Light Bulb Limiter

[J M Fahey]

Some images:



I don't like the switch bypassing the bulb shown in the lower image, simply because it's too easy to switch it on by mistake.
Suggested bulb: 25W for small SS amps (10/30W) ; 100W for high power ones (60/200W); a tube amp might require a 150W to 200W bulb, simply because it eats too much on idle, because of all those filaments.
Most sites state that the bulb burns if a short is present; it does not; worst case it shines brightly, with full power.
If the amp is fine, the bulb usually lights barely visible red.
It allows to measure many things (do I have power rails? do I have DC on the speaker out?) which a fuse would not let you to, because it burns in a millisecond.
Remember all voltages will be low, the bulb is eating part of the wall voltage, do not aim for "schematic perfect" voltages but a reasonable proportion.
Example: power amp /main PSU states +/-40V DC
If I have a dark red or weak orange bulb, measure +/-25V, have no DC on output jack, amp is probably fine.
If I measure +/- 1 or 2V rails or +/-25V but have 24V on speaker out: bad sign.
In this last case, if I have 49V across the lower output transistor and 0.12V across the upper one, I can be pretty certain that the upper one is shorted, which will be confirmed later with an ohmmeter test with the amp off .
That's the general idea.
Much faster and better than:
replace one transistor -> power on -> blown fuse ... replace another -> power on -> blown fuse ... (repeat 25 times) power on -> nothing burns ... so THIS was the bad transistor !!!! 

[joecool85]

Thanks Juan, I think I will rig something like that together for testing purposes.  In all reality I probably should have done this long ago but I've been lazy - time to get my butt into gear

[joecool85]

Alright, I finally built one.  I've attached the pics.  It is an old "spot light" interior lamp that I had while in college and hadn't used in several years.  I disassembled and then reassembled as necessary.  Now the "globe" the bulb used to rotate in to aim it is now a shield to keep the bulb from getting broken by my dog and my future children.  Works really well.  Just barely glows with a soldering iron in, blips for a second when my Vox turns on and shines brightly when shorted with a piece of wire.  I used a 75w bulb since that's what I had on hand.

[J M Fahey]

As the cat said: "it's purr-fect"

[JPHeisz]

Nice!
I built mine into my homemade lab supply. There is a plastic tube that channels the light to the front panel:

[joecool85]

Nice!
I built mine into my homemade lab supply. There is a plastic tube that channels the light to the front panel:

How many watts is that bulb?

[JPHeisz]

It's 60 watts - good for most everything I do. Convenient and out of the way, it's always ready. It feeds an outlet on the front of the supply, top one is limited by the bulb, bottom one has unlimited mains voltage.

[Casey4s]

This is just my second post here, but I couldn't help notice this thread.   I have used a ligt bulb limiter several times, it's a quick easy way to safely control startups and testing.

Here's my take on the limiter...

[rowdy_riemer]

Sorry for the late comment, but this is cool, JM. 

[DrewV]

I built one a few years ago. Works like a charm. I wouldn't want to make smoke without it! dv

[polo16mi]

It´s a great stuff this bulb. Saves me (at least) several fuses today. . I see trouble for it at future.... no more regular bulb in our country, ..just compact fluorescent lamp...

[joecool85]

It´s a great stuff this bulb. Saves me (at least) several fuses today. . I see trouble for it at future.... no more regular bulb in our country, ..just compact fluorescent lamp...

I don't know about where you are at, but here in the US they are getting rid of standard incandescent bulbs, but not halogen bulbs.  There is no reason a halogen wouldn't work just fine.  Also, if you can't find a "light bulb", buy a "heat lamp".  Basically the same thing except a heatlamp is red and won't be outlawed because it isn't a "lighting appliance".

[DrewV]

You could probably stock up on a 20 year supply of incadescents for not much $$

[teemuk]

Someone also instigated a rebellious act to sell those light bulbs as "heat balls" ...or something in that manner. Really a genious idea to get around the loopholes.

I'm all for protecting environment and saving energy but I don't support total ban of something that in the end is just a drop in the ocean when considering the big picture. The electricity consumed by my house's light bulbs is peanuts compared to street lights not to mention some industrial plants that alone may draw as much as electricity as mid-sized towns.

[Tann-74]

Hello to all...


we used to do the bulb limiter thing when working on tv's to check the ht feed to the loptie..

It's a handy plus cheap way of limiting the ac voltage in to a peace of equipment being worked on.