It seems that when choosing ESD/TVS diodes, MOVS and GDTs that while their clamping voltage is rated in Volts, their protection is rated in Watts or Joules.

For example, the LittleFuse SMCJ250CA is 405V Clamping and 3.7A Current. On the front page it states "1500W peak pulse power capability at 10/1000μs".

This makes sense I suppose.

But then the standards I have been reading only talk about transient/surge voltages. For example: 1kV Surge at 1,2us/50us (8/20).

Since the standards I have read are only referencing voltage not current or power. How do I use this information to select a protection device which talks about Watts? Is there some standard current values to assume?


  • \$\begingroup\$ That is a good question. Could you tell what standard that is? \$\endgroup\$
    – jay
    Sep 2, 2021 at 1:21
  • \$\begingroup\$ The standards will specify a source impedance or other information so you can determine the available energy of the surge. As to how much energy is dissipated in the load (ie protection device) depends on the load and the time. The wattage of the protection device is effectively a thermal rating. Too many Watts for too long and the device will overheat. Way too many Watts will melt the silicon or bond wires. \$\endgroup\$
    – Kartman
    Sep 2, 2021 at 2:12

1 Answer 1


For example: 1kV Surge at 1,2us/50us (8/20).

That sounds like EN 61000-4-5 (indirect lightning surges) and, if so then the output impedance of that surge will be specified. In most situations, this will be 2 Ω hence, the peak current into a short circuit could be as high as 1 kV / 2 Ω = 500 amps.

The worst case power delivery is when the suppression device looks about 2 Ω - it will dissipate a peak power of half the volts squared divided by 2 Ω. That's 125 kW. OK, it's only for a short duration but, your device (SMCJ250CA) or, any device in that range is incapable of surviving that for a 50 μs surge duration: -

enter image description here

You need to have a much heftier TVS to survive EN 61000-4-5 if you are trying to protect a power port (2 Ω source impedance) from surges.

choosing ESD/TVS diodes

If you are only interested in ESD then you might find it works but I'm not going to guess what specification you might be considering.

  • \$\begingroup\$ "That sounds like EN 61000-4-5 (indirect lightning surges) and, if so then the output impedance of that surge will be specified" - just to be clear, by that do you mean that EN61000 specifies the 2 ohm figure you are to use to calculate component ratings to use? \$\endgroup\$
    – Sean
    Sep 2, 2021 at 7:45
  • 1
    \$\begingroup\$ @Sean for power ports 2 ohms is the figure stipulated in 61000-4-5 \$\endgroup\$
    – Andy aka
    Sep 2, 2021 at 7:47

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