DIm Bulb Tester - Current Limiter

I often work on repairs of different AC appliances and devices and after each repair, when the time comes to test the device, I always have that fear that I've missed something and that the device will cause a short circuit and a mini-explosion on my bench.

I happened a few times already and I almost always double-check that I don't have a short anywhere, but I wanted to make a device that can bring additional peace of mind and safety when doing repairs.

So I decided to build myself a Dim Bulb Tester and in this Instructable, I will show you how.

The tester adds an incandescent light bulb in series with whatever device that you are working on and it serves as a current limiter where it prevents a large amount of current to flow through the device.

• Conversion plug with switch - https://s.click.aliexpress.com/e/_9QAetj
• E27 light socket - https://s.click.aliexpress.com/e/_ADw4l3
• Soldering station - https://s.click.aliexpress.com/e/_AWSfjb
• Screwdriver set - https://s.click.aliexpress.com/e/_AMjsIl
• Multimeter - https://s.click.aliexpress.com/e/_ATaQop

I've started by first opening up the wall socket to inspect the inside.

The socket was closed with "safety" screws, the type where you have a middle piece in a flat head screw, so because I didn't have such a screwdriver, I made a notch in one of my regular flat screwdrivers to open it up.

On the inside, one of the pins is directly coupled with the output, and the other one is connected through the switch so whatever device si plugged in can be turned off.

The switch has an additional wire that is connected to the other pin so it can light a neon indicator that shows that the device is turned on.

Depending on the switch type, you may have different options for mounting the light bulb but since this is a small plug, I decided to mount the bulb externally and for that, I drilled a hole in the plug casing so I can then thread a wire on the inside.

I first wired the light socket and I've then threaded its wire through the hole that I made earlier.

The two wires coming in from the light will then be soldered across the switch contacts that it terminates.

Since they were chunky, the soldering iron struggled a bit to heat them up but eventually, they were attached with the help of some fresh solder.

When attaching the wires, make sure that you attach them to the wires that the switch breaks and not to the indicator wire as that will wire the light bulb in parallel to the plug.

With the wires attached, you need to make sure that you find the proper place for them to be routed inside the plug.

This means that the wires should not obstruct the normal operation of the plug, nor they should be pinched by any of the plastic parts or screws.

For me, there was a convenient space on the left of the plug so I secured them there and I then closed the plug.

Now to test that the plug works as expected we need to plug it in, and with a multimeter, we need to test the voltage in both switch positions.

If you did everything right, you should see the full AC voltage in the plug both with the switch on and with the switch set to the off position.

Working with mains AC can be dangerous so extra precautions need to be taken. The images with the exposed wires are for demonstration purposes and they should never be left exposed if you have similar connections. Mains electricity is dangerous and can easily kill you if you are not careful.

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Whenever you want to test a device with the tester, you need to plug that device in the tester and make sure that the switch is in the off position.

Now, when the plug is plugged-in, the current will have to pass through the light bulb and then to the device under test. In the worst-case scenario, if the device is short-circuited the worst that can happen is to have the bulb turn on at full brightness, but there won't be any explosions or serious damage to the device under test.

If the bulb is not lighting or if it is only dimly lit, that means that the device is not drawing too much current and you can proceed to check its functionalities.

If you are certain that there is no fault with the device, you can then turn on the switch on the tester and bypass the lite so the device can be tested directly without the current limit.

I hope that this device can be of great use on your workbench, and I'm sure that it will be on mine.

For more of my projects, you can check out the other Instructables that I have or you can subscribe to my YouTube channel for more electronics content.

Thanks for reading!