I am looking for a suitable series resistor (220k) for a 230VAC optocoupler input.

The device must meet requirements from EN60664-1. I am still trying to understand the final requirements, but from what I figured out so far, it must withstand a transient over-voltage of at least 5000V.

I was looking at some data sheets like the PO Series from VITROHM. For short transients, there is a maximum pulse voltage of 2000V specified.

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Is using this resistor really a no-go or what exactly happens when this voltage is exceeded? If it was just about power dissipation, it would depend on the resistance.

I was not able to find any resistor with a higher rating. I have kind of a reference device (no schematic) and there are just some power resistors (unknown type) placed on the opto-coupler input as well, without any other protection/components.

Any suggestions how to design this?

  • \$\begingroup\$ I wanted to measure the pulse energy going into a xenon flash lamp when fired, so made a ladder of twenty 2W one-ohm film resistors in parallel (0.05Ω 40W.) Dumping kilowatts into it worked about a dozen times... before the metal film started cracking and blowing off. \$\endgroup\$
    – rdtsc
    May 2, 2018 at 12:25

1 Answer 1


The PO596 is quite small and is suitable for 1400 volts so maybe use 4 in series to give you the 5kV requirement. Clearly, the resistance for each has to drop to 55kohm or thereabouts. All four resistors should be nominally identical in value.

Without the relevant section of EN60664-1 spelled-out anyone reading this question is probably going to be "in the dark" but maybe you have an internet source?

The name of the spec is: -

Insulation coordination for equipment within low-voltage systems. Principles, requirements and tests

Based on this I would say that your interpretation may be flawed - the 220k resistor in series with the opto is, presumably connected AC wire to AC wire (live to neutral) and this has no relevance as "insulation". It isn't an insulator.

  • \$\begingroup\$ I only have the German DIN 60664 at hand. I thought that the insulation testing would probably require some burst/surge testing on all inputs. I just wanted to design it with that in mind. Its an electrical meter, so it is also potential subject to transients directly from the supplier net (its in a higher class than normal household devices). You may be right that the before mentioned EN does not specifically require the live to neutral insulation. \$\endgroup\$
    – Rev
    May 28, 2015 at 11:05
  • \$\begingroup\$ And you are right about the resistor in general. I was looking for 0.5W-1W resistors mainly, because thats sufficient for the application. However, it seems one has to go above that to also get higher voltage ratings. \$\endgroup\$
    – Rev
    May 28, 2015 at 11:08

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