Combination Isolation Transformer, Bulb Limiter with Bulb Bypass

[TomCarlos]

MEF members….

MEF has several posts on Isolation Transformers and Bulb Limiters. I wasn’t sure which would be the best post for this follow up. But since it is a combo device, I decided to open a new thread. If you want more info on either device, please look for those threads.

I wanted to share something I built, based on prototypes from Terry Dayton’s (How to Modify Re-Purpose Heathkit IP-12 DC Power supply into a bench AC Variac, D-Lab Electronics) and Ron’s Vintage Electronics “AC Variac and Dim Bulb all in one Episode 1 and Episode 2.

DISCLAIMER – Like many of you, I am a hobbyist. So use extreme caution when working with these types of devices. Please view the Todd Fun video on Isolation Transformer modifications. There are other YouTube videos on the same subject (Uncle Doug, Mr. Carlson’s Lab, and a few others). BE CAREFULL !!!

The end result of my project is a single box that contains an (1) Isolation Transformer and (2) a Bulb Limiter with a bypass switch. Since I already have a Variac, I left that unit as a stand-alone and this is where I connect the Iso/Limiter unit. I had an Iso Transformer that I salvaged from a non-working Viz unit. The variac part was bad, meters not working, etc. But the Iso Transformer was ok. You can read my notes in the Schematic Diagram I posted.

Having the analog Voltage and Ammeters is very helpful. The meters are not exact, but they do give you helpful information for the device output. Adding the bulb bypass switch is also helpful. That way, I do not need to keep disconnecting and reconnecting the Device Under Test to and from the Bulb Limiter.

Sorry for the messy schematic, but I think you get the idea!!

[The Dude]

That Heathkit reminds me of the IM-5284 analog multimeter I built in tech school. I still have it!

[TomCarlos]

I should add, I removed the original faceplate from the Heathkit and replaced it with a plastic one, something I could easily drill and cut into.

[TomCarlos]

MEF members and site visitors.....

I am helping a friend build a similar unit to what I show below (image and schematic). And I thought it would be a good idea to cross reference other posts related to Bulb limiters and some specific comments. I hope the links will work!! So here we go...

MEF site: Light bulb current limiter behavior

MEF site: I've been reading about using a light bulb limiter

Marshall Forum site: Dim bulb tester or limiter

An FYI to level 1 techs and hobbyists (including myself) ... On each of the threads, you will find some suggestions regarding the type (bulb power in watts) of bulb that could be used for different amplifiers. For example, Jon Snell posted the following on the Marshall site:

A 40W lamp covers all valve amplifiers with the valves unplugged ; checks for short circuits.
A 100W lamp covers most amplifiers.
A 150W lamp is for larger power amplifiers.
Bearing in mind, the amplifiers cannot be run up at high power.
If running up a Class D or Switch Mode Power Supply, the light bulb limiter cannot be used as damage will occur.

Also on the Marshall site, pdf64 posted the following:

I suggest to have a range of bulb wattages available, start as low as is feasible for the application, and work up.
I think it may have been me that put the ‘bulb wattage twice amp wattage’ guideline out there. It should workout ok for most typical class AB amps.

So if you have specific questions, ask. And you may want to experiment with the usage of different bulb wattages.

[DrGonz78]

Look at post# 54 by Roly. He was an Australian man with a ton of knowledge. Sadly he was diagnosed with terminal cancer around this time. Juan M Fahey started the thread back in 2011! Uncle Doug even updated his video with a note about using lower bulbs with Solid State gear. Still a 300 watt bulb on a limiter is not protecting much. I have bulbs for every occasion. 40-150 watt bulb and I have all I need. But. What if I get some 12v bulb to limit a lower voltage 12v rail that is blowing those low voltage fuses? Limiters are great

[Greg Robinson]

I find juggling a collection of bulbs rather impractical, especially now they're increasingly difficult to find and purchase, as well as their fragility. Instead, I have a whole bank of 75W halogens inside a 2U 19" rack, with switches to select how many are in circuit, a master on/off switch and bulb bypass. I've also got a "killawatt" type monitor which displays voltage/current/wattage/power factor. I modified the killawatt, as they typically use the same input for self-powering and voltage measurement, it was simple to seperate those and feed the raw 240V into the power, and the measurement input from post-lightbulbs, else the killawatt may not turn on or lockup if the output voltage is too low. The rack is not perfectly sealed so plenty of light escapes for me to see if a bulb is bright (plus I have the killawatt to tell me what's going on). People often ask about heat generated by the bulbs in an enclosed space, but if a bulb is bright and generating heat, you've got a short circuit and should turn off the device under test anyway, so it's not a problem.

[nevetslab]

Very cool box! I've been meaning to build something like that, but with so many ongoing projects and repairs, I just never seem to get to the list I have for 'laterman'. I did recently inherit a 120V to 240V Step-up/Step-down transformer that a client brought along. It was partially broken inside and was a simple fix to retore. Low current, suitable for 40-60W Combo amps and such. I did have a Superior Electric 0-280V 15A 'Variac' but ended up selling it. Probably should have kept it but was strapped for funds back then. Still have an Elgar 1127 International AC Power Source 0-260V/5-10A 47-500Hz power source that I've still yet to get around to repairing (also on the Laterman list). Picked it up at a Radio Amateur's Technical Swap Meet at TRW in Redondo Beach, in the days before ebay. Bought it in 1994 for $25. Front panel was fitted with 240V outlets common to European and other countries around the planet.