}www.zerobeat.net] *On Behalf Of *k5vkl{at}juno.com
>>*Sent:* Thursday, March 03, 2005 2:54 AM
>>*To:* drakelist{at}www.zerobeat.net
>>*Subject:* Re: [drakelist] AC-4 Cap values
>>
>>Carl,
>>
>>Now might be a good time to review some slow-start feature design reviews,
>>
>>looking forward to incorporating one into the turn-on function of the power
>>
>>supply. When you have a "thump" from turning on a power supply, it
>>
>>generally is indicative of too great of a surge current going into the filter
>>
>>capacitors and stressing them unnecessarily. It doesn't help the rectifiers,
>>
>>either. Also, I'm not sure that it is a "leaky" capacitor
that causes the
>>
>>"thump" you heard at power supply turn-on....I've always
been led to
>>
>>believe that the "thump" was from the large flow of current into an

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