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Control panel 3 phase sequence indicators

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dr pepper

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Mar 15, 2010
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I'm a industrial controls electrican.
Years back when I started this kinda work some control panels had 3 neons on one end, which flashed in sequence, I'm not sure if they just indicated the presence of the 3 phases or actually indicated correct sequence.
Does anyone remember these?, the circuitry was probably simple using just R's & C's, I'd like to re create one maybe a low voltage version with leds, probably ott I was thinking of 3 logic counter dividers to flash leds at much reduced frequency, and reset 2 of them when one phase goes through zero.

Are you talking about synchronising two 3 phase systems ? e.g. generator and mains - if so the " 3 lamps " method is very good for checking operation of automatic synch & close systems ...

This is done with one lamp U-U, and the other 2 V-W and W-V

at steady state synch, the U-U is effectively off ( same volts both sides ) and the other two are max brightness.
at near synch if placed in a triangle the light appears to revolve slowly ...

neons showing zero cross will flicker at 100/120Hz - so no earthly use to human eyes ...

3-stage chaser or sequencer circuit turns led's on and off. Self-oscillating.
Uses 3 NPN, 3 capacitors, 6 resistors. Low voltage.
(My simulation has 2 led's lit at a time. By mounting led's across transistors you can have 1 led lit at a time.)


Up to 3 neon bulbs arranged in simple chaser circuit. Power supply is 90 VDC.
Uses 3 resistors, 2 capacitors.

Never seen them used with gen sets, but I spose you could.
It seems these were not all that popular.


Similar ideas, for LEDs, not thought about details or implemenbility much..., what about dividing down AC phases, and isolating with optocouplers, then:
a) quad comparator, three triggering on zero crossings, fourth resetting other two.
b) three 555s as one-shots triggered by falling edge of sine waves and outputs lasting e.g. 333ms, lighting LEDs momentarily at least, using third 555 to reset the other two maybe, or a bjt with CR on base to deliver short reset signal.

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