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troubleshooting with bulbs?

Discussion in 'LEDs and Optoelectronics' started by mbot, Nov 19, 2010.

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  1. mbot


    Jun 27, 2010
    hi, i have a discussion going with someone about repairing a strobe tuner in another thread ( ) and they brought up the idea of using a couple bulbs in place of resistors on a motor control board that keeps frying in order to help save the components while looking for the problem. i've tried to look this up as a troubleshooting technique but i can really only find similar applications where prople are using bulbs to limit current off the main power like soft starting amplifiers and stuff. i was hoping someone would be able to explain to me how this would work and how you would go about choosing the right bulb for a particular circuit.

  2. shrtrnd


    Jan 15, 2010
    I used to use bulbs for cleaning the probes inside magnetic flowmeters, so I wouldn't have to open them regularly.
    In your case, it seems to me that all you'd be doing with an incandescent bulb, is using it as a fuse. (Unless you're using a bulb rated for a voltage much higher than what you would see during a failure of your motor control board.
    In your case, I'd consider using a NEON bulb. When I was troubleshooting TV's years ago, I used to use neon bulbs so I wouldn't keep blowing fuses during troubleshooting.
    The neon bulb has the characteristic of glowing weakly, but going to high intensity when the voltage/current exceeds the circuit norm. Since the circuit is to the two elements in the bulb, through the neon gas, instead of a direct connection through a filament, as in an incandescent bulb. The higher voltage/current, just causes the neon gas to glow brighter. So you get a glowing lamp, instead of a direct short, causing circuit damage.
    Just a thought to consider.
    I really don't think an incandescent filament is the way to go. I'm not an expert on bulb physical properties, but it might not offer much in the way of resistance to the circuit (of the resistor you'd be replacing it with). Maybe an expert on this site would know better.
  3. Lenp


    Sep 8, 2009
    Here's the theory,
    The lamp is in series with the power line and could replace the fuse. The lamp wattage is determined by the typical current draw of the device. Let's say you have an amplifier, rated at 70 watts at 120 vac. I would use a 100-150 watt lamp. When the amp is first turned on the inrush current will cause the lamp to flare up in brightness, then as the power supply capacitors charge the lamp dims again. You will find that the circuit voltages at idle, are surprisingly close to their 'normal'. As the amp starts to produce sound the current draw increases, the bulb brightens and the voltages may start to droop. If there was a serious overload or a short, the bulb would go to near full brightness and save a bucket full of fuses.
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    I guess the point is that a fuse limits the current by going open circuit under overload conditions. It attempts to prevent (or at least limit) damage by removing power from the affected circuit.

    A light bulb limits the current by increasing its resistance under overload conditions. It attempts to prevent (or at least slow the onset of) damage by reducing power available to the circuit.

    With a light bulb, you can still smoke stuff, but at least you may be able to see what's smoking, and perhaps you'll pick up extra heat before that, and maybe you'll limit damage to one component.
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