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I am having a question about inrush current for a simple AC/DC converter.

I am using a transformer from 230 to 36 V,followed by a full bridge rectifier and a filter capacitor of 1000 uF 100V.

When switching the power on, can I expect a high inrush current destroying the transformer or the full bridge rectifier?

After rectification and filtering the DC voltage should be around 50 V. The load will draw around 300 mA at 50V.

Transformer I will use

The rectifier is a "GBU8K"

Looking at the the rectifier, it seems it can handle a large forward current peak. As for the transformer I have no clue. It seems it has a DC resistance of 6 ohms. Is it safe to assume that for fast inrush currents, the inductance of the transformer will have a high impedance and therefore it will be not a problem?

If this is a problem I think I will have to add a series thermistor I also have a hard time finding out which one would be suitable.

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When switching the power on, can I expect a high inrush current destroying the transformer or the Full bridge rectifier?

There are two inrush currents associated with this type of power supply: -

  • Charging the reservoir capacitor
  • Transformer magnetization current overload causing significant inrush

The 2nd one doesn't affect the bridge rectifier because it only affects the transformer primary magnetization current so if you need to worry about your bridge surviving a power up event just rate it accordingly for supplying a high current for the first half cycle of applied AC into the reservoir capacitor.

As for the transformer inrush due to core saturation, if it's designed to run on the voltage you use then there shouldn't be a problem either providing it's a device from a reputable source. The inrush will disappear after a few cycles.

As for the transformer I have no clue. It seems it has a DC resistance of 6 ohms. Is it safe to assume that for fast inrush currents, the inductance of the transformer will have a high impedance and therefore it will be not a problem?

No you can't assume the primary magnetization inductance will protect it because, it is this inductance that plummets during an unfavourable inrush situation; it causes the inrush. But, transformers are a rugged piece of well-proven tech and chances are very high that it will survive without a hitch (if it's a reputable component) for many years.

So, I don't think you need to worry about adding a series thermistor unless of course, your upstream fuse blows occasionally when the transformer primary is connected to AC.

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  • \$\begingroup\$ Thank you for the awnser! I will give it a go as it is! \$\endgroup\$ – WillyBogard Nov 16 '20 at 17:18
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    \$\begingroup\$ btw very good explanation thank you for your time and effort \$\endgroup\$ – WillyBogard Nov 16 '20 at 17:18
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    \$\begingroup\$ It's worth a try and remember that the only thing that usually dies is the fuse if it's a little under-rated. \$\endgroup\$ – Andy aka Nov 16 '20 at 17:19

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