lightbulb trick

[buncey]

hi there this is my first post here, hello everyone,

im working on a power amp, done a few repairs and noticed people say use the lightbulb trick when you test,

well can someone tell me exactly what i do?

does someone have a diagram how i do this, just so i know how to get it right,

thankyou

[zox2003]

Hi,

If i remember thats very simple.
You basically take 4 bulbs (12V) and you conect them in series and you are using them instead of fuse in each rail.
Those bulbs create resistance and they protect amp if anything is wrong in circuit.Plus visually if they fully light on you know that something is wrong with circuit.
Used long time ago and worked good.

Diagram?

+ Input---------BULB-----BULB------BULB-----BULB----- + OUT

Same for - rail.

[Netlist]

You would insert a lightbulb in series with one wire of the power line.
Think of a fuse in a circuit.
The lightbulb will dramatically reduce the current flowing through the circuit and so preventing the device to run excessive current in case of a failure.

Be careful when wiring things up, as you’ll mostly deal with lethal voltages here.

/Hugo

[K-amps]

Worth a thousand words.....

[zox2003]

I would not do it in AC line (220V or 110V).I was talking about DC voltage that you soplly amplifier with.That is how I used it.

So,

You take fuses out of your +/- amplifier power supplies and replace them with two of these sets.



+ Rail P.S -------Bulb----Bulb----Bulb-----Bulb----+ Rail Amp.

It works good.

[tool49]

Just a little note regarding K-amps' picture; 250W is a bit high when you start debugging. I usually start by a 25W bulb and then a 40W bulb and if needs be (for class A mostly) a 100W bulb. This limits the current a lot lower thus reducing the chances of burning something.

Hope this helps!
Sébastien

[Netlist]

I think both ways are possible.
Only with the bulbs in the DC line you don't catch faults in the primary section of the amplifier or whathever DUT.
The advantage is that a more safer working condition is created.
You could also integrate the light bulb circuit into the workbench,
being a fixed system. You could even build one with different light bulbs and switches to activate whatever wattage you need.
That being something for the “fearless lightbulb workbench fanatics”.

/Hugo

[K-amps]

The 250w was in the diagram I did long ago for someone, hopefully whoever uses the idea knows what to choose, the diagram was to illustrate the schematic moreso than the component values.

I'd would not use a bulb below 100 watts (in a 120vac scenario) because in some receivers with pre-power triggering, the PSU goes into oscillations. Also some amps without a delay relay also go into oscillations.

100w is a good candidate.

For Class-A I use higher rated bulbs, I have a separate board built and plug my amps in there so I dont have to mess with having series bulbs on my DC fuses... its messy and can cause shorts in the amplifier PCB if a quick and dirty install is done.

Also with DC, if one rail is not connected properly, you can blow the amp too without significant current passing through the other rail...

tool... you use 100watt bulb with class-A amp??? Is the bias turned down all they way?

[tool49]

I never had any problems with oscillation even with smaller bulbs, but it could occur. Then again 100W is a good measure. And yes 100W for a single 10W class A is a good measure even with regular bias adjustments. Of course for a bigger amp, you need to scale the bulb!

Hope this clarifies things a bit!
Sébastien

[K-amps]

Sure it clarifies. I have never worked on a 10w amp before hence the disjoint.