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Deleted member 105166

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Was at a small event recently providing generator power & distribution to the usual line-up of PA system, bar refrigeration, coffee stall, etc. plus a photographer's tent. The photographer had two budget Godox studio strobes.

I place a multimeter at the distro on AC voltage min/max and check periodically for early warning of any irregularities.

I understand this type of flash strobe works by discharging a voltage of circa 1,000V for a small fraction of a second, and would imagine a diode protects the supply from being back fed.

Each time the strobes were discharged, I was metering a momentary spike of circa 30V at the distro.

I've given power to photographers previously and never encountered this... my thinking is that one (or both) of these budget strobe units had poor/leaky diodes allowing the capacitor voltage to back feed.

Does anyone have any other thoughts as to the cause or similar experiences with this type of equipment?

The photographer was blissfully unaware and didn't really understand the problem - I told him if I metered >265V, that I'd disconnect, fortunately for him, it didn't - quite! The PA system circuit was feeding a power conditioner and the other circuits didn't support anything sensitive, hence setting 265V in my mind as the threshold I'd allow.

Deleted member 105166

  • Thread Starter Thread Starter
  • #3
What's a distro?
What's the issue with a momentary spike with equipment being switched on?
Hi Charlie

Distro - temporary distribution board, like pictured in my avatar

The spike was occurring every time the photographer flashed and was increasing the supply voltage on other users’ circuits

Lucien Nunes

What were you measuring with, a multimeter? What size genny with what type of AVR? I'd say one of two things:

If the genny is smallish, and the power supply in the flash units is very crude with a low power factor (either capacitive or distorted) it might interfere with the regulation. A strongly leading power factor causes an excitation boost, and a half-wave rectified input current can cause unpredictable effects. But for this to happen, I would think the flash input power would have to be a sizeable fraction of the total genny load.

Alternatively, what you were seeing was not really an increase in AC voltage but your meter being disrupted for a moment by an impulse of interference, possibly only microseconds long, radiated by the flash due to skimpy filtering. If you had a filament lamp connected, its behaviour would have suggested whether there was a meaningful change in the voltage. Other than that you would probably have had to watch it on a scope to understand what was happening.

I don't think 'back-feeding' as such is a thing. If the flash unit does charge a reservoir cap directly from the mains, any failure of the input rectifier would be instantly self-destructive and the unit would not function.


This is a prime example where a digital multimeter has you looking for a non existent problem and highlights the test equipment's limitations
The trouble is the instruments more suited to this task are much more expensive

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