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Selecting a NTC thermistor inrush limiter

gadget73 said:
I really wouldn't be overly concerned about protecting solid state rectifiers. They're cheap as chips, even if you use fancy ones.
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From my perspective, the reason to use them with SS rectifiers is because SS rectifiers tend to fail short. It's not about protecting the rectifier, it's about protecting the power transformer.

But considering the rating of the diodes I'm using, perhaps I'm being overly-protective, and just shouldn't bother.
 
I would suggest putting all power transformers together an putting the thermistors then as 1 for all tranny's.
(that way the thermistor sees the whole load, and actually limits the inrush better along with output voltage)

And the way I've done it on all the units I do is 1 CL-80 on each leg of the incoming AC, just for reference.. :)
 
thorpej said:
From my perspective, the reason to use them with SS rectifiers is because SS rectifiers tend to fail short. It's not about protecting the rectifier, it's about protecting the power transformer.
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Thats what fuses are for. A thermistor is to reduce inrush surges, its not meant to protect against an over-current condition.



I used a CL90 on my Fisher 600. Its probably running close to it's rating, but it seems to be doing OK. The Pilot SA-260 got a CL-80. The 260 originally had no power switch, but it had 2 outlets on the back. I found a nice heavy duty slide switch that fit into that hole perfectly, and it was an SPDT switch. The third terminal is used to mount the CL-80. Its up in free air so nothing around it gets hot. It went from having a distinct "thunk" on powerup to being absolutely silent. I have no special love for slide switches, but it was the least invasive option, and one that involved no permanent modifications to the amplifier chassis.
 
a typical example of incororating an unecessary component (thermister) into a ciruit,

whereas a small heatsink set-up would work just fine. Minimizing component count

will ensure a much better (more accurate & true) signal flow , thus having a higher

degree of stability. “Keep It Simple”.
 
I use CL90's and install on the hot side along with the fuse and power switch if there is one. I find with vintage equipment the CL90 provides just enough voltage drop to bring the B+ very close to where it should be vs not using it.

I've also read they (say a CL140) can be installed on the centre tap of the filament portion of the transformer.
 
I like the idea of 2 in series, since I only switch 1 leg of AC. We're talking millisecond delay? Which could take the jolt of start up. Also, it could help keep the cold filiments from start up shock.... I feel primary side will help the whole amp. This could be debated forever....
 
If the voltage on the primary side ramps up slowly, the voltage on the secondary side does too. Having them on both sides of the transformer I just don't see a benefit to.

Its also a lot more than a millisecond. From power-on to full brightness of the panel lights on my Fisher 600 its at least 2 seconds. Its extremely obvious that the voltage is ramping up.


What I get out of them is improved life of the power switch. This is useful on things with goofy switches that you just cannot get easily. I suppose you might also be able to run fast blow fuses and a slightly reduced current rating on the fuse since it doesn't have to deal with that inrush current.
 
Ah, yes ... and thanx for the reminder.

I just rebuilt an Eico HF12A and forgot to add a thermister. Those are known for power switch failure, and that's an addon to the treble control and difficult to find. No telling how many cycles on it now, but every little bit helps!

eico-death-cap.jpg


(Ya, I know, it's also still got the "death cap". Still debating whether I want to replace or remove that. Certainly not losing any sleep over it. Zap! )
 
the CL60 isn't mentioned but it's a 5 amp inrush limiter and is only 10 ohms instead of 47 ohms like the CL80.
and it's less expensive and the lower resistance is a plus.
 
Just a recommendation for inrush current limiters. I came across Ametherm a while back,and have used their products exclusively ever since. I find that they have a greater variety,and prices are extremely competitive. Available from all of the usual suppliers.

I find that their MegaSurge line covers most of the needs for vintage tube gear. They have many other lines for other applications.
( R= resistance at room temp,and SSI is the allowable continuous current )

https://www.ametherm.com/inrush-current/megasurge-inrush-current-limiters.html
 
Agree. There are now better choices than CLxx.
Mouser as some at 220R @ 1A, perfect for some other B+ application.
 
I know this is an older thread, but I have three questions regarding placement of thermistors. 1) Do I need thermistors in both the hot and neutral, or is one just in the hot leg enough? They do add heat, and therefore assume that limiting this heat (whenever the amp is running) would be beneficial - therefore one, if it provides reasonable inrush current protection. 2) for the thermistor in the hot leg does it matter where it is in the series between the line into the amp and the transformer primary? e.g.: fuse, switch, thermistor, transformer. 3) Would it make any difference if I switched the sequence of the switch and thermistor to fuse, thermistor, switch, transformer? In a MkIII I am restoring, it will be physically easier to put the thermistor between the fuse block and the switch, with the other terminal on the switch going directly to the transformer.
 
sabrefencer said:
1) Do I need thermistors in both the hot and neutral, or is one just in the hot leg enough?
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As per a response back early on in this thread, you should try very hard not to put any components on the neutral side of the line, so that if a fault occurs, there is nothing in between. I suppose you could series-connect two thermistors on the hot line if you wanted.

sabrefencer said:
2) for the thermistor in the hot leg does it matter where it is in the series between the line into the amp and the transformer primary?
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With respect to inrush limiting, it doesn't matter.

sabrefencer said:
3) Would it make any difference if I switched the sequence of the switch and thermistor to fuse, thermistor, switch, transformer? In a MkIII I am restoring, it will be physically easier to put the thermistor between the fuse block and the switch, with the other terminal on the switch going directly to the transformer.
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Seems fine.

