Lamp glowing faintly between exposures

Owned by Break/Fix: the timer that lies and by switching lamps safely, which carries the arithmetic this entry summarises.

This is a fault in a bought instrument, not in anything you built. The course builds no relay and switches no mains: its timer drives a low-voltage LED head, and where an enlarger’s mains must be switched the course sends you to a certified enlarger timer with its own switched socket. So the device that can leak here is one you bought, and the fix is not yours to make.

What you see

The enlarger lamp is not quite dark between exposures. In a properly dark room, after your eyes have adjusted for several minutes, the filament shows as a dull red or orange thread rather than as light falling on the baseboard. You will usually notice it when you stop looking at the easel and look at the lamphouse.

What you will not usually see is a print that proves it. That distinction is the whole entry.

Likely causes

• A solid-state switched output doing exactly what its datasheet says. Every SSR passes a small current when commanded off: Omron quotes 1.50 mA at 200 V ac for the G3MB and 5 to 10 mA for the larger G3NA.
• A device that has failed, passing far more than its rated leakage. A datasheet describes a working part.
• A load that is not a tungsten filament. An LED driver, a neon indicator or a small transformer is not a resistor, and may respond to milliamps in ways the filament arithmetic says nothing about.
• A neon indicator wired across the switch, which is a design choice rather than a fault, and which puts a small current through the load by construction.

What is happening: the electronics

A solid-state relay switches through a semiconductor, and a semiconductor commanded off is a very large resistance rather than an open circuit. Omron’s own precaution matters more than the number: the leakage flows through the device’s snubber circuit even when there is no power at its input, so the load side is treated as live until the supply itself is proved off.

Whether that leakage can fog paper is a separate question, and Part XVII does the arithmetic because the claim is repeated everywhere. A 150 W lamp at 240 V draws 0.63 A and has a resistance at its operating temperature of 384 Ω. A leakage of 1.5 mA develops 0.86 mW in it; 10 mA develops 38 mW. Against 150 W those are 6 and 256 parts per million, and a cold filament’s resistance is lower still, which makes the dissipation smaller rather than larger.

=The arithmetic says no, and the arithmetic is not the last word

On the makers’ own figures a tungsten enlarger lamp on a switched-off solid-state relay receives tens of milliwatts at most, and does not reach a temperature at which a filament emits visible light. That is this course’s calculation from published figures, not a measurement, and not a general rule about every product.

It can still be wrong on your bench, for the two reasons above: a different load is not a 384 Ω resistor, and a faulty device is not a datasheet. So the test below is a sheet of paper and not a meter.

Diagnostic questions

1. Is the glow visible on the filament, or on the baseboard? A visible filament and no measurable density on paper are consistent with rated leakage. Light on the baseboard is not.
2. What is the load? If the enlarger has an LED head, a driver or a transformer, the tungsten arithmetic does not apply to it and the paper test is the only evidence you have.
3. Does unplugging the enlarger from the timer stop it? That separates the switching device from everything downstream of it.
4. Is there an indicator lamp across the switch? Check before condemning the switch.

Corrective action

Prove it with paper, not with an eye. Lay a sheet of the paper you print on under the lens, cover half of it with card, and leave it there for the length of a real printing session with the timer powered and the lamp commanded off. Develop it as you would a print. A step you can see between the covered and uncovered halves is the answer; no step is also the answer, and it is the one the arithmetic predicts.

If it fogs, take the enlarger’s plug out of the timer between sessions. That is the fix. Do not open a certified mains appliance to modify its switching, and do not substitute a device of your own for its output — switching lamps safely sets out why the course routes mains switching to a certified product in the first place.

If the paper test is clean, the glow is a cosmetic annoyance and you may ignore it. Recording that you tested it is worth more than the test, because the next person to notice the glow will otherwise run the same session again.

Prevention

Unplug rather than trust an off state you have not tested. Where the enlarger is a low-voltage LED head, the course’s own timer switches the low-voltage side and the question does not arise.