I have seen many 3 phase AC/DC flyback power supply specially in energy metering field.

most/all of them have a series resistor in the range of 50 Ohm between the AC lines and the

rectifier bridge, few of them uses only one resistor after the bridge rectifier.

by the way the flyback uses a cascode MOSFET configuration.

Does anyone knows why these resistor(s) are there?

My first consulting job was for a metering application. It's a little scary when you connect to a 480 VAC bus for the first time.

When you are hooking up a little flyback to unlimited amount of power such as this, it's not a bad idea to put a current limiting resistor in place. If things go wrong, the resistor is going to act as a fuse as well that may be another reason.

In metering applications, the line can be very raw and surges and spikes higher than in typical domestic applications. The 50 ohm resistor will help filter some of that.

Thanks a lot Ray, the problem is that sometimes when you connect the power to the meter these resistors burns out which could happen in the field and the meter will completely stop working. this very rarely happens with single phase.

anything special with 3 phase make this happen?

How to avoid meter run out because of such resistors?

Many thanks

Hi Mohamed,
the resistor (better in the order of 10-15ohm) is used to comply EMC (CE marking) testing (Surge).
Michael.

Hi Michele,

Would you explain more about how this is related to surge?

Thanks
Mohamed

is it better to use a PTC instead of resistor?

You should be careful about considering the input resistor a fuse at those voltage levels. You'll have to get special "fuzeable" resistors for that purpose. I've has resistors actually short under high overload condition and turn the resistive element into a ball of molten goo.

Mainly those resistors are for inrush current limit when the knife switch is used to apply voltage to the input. Otherwise you step charge the input capacitance and possibly damage them with high peak current.

mohamed at time a resistor like that of 50ohms is used to cut voltage spikes found on the power lines,but in most cases its resistance is higher than 100 ohms, or that resistor is a high voltage protector.

Are you using normal resistors rated for 200 or 300v ? Try using two of them in series. Also do a measurement of peak power dissipation to confirm margins.