===
Note however, and assuming this is not already obvious to you, a thermistor will not provide any current surge protection against a hot power cycle. Under hot power cycle conditions, the thermistor is hot already and therefore offers little to no resistance for current surge reduction. Rather it only works as a current surge limiter if the thermistor is cold.
 
Thanks. This answers my original question. To summarize - nothing in the neutral leg, the neutral leg of the power cord will go directly to one leg of the transformer primary. The hot leg of the cord will go first to the fuse, then to the thermistor, then to the switch, then to the other leg of transformer primary (in this particular amp). My preference would still be fuse first, then switch, thermistor, transformer, but as stated above, it does not make much difference.

Yup, I understand the limitations of the "limiter". When the thermistor is hot (temperature) it has very little resistance and therefore cannot provide its cold resistance rating against a re-start.
This brings me to an extension of the thermistor topic. Does the thermistor need to be inside the unit at all? Why couldn't it be in an external box, which would keep the heat it generates out of the amp. It could be housed with a bucking transformer, so one unit could provide proper voltage reduction, and inrush current limitation. This allows restoration purists to keep things (more) original, while still protecting equipment.

Since the internal space of the amp is no longer a limitation, you could add a circuit to bypass the thermistor after it heats up. So, after it is done limiting inrush current, switch it out of the power path. It would then cool down, and if the unit were hot switched, it could be in the path and ready to do its current limiting job again. This would require some sort of temp sensor or timer to initiate switching the thermistor out of the line, and the shutting off power would switch the thermistor back into the line. This one unit could provide voltage reduction, and inrush current limitation, even for hot switches (unless some idiot rapidly cycles the switch), and it keeps the heat out of the amp. Has someone already thought of this, and does it exist, or there are reasons I have not thought of why this is a bad idea?
 
Wrt to varistor dissipation - although the NTC needs to maintain a high temperature, it is a small part and doesn't dissipate much power, so as long as it is not touching anything and is in the 'power' section of the amp then I reckon that is the safest spot for it. If heat is an issue then don't use valves.

Many seem to want to add an NTC because others did it, or due to some intangible hand-waving benefit. For sure if there is a tangible reason, and the part is designed for the application then fine. Imho, adding complexity by way of timers and relays also should be consistent with the base benefit of adding an NTC. Perhaps best to KISS and reason why all previous amps didn't use an NTC, and hence is there really a tangible benefit now.

An NTC should be designed for an amp, so if adding it to an external box then there is a risk if the box is not 'tied' to the amp.
 
sabrefencer said:
Thanks. This answers my original question. To summarize - nothing in the neutral leg, the neutral leg of the power cord will go directly to one leg of the transformer primary. The hot leg of the cord will go first to the fuse, then to the thermistor, then to the switch, then to the other leg of transformer primary (in this particular amp). My preference would still be fuse first, then switch, thermistor, transformer, but as stated above, it does not make much difference.

Yup, I understand the limitations of the "limiter". When the thermistor is hot (temperature) it has very little resistance and therefore cannot provide its cold resistance rating against a re-start.
This brings me to an extension of the thermistor topic. Does the thermistor need to be inside the unit at all? Why couldn't it be in an external box, which would keep the heat it generates out of the amp. It could be housed with a bucking transformer, so one unit could provide proper voltage reduction, and inrush current limitation. This allows restoration purists to keep things (more) original, while still protecting equipment.

Since the internal space of the amp is no longer a limitation, you could add a circuit to bypass the thermistor after it heats up. So, after it is done limiting inrush current, switch it out of the power path. It would then cool down, and if the unit were hot switched, it could be in the path and ready to do its current limiting job again. This would require some sort of temp sensor or timer to initiate switching the thermistor out of the line, and the shutting off power would switch the thermistor back into the line. This one unit could provide voltage reduction, and inrush current limitation, even for hot switches (unless some idiot rapidly cycles the switch), and it keeps the heat out of the amp. Has someone already thought of this, and does it exist, or there are reasons I have not thought of why this is a bad idea?
Click to expand...
One benefit of installing a thermistor in a 50/60 year old tube amp is that even after its warmed up, it still provides a drop in line voltage of a couple/three volts.
My line voltage is normally 120+/- 1V. This keeps me close to the design center of 117V that most of this equipment was designed around. The higher B+ can be stressing the ancient insulation inside transformers and aging capacitor dielectrics if the thermistor is bypassed after it's warmed up.
I've seen where some folks are putting two thermistors in series to cope with higher line voltages.
 
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