Realistic STA 200 Blackface find, a question,,,,,

At 13.5V / 5 Watts, that is 0.37037 amps each. Three times the amount of the original bulbs x 4 bulbs = something going to burn up! It's almost like shorting the bulbs out and the resistor is seeing way too much current.
Since this is DC, why not scrap the resistor and put a LM7812 regulator in it's place? You can then use 12V 0.15 fuse lamps.
OK, I can do this. all the parts were easy to find on mouser but I have a question. I remove the cement resistor, but then the 7812 regulator has 3 legs, so how do I solder it in?
 
Look at the data sheet. One leg is input. One output. One ground. I think they want a couple of ceramic disc caps at the input and output too..
I wish someone would chime in and see what they think of this scenario. I'm not sure if you need a heatsink or not.
 
Look and see if the 12v dome lamps available at auto supply stores would work. Be sure to check the ma rating of them as I cannot remember what they were offhand. They were used a lot in the 70's for automotive applications (13.6vdc)
They look like fuses but are for the overhead interior lighting lamps to clarify.
I tried the dome bulbs but end up overheating the cement resister. It was recomended I use a regulator instead,,,,, please browse the last few posts in this thread and see what you think,,,,,,,, we need a second opinion.
 
For lack of knowledge, I feel compelled to ask if I could simply replace the resistor with a different size to be able to use the 13.5v 5w bulbs I have?

No? Yes? please explain somebody.
 
Short answer first. You could use a 7812 regulator but would involve some fussing about having to add a diode, cap between legs 1 and 2, a heat sink etc etc, not hard but fiddly. On the other hand changing the resistor value is relatively simple, don't have time this evening to figure out the value however if you look back a couple of posts redk was kind enough to show you the way using ohm's law. Give it a try and see what value you get and someone will check later on :)
-Lee
 
Short answer first. You could use a 7812 regulator but would involve some fussing about having to add a diode, cap between legs 1 and 2, a heat sink etc etc, not hard but fiddly. On the other hand changing the resistor value is relatively simple, don't have time this evening to figure out the value however if you look back a couple of posts redk was kind enough to show you the way using ohm's law. Give it a try and see what value you get and someone will check later on :)
-Lee
Thanks Lee, I was hoping you'd say that, and at the same time was afraid you'd say that, because, I already looked into what redK said (thanks redk), and attempted the calculator but got lost in it,,,,,,,,, I'm afraid my talents are much more inherent to machinist work than ohm's law. I'll try again for sure though,,,,,,,,
 
Yea, no luck. I am having no luck applying the formulas into anything I can understand.

I have approximately 19v power source to the board,,,
I have 4 each 5w, 13.5 volt bulbs, and so I THINK they are .3703 amps each,,,,,,,,

beyond that, I have no clue

These may be terrible choices for bulbs, and thats ok. I'm trying to figure out what resistor I would need to make them work, or, how to substitute say a more common bulb size like 12v .15ma and resistor combination to make this circuit stable. As you can tell I have zero training in the use of ohm's law. The formulas are easy enough to use, knowing which variables to use when, not so much.

I have a given feed, and need 4 light bulbs, in parallel,,,,,,,,, what is the formula to find the proper resistor when calculating various bulb choices, (Vots and watts, or ma)
 
With the unit off and light bulbs removed would you measure the resistance (ohms) across the cement resistor. Need it for calculations. Looks like a 10 ohm 5 watt resistor but that k after the value has me scratching my head as 10,000 ohm cement resistors are not common.
-Lee
 
And a second reading, measure the voltage from one leg of the cement resistor to ground using first the ac scale on your meter then the dc scale . Report readings as xxxAC and xxxDC where xxx is the voltage reading value. Need that to be pinned down to whip up a 7812 regulator circuit if you decide to go that way :)
 
With the unit off and light bulbs removed would you measure the resistance (ohms) across the cement resistor. Need it for calculations. Looks like a 10 ohm 5 watt resistor but that k after the value has me scratching my head as 10,000 ohm cement resistors are not common.
-Lee
OK, on the 200 scale it reads 10.6 - 10.7ohms
 
And a second reading, measure the voltage from one leg of the cement resistor to ground using first the ac scale on your meter then the dc scale . Report readings as xxxAC and xxxDC where xxx is the voltage reading value. Need that to be pinned down to whip up a 7812 regulator circuit if you decide to go that way :)
AC 200 scale reading=.4
DC 20 scale reading=.34
 
We are dealing with fractional differences once I sat down and worked out the numbers. A 12v 150ma bulb time 4 in parallel yields a total resistance of 80 ohms or 20 per bulb. So, 20 bulb resistance divided by 30 (bulb plus 10ohm cement resistor) times 19v gives 12.6v which should be within the bulb tolerance range. However to be sure if you replace the 10ohm cement resistor with a 12ohm cement resistor readily available you will drop the overall voltage seen at the bulbs to 11.9v. Now, having worked through this whole thing I am guessing that just replacing those original bulbs with 12v 150ma bulbs from parts express would work just fine and we fussed over not very much after all :)
-Lee
 
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It's as bad of a design as the fuse lamps in my JVC. It has a 24VDC power source driving three 8V zener diodes in series. Each diode has a fuse lamp in parallel to it. When a lamp burns out, the diode still completes the circuit. I agree with switching to a 12 ohm resistor for 12V .15A bulbs. But, if we are going through the trouble to do that, why not just switch to a 12V regulator. That way, when a bulb burns out, the circuit is not out of balance. If I am thinking correctly, when one bulb burns out, the resistance changes to 26.67 ohms. Now the voltage across the remaining three jumps to 13.104 volts. Another burns out and the resistance changes to 40 ohms. Voltage jumps to 14.615V on the remaining two. At that point they all will likely go pretty quick. I guess it doesn't really matter, because you would likely change all at once anyway.
 
Good point sir, could do that except we have to convert the raw ac into dc with the correct diode, find an off location mount for the 7812, explain how to tie two small caps across the feed and output leg etc etc which although do-able may be beyond the scope of the abilities presented by the op :)
That 7812 with over 600ma of draw gonna try and get toasty.
-Lee
 
Ok so here is what I did today, I went over to Radio Shack with the 12v .15 bulbs and a 12 ohm cement resistor on my shopping list. They didn't have either. So I'm browsing through the cabinet drawer with lamp bulbs in it and I see 14v ma bulbs, but, they are E- style miniature screw in bulbs. Looked around for anything in fuse style lamps and nothing, so just out of curiosity I find some E-10 receptacles. It was the only thing they had, and it was the same voltage and ms of the originals, so if I can figure a way to mount it, nothing else to change. So I bought 4 bulbs, 4 receptacles and headed home. At home I'm lookinng at the stuff and the board and figure I can remove, or bend the little mounting screw tab, and put it in between the + & - fuse clips, and solder the receptacle + & - tabs right to the fuse clips. This worked very well, and had just enough room to get the board back in place, even though the bulbs are alot higher now, but it it. Powered up and walla, everything lit up and the cement resistor gets warm, but not severe heat, just warm but not hot. So that's pretty good, but with the bulbs closer to the panel now I get bright spots at each bulb. I needed to redirect the light somehow but didn't have anything to do it with so I decided to pull the bulbs out and Black out the clear glass so I taped up the bulbs and painted the end with flat black High Temperature spray paint. Just got it back togetherafter letting the paint dry quite a while and I must say it looks pretty good to me. In a way I feel like I kinda McGyvered it, but on the other hand I can still use original value bulbs, that are readily available and they just screw in, so I'm over all pleased with it. Here is how she lights up now.
20160815_194654.jpg
 
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