Building a Dim Light Tester


Its time I addressed the question of safety when working on valve technology. These pieces of equipment can be very dangerous, the voltages involved are substainial and the design of the equipment, whilst being satifactory for the standards of the day, would not be considered Ok by todays standards. There are hazardous voltages everywhere in these devices, including mains level AC voltages and very high DC voltages. Some equipment, although not common in Australia, does not include a mains transformer so the circuit can be at mains potential.

If you do not know what you are doing, DO NOT WORK ON THIS EQUIPMENT, you do so at your own RISK.

OK so to this project, this project is designed to improve the safety for both myself and the devices I am working on.

Protecting myself

So for protecting myself, I have included a mains isolating transformer which is basically a 240 VAC to 240 VAC transformer. The effect of including this transformaer is it removes the reference to earth from the mains potential by sitting between my bench mains outlet and the device under test. Without this, if I was to accidentally touch a terminal connected with mains active, I could be in very serious trouble. With the transformer both the mains wires to the device are floating so it affords a degree of safety. 

It should be understood that there are still risks, if you put yourself across the output of the transformer you are at very serious risk and this could be fatal!!!. The general rule of thumb is still assume everything can kill you and only touch the device under test with one hand at a time. This makes it very difficult to place yourself across the mains feed to the device.

Protecting the Device

So for protecting the device, this is where the dim light component comes into play. There are plenty of other circuits available on the web and You Tube however they are all basically the same. To afford a degree of safety for the device under test we basically limit the voltage and current to the device by placing a light globe in series with the mains feed to the device. In my case I have provided for two different wattage globes, thius in effect gives 3 levels of limitting, wither one ot the other glode in series, or both. If an additional level of limiting is required, it is simply a matter of replacing one of the globes with a different wattage.

The beauty of using light gloges is it firstly very simple and secondly provides excellent feedback on what is going on in a visual manner. For instance, lets consider there is a direct short in the device, as soon as the power is applied all the voltage will be across the light globe and it will glow at normal brilliance. Right away you know something is very wrong and can shut off the power. The brilliance of the globes gives instant feedback on how much current is being drawn so you can make quick decisions.

PK so the completed circuit is below and I recommend you take a look at my You Tube video, link below, as I describe the circuit in detail. I also cover off on the construction in the video and show it operational.


Checkout my You Tube video for more information and how it was constructed.

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