Limiting inrush to a variac.

awkwardbydesign

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I have already asked this over on diyAudio, and have had every question answered except the one I asked! Plus plenty of reasons why I should do something different.
https://www.diyaudio.com/community/threads/limiting-inrush-to-a-variac.392028/So, I am giving the Wam massive a chance to do the same. :D


I have 15A 240V variac, and I want to limit the inrush current. It seems an NTC thermistor is a simple way to do this. Unfortunately I am even simpler than the calculations needed!
At the moment there is nothing connected to it, but it could be used for powering a large Japanese 100V amp.
So, any help would be welcome.
 

zeta4

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I have already asked this over on diyAudio, and have had every question answered except the one I asked! Plus plenty of reasons why I should do something different.
[URL][URL]https://www.diyaudio.com/community/threads/limiting-inrush-to-a-variac.392028/[/URL][/URL]
So, I am giving the Wam massive a chance to do the same. :D


I have 15A 240V variac, and I want to limit the inrush current. It seems an NTC thermistor is a simple way to do this. Unfortunately I am even simpler than the calculations needed!
At the moment there is nothing connected to it, but it could be used for powering a large Japanese 100V amp.
So, any help would be welcome.
You could cheat like i did on the calculations and buy an inrush limiter off Aliexpress. Its on its way on a "slow boat from china " so I dont know how well it works. Its an NTC which is bypassed by a timer delayed relay. I ts pretty cheap so I will have to check out the components when I get it. Have a look if your interested there's loads on there
 

toprepairman

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You could just connect a big resistor in series (say 22 Ohm, 10W) and then put a switch across the resistor to bypass the switch once the variac is on.
 

awkwardbydesign

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Is 22 ohms/10W a suitable resistor? Considering this is a 15A variac. And a manual switch is no use, remember it has to be wife proof!
I'm pretty sure I will go for time delay relay (from CPC), it's just the limiter I need to get right.
 

rabski

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Fixed resistor doesn't seem a great idea to my mind as the rating will depend on the current draw but will still be large. 240 volts, 5A and 22 ohm resistor would need 500 watt rating constant, though of course it won't be constant. Adding a timer means complication and issues with reliability, plus as it's inrush, it only needs a very short time. Timers are generally not good at being accurate for very short durations, so you'd need a seriously heavy rated resistor in series.

If you really do need this, then an NTC is the better bet, but it's not perfect. NTCs still have some resistance at their stable operating point. Usually just an ohm or two, but they still need to be rated accordingly based on the current draw and you're still losing efficiency and generating heat. An NTC belongs in the input line of the equipment inside its casework for safety.

I realise you don't want 'alternative' solutions, but I don't really understand why to be honest. If you want safe, reliable and 'wife proof' then a variac would be a long way down my list and far from ideal for simply dropping voltage like this. Just use a step-down transformer and there's no need for inrush limiting, resistors, timers, switches or anything else.
 

Jazid

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Depends on inrush, is it very short duration or longer? For the latter you could use a light bulb, simple, basic, works? Also, it "resets" much faster than an NTC thermistor so rapid on/off/on won't affect performance.
 

awkwardbydesign

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Well, as I have the variac, that is what I will use. Calculating the NTC thermistor needed is exactly what I want, I just find it hard to do myself.
A lightbulb won't work, as it has to be inside the case, and there won't be rapid on/off. It would stay on for the whole evening, generally.
So a thermistor bypassed after the surge is what I want..
I appear to be getting the same don't do it like that, do it like this, as on diyAudio.
 

Jazid

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Well if its maths you need doing, add the parameters: idling current and start up draw (if known). Difficult to calculate without...

I am inclined to unhelpfully opine that if the variac is so marginally specified that it won't take the start up current, then unless unusual it won't take the steady state current either since they usually have plenty of margin. If this is the case then perhaps your venture is an accident waiting to happen. Is this muting people's wish to help?
 

awkwardbydesign

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Well if its maths you need doing, add the parameters: idling current and start up draw (if known). Difficult to calculate without...
Yep, them's the ones I want help with! And it's not the variac I want to protect, but the circuit breaker.
 

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