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Homemade Power Box

By on August 2, 2014
Home made power box







If you have read Gerald Musy previous article about Light Bulb (click the below link)

I’m sure you have noticed a power box was used to plug in the load (light bulb) and gain access to voltage and current test points. What is exactly inside the power box and how it was made? I have emailed him and he kindly shared out his experiences on how to built the power box. Let us check out his article below:


homemade Power Box

Time to time we need to measure voltage and current from mains connected equipment (240V here in Malaysia). I built the above box to make this operation as easy and as safe as possible. We know that mains electricity is dangerous and stripping wires can be hazardous. This power box proved to be so useful that it won a permanent position on my workbench…

The load to be measured plugs into the mains socket. The red and black jacks (in parallel but also controlled by the switch) provide an easy connection for a voltmeter. On the right hand side a jumper made of the two green banana jacks and a wire loop is connected in series. Here is a picture of the wiring inside:

 home madepowerbox

I added a 1A fuse and an extra pilot lamp. The extra lamp is to indicate presence of a voltage because I found the lamp built into the mains socket too dim…

The serial jumper with a wire loop allows measuring the current using a clamp meter:

Home made power box

Alternatively we can use an ampere meter. In such case the jumper needs to be removed. Be careful to disconnect the power box before doing this as the green banana jacks are not controlled by the switch:

 homemade powerbox

We can also connect a light bulb instead of the jumper for the famous “light bulb trick”, just plug the power supply to be tested into the mains socket:

 home madepower box

Please note the following:

1)    I drilled a hole into the back of the mains socket to connect the red banana socket after the switch. So there is no voltage when the switch is off.

2)    However, the green banana jacks will be life, even when the switch is off. I had no choice because the switch is built in the mains socket and soldered directly to the life contact. A safer alternative would be to use a separate switch. I might do this one day… For time being I am careful to unplug the box before connecting an ampere meter.

homemadepower box

In any case, be careful.







This article was contributed by Gerald Musy from Penang, Malaysia.


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  1. yogesh panchal

    August 2, 2014 at 4:53 pm

    Verry Informative and the tool that every should have on the bench
    thanks for sharing.

    • Gerald

      August 3, 2014 at 11:13 am

      Thanks Yogesh for your kind comments.

      If you build one please look at Erik suggestions below. Make it better and safer.


  2. Waleed Rishmawi

    August 2, 2014 at 5:17 pm

    I really like the idea. thanks for sharing. I like the light bulb trick you can actually plug a device in the machine and see if that machine shorted on not..wonderful.

    • Gerald

      August 3, 2014 at 10:49 am

      Hi Waleed, thank you for your comments. I usually read all your articles and comment and appreciate greatly your active participation to this blog. Always very good inputs.


  3. marco tapia

    August 2, 2014 at 5:27 pm

    I want to thank you and Gerald musy for this article and as always inspires me into following in your footsteps cause I feel you are amazing in electronic repair so thanks again for sharing your precious gift with us. 🙂

  4. Luciano Khware

    August 2, 2014 at 5:29 pm

    Mr. Gerald Musy,
    I am fascinated by your articles mostly because of their practical element! I surely learn a lot from you.
    Keep it up!

  5. Mark

    August 2, 2014 at 6:00 pm

    Hey Gerald,

    I was wondering about the box in the globe article.

    Can you tell me where you got the banana connector current loop from? I'm not sure what it is called and can't find one. I would like to make something similar to the box you have made.

    Keep up with the excellent articles - I always learn a lot



    • Gerald

      August 3, 2014 at 10:55 am

      Hi Mark,
      Thanks for your comments. The banana jacks come from the local electronic store. They are the fully isolated type.
      For the loop I use a two pins mains plug I bought from an electrical hardware store, shorted with a wire loop so we can use the clamp meter.


  6. Paulo Barbosa

    August 2, 2014 at 6:25 pm

    Very good job! However to be absolutely perfect you should have mounted permanently with nuts and bolts the lamp's socket on the top of the box with a bypass switch in parallel with it.

