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Dim bulb tester reactions

FRANKPIETTE

FRANKPIETTE

Member
Hi everyone,

I got a Kenwood KR-4600 receiver. First i just tested it on my dim bulb tester (60w) to power it On, and a slim dimmed of the DBT remains. Since on DBT i hooked a CD player to Aux., then Tape In for the same CD player. On both case the dim bulb tester reacts/follows to the volume of signal like a Vu meter, so getting way brighter when signal/volume way higher. Is it normal? If not, anybody had the same situation and solved it? Thanks H.A.N.D.
 
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You aren't supposed to run audio through while on dim bulb. Just use it to check for shorts, then full power.
Of course it would light up brighter because the amp is using more current. You are starving the amp of power running it like this.
 
BmWr75 said:
Think that is normal DBT behavior. Louder volume draws more power and the brighter bulb reflects that.
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Thank you so much for having taken the time and especially for the answer that i hoped for Thanks again dear BmWr75
redk9258 said:
You aren't supposed to run audio through while on dim bulb. Just use it to check for shorts, then full power.
Of course it would light up brighter because the amp is using more current. You are starving the amp of power running it like this.
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Redk9258 when you mention full power test, is it with signal or none at all? Thank for your reply. Another little thing when i tested it for shorts no signal on aux, the wire in the bulb was off, when i moved function selector from Aux. to Tuner (AM & FM) a very low dimmed orange color on the bulb filament appears, and opposite getting back to Aux. Was it normal too?
 
Yes - the idea of a dim bulb tester is that the lamp serves as a current limiter in the case of a near dead short in the device under test. It can also act as a current limiter if large electrolytic filter capacitors are badly formed after being unused for a very long period. If the latter occurs, you may see the lamp lighting relatively brightly as power is first applied, then dimming out as the caps reform.

The normal use is to plug the DUT into the tester and turn it on, observing the behavior of the bulb. Note that the bulb should be sized with the expected load in mind - devices that draw more current will require a higher wattage bulb than smaller ones. (I have seen a list of recommended lamp sizes for various loads, but don't have it readily at hand at the moment) If the bulb lights brightly and stays lit, your device has issues. "Normal" behavior may be anywhere between next to nothing from the lamp to a bright flash that then dims out or almost anything in between. The main thing is you don't want the lamp to light brightly and stay lit. Once you've established that the DUT isn't shorted, the dim bulb tester is usually removed from the circuit before further testing.

The reason that you're seeing the behavior described above (dark when on Aux with no signal, but showing a dim glow when switched to Tuner) is that in aux with no input the unit is drawing minimal current, and when the tuner is activated it is likely passing at least some signal (not to mention that the tuner circuitry may be unpowered when it aux, but powered when in tuner). Hope this helps to clarify.

-Pat

<edit to fix typos>
 
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I just found the post I'd made a while back on another forum; it's pasted below. It was originally in response to someone asking if LED bulbs could be used in a dim bulb tester.

As you've already found, a dim bulb tester only works with incandescent lamps.

To elaborate a bit on the why, it relies on the positive temperature coefficient of the lamp's tungsten filament - it has a low cold resistance, and a much higher (relatively) hot resistance. When in series with a DUT (Device Under Test), when the DUT is first turned on the low resistance of the lamp allows nearly the full current the device might draw to flow (a 60 W bulb has a cold resistance of ~17.5 Ω, so it will allow an inrush of nearly 7 A). The 60W bulb's hot resistance is about 240 Ω, so if the DUT is a dead short it will limit the current to about a half an amp and the lamp will burn brightly. If the DUT is not shorted, then once the current draw stabilizes, it will act as if it has a resistor between the 17.5 Ω cold and the 240 Ω hot resistance in series with it, depending on how much current it draws and where on the resistance curve of the filament the resulting temperature puts it. This will also determine how bright the lamp is - at low current draws it may barely illuminate at all once the DUT has stabilized.

The testers usually flash on initial power up of a DUT because things often draw a relatively large inrush current as the power supply capacitors charge; this then tapers off to the normal operating current once the caps are charged up.

Courtesy of an article on EDN: https://www.edn.com/hot-resistance-vers ... esistance/ here is a table of approximate hot and cold resistances of various wattages of incandescent lamps:
Image

-Pat
 
Been looking around in my area, and incandescent light bulbs 60/75/100 watts are getting very hard to find.
 
Is it 1 watt for 1 watt???????
If you have a 260w per channel amp, do u need a 520w lamp. Technically they are lamps~:rolleyes:
 
P2200 said:
Is it 1 watt for 1 watt???????
If you have a 260w per channel amp, do u need a 520w lamp. Technically they are lamps~:rolleyes:
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No, there is no direct correlation, remember that the use of a DBT is only to check for shorts and high current draw situations in the case of a fault, where being able to power up the faulty unit may be helpful with finding the fault. You should not feed a signal into an amplifier on DBT as this could push the amplifier into unknown (potentially damaging) PSU and bias situations.

If you have no faults, you do not need to use a DBT. Although some people with gear that hasn't been used for some time, or is in unknown condition use a DBT to power up a unit for a brief test initially.

I recommend a 100w bulb (as a starting point) for a 260wpc amplifier, you may need to go up to a 150w bulb, but you shouldn't need to go higher.
 
Thank-you Hyper~ I got most of my components to build my tester with a 200w lamp. Wondering if I want to add a variac and/or isolation transformer ? Is 200w too big?
 
Yes already been there~ I read some of the compendium. It is so elaborate that it is confusing, & several slightly opposing opinions. Overall it is a very good piece :bigok:
 
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