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Light bulbs as dummy load for Stereo ?

techforce

Dec 26, 2008
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Was curious if anyone has done this before, instead of using resistors or other test gear as the load in place of actual speakers to protect the speakers?
 

Alec_t

Jul 7, 2015
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in place of actual speakers
If the bulb replaces the loudspeaker, i.e acts as a dummy load, then clearly the loudspeaker is protected :confused:.
Did you mean use the bulb as a dummy load, or use the bulb in series with the speaker to limit current?
Bear in mind that the bulb rating specifies wattage at the rated voltage when the bulb is hot. The bulb's cold resistance is a lot lower than its hot resistance.
 

dave9

Mar 5, 2017
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You mean something you built yourself, or repaired, and question its proper function? I usually just measure to be sure there's no DC on the output (if applicable), then first hook the amp up to crappy low value speakers, actually just a loose driver I got out of something or other a long time ago.
 

Martaine2005

May 12, 2015
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Why would you want to put a bulb in place of a speaker?,
A 120v, 60w bulb is 240 Ohms....

Martin
 

dave9

Mar 5, 2017
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Series parallel array of incandescent bulbs in a freezer with a thermal detection liquid nitrogen spray for the win!
 

techforce

Dec 26, 2008
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Yes as a dummy load. I was thinking a100w floodlight maybe. Just to protect the speakers from possible excess DC when
doing repairs. I have something like 80mv on one channel as DC offset, and the other is about 0v , doing the tests with
nothing attached to the spkr terminals. Its a Pioneer SX-240 Stereo and the protect relay keeps clicking out at about 33% volume.
There was alot of cracked solder on the spkr terminals, and one on the coil for the relay.


If the bulb replaces the loudspeaker, i.e acts as a dummy load, then clearly the loudspeaker is protected :confused:.
Did you mean use the bulb as a dummy load, or use the bulb in series with the speaker to limit current?
Bear in mind that the bulb rating specifies wattage at the rated voltage when the bulb is hot. The bulb's cold resistance is a lot lower than its hot resistance.
 

dave9

Mar 5, 2017
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Won't a 12V bulb be too near the amp output voltage to keep the resistance anywhere near constant?
 

Alec_t

Jul 7, 2015
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An electric kettle element would run at a lower temperature than a bulb filament, so the hot/cold resistance variation would be less.
 

techforce

Dec 26, 2008
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Interesting idea. Maybe a spare electric stove coil would also do the same thing.

Good idea @Alec_t about the bulb in series with the speaker.

An electric kettle element would run at a lower temperature than a bulb filament, so the hot/cold resistance variation would be less.
 

majoco

Nov 10, 2019
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A 10ohm 5watt resistor is probably cheaper than a lamp.
 

duke37

Jan 9, 2011
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I go to Rich who saves me headlight bulbs with one broken filament.
 

WHONOES

May 20, 2017
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If all you want to do is protect your speaker from excessive dc from the amplifier output then, connect a largish capacitor in series with the speaker. BTW, 80mv offset is not unreasonable but, if it increases during usage then there is something wrong. There could be something wrong with your offset detection circuit.
 

bertus

Moderator
Nov 8, 2019
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Hello,

As @WHONOES said, 80 mV is not that bad.
If it changes, it could be a heating problem.
Heat can make the setting go haywire.

Bertus
 

WHONOES

May 20, 2017
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Offset detection circuits tend to be something like a window comparator (at least that's what I use). They have a largish value resistor on the input with an even larger capacitor connected from input to 0V to bypass the ac from the amp when it is up and running. If that cap' is below par, it could be causing your problem.
 

CircutScoper

Mar 29, 2022
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.... but maybe < 50 Ohms when cold.

Probably more like 6 Ohms. A bulb filament when ON runs around 2000C, so a 0.5%/C tungsten tempco produces a 2000 x 0.005 = 10:1 increase in resistance hot vs cold.
 

CircutScoper

Mar 29, 2022
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An electric kettle element would run at a lower temperature than a bulb filament, so the hot/cold resistance variation would be less.
True -- with clusters! Less because of the lower temperature, and less again because heating elements are typically made from wire (e.g. nichrome) with a much lower tempco (e.g., ppm/C vs ppk/C) than tungsten.
 
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