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


Carl Strode  made an utterence to the drakelist gang
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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
>
> uncharged capacitor, although I've never had it proven to me that this
>
> was the actual cause.
>
> You could, however, benefit the power supply considerably, by inserting a
>
> resistor of the proper resistance and wattage, into the 650 volt output,
>
> between D2 and the connection to R1. This would also serve to bring your
>
> 650 volt HV (now approx. 700 volts, as you indicated) back down nearer to
>
> the correct 650 volt value.
>
> Your could also do the similar action with a resistor added between D5 and
>
> the connection to C3, and also reduce the 290 volts down to the original
>
> design value of 250 volts.

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