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I have the diagram below. Wire 4 has a phase of 120VAC and N is the neutral. I need to test several lights that turn on individually each one using different contactors that are activated by sensors (simply switches) using a single switch called lamp test that when pressed turns on all the lights. Diodes are used in the diagram, but as I understand this, as it is in the diagram, it should not work because the alternating current is not constant but varies in time in a sinusoidal way.

How can I solve this so that the lights turn on with the test button?

enter image description here

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  • \$\begingroup\$ That's a clever circuit. \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented Nov 7, 2022 at 16:06
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    \$\begingroup\$ Looks like an exam question to me... \$\endgroup\$
    – Steve
    Commented Nov 8, 2022 at 3:39

3 Answers 3

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The lamp test button will work in your example schematic, however the diodes will only allow the positive half of the AC cycle through, so the lamps will not be at full brightness when being tested.

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    \$\begingroup\$ And the diodes will block the signal to one lamp from lighting any other lamps, because there are always two diodes of opposite polarity between any two lamps. \$\endgroup\$
    – Simon B
    Commented Nov 7, 2022 at 15:58
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    \$\begingroup\$ A lower brightness will probably be sufficient to test the lamps, if they are incandescent or of a similar type. If your lamps are common household LED E27-socket types ("non-dimerizable"/"não dimerizável"), then you might face some problems. Also remember to be careful when selecting the diode: it must support the voltage and current (with some margin). \$\endgroup\$
    – Ronan Paixão
    Commented Nov 8, 2022 at 1:04
  • \$\begingroup\$ +1@RonanPaixão - I suspect these are little indicator bulbs for which drop-in LED replacements have fairly recently become available, rather than room-illuminating bulbs. While we might hope that the manufacturers of said indicators have taken unusual circuits into account (a dimmed state to indicate idle/standby etc. isn't uncommon on panel indicators, and diodes like this could easily be used for that) I wouldn't be optimistic. Still, you should get some light, even if the brightness might not be well controlled or they might flicker; brief testing shouldn't appreciably shorten lifetimes \$\endgroup\$
    – Chris H
    Commented Nov 8, 2022 at 8:46
  • \$\begingroup\$ ... the originals could even be neon bulbs \$\endgroup\$
    – Chris H
    Commented Nov 8, 2022 at 8:47
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As previous answers have stated, diodes are used to block voltage between lamps when lamptest is not closed, and you don´t have so many individual lamptest contacts for each lamp. The setup works fine in DC, but AC supply will only feed half wave to the lamps, they will light at lower brightness when lamptest is on.

Following circuit is an example for 3 lamps, that will work in AC. 2 normally open contacts are needed for the lamptest pushbutton, and 2 diodes for each lamp. enter image description here

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If you have a single test contact to exercise all the lamps simultaneously then, you cannot directly connect the lamps together because, when you want to operate them separately and individually, they will be linked together and will all come on together. This is why diodes are used.

Of course, if you provided a test contact for each lamp then they will only be connected together during the test but, that's a lot more cost and trouble than using diodes.

If, as suggested, the power is AC then for sure, during testing, the lamps will be at half power but, that is not the main reason behind using steering diodes.

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