Inrush limiter needed for 1kVA balancing transformer

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thommy

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#1
Couldn't find a better place for this than the General board.

I'm building a balancing transformer to give me a smoother mains supply. So far it's just the transformer and a circuit breaker to limit things in case of meltdown but I plan to add some filtering in there too. It sounds good and I am happy with the idea overall.

My problem is that when I switch on the transformer the RCDs in my house trip out because of the inrush current.

I have searched for inrush limiters but just keep finding thermistors. Considering the load on this huge transformer and the nice thick copper power cables I don't want to put a tiny little thermistor in series on the incoming mains.

I could use a transformer soft start, something like the link below but wonder if that is really necessary:

220V 1000W Transformer Delay Power Soft Start Protection Board for Amplifier AMP

Can someone suggest a better inrush limiting scenario?

Thanks
 
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#3
I've repaired a few soft-start cards (for 1980's amps with 2x 1kVA transformers) - most recently this weekend. Little cap-charging circuit triggers opto-triac after ~2secs, which then operates big triac to short series resistor in neutral return.

Straightforward enough, and usually works - but I'm amazed at how many ways they've found to fail (and tax my troubleshooting skills). Everything from slightly degraded zeners to measure-OK but actually-not X2 film caps. Every board I've seen is charred (but these beasts have had a long, tough life on the road and are with me for a well-deserved retirement).

Caused me to dig a bit; Rod Elliott's 'inrush' page (as usual) is good, shows why this seemingly simple problem is not at all.

I'll step out, as unqualified to advise or comment further. But I wonder whether e-bay's hj_021tongxian really solved all these problems for £6.51...?

Be careful - it's mains.
 
#4
A transformer soft start is a current limiter that gets switched out after it has done it start up job.

A resistor or thermistor in the primary feed that is bypassed by a mains relay after a very short delay.
It really is that simple.
 
#7
Thanks guys, but I was basically looking for a better product than the one I suggested, rather than theory on how to solve the problem of inrush.

I'm going to select a slightly better quality soft start board for this project. There are 3x 1kVA transformers total, one each for left & right amps plus another for sources so I think will choose a soft start design that employs a 555, and modify the resistor values on two of them to stagger the timing and reduce the overall load, unless someone has a better suggestion that is going to be as easy to implement.
 
#9
thommy said:
Thanks guys, but I was basically looking for a better product than the one I suggested, rather than theory on how to solve the problem of inrush.

I'm going to select a slightly better quality soft start board for this project. There are 3x 1kVA transformers total, one each for left & right amps plus another for sources so I think will choose a soft start design that employs a 555, and modify the resistor values on two of them to stagger the timing and reduce the overall load, unless someone has a better suggestion that is going to be as easy to implement.
Click to expand...
fuse each transformer separately. You should find each will soft start with a T3.1A or T4A mains fuse.
Your plug top fuse should be 13A
Use a separate relay contact for each transformer if you time them all to switch concurrently, or use two or three relays if you want to stagger the bypasses. There is a small current peak as the bypass makes contact.
 
#10
Hello

I need a advice I want to get me a 600VA toroidal transformer and the mains where I live in Puerto Rico is 120V same as USA from the outlet well I went online and search for soft turn on Elliott website with the formula results I have about 12Ω for but with 33Ω x 3 in parallel I got like 12Ω so is like this:

I = 600/120 = 5 (2) 600 is the VA desire toroidal I want
R = 120/10 = 12Ω

I just want to be sure that this is correct before I jump into deep water, any advice I will really appreciate

Best Regards
Juan
 
#11
thommy said:
I'm going to select a slightly better quality soft start board for this project. There are 3x 1kVA transformers total, one each for left & right amps plus another for sources so I think will choose a soft start design that employs a 555, and modify the resistor values on two of them to stagger the timing and reduce the overall load, unless someone has a better suggestion that is going to be as easy to implement.
Click to expand...

A rotary switch to successively connect the transformers via thermistors/resistors then finally directly
 
#12
vargasmongo3435 said:
Hello

I need a advice I want to get me a 600VA toroidal transformer and the mains where I live in Puerto Rico is 120V same as USA from the outlet well I went online and search for soft turn on Elliott website with the formula results I have about 12Ω for but with 33Ω x 3 in parallel I got like 12Ω so is like this:

I = 600/120 = 5 (2) 600 is the VA desire toroidal I want
R = 120/10 = 12Ω

I just want to be sure that this is correct before I jump into deep water, any advice I will really appreciate

Best Regards
Juan
Click to expand...
600VA needs a T5A mains fuse.
That 5A fuse will tolerate a 10Apk start up current.
A 12r resistor limits your peak start up current to 10A.
You will probably find that the mains fuse will tolerate a higher start up current and thus allow an even lower value resistor. But I have no idea how long a life you would get from your fuse if you adopt a lower resistor.
You may find you get an acceptable life using a peak start up current of 15A from that T5A fuse.
 
#13
thank you AndrewT, on the scenario I have this new "prototype" unregulated power supply that I design with Mr. ST, it has 6 x 15000uF 100V caps I have plans to use only for 8Ω no 4Ω load, so 12Ω will be just right ? I just want the inrush current to be slow down efficiently and keeping the fuse on a save range (prolonging the life of it) and of course I'm gonna used a relay circuit to bypass the 3 resistor combination that in parallel will give me about 12Ω

Best Regards
Juan
 

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#14
I'm gonna used a relay circuit to bypass the 3 resistor combination that in parallel will give me about 12Ω
Click to expand...
I have proposed this hypothesis in the past.

A higher value resistor will use a a longer length of thinner wire, than a lower value resistor.
Let's use your 12r as our example.
Three 36r power resistors will use a very thin wire compared to three 4r power resistors.
It is my belief (no evidence - just intuition) that the thick wire 4r0 will survive abuse due to over current much better than the thin wire 36r

At start up the three resistors pass 10Apk and will dissipate an instantaneous peak power of 1200W, or 400W (instantaneous) per resistor for a 5W resistor.

I use many 5W power resistors wired in series to achieve the required added resistance, because I think they will be more reliable.
 
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#19
thommy said:
Thanks guys, but I was basically looking for a better product than the one I suggested, rather than theory on how to solve the problem of inrush.

I'm going to select a slightly better quality soft start board for this project. There are 3x 1kVA transformers total, one each for left & right amps plus another for sources so I think will choose a soft start design that employs a 555, and modify the resistor values on two of them to stagger the timing and reduce the overall load, unless someone has a better suggestion that is going to be as easy to implement.
Click to expand...

A shameless plug for my PCB, uses a 555 timer and has onboard transformer.

2pcs Blank PCB: Universal Soft Start / Power On Delay / Inrush Current Limiter | eBay
 
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