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Hey,

For my integrated (dual on one chassis) Mark III build, I'm wondering what the current (inrush) limiter value should be. I bought two CL-90's. Is one likely to be sufficient or should I use both in series?

They way I figure, whatever would be used for a Mark III monoblock times two should be an approximate value. Or is this too much of a WAG?

Any thoughts?

Thanks,

Chris

Shannon posted the pdf for IRCL specs here:
http://www.diytube.com/phpBB2/viewtopic ... ircl#14684

You want to size the thing based on the expected Steady State Current (amps). Then you're looking for a high resistance at cold (25 deg C) and a small resistance at "Under Load at % of Max Rated Current".

The CL-90 is designed for a 2 amp draw. It is 120 ohms cold, and goes down to less than 2 ohms at 75% of load. The tricky part is figuring out the current draw at the power transformer. I usually screw up the math here somewhere. What is it for a single Mark III? Say it's about 440VAC @ 150 mA, plus 5V @ 2A, plus 6.3V @ ~4.15A. That's a whopping total of just over 100 VA. There's going to be some efficiency losses in the power transformer - let's pretend it is 65% efficient? Then we need somewhere in the neighborhood of 150 VA going into the power transformer. At a line voltage of 120 VAC, we would need about 1.25 amps going into the primary winding.

Multiply that by two for a pair of Mark III, and I'd guess that a single CL-90 won't have the necessary current rating. Putting them in series won't work either, since all the current would need to go through each one. You might get away with wiring them parallel to each other, but I don't know how they're going to split the load since their resistances are going to vary as they heat up. You'd probably be best going to a CL-80 for its higher max current rating.

Find someone to check my math. I probably made a wrong assumption in there somewhere.

Ty_Bower wrote:Shannon posted the pdf for IRCL specs here:
http://www.diytube.com/phpBB2/viewtopic ... ircl#14684

I'd guess that a single CL-90 won't have the necessary current rating. Putting them in series won't work either, since all the current would need to go through each one. You might get away with wiring them parallel to each other, but I don't know how they're going to split the load since their resistances are going to vary as they heat up. You'd probably be best going to a CL-80 for its higher max current rating.

Thanks for the link and the thorough response. Of course, now that you made me think, series can't work. You answered my question and now my lazy mind will just have to work it through...probably what I should have done in the first place.

Happy Fourth of July and regards,

Chris

About inrush current limiters in parallel - google led me to this -

"Inrush current limiters must not be connected in parallel.
If connected in parallel, the NTC thermistor with the
smallest rated resistance consumes almost the entire
current in the circuit, and the resulting overload could
destroy the component."

and

"If two paralleled devices had precisely identical characteristics, it would work. But assuming that the
initial cold resistance was not matched, the unit with the
lower resistance would pass more current
than the other, so it would get hotter, making its
resistance lower still, and it would remain the dominant
path, not sharing the current."

Quad wrote:About inrush current limiters in parallel - google led me to this -

"Inrush current limiters must not be connected in parallel.
If connected in parallel, the NTC thermistor with the
smallest rated resistance consumes almost the entire
current in the circuit, and the resulting overload could
destroy the component."

and

"If two paralleled devices had precisely identical characteristics, it would work. But assuming that the
initial cold resistance was not matched, the unit with the
lower resistance would pass more current
than the other, so it would get hotter, making its
resistance lower still, and it would remain the dominant
path, not sharing the current."

Thanks, Quad. I don't know how we ever did anything without Google! I found this:

"For inrush current limiting the NTC must be connected in series with the load circuit. Several inrush current limiters can also be connected in series for higher damping. Inrush current limiters must not be connected in parallel.

If connected in parallel, the NTC thermistor with the smallest rated resistance consumes almost the entire current in the circuit, and the resulting overload could destroy the component."

http://www.epcos.com/inf/50/ap/ntc.pdf

Sounds like the info is a bit all over the map. More reason for me to calc the right value and buy the right part.

Regards,

Chris

Technically you are not supposed to parallel current limiters
but I paralleled two CL90s when I ran out of CL80s on one occasion.
I made sure that both units were in intimate contact so heat was equalized between them plus I wrapped a layer of fiberglass/silicone tape around them as a unit. That way they heat together and stay in better match.
I am not recommending it however. I just did it in a pinch and when I was sure I would not be overloading either of them.

mesherm wrote:Technically you are not supposed to parallel current limiters but I paralleled two CL90s when I ran out of CL80s on one occasion.I made sure that both units were in intimate contact so heat was equalized between them plus I wrapped a layer of fiberglass/silicone tape around them as a unit. That way they heat together and stay in better match.

Thanks for the info. Makes sense if temperature can be evenly maintained between the two.

For some inexplicable reason, it made me think of encasing multiple ICL's in a small sealed can filled with oil and selling them as boutique components....

Chris