Okay, a few weeks ago I bought a Farfisa Compact Deluxe organ from the 60s. At first glance, it’s a substantial project. There’s a fair amount going on with it. Now, when switching on older equipment where I wasn’t sure everything was in good order, I’ve always approached this somewhat differently depending on the device. I was looking for a controlled way to power up a device for the first time without causing consequential damage. You never know what might be broken or whether there’s perhaps already a short circuit. Even after a repair, it can be useful to test a device in a controlled way to check, for example, that you haven’t installed any capacitors backwards. With a dim bulb tester, the lamp limits the current going to the device under test to a safe level, instead of immediately getting smoke, a blown fuse, or worse.
How does this work?
A dim bulb tester reduces the current to a test device by acting as an automatic safety resistor in series with that device. This causes the voltage to divide, and the maximum current is physically limited by the incandescent bulb.
Below is a diagram of how you could build one yourself. I used this simple design and made my own dim bulb tester. A simple little project to do yourself, and I think it’ll pay off a lot for me.
Some tips:
Bulb type: use an old-school incandescent bulb it never an LED or CFL bulb, since they don’t behave as a simple resistive element and won’t limit current the same way.
Clear vs. frosted: a clear bulb lets you see the filament brightness clearly, which makes it much easier to judge current draw at a glance than a frosted/matte bulb.
Wattage selection: choose the bulb wattage based on the device’s power rating. Too low a wattage will starve a normally-operating device of power, too high won’t limit fault current effectively.
