Working on an compact electronic ballast design (~5 W load), I see the need for an over-voltage protection circuit that has a 'slow-blow' characteristic. When a lamp is turned on, a striking voltage above steady-state is required for ignition, say for a few milliseconds. But when the lamp is at end-of-life or not connected, the open-circuit voltage persists indefinitely.

ON Semi's AN1543 provides some examples on page 15, the one below being the simplest.

ON Semi's over-voltage circuit

Question: is there a single device, or simpler implementation, for this application? An answer has already been given recommending varistors/MOVs, but all that I find online seem to be instantaneous. I assume that if there were a single device then it would be more common, but I'd like to ask anyway.

I thought that perhaps a slow-blow fuse plus resistor combination in parallel with the lamp might do the trick. But deviating from protection devices' intentional use gives me hesitation.


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ Is an ANSI 59 time overvoltage protective relay overkill? \$\endgroup\$ – schadjo Aug 1 '18 at 20:43
  • \$\begingroup\$ @schadjo Unfortunately, yes, it is overkill. These ballasts will be about 6 cm^2 slash 4 in^2, and hopefully pretty low-cost \$\endgroup\$ – calcium3000 Aug 1 '18 at 21:17
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    \$\begingroup\$ Considering the arc threshold and V vs Temp characteristics with controlled impedance changing wildly, and a need for arc strike to be stable before alternating currents being regulated, I doubt there is a simpler method. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Aug 1 '18 at 22:56

Unfortunately this circuit is not an overvoltage protection.

During regular operation it uses additional energy.

When the fuse triggers because of overvoltage, nothing will be protected. You'll simply have a broken fuse, at least it will indicate that an overvoltage situation has occured.

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