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Old 23rd April 2007, 03:00 AM   #1
jarthel is offline jarthel  Australia
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Default using light bulbs when testing new DIY project

I've seen replies here and in AA where people suggested the use of light bulb when testing new DIY project. Is the light bulb used because it limits current?

Also, do I need to use the light bulb all the time when testing?

Thank you.
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Old 23rd April 2007, 09:06 AM   #2
Shaun is offline Shaun  South Africa
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I have used light bulbs to load an amp's power supply. Provides a quicker discharge path for the reservoir caps when powering down. You do not want to work with charged capacitors as they can inflict serious damage to the amp should there be an accidental short-circuit.
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Old 23rd April 2007, 10:55 AM   #3
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apply 10-20 ohm/10W resistor will do the job too.
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Old 23rd April 2007, 11:11 PM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
the light bulb provides an automatic limiter if there is a wiring fault.
Once the equipment is up near full voltage and proved to be correctly wired, the tester bulb & it's plug top/socket is removed. It should be re-inserted each time you modify part of the equipment.
You could fit a bypass switch but this should more safely be a hold to make type rather than a flip over (latching) in case one were to forget to reset the bypass to off ready for next test.
The bulb works in opposite fashion to a thermistor.
At low currents it provides little resistance and let's most of the voltage through, but if the current is high enough to warm the filament the bulb starts to limit and becomes even warmer starting to glow or in worst case becomes bright indicating a fault when it absorbs almost all the mains voltage.
Do not try to set bias in an amplifier with the bulb in-line. The low voltage after the bulb will upset the amp and all the setting you are trying to achieve.

ps.
the fixed resistor referred to in post 3 will burn out if the transformer were wired out of phase and would be at mains voltage creating an additonal danger.
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Old 24th April 2007, 12:19 PM   #5
PCC88 is offline PCC88  United Kingdom
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Rather like a poor-man's variac.
Some info here:-

http://www.vintage-radio.com/projects/lamp-limiter.html
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Old 24th April 2007, 01:23 PM   #6
Shaun is offline Shaun  South Africa
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Oops!

I seriously misread the original poster's question. I use 10 ohm resistors in series with the supply to the output transistors (in place of the onboard fuses). Bias starts off at minimum, and is slowly increased until the Vdrop reflects to the required quiescent current. Of course, if the resistors start to smoke, then there's a fault somewhere....
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Old 24th April 2007, 01:43 PM   #7
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There is some great information on the site mentioned by valve power - well worth a look.

Some pictures would have been nice though........
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Old 2nd May 2007, 11:01 PM   #8
Nordic is offline Nordic  South Africa
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Quote:
Originally posted by AndrewT

Do not try to set bias in an amplifier with the bulb in-line. The low voltage after the bulb will upset the amp and all the setting you are trying to achieve.

Missed this before, and it may be part of my problem....Can it damage anything though???

My amp wouldn't work...(tried with the bulb setup) can't find any realy broken components except for a driver with too high hfe, and a resistor to the base of the output pair off by a decimal point ... i.e. 10 times less current to the base... one bias resistor showed 10v/10R the other rail showed something like 5mV...
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Old 3rd May 2007, 08:03 AM   #9
AndrewT is offline AndrewT  Scotland
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Quote:
one bias resistor showed 10v/10R the other rail showed something like 5mV...
nothing broken, you must be joking.
That indicates an amp is flowing. Where from and where to?
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Old 3rd May 2007, 11:02 AM   #10
Nordic is offline Nordic  South Africa
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cant find anything that looks shorted... only tested hFE in the transistors.... and that looked good except for 1.
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