I have a 220V - 100V transformer, to power some home electronics from Japan. It's a massive beast, and when I flip on the power to the transformer, the breaker pops. So does the one out in the hall feeding the entire apartment, so I can't just replace my own with a slow-blow breaker. If I reset the breakers and hold them, past the initial inrush, the setup works fine after that, since the electronics draw very little power - a few dozen watts total.

I can't find anyone here with a sensible-sized 100V transformer, so I would like to build something that will limit the inrush current into the one I have. My idea was an inline power resistor of around 20 ohms, which would limit the current from the wall to around 11 amps. The breakers are rated to 16 amps, so that should be fine. I would then have a relay that would short across the resistor, after about a second, so the resistor would only be in the circuit for the initial power up, then power would flow directly to the transformer with nothing in the way.

I don't have all that much experience with AC and power supplies. Does this sound reasonable?

 

Hi pd,
What type of equipment loads do you have connected to the 100V transformer output?
A fixed 20R resistor would create a voltage drop on the load voltages.
E

 

Home electronics - music gear, like amplifier, cassette deck, equalizer, etc.

I know the resistor would create a voltage drop, that's why the relay to shunt around the resistor, but only after the initial inrush has been curbed.

 

Hi,
If you want to limit the inrush current to say 10Amps by using a series 20R resistor for a period 1 Second, then the wire inside the resistor should have fusing current of at least 20A, otherwise the resistor could act as a fuse.
Also, the instantaneous voltage across the 20R would be approx 200V.
It will be substantial wattage rating.
E

 

I have a 220V - 100V transformer, to power some home electronics from Japan. It's a massive beast, and when I flip on the power to the transformer, the breaker pops. So does the one out in the hall feeding the entire apartment, so I can't just replace my own with a slow-blow breaker. If I reset the breakers and hold them, past the initial inrush, the setup works fine after that, since the electronics draw very little power - a few dozen watts total.

I can't find anyone here with a sensible-sized 100V transformer, so I would like to build something that will limit the inrush current into the one I have. My idea was an inline power resistor of around 20 ohms, which would limit the current from the wall to around 11 amps. The breakers are rated to 16 amps, so that should be fine. I would then have a relay that would short across the resistor, after about a second, so the resistor would only be in the circuit for the initial power up, then power would flow directly to the transformer with nothing in the way.

I don't have all that much experience with AC and power supplies. Does this sound reasonable?

That works. Occasionally the relay fails to close and blows up the resistor.

Will the equipment work at 110V? If so you could use a power tool transformer, which has the added advantage that it has a balanced output which minimises the field from the mains cables.

Otherwise, just look for a 50-0-50v transformer, which would be fairly common for power amplifiers. You could then keep it as 50-0-50 which would give you a balanced output with no net field, but you would need double pole breakers, or you could use it as 0-100, but don’t forget to earth one side of the secondary.

 

Hi,
If you want to limit the inrush current to say 10Amps by using a series 20R resistor for a period 1 Second, then the wire inside the resistor should have fusing current of at least 20A, otherwise the resistor could act as a fuse.
Also, the instantaneous voltage across the 20R would be approx 200V.
It will be substantial wattage rating.
E

Yes, it would be a lot, but only for an instant. Not nearly even the full second I wrote - just the momentary cavalry-charge while the transformer initially builds its magnetic field, then the current would drop to whatever the downstream electronics draw - again, only a few dozen watts total. I was thinking one of those big wire-wound, baked-in-ceramic jobs. They can usually be found fairly cheaply, and they can take a lot of power. Should be no problem handling a single spike of a fraction of a second duration.

 

That works. Occasionally the relay fails to close and blows up the resistor.

Will the equipment work at 110V? If so you could use a power tool transformer, which has the added advantage that it has a balanced output which minimises the field from the mains cables.

Otherwise, just look for a 50-0-50v transformer, which would be fairly common for power amplifiers. You could then keep it as 50-0-50 which would give you a balanced output with no net field, but you would need double pole breakers, or you could use it as 0-100, but don’t forget to earth one side of the secondary.

No, I don't think it will. I've talked to people about it, and they all say the stuff is fairly fussy - a 10% overvoltage could smoke the lot. I can get a 220 - 110 transformer fairly easily - Europe to US convertors are plentiful, but Japan has a rather fractured electrical grid. They use 100, 110 and 220, all at both 50 Hz and 60 Hz, depending on where you are in the country. Modern gear can handle all of that with ease, but this stuff is older and has no provisions for any power other than what is specified.

I'm in Prague, which is a fairly large city, and I would have thought that a decent electronics supply house would be a standard feature in a city of this size, but nothing. There is one sort of hobby store, with bubble-wrapped individual components, and sales personnel that should be selling fries at McDonalds, for all the knowledge they have of their store's merchandise.

Maybe a business opportunity...

And why would the relay occasionally fail to close? Everything can fail, obviously, but what you write makes it sound like something that I should expect to happen regularly.

 

And why would the relay occasionally fail to close? Everything can fail, obviously, but what you write makes it sound like something that I should expect to happen regularly.

Depends on where the relay gets its power from. If it is directly off the mains, then miscalculations about how much power it needs, or failures to trigger on startup can cause problems.
Tends to more of a design problem than a component reliability problem.

 

https://www.tme.eu/gb/katalog/?queryPhrase=55188
not quite in your neck of the woods, but nearer.

 

One other option, which is much less elegant but more gentle, is an array of parallel connected incandescent light bulbs, connected in parallel a few at a time. The benefit would be a gentler increase in current and less likely to burn up, although that will still be a hazard with 220 volts and 120 volt bulbs.
Certainly that 100 volt rating is a pain, but with a number of bulbs to adjust the output you will have some ability to adjust the voltage.

