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Test Equipment Isolated Variac lamp limiter

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Evening all,

For quite some time now I had wanted to have an "all in one" solution for an isolation transformer, variac and a lamp limiter, not having owned or used a lamplimiter before, they sound like a good idea. I have always used the "bring it up slowly" method with the variac while keeping your eyes peeled for anything going wrong. I also wanted to have some meters built in so I can monitor whats going on without using my bench meter that may well be being used for something else.

I will state from the start that this project is probably a "way over the top" design, but I was doing it for myself and using parts I already had in stock, so it was mainly designed around those parts. I did still have to buy some things in, but nothing like the cost it would of been if starting from scratch*.

As building and designing test rigs is my day job, I thought I would use my skills to design something for myself. I made a list of basic requirements that I wanted to have and started from there. These were as follows:

  • A variac for adjusting the voltage supplied to the DUT.
  • Volt meter to measure outgoing voltage.
  • Ammeter for measuring current drawn by the DUT.
  • Isolation transformer.
  • Switchable Lamp limiting.
  • 13 Amp socket and Safebloc for output connections.
  • Able to supply 3.5 to 5 amps.

I set about drawing some block diagrams and basic schematics, but this was a bit of a pick it up/put it down task as and when time allowed. After some time I had completed this piece of work. As with most things like this, the more you think about it the more things get added to the mix and by the end of it, the list had grown somewhat. The list of features are now as follows:

  • A switch to select between live mains, isolated supply and off. Mains earth is also disconnected durring isolation mode.
  • A variac ouput switch to select between 100% or 112% output tapping.
  • A variac bypass switch to remove the variac if required.
  • A lamp limiter with 6 lamps that can be switched in any combination of 25, 40, 60, 100, 150 and 200W.
  • A lamp bypass switch to remove the lamp limiting if required.
  • A voltage/frequency meter to monitor incoming supply.
  • A volt meter to display the voltage from the variac.
  • A volt meter connected after the lamp limiter so the volt drop/difference between the variac and what the DUT is actually getting can be monitored. This only shows a different voltage if the limiter is in circuit.
  • A ammeter to measure the current drawn bu the DUT. Connected via a shunt.
  • Various panel indicators to show what is selected.
  • Relays for switching of the lamps in the limiter.
  • Isolated PSU's to power the meters and relay circuits.
  • Fuses for the variac, isolation transformer, metering and output.
  • A fascia panel with a clear legend for all the switches, indicators, meters etc.

While I'm aware I could of built all this using point to point wiring, I decided to make a PCB for the connections to the various parts of the circuit and for the relays and relay drivers circuits. The PCB is a double sided design with all the mains/high voltage tracks on the bottom side and all the low voltge tracks on the top side. All connections are made via soldered connection to terminal pins. Any pins carrying high voltages only have pads on the bottom side. All connections are soldered and are either heatshrinked or sleeved.

The nex job was to work out and design how big the box would need to be and where to place all the parts inside and to give me a layout that I liked. I used Design spark from RS for this task along with the PCB and later the fascia panel design. I find Design spark a very useful bit of software (not just for PCB layout and schematic capture) and I've yet to find anything I can t do with it and it's free.

I measured and drew all the components so I could move them around and place them. Once this was done and I had a layout I liked, I had the dimentions for the box.

I had originaly wanted the lamps for the limiter mounted on top of the box, but now knowing the size, I wasn't going to have room for this, so I decided to mount the lamps on a seperate box that could be wall mounted and connected via an umbilical cable with a 12 pin Burndy plug and socket so it can be disconnected if needed.

Now with me not being very good when it comes to cutting wood with any great accuracy, I decided to farm this job out and found a company on the web for this and I went for 15mm plywood.

With the weight of the variac (8Kg) all being placed on the front panel I went for a box that was in two pieces. Base, front and left side making up one part and the rear, top and right side making up the other. This gave me strength for the front panel to support the weight of the variac and allowed everything to be fitted in the box and wired up with the ability to remove the covers and leave it in a working state should the need arise.

While I was waiting for the wood to arrive I could work on the layout for the fascia panel, with reference to my previous drawing of where everything was to be placed. Design spark was pressed back in to action for this job. It wasn't long before this was finished. The Gerber files for the PCB and the fascia panel were sent off to be manufactured.

Upon receiving the plywood, fascia panel and the PCB the build could begin. The first task was to use the fascia panel as a template to mark out the front the panel ready for drilling of all the holes and rebating. I used transfer punches to mark the centers of all the holes and once drilled I could rebate enough material on the rear side to allow the switches, fuse holders and indicator lamps to be mounted through the front panel and then be secured to the fascia panel. An area was removed completely for the panel meters to be mounted to the fascia panel only. With this completed I could glue and screw the two halves of he box together. While the was going off I fitted all the components to the PCB.

With the box dry the first job was to mount the variac to the front panel, followed by the fascia panel, switches, fuse holders, meters, idicator lamps 13A socket and Safebloc. Then inside the box the isloation transformer, current shunt and PCB were mounted. Now the wiring can begin. I used 1.5mm2 cable stripped out from old mains flex for all the high voltage/current carrying and 16/0.2 mm for all low voltage wiring.

With the wiring complete and cold checks done, fuses were fitted and it was time for the first power up and switch on! (If only I had a variac to bring it up slowly 🤣) Any way all went well and I was quite surpised and delighted that it all seemed to work as expected.

The eagle eyed will notice that the Safebloc is mounted upside down (so to spaeak) this was done for a safety reason, but ended giving a second benefit.

  • It stops accidental closure (the main reason for mounting it that way).
  • It makes it easier to connect wires to the terminals.

After using it/playing around with it, a couple of minor bugs needed addressing, nothing too serious but I wanted them fixed.

  • Primary side of the isolation transformer was live regardless of the main switch position.
  • The meters stayed powered up even when in the off position.

Both of these issues were quickly fixed with a small modification to the wiring around the main switch, all is now well.

After a bit of use one more bug/issue/metering problem has shown it's self and I'm currently working on a solution for that. There is a discrepancy with the output meter and an external dmm if used on the supply to the DUT when the lamp limiter is in circuit and you are driving a half wave rectified load and I believe its down to the meter in my box being of an averaging type that is setup to maeausre RMS volts, when on load the AC waveform is distorted due to the rectifier and is no longer a sine wave. So I'm looking at adding a true RMS circuit to the meter.

*Since building this a few pound could be saved by using a much cheaper relay and the meters I used are now available with an isolated power supply built in, so the need for seperate supplies is not required now.

Anyway I have rambled on far too long. Here are a few pictures of what I have built.



1 Original block diagram
Placement drawing
Pile of Ply
Box mock up
Fascia panel
Front cut out
Ready for wiring
Let there be light
Final Block

BVWS Member

Posted : 04/03/2024 3:54 am
slidertogrid and Lloyd reacted
Posts: 552
Honorable Member Registered

A lot of work in that. Very impressive.

I guess the monster isolation transformer is needed to handle TVs with half-wave rectifiers.

We are SO fortunate here in Oz that we don't need them!

I just plug different bulbs into my extension lead type dim bulb! And I no longer own a variac.

Posted : 04/03/2024 9:21 am