# Choose a bulb for a current limiter



## Greg Ellis (Oct 1, 2007)

I've been gathering parts to build a light bulb current limiter. I think I've got most of the bits and pieces now.

I'm curious about how to choose a bulb.

If I understand right, a light bulb is designed for parallel operation on house voltage, so roughly 125V AC.

100W = 125V x 0.8A
150W = 125V x 1.2A
etc.

Moving this bulb into a series circuit will limit the current available downstream of the lightbulb to the max current that the lightbulb is designed to pass?

Is it really that simple? 

So a 100W bulb will not allow more than approx 0.8A of current to flow, and will also provide a visual indication, based on brightness, of how much current is flowing.

My brain hurts a little bit trying to understand how there could be any voltage LEFT on the downstream side of the bulb, since it's designed to use up the whole 125V.

What size bulbs are you guys using for amp test purposes?


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## loudtubeamps (Feb 2, 2012)

Greg:this is what I have been using for 20 + years... A 100 watter would be ok as well.


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## nonreverb (Sep 19, 2006)

loudtubeamps circuit is the one. The only addition I have is a Variac at the wall.


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## oldjoat (Apr 4, 2019)

the bulb effectively reduces the Voltage to 1/2 the wall value ( 120 down to 60 volts )
the internal voltage values become 200- 250V instead of 400-500V 
the caps are given time to re-form at lower volts ( less chance of arc over internally )


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## jb welder (Sep 14, 2010)

oldjoat said:


> the bulb effectively reduces the Voltage to 1/2 the wall value ( 120 down to 60 volts )
> the internal voltage values become 200- 250V instead of 400-500V
> the caps are given time to re-form at lower volts ( less chance of arc over internally )
> 
> any bulb will do the trick .


Sorry but this is incorrect. 
Voltage drop across the bulb will be completely dependent on the current through the circuit. And the bulb filament will act as a dynamic limiter, the bulb resistance changes depending on the current (heat). The principle is the same as tweeter protection bulbs (dynamic limiting), and wattage is definitely important.
Also, you are counting on the bulb to give a visible indication of a fault, so the higher the wattage, the less it will light up.
The most general rule of thumb I've heard (for tube amps) is use a bulb with a wattage of around the amps power output. That being said, I'd never go below a 60W bulb.
There's somebody's uncle  doing videos that recommended a 250W bulb. This will allow a lot of current before it lights up and tells you anything, I don't see the point in using it at all. He got a lot of criticism for that one, not sure if he changed it or not.


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## DavidP (Mar 7, 2006)

Yup, just get a 60W bulb and yer good to go! Rona still has them for cheap.


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## oldjoat (Apr 4, 2019)

"correct" use a 60 watt incandescent for this trick ( and most amps )


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## nonreverb (Sep 19, 2006)

Funny but I rarely ever use mine on a tube amp. I find tube amps far more forgiving when faults are present. Mine gets used often with SS amps.


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## Greg Ellis (Oct 1, 2007)

jb welder said:


> There's somebody's uncle  doing videos that recommended a 250W bulb. This will allow a lot of current before it lights up and tells you anything, I don't see the point in using it at all. He got a lot of criticism for that one, not sure if he changed it or not.


If I'm calculating this stuff correctly, Uncle Doug's 250W "photo flood" bulb would let something like 2A flow. 

That's a lot of current, and maybe not a great idea for an application like this. 

If the amp is actually working correctly, 2A is not a crazy-high value. You might find a 2A fuse on a a lot of ampls, maybe even 3A. 

But yeah, to quickly detect a fault like a short, I can see how a smaller wattage makes sense.

I'll see what I can find hidden away in the house. There ought to be some 60W bulbs and maybe 100W.

Incandescent bulbs are getting harder to buy these days. Everything incandescent in the stores tends to be fancy decorative thingies now.


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## jb welder (Sep 14, 2010)

Greg Ellis said:


> Incandescent bulbs are getting harder to buy these days. Everything incandescent in the stores tends to be fancy decorative thingies now.


They're easier to find if you search for 'appliance bulbs' or 'rough service bulbs'. Those are 2 of the allowed uses, I believe. Can.Tire has 4-packs of 60W appliance bulbs in their incandescent bulbs section.


