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Lamp Limiter / Dim Bulb Tester

Lamp Limiter / Dim Bulb Tester


One piece of test gear I’ve never had is a lamp limiter or dim bulb tester. 

They’re ideal for s-l-o-w-l-y awakening our vintage electronics up from years of slumber, without too much drama. 

I’ve been helping my neighbour, Steve, build a pub in his back garden, and while we were testing out some of the electrics, he donated a box of three incandescent 60w lamps to the cause. Ideal. You can’t use LED lamps, and halogen lamps are far from ideal.

Here’s the schematic.

It’s a simple enough circuit. The live side of the mains is supplied via two 60w lamps (in series), each lamp can be switched out as required. This will limit the current and give us a (very) visible sign if there’s any issue! 

I purchased a cheap energy monitor from eBay. With both lamps switched out it’ll give similar functionality to the Hopi or Killawatt testers. 

I got it from eBay from eBay seller topwoodfly, and it arrived promptly. It has it’s own current transformer, as you can see in the picture. 

It’s mounted in a box, along with the lamp holders and switches. 

I’ve put a change over switch on the input to the energy monitor, so we can measure the output voltage. I expect if the output voltage falls too low the energy monitor will stop functioning. I’m not expecting any miracles of accuracy here!

All wired up in a jolly shade of yellow, just to make fault finding on the unit as difficult as possible.

The neutral wire from the mains is passed straight from the input to the output, via the centre of the
current transformer. 

Here’s the finished box, running off the bench variac at about 138V

There’s a short circuit across the output, just for demonstration purposes!

I must say I’m actually quite impressed by the accuracy of the energy monitor!

Quite a useful addition to the workshop 🙂


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