Aleph 5 inrush current protection

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vossie

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Joined 2002
stephanphotography.com
#1
Hi,

I'm in the process of building an Aleph 5 and the BOSOZ preamp both at the same time (got a serious bite from the diy bug). I have a dear friend who recently build the Aleph30 but had problems with the startup inrush current to get the psu caps charged. He blew a few fuses :xeye: until eventually putting in another thermistor in the primary winding circuit on the toriod.😱

I am using a 625VA 28-0-28 toriod (with 120000uF caps) for the Aleph 5 and two 50VA 30-0-30 toriods for the revised BOSOZ. My question is this: What is the best way to handle the startup current. (you will have to give me a circuit or explain slowly please)

Thanks in advance,
SV😀
 
#2
The best way is thermistor. if one is not enough, put 2 in series. I'm using 2 x CL-70 on my Aleph X and they work fine. The reason I used two, was that I had some bridge failures before and I wanted to be sure that this time my discreet soft recovery diodes won't blow.😉

I tried fixed resistors, but because of the nature of the amp (constant high current draw), the higher the resistor's value the higher voltage drop on a resistor which prevents the full charge on PS caps. Thermistors, OTOH, behave more dynamic and seem to be a perfect solution for the inrush problem (and the easiest as well).

Some people use a relay to short thermistor later, I don't think it's necessary though.😉
 
#3
I had heard that just adding a thyristor based soft start in the circuit adds noise both up and down the power path and using a relay circuit in conjunction with the thermistor will help prevent the noise. I am building a Borberly DC100 and putting the power supply together right now so this would be nice to know.
 
#6
pretty slick, that one is much more of the KISS line of thought than the one described in Randy Slone's sourcebook. That one has twice the amount of parts and much larger. What is the duration of time delay with C2 and C3 at 470uF?
 
#8
The problem with thermistors is they don't work if they are already hot. If the thermistor is cold is supresses the inrush. But if the thermistor is hot, and you turn the amp off, and the power filter drains, and you restore power, the thermistor does nothing. I am partial to the solution of a relay which shorts out a resistor bank.

You may say that you should just avoid turning the amp on and off, but sometimes the power company doesn't give you the choice.
 
#11
Just got the tiny PCB from elektuur (the dutch version of elektor) for the promitheus inrush protection.

Gonna build (all those parts 🙂 ) this weekend, I'll use it for an Aleph 5.

I'll still use thermistors but in the CRC filter
 
#13
rwagter said:
Just got the tiny PCB from elektuur (the dutch version of elektor) for the promitheus inrush protection.

Gonna build (all those parts 🙂 ) this weekend, I'll use it for an Aleph 5.

I'll still use thermistors but in the CRC filter
Click to expand...

It won't work well for Aleph. Too much voltage drop on resistor (depending on resistor value, but may be up to 60V), not allowing caps to fully charge and when relay closes, you'll get inrush anyway. Don't even play with the delay time, because it's useless. Everything happens in first second. I tried this circuit and chose the thermistor.😉

This circuit might be good for a preamp or class AB amp, but not current hungry Aleph.
 
#17
It all depends on primaries current draw. You might have to use really small resistor's value and then inrush protection feature is gone. I wanted to use at least 20 ohm resistor, but since the voltage drop was so high, I opted for 2 in series connected CL70 thermistors. And I didn't have to use any additional circuits.

If you want to use it with Zen 4, just connect series resistor on your modified power cord and see what happens, might work for you, might not. I was also working with 120V wich probably makes a difference when comparing to 240V.
 
#19
Anything that has fixed resistor in series with primary will not work well. TSRL is using thyristor, but still I don't know how it works. When you use fixed resistor, you will get permanent voltage drop and with current of 1-2A you can figure out what this drop will be.

Why don't you use thermistor. Nelson is using it in his amps and says that there were no failures. To protect amp from short power disruptions you can use a relay which will short thermistor after 3 sec.
 
#20
Peter,

I also use the elektor soft start circuit on my Aleph 5. It starts up two 600VA transformers and 160.000 uF of caps (not counting the 240.000uF after the chokes).

Untill now it works just fine although it is true that when the second relais closes a second inrush of current occurs (you can hear the transformers).

The max current is certainly lower than with the thermistors used in Pass amps. Both the Aleph Os and X150 had the lights flackering:bigeyes:

william
 
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