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I have a 230v to 110v transformer which I use to power some equipment. I saw on the internet that some power transformers need a soft start circuit. The power of my transformer is 200VA. On the internet is specified that only transformers with a power over 300VA require a soft start circuit. I also read on the internet that if I connect the transformer to 230v there is a high current which goes through the primary winding for a short period of time.

My questions are: Do I need a soft start for my 200va transformer?

What happens if I connect some equipment (for example a soldering station or an oscilloscope) to the secondary of the 200va and after that I connect the transformer to 230v?

Is possible to appear in the secondary a voltage bigger than 110v which can damage my equipment?

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Transformer primary windings can take an inrush of excessive current when voltage is supplied. If the AC voltage is activated at the peak of the waveform then there is no problem; if the AC voltage is activated at a zero cross there can be problems: -

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It only affects the transformer primary current and is due to the magnetization of the core. It won't increase the output voltage i.e. it won't generate an excessive secondary voltage so there is no worries in this respect but, it can blow primary fuses occasionally.

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  • \$\begingroup\$ Why will high primry flux not generate a high secondary voltage? \$\endgroup\$ – analogsystemsrf Feb 13 '18 at 15:20
  • \$\begingroup\$ @analogsystemsrf the secondary induced voltage is proportional to rate of change of flux (N \$\frac{d\phi}{dt}\$) and this will remain the same p-p value. For a lossless transformer the current will never go below 0 amps but the shape will be the same as if voltage was applied at the peak. \$\endgroup\$ – Andy aka Feb 13 '18 at 15:23
  • \$\begingroup\$ Do you think NTC will help reducing it? \$\endgroup\$ – Unknown123 Apr 3 '19 at 7:08
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    \$\begingroup\$ An NTC thermistor can sometimes help but be aware that once it is warmed up it won't provide inrush limiting on drop-out voltages. \$\endgroup\$ – Andy aka Apr 3 '19 at 8:43
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    \$\begingroup\$ When there is an AC supply anomaly and the voltage drops away for a few tens to hundreds of milli seconds \$\endgroup\$ – Andy aka Apr 3 '19 at 19:44
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  1. Are you tripping your fuse when you connect said transformer? If not, then no.
  2. It should fire up normally, with slightly more inrush current than before.
  3. Not really.
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For something as small as 200VA, the only starting precaution you need is to avoid using a fast mains fuse, use a slow one.

Having a load on the transformer when you connect it will make no significant difference to the starting current.

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  • \$\begingroup\$ Thanks for the reply @Neil_UK I want to use the 110v/200va transformer for an GDS 1052 U scope, because I think that the mains voltage is too high (247vac) and I am wondering if I can use the transformer without soft start to power my scope or the scope can be used without transformer ? Because in the manual it says that the input power voltage can be maximum 240vac and it accept an 10% fluctuation. \$\endgroup\$ – beard999 Feb 13 '18 at 9:46
  • \$\begingroup\$ by my reckoning, 240v +10% is 264v. Now, is that more, or less, than 247v? \$\endgroup\$ – Neil_UK Feb 13 '18 at 12:11
  • \$\begingroup\$ 247Vac is less than 264Vac. I was asking because I thought that there are some things that I do not know. I was thinking that 247Vac is not fine for my oscilloscope. If I use the scope at 247Vac, is there the posibility to damage my scope after a period of use ? For example after 3 months or 3 years ? @Neil_UK \$\endgroup\$ – beard999 Feb 13 '18 at 13:47
  • \$\begingroup\$ If your scope is designed to be connected to mains, then it has been designed to withstand the frequent 1500v spikes that occur on it. Look at the spec of (for instance) X2 capacitors that are designed for direct mains connection, to see what they must withstand without failure. If the documentation for the scope says it can be used at 240v +/- 10%, then why don't you believe it? If you are still troubled, you could use a smaller transformer to buck the voltage down a little, for instance a 20VA 20v transformer would take the nominal 240v down to 220v at 1 amp, and give you some margin. \$\endgroup\$ – Neil_UK Feb 13 '18 at 14:28
  • \$\begingroup\$ It is ok if I use a transformer with primary at 230V, the secondary at 110V and a power of 200VA ? Or do I need an soft start circuit for this transformer ? If I am not using an soft start for the transformer can it damage the scope ? @Neil_UK \$\endgroup\$ – beard999 Feb 13 '18 at 15:10
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I have just bought a 220-to-70V 400VA torroidal transformer and I have encountered the same issue. The automatic 10A switch-fuse goes of when I give power to the primary winding, given that according to the tranformer's characteristics it is not supposed to draw more than 4 Amperes. I resolved the issue without using a soft-starter. I just use a 2-stage powering circuit. Firstly I give power to the transformer using a 16 Ampere automatic-fuse-switch and after I close the circuit that powers my motor using a 6 Ampers automatic-fuse-switch. I dont't know why but it works.

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    \$\begingroup\$ Not a very useful answer. The question is over a year old, and you say yourself that you don't know why your solution works. \$\endgroup\$ – Puffafish Apr 3 '19 at 8:07

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