Home Made bench test equipment

[Theodosea]

I thought members would be interested in some bench test equipment I made up last year,

I picked up a bare 6Amp variac at a radio rally and made up this enclosure  - the steel cabinets are from TLC Electrical wholesalers and are around £30 - well made and perfect for these types of project.

The isolation transformer - using 1:1 toroidal transformer, for powering up unknown repair items safely, when I first built this I had a few problems with the transformer tripping my consumer unit - these toroidal transformers do create quite a surge on start up so I fitted a BC socket inside the enclosure with a 40watt incandescent lamp which I can switch in series with the mains incoming for first switch on, it absorbs the surge, I then  throw the switch and this shorts out the lamp ready for use. (it works well as long as I remember to set the switch!)

Also built a current absorption device - lightbulb! this is useful for test items that may be short-circuited.

I recently acquired an AVO RF135 Audio/radio frequency signal generator for a recent FM Radio repair and set up, but prob wont ever use it again as I have a Gould AF sig gen for general use. this is available for sale if someone can make use of it.

[Theodosea]

Re My last post - I did upload some photos but these apparently are too large? not sure what I can do about that?
Nick

[ppppenguin]

The lamp can work both as a surge limiter and a lamp limiter for powering suspect kit.

Don't forget that an isolating transformer removes any protection given by an RCD. Mark Hennessey, the forum admin, has written about this more than once.

In Windows and Linux there is plenty of photo resizing software available. In Linux Mint which I use you simply right click on the file name and there are resizing options. Free software such as Irfanview can resize images. I don't know about Mac. I think both Android and Iphones can do resizing.

And save as a JPEG file - PNG always gives large files with photos.

[Theodosea]

Thanks Jeff,
I think I have worked out how to reduce these photos! but it is only allowing me to upload four photos,

Regards Nick

[ppppenguin]

I've unpacked one of your photos from the zip, re-szied and re-attached it. Your original photo was 4.8MB. The resized version is 317kB, less than 7% of the size. The quality loss is minimal.

Nice, neat piece of homebrew kit.

[Kalee20]

Very nice, neat, piece of kit.

As for the switch-on surge, why don't you throw away the switch bypassing the lamp, and replace it by a relay - the coil of which is connected directly to the isolation transformer?

At switch-on, the contacts are always open, and the lamp in circuit, but as the surge disappears and the full voltage appears across the toroidal transformer primary (typically in a few cycles) the relay energises and bypasses the lamp. No operator intervention necessary!

[Mike Watterson]

A lamp is poor to abysmal for switch on surge as it can be 1/10th resistance when cold. This is why valve TVs had a thermistor that was highish resistance till it heated as the series valve heaters had a low cold resistance. Some older radios with a barretter (iron wire in a hydrogen atmosphere) to regulate current with varying mains voltage when the C series heater chain was hot also had a regular thermistor to stop cold surge.
A tungsten or halogen lamp limiter is simply a modern replacement for a barretter.

More compact thermistors that look like ceramic disc capacitors are sold to limit inrush surge on a SMPSU as these may have a bridge rectifier feeding 100 uF to 400 uF on the larger PSUs (100W to 600W).
I used to fit those in the light switch feeding 100W and 200W halogen lamps.

The thermistor for the degaussing coils on a colour shadow mask CRT usually has three terminals, because it is is very low resistance to the coils when cold but has a resistive heater to make the thermistor go high resistance, so a sort of turbo barretter and useless to limit in-rush currents.

[Theodosea]

Here is my Lamp Tester, and also photos of recently refurbed AVO HF135 signal Generator and Hameg 203-5 which are both available for sale if any one is interested.

Last year I constructed a two-valve radio from scratch - I will put up a post for this project when I have re-sized the photos! just learned how to do this!

Regards Nick

[boater sam]

Some protection for that now irreplaceable filament lamp would be a good idea. I have a tin can around mine.

Sam.

[Radio Fixer]

All nice neat construction and refurbs. Well done.

I never have bothered with a lamp limiter, just got stuck in replacing all the bits likely to go bang and then switched on. All been OK.

Gary