Series Resistors with Incandescent Bulbs?

Thread Starter

PropForge

Joined Mar 5, 2016
28
Background: I'm attempting to reverse engineer some old lighting circuits that were built in the early 80's in the hopes of replicating them. The circuit in question uses a 5V regulator, a timer, a counter, a few ULN2003A's, and some 22 Ohm 1W 10% carbon film resistors, all to "animate" a set of small 6V lamps. I believe a 6V motorcycle battery was used to power the circuit, the lamps powered directly, and the ICs through the 5V regulator.

Question: What would be the most likely use for the resistors? I know incandescent bulbs don't require them, but from what little bit of information I've been able to find, they may decrease the speed of the filament heating up and therefore help with bulb lifespan?
 

Thread Starter

PropForge

Joined Mar 5, 2016
28
Hello there. :)
This is a simple lamp flasher circuit, working on the principle of regenerative feedback. On connecting the circuit to power, the 6V DC acts as a control signal to the base of the transistor Q1.
I hope it helps.
https://circuits-diy.com/lamp-flasher-circuit-using-transistors-diy-project/
I'm not looking to design a new circuit, but replicate an old one. I'm just curious as to the function of a resistor in series with an incandescent bulb, like you would do with an LED.
 

Thread Starter

PropForge

Joined Mar 5, 2016
28
hi prop,
What is the current or wattage rating of the 6V lamps.?
E
The lamps should be 6V 0.2A 1.2W

I should note that arrived at the 1W resistor value by scaling off photos, using a TO-220 as my reference. In this case, they're brown, cylindrical ones, with the color bands appearing to have been roughly painted.
 

ElectricSpidey

Joined Dec 2, 2017
1,971
They could be in there just to dampen the surge current of the lamps, but that is just a guess usually when you have a resistor in series with a load it's there to drop voltage or reduce current.
 

Thread Starter

PropForge

Joined Mar 5, 2016
28
hi prop,
A 6v 0.2A tungsten lamp can draw about 1 amp from cold, the UL2003 can only tolerate 600mA peak, so the 22R is to limit the cold lamp current.
E
Good to know. I'm assuming ULN2004s would have the same sort of issue, given they're rated the same for their outputs? The same 6V battery would be used, but without a 5V regulator.
 

MrSalts

Joined Apr 2, 2020
733
If you have a 0.2 amp @ 6V bulb, while at full temp, the bulb is equivalent to a 30 ohm resistor. Adding a 22 ohm resistor in series will cut the current and the bulb temp and lower the operating temp of the bulb which decreases the Dc resistance of the bulb. Let's say the bulb on resistance is down to 22 ohms when powered. The current will be down to 0.136 amps and half the voltage drop will be on the bulb (3v) and half on the series resistor. Power of the lamp will be down to 0.136 x 3 = 0.4W watt but (BUT!) the the bulb will be less efficient (less lumens per watt when not run at full power) - so you'll see less than 1/3 the lumens.
 

Thread Starter

PropForge

Joined Mar 5, 2016
28
hi,
You don't need a 5V reg for that project.:)
E
There's also EPROMS (2716) on the same board that I don't (currently) think are associated, but they are TTL, so the regulator may be for them.

It doesn't help that there may be unused, superfluous parts that were just left on the board. The deeper down this rabbit hole I go, the weirder things get.
 

ericgibbs

Joined Jan 29, 2010
14,189
hi prop,
The EPROM is used for sequencing the lamp lighting pattern, so use the 5V for the EPROM.
E
It's an Alice in Wonderland of electronics, remember that rabbit hole story.?;)
 

ElectricSpidey

Joined Dec 2, 2017
1,971
Another consideration by the designer could have been not exceeding the total package wattage. But without knowing how many bulbs are being driven by each chip and how long each bulb would be lit, that would also be a guess.
 

MisterBill2

Joined Jan 23, 2018
9,844
A series resistor was often used with some incandescent light bulbs to reduce the brightness, and also often used to reduce the current and heating. I have used series resistors to allow six volt bulbs to be used with 12 volt sources. No, it is not efficient, but certainly it is effective.
 

Thread Starter

PropForge

Joined Mar 5, 2016
28
Yes, it's a movie prop from the 80's, that hasn't exactly been curated incredibly well. Unfortunately, I can't post photos or too much information.
 
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