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## Cardamonfrost (Dec 12, 2018)

What's the purpose of this? To stop a big initial surge?
For something that uses a light bulb, I'm in the dark here!
C


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## Greg Ellis (Oct 1, 2007)

When troubleshooting an amp, it's mostly to provide a warning and some level of protection against excessive current draw (which would suggest a fault of some sort).


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## jb welder (Sep 14, 2010)

Cardamonfrost said:


> What's the purpose of this? To stop a big initial surge?
> For something that uses a light bulb, I'm in the dark here!
> C


In most basic terms, it's for troubleshooting faults that cause fuse blowing. Instead of going through fuses, the bulb lights up. You save fuses, and save the amp from further damage.


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## Lincoln (Jun 2, 2008)

jb welder said:


> Sorry but this is incorrect.
> Voltage drop across the bulb will be completely dependent on the current through the circuit. And the bulb filament will act as a dynamic limiter, the bulb resistance changes depending on the current (heat). The principle is the same as tweeter protection bulbs (dynamic limiting), and wattage is definitely important.
> Also, you are counting on the bulb to give a visible indication of a fault, so the higher the wattage, the less it will light up.
> The most general rule of thumb I've heard (for tube amps) is use a bulb with a wattage of around the amps power output. That being said, I'd never go below a 60W bulb.
> There's somebody's uncle  doing videos that recommended a 250W bulb. This will allow a lot of current before it lights up and tells you anything, I don't see the point in using it at all. He got a lot of criticism for that one, not sure if he changed it or not.


In Uncle's latest video, his bulb just starts to glow slightly on a large Magnatone amp with lots of tubes. He puts an amp meter in line just to check and it measures 1.25 amps.


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## nonreverb (Sep 19, 2006)

Lincoln said:


> In Uncle's latest video, his bulb just starts to glow slightly on a large Magnatone amp with lots of tubes. He puts an amp meter in line just to check and it measures 1.25 amps.


Where in line? On the secondary or primary side of the power transformer Lincoln?...and if on the secondary, on the 6.3V supply?


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## Lincoln (Jun 2, 2008)

nonreverb said:


> Where in line? On the secondary or primary side of the power transformer Lincoln?...and if on the secondary, on the 6.3V supply?


On the primary. he's got a bulb setup/current limiter thing like we're talking about. When he measured the primary, the amp was pulling 1.25 amps @ 120 volts. 
Start watching at 16:00 minutes in.


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## nonreverb (Sep 19, 2006)

He's using a 200 or 250 watt bulb by the looks of it. That's a fairly significant draw with no signal....lots of filaments to supply!


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## jb welder (Sep 14, 2010)

This is the problem. The 250W bulb is way too big. It will allow a lot of current before it lights up and starts limiting what gets to the amp. If there is a fault, the bulb is not offering protection or a fast enough indication that there is a fault.


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## High/Deaf (Aug 19, 2009)

The real trick (or is it magic) with incandescent bulbs is that their impedance isn't linear with the current being drawing through them (as mentioned by @jb welder on the previous page). At small currents they can appear to be a short circuit (0 ohms). At higher currents, the impedance goes up. The amount the impedance increases changes from one bulb to another.

We used bulbs as protection on a large Clear-com system spread through an arena I worked at. A normally working CC pack drew about 1 amp. The lightbult connected in series to each circuit would not light up at 1A and were near 0 ohms impedance so no impact on the working CC pack.

If something happened to the circuit/building wiring (pinched wires or someone jamming a tinfoil gum wrapper in the XLR jack - hey, it happened) the circuit would draw lots of current because of the short circuit (multiple amps, depending on the power supply). And the short circuit would drag the whole system's power supply down, killing all the Clear-com packs, not just the one on that circuit. This was enough current to light up the bulb, which would then have an increased impedance and voila, no more short circuit, and the Clear-com system kept working. The bonus was - the lit bulb told us which circuit / wiring was shorted and allowed us to find it quickly.


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## nonreverb (Sep 19, 2006)

The light bulb protection is only useful in tube amps to prevent blowing a handful of fuses....and in a tube amp, there are only a small number of faults where that would occur. A tube amp by design is fairly forgiving compared to a solid state amp. That's where the bulb limiter can save a lot of headaches.....and lots of transistors. 
jb welder is correct. You're not going to save much with a 250 watt bulb in there. In solid state service, 60watt or 100watt max depending on the quiescent current draw of a given amp.


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