  7. Robert

    August 2, 2014 at 6:53 pm

    Thanks for sharing the box with us Gerald. That's a good tool to have.

  8. eRIK

    August 2, 2014 at 8:42 pm

    Look very nice your powerbox.
    Although i have 2 qwestions.

    You have to isolate all the 220V contacts with shrink-wrapping.
    I think it is better to build in the digital 240V - and Amp. meter.
    Because 220v on the bananaplugs and - busses is very dangerous
    Is it nicer to build in a 220v 50W Halogen lamp G14 model.

    regards erik blankers
    from Netherlands

    • Gerald

      August 3, 2014 at 11:10 am

      Hi Erik,

      Thanks for your comment and suggestions.

      Yes an inbuilt ampere meter and voltmeter would be the ideal solution. Mine is still the cheap version 🙂 But as I am the only one operating it I am very careful. This is also the reason why I am using a transparent mains plug with the light inside so I can see when it is powered from the mains. The plastic box is normally closed so no one can touch the life parts. But yes, shrink wrapping them is the way to be even safer. I will do that too.

      I will anyway put a separate switch that insulates the whole box. Probably this weekend... We are never too careful.


  9. Hank

    August 3, 2014 at 5:30 am

    I would like to add to this article that a wire (wrapped 10 times) and insulated will act as a (approxm.) times ten multiplier in addition to the standard wire - for those low current operations - a HVAC trick that I learned many years ago.

    • Gerald

      August 4, 2014 at 6:35 am

      Thanks Hank, excellent input.


  10. mike retz

    August 3, 2014 at 7:10 am

    great project!! you explain very well.. super usefull. measuring ac current is an art the electric companys dont want you to know.. there shoul be a meter on every appliance that draws a heavy load such as air conditioners hsat pumps stoves etc.. that way you could kind of tell what our electric bill would be..but no one could figure out the black magic formulas the electric companys use...thanks for a great wonder box.. mike retz dallas texas

    • Gerald

      August 4, 2014 at 10:03 am

      Hi Mike,

      Many thanks for your comment, I totally agree. There are now interesting devices to check how much electricity you are using. Here is one:


  11. Gopal Sharma

    August 3, 2014 at 10:34 am

    Useful article. But i simply use series bulb trick, which is sufficient for diagnosis of excess load current. This article is useful for experimental purpose like voltage, current, watt measurement at different load condition.

  12. Gerald

    August 3, 2014 at 11:15 am

    Thank you all for your kind and constructive comments. This is the way we can improve things.


  13. FredPick

    August 5, 2014 at 5:05 am

    You might look at a product called Kill-a-Watt I plug it into my vari-ac adjustable transformer you should use the isolated version,
    It reads out on a digital scale volts,amps,watts,and Hz and it inexpensive!
    Have fun!

  14. Joshua oloo

    August 5, 2014 at 3:02 pm

    Thanks for the Article Sir.going through your Article inspires me alot.Regards.

  15. K SGoh

    August 7, 2014 at 10:34 am

    The concept is good, but potential dangerous as LIVE voltage is easily accessible through two of the banana sockets.

    Suggest modifications (built-in lamp and a.c. voltmeter, or alternatively, use sheathed banana socket/plug sets) as suggested by eRIK.

    Other minor modifications:
    1) incorporate a separate siwitch to isolate line voltages from the connections.
    2) incorporate a switch to toggle between the built-in lamp or a short along the LIVE line (this saves the hassle of manually inserting the lamp or the short-circuit plugs)

    Overall, these modifications will minimise the risk of electrical shocks.

  16. Humberto

    August 19, 2014 at 7:38 pm

    Hi Gerald Musy. A very instructive article, as usual. Congratulations.

  17. Raymundo Saura

    August 27, 2014 at 4:11 pm

    great article very informative thanks sir


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