WinterDeere
Elite Member
Okay, here's one. Most people know by now that typical mini bulb light strings have bulbs rated 2.5 volts, arranged in series clusters of 50 bulbs, for a nominal 125 volt rating. What's less obvious to most when purchasing replacement bulbs is that not all 2.5 volt bulbs are the same, they are commonly made and sold in 100, 170, or 200 mA ratings, corresponding to 0.25, 0.43 and 0.50 watts per bulb.
The trouble with this is that, with the way they're series connected, using one bulb rated differently than the rest can really overtax that bulb, to the point of very quick failure. Take an older string populated with 200 mA bulbs, and pop a modern 100 mA bulb into it, and that bulb is going to dissipate nearly 4x its intended wattage, by I^2*R. It's counter-intuitive to most, that the sole "0.25 watt" bulb in the string is nearly 4x brighter than the "0.50 watt" bulbs that make up the rest of the string, it's just a result of all bulbs on the string operating at the same current.
If you've ever bought 2.5 volt bulbs, and noticed the replacements are way brighter or dimmer than the rest of the string, look for the current ratings. In most cases, a 100 mA bulb on a string otherwise populated with 200 mA bulbs will blow immediately, whereas being just one size off (100 vs. 170 or 170 vs. 200 mA) will usually just result in bring too dim or bright, depending on direction of the shift.
Sadly, most strings don't tell you which current bulb to use. But knowing that 2.5 volt bulbs are arranged in clusters of 50, simply divide the total string current (always listed) by N/50, where N is total string bulb count. In other words, a 150 bulb string rated 0.5 amps has three clusters each pulling 170 mA. If you shop bulk bulbs on Amazon or elsewhere, you will see they commonly list 100, 170, or 200 mA flavors.
The trouble with this is that, with the way they're series connected, using one bulb rated differently than the rest can really overtax that bulb, to the point of very quick failure. Take an older string populated with 200 mA bulbs, and pop a modern 100 mA bulb into it, and that bulb is going to dissipate nearly 4x its intended wattage, by I^2*R. It's counter-intuitive to most, that the sole "0.25 watt" bulb in the string is nearly 4x brighter than the "0.50 watt" bulbs that make up the rest of the string, it's just a result of all bulbs on the string operating at the same current.
If you've ever bought 2.5 volt bulbs, and noticed the replacements are way brighter or dimmer than the rest of the string, look for the current ratings. In most cases, a 100 mA bulb on a string otherwise populated with 200 mA bulbs will blow immediately, whereas being just one size off (100 vs. 170 or 170 vs. 200 mA) will usually just result in bring too dim or bright, depending on direction of the shift.
Sadly, most strings don't tell you which current bulb to use. But knowing that 2.5 volt bulbs are arranged in clusters of 50, simply divide the total string current (always listed) by N/50, where N is total string bulb count. In other words, a 150 bulb string rated 0.5 amps has three clusters each pulling 170 mA. If you shop bulk bulbs on Amazon or elsewhere, you will see they commonly list 100, 170, or 200 mA flavors.
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