Basic electronics/circuit question

Uncle Bambi

Funky Tut
I have a receiver with a burnt out dial display bulb. The bulb is rated at 28v/100ma, although actual measured voltage across the bulb is 24v

Since I do not have a 24v bulb but I do have a 12v one, I thought I'd try putting a resistor in series with the 12v bulb in order to drop the voltage and use the 12v bulb. I did this with a 50 Ohm 5w resistor and it worked........ BUT the resistor gets too hot to touch in a matter of less than a minute. I tried as low as a 25 Ohm 5w (too bright), and a 100Ohm 5w (too dim). In all cases, the resistor gets too hot to touch inside of a minute.

What's going on here? What simple thing am I missing?
 
Nothing seems too out of whack to me. If you're dropping 12 V across 50 ohms then your bulb current is 240 mA. I'm pretty sure that's what a bunch of fuse-style dial lights I bought were rated at. The resistor has to shed 2.88 watts and maybe it just gets that hot while doing so.

Side Note: this is why linear voltage regulators get hot - they're basically dissipating away the input / output voltage difference.
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wondering what might happen with a resistor in parallel ..
If you're wondering about a parallel combination instead of a series combination several unpredictable things could happen - one thing's for sure - if the supply holds up you now have 11.52 watts that the 50 ohm resistor has to dissipate.
 
If this bulb is a standard bayonet type 28v 100ma they ar pretty common with some being offered on "the Bay" as we speak. Very reasonablly priced. I would go for the right bulb.
 
Bulb doesn't have linear resistance, because it's very dependable of temperature. In that case calculations are not that simple. Obviously temperature should be taken very seriously into calculation.
 
If this bulb is a standard bayonet type 28v 100ma they ar pretty common with some being offered on "the Bay" as we speak. Very reasonablly priced. I would go for the right bulb.

It is a wedge base bulb. I found some 28v flashlight bulbs at RS that has bayonet bases. I'll try one of those.

Here's a question - how about a 12v zener, a LED, and an appropriate resistor in series with the 12v bulb? Just spit-balling....... I can try the two 12v bulbs in series, but that would result in an internal heat source (the second bulb) and possible light leakage from the case.

I know, I know..... buy the correct lamp. :)
 
I know if you measure the resistance cold it will not be the same as when it is powered up. However there are ratings on the bulb and you can use them.
 
It is a wedge base bulb. I found some 28v flashlight bulbs at RS that has bayonet bases. I'll try one of those.

Here's a question - how about a 12v zener, a LED, and an appropriate resistor in series with the 12v bulb? Just spit-balling....... I can try the two 12v bulbs in series, but that would result in an internal heat source (the second bulb) and possible light leakage from the case.

I know, I know..... buy the correct lamp. :)
Any solution would make additional heat. And what if you would use simple voltage divider? I mean two same resistors, connected in series attached to 24V. That would "create" 12V, but, then you would have parallel connection to just one of that resistor, and if you do that, resulting resistance would be smaller than any of resistors that are connected in parallel, in this case resistance of bulb, and one resistor.
 
It is a wedge base bulb. I found some 28v flashlight bulbs at RS that has bayonet bases. I'll try one of those.

Here's a question - how about a 12v zener, a LED, and an appropriate resistor in series with the 12v bulb? Just spit-balling....... I can try the two 12v bulbs in series, but that would result in an internal heat source (the second bulb) and possible light leakage from the case.

I know, I know..... buy the correct lamp. :)
Whatever dissipative scheme you come up with you're just moving the heat sources around. You're always gonna have a voltage drop and current going through it. IIRC I think I've heard of hams using a series capacitor to drop voltage in a "cool" way - but the source has to be AC - not uncommon in the vintage world.
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