Transformerless power supply: Inrush current limiting resistor placement

Hi All,

I'm studying transformerless power supply systems these days and noticed that basically in every circuit I found on internet has a inrush current limiting resistor connected to the supply line where capacitor is not connected.
That is, If the dropper capacitor (and its parallel discharge resistors) are connected to the live, then the inrush limiting resistor is connected to the nutral.

I could only find one legit place that shows a circuit connecting the inrush limiting resistor in the same leg as dropper capacitor, for all other they were using it in the other leg.

Is there any requirement that we "must" add the current limiting resistor to the other leg? logically I dont see why one need to go for the other leg, and doesn't matter if we connect it in either leg, but there could be a reason that I don't know yet

**broken link removed**



Following is from hackaday, which has resistor in series with the dropper cap


Thank you very much

Makes no difference which leg it's in - however, I would strongly advise against using transformerless power supplies!.

Nigel Goodwin said:

Makes no difference which leg it's in - however, I would strongly advise against using transformerless power supplies!.

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Hi Nigel,
Yes you are right, I also avoid TPS whenever possible., but due to size constraints I'm having, I'm unable to go for a transformer based solution. and smps solution is not feasible either

aruna1 said:

due to size constraints I'm having, I'm unable to go for a transformer based solution. and smps solution is not feasible either

Click to expand...

Have you seen the tiny modules that are available, such as these? SIL type or approximately "24 pin DIP" size:


There are many variations of voltage and power ratings in that type of thing, and they are cheap!

rjenkinsgb said:

Have you seen the tiny modules that are available, such as these? SIL type or approximately "24 pin DIP" size:

RS PRO Switching Power Supply, 3.3V dc, 600mA, 1.98W, 1 Output, 70 → 420 V dc, 85 → 305 V ac Input Voltage | RS

uk.rs-online.com

MEAN WELL Switching Power Supply, IRM-01-5, 5V dc, 200mA, 1W, 1 Output, 120 → 430 V dc, 85 → 305 V ac | RS

uk.rs-online.com

There are many variations of voltage and power ratings in that type of thing, and they are cheap!

Click to expand...

wow, didn't see these before.

but unfortunately as I'm living in Sri Lanka, obtaining these would be very expensive due to ruthless shipping charges

Do not make one of these. Use tiny transformer or smps. This is really bad design!
Hack smartphone charger instead

In the first diagram with the bridge rectifier, the current limiting resistor needs to be in the leg opposite to the capacitor dropper, for safety reasons. If the negative side of the power supply output is accidentally grounded, the resistor will act as a fuse and open the circuit.