 

Maybe I'm missing something here but you can get motor soft-start devices from Ali for a few dollars, probably just thermistor-based, or more rigorous ones like this for a few tens of dollars, you can set the time and ramp-up on the latter type.

 

One other option, which is much less elegant but more gentle, is an array of parallel connected incandescent light bulbs, connected in parallel a few at a time. The benefit would be a gentler increase in current and less likely to burn up, although that will still be a hazard with 220 volts and 12 volt bulbs.
Certainly that 100 volt rating is a pain, but with a number of bulbs to adjust the output you will have some ability to adjust the voltage.

I'm afraid I don't know how to do that, but thank you for the idea.

 

Maybe I'm missing something here but you can get motor soft-start devices from Ali for a few dollars, probably just thermistor-based, or more rigorous ones like this for a few tens of dollars, you can set the time and ramp-up on the latter type.

That looks promising - many thanks. Again, there is ONE semi-decent electronics store in this city, and all I got from anyone there was, "Maybe there are such things, but I've never heard of them." About as helpful as sand on a skating rink. I'll give this place a look - sounds easier and safer than trying re-invent my own wheel.

 

First, an electric motor soft-start package of adequate rating will cost more than the smaller transformer you would use.
For a series resistor another set of ideas would include an electric heated flatiron, or just an electric heater of the type without a blower.
I did not see any mention of the rating for the transformer other than it was "very big." That is not much information to help with any suggestions. And if the TS did not understand what I was suggesting with the lights, I am concerned about their safety working with 220 volt mains in general. Not only are 220 volt shocks more hazardous, but also 220 volt circuits are very unforgiving when you make an error.

 

The probably-thermistor-based ones are a few dollars, they're so cheap I got several of them in case I fry one or need more later. The more serious ones in the second link I posted are a few tens of dollars, but they're also pretty flexible and functional, all you need to be able to do is mount it in a case with appropriate insulation.

For the OP, I'm not sure where you're located but there's probably also a ton of these on Amazon if you're in the US and want one quickly, the smaller ones are standard for name-brand power tools like Makita so you can either buy the originals or the Chinese clones at a fraction of the price. Just be careful if you go with Amazon because most of the sellers will be dropshippers and you're better off getting them direct from the source on Aliexpress.

 

You need to place an NTC device – something like this in the transformer primary.

Having a negative temperature coefficient, as it warms up due to the power dissipation in the device, reduces the resistance. You need to select one with suitable rating values for your circuit, inrush current and cold resistance (normally quoted for 25⁰C).

https://www.amazon.com/5D-20-Circuit-Protection-Power-Thermistor/dp/B0BR4HZQJC

 

I suggested the array of incandescent light bulbs as a safer scheme, and please consider that the TS claimed to not understand what I described. So the "Other Engineer Type" does not have access to as many resources as we might assume.

 

First, an electric motor soft-start package of adequate rating will cost more than the smaller transformer you would use.
For a series resistor another set of ideas would include an electric heated flatiron, or just an electric heater of the type without a blower.
I did not see any mention of the rating for the transformer other than it was "very big." That is not much information to help with any suggestions. And if the TS did not understand what I was suggesting with the lights, I am concerned about their safety working with 220 volt mains in general. Not only are 220 volt shocks more hazardous, but also 220 volt circuits are very unforgiving when you make an error.

I don't know much about the transformer, except that it converts 220 to 100, and pops a 16-Amp circuit breaker on start-up. If you can tell anything from that, like a minimum milli-Henry rating, great. I don't know enough about AC behavior to even guess.

The soft-start modules I saw start as low as fifty cents, up to around a hundred dollars for the fancy ones.

As for your idea, I don't know what you mean by 'a few at a time'. Like multiple switches manually turning on banks of lights? That seems quite complicated, and a panel necessary to hold all those bulbs would be large, and the heat and light given off would be a PIA. But maybe I'm not clear on what you mean. Possibly a circuit sketch would help.

 

The probably-thermistor-based ones are a few dollars, they're so cheap I got several of them in case I fry one or need more later. The more serious ones in the second link I posted are a few tens of dollars, but they're also pretty flexible and functional, all you need to be able to do is mount it in a case with appropriate insulation.

For the OP, I'm not sure where you're located but there's probably also a ton of these on Amazon if you're in the US and want one quickly, the smaller ones are standard for name-brand power tools like Makita so you can either buy the originals or the Chinese clones at a fraction of the price. Just be careful if you go with Amazon because most of the sellers will be dropshippers and you're better off getting them direct from the source on Aliexpress.

I'm in Europe - Prague, Czech Republic. A large city, but the electronics supply houses that were common in my younger days are disappearing even from US cities. In any case, I don't think it's a problem for any large company to ship here. The modules on AliExpress are listed for around fifty cents into offer, although I'm not sure what they would say about just one. But sometimes you can even get things like that free as engineering samples.

 

You need to place an NTC device – something like this in the transformer primary.

Having a negative temperature coefficient, as it warms up due to the power dissipation in the device, reduces the resistance. You need to select one with suitable rating values for your circuit, inrush current and cold resistance (normally quoted for 25⁰C).

https://www.amazon.com/5D-20-Circuit-Protection-Power-Thermistor/dp/B0BR4HZQJC

That specific one is out of stock, but they have similar ones. I'm not sure about the ratings, but if I assume that the transformer looks like a dead short on start-up, and do my max current calculations from that, it should be safe, no?