# Does this Circuit make sense?

Discussion in 'General Electronics Discussion' started by Raul, Sep 25, 2012.

1. ### Raul

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Sep 25, 2012
I'm building a doll house and I dunno nuthin' bout elektrisity
Well I can wire a real house, but electronics are a different thing. Calculating loads and what not and figuring power supplies and resistance just gives me the Heebees, But then I am dyslexic as hell so anything with math gives me the heebees

Any way here are four units of series wired LEDS together run parallel from a power supply.

Next I gotta figure out how I'm going to run the incandescent bulbs. They are 12 volt bulbs. I think I'll have 5 or six units and I don't know if I have to play the same game as I do with the LEDS or if I can just get a 12 VDC Power Supply and run 'em off it willi nilli like I might a real house.

Here is the LED circuit, wha-da-ya think?

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Jan 21, 2010
3. ### BobK

7,682
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Jan 5, 2010
12V incandescent bulbs can just be connected in parallel to the 12V supply.

Bob

4. ### CocaCola

3,635
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Apr 7, 2012
If it was me I would skip the incandescent and just go with all 3mm warm white LEDs as they won't build up heat and they have a much longer life span...

5. ### KrisBlueNZSadly passed away in 2015

8,393
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Nov 28, 2011
Mostly looks good, but your LEDs are connected backwards (reverse polarity) and they won't run at quite 20 mA.

Each series LED chain has two LEDs, which drop 3.2V each for 6.4V total. That leaves 5.6V across each current limiting resistor (assuming your 12V power supply is actually 12V not 13.8V). To get 20 mA current flow, R = V / I which is 5.6 / 0.02 which is 280 so I would use 270 ohm resistors (or two 560 ohm resistors in parallel) for each current limiting resistor. Power dissipation in each resistor will be 5.6 * 0.02 which is 0.112 watts so they won't get very hot.

6. ### Raul

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Sep 25, 2012
The units I built to take the LEDs are ceiling units intended to illuminate and take only two LEDs each. ERGO they are series wired units of two each on the parallel chain

My symbols are reversed. OK Got it.

Don't want to skip the incandescent because doll house lighting fixtures in LED are brutally spendy and the incandescent ones are what the little girl picked out

I suppose I could re wire them to take the LEDs But that's a project for another time and I don't know if it's do-able.

Hook the incandescent up in parallel or series? Cool Thanks

Still at 1/4 watt each but 270 Instead of the 330 ohm ones?
like so?

Or with 560 1/2 watt resistors like so?

I'll buy the resistors after I get the PS and check it.

Last edited: Sep 26, 2012
7. ### KrisBlueNZSadly passed away in 2015

8,393
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Nov 28, 2011
The first drawing is right. Each chain should have one 270 ohm resistor.

BTW your comment "LED = FW Current 3.2 v" should say "LED forward voltage = 3.2V".

If the incandescent bulbs are rated at 12V and you have a 12V power supply, they should all be connected across the power supply, i.e. in parallel.

Also make sure your power supply's current rating is adequate. The power supply in your diagram is rated at 12V 6W which means it can supply up to 500 mA (this is calculated using I = P / V). The LEDs draw about 20 mA per chain, so that's 80 mA total, leaving 420 mA for the incandescent bulbs. Add the currents for all the incandescent bulbs together and make sure the total is less than 420 mA. You may need a more powerful supply.

Also, incandescent bulbs draw more than their rated power when you first turn them on, before they heat up, so the power supply should be over-rated. It's also a good idea to over-rate the power supply to give a safety margin.

Last edited: Sep 26, 2012
8. ### Raul

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Sep 25, 2012
Cool.

My inexpert abbreviation

OK.

I dunno what I = P / V means. Googling it I get:
I = Amperes
P = Watts
V= Volts
Is that right?
6 / 12 = 0.5
0.5 is 500 milliamps ?
so 0.1 is one hundred milliamps?
Is not milli a thousand? yet 0.500 is 5-hundreths ?
That is confusing.
In Mechanical terms, 0.005" is five thousandths of an inch. 0.5 is five hundreths
A mil is engineering slang for one thousandth.
See my confusion?

A milliamp is one thousandth of an ampere. Yes?
I don't see how 0.5 is 500 milliamps
How is it that five hundreths is five hundred thousandths?
What am I missing?

Now I know what Joey felt like when he saw the penis cake. I get like this whenever I try to do math.

Do I determine the mA for a incandescent bulb by switching my multimeter to Ohms and placing the black lead to the base and the red to the socket body?
When I do this to a regular 120 VAC bulb I get a reading of about 9.8 which is pretty much what they ought to be. But, I dunno if this is just coincidence or not and whether Ohms is not even the correct thing to be looking at.

However I hadn't thought to hook the incandescant bulbs to the little PS I planned for the LEDs. I was going to get a separate PS unit for the incandescants .

9. ### CocaCola

3,635
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Apr 7, 2012
Yes

1 A = 1000 mA
½ A = 500 mA
½ = 0.5
0.5 A = 500 mA

Last edited: Sep 26, 2012
10. ### Raul

33
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Sep 25, 2012
Even a crippled dyslexic like me should get that huh?

One of the lamp fixtures tested across the leads yields 27 ohms Does that inform what mA rating they are? Is that mA load or draw?

Last edited: Sep 26, 2012
11. ### CocaCola

3,635
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Apr 7, 2012
Another way to help clarify it is to always carry out the 'Amps' to three places by padding with zeros...

examples...

0.350 A = 350 mA
0.035 A = 35 mA
0.100 A = 100 mA
0.001 A = 1 mA

so

0.500 A = 500 mA

12. ### BobK

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Jan 5, 2010
No, 0.5 is 5 tenth's.

See the pattern:

0.005 is 5 thousanths
0.05 is 5 hundeths
0.5 is 5 tenths

Bob

13. ### KrisBlueNZSadly passed away in 2015

8,393
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Nov 28, 2011
Yes, you're exactly right so far.
Yes, 0.1 amps is one hundred milliamps.
No, that's not the best way, because that measures the COLD resistance. When the filament is glowing, its resistance is higher.

Connect the bulb to the power supply, and insert the meter, set to CURRENT range, in the circuit. You should probably use the 10A range on your meter, at least to start with. If the current is too low to measure accurately on the 10A range, you can try the next range down, which is often 200 mA, if the current you measure on the 10A range is less than 0.2A.
There's no reason to have two separate supplies, but you'll probably need a power supply with a higher rating than the 500 mA one you currently have.

14. ### CocaCola

3,635
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Apr 7, 2012
Yeah, those mini incandescent dollhouse bulbs generally pull at minimum about 50-60mA a piece some upwards of 200mA... They will gobble up the available Amps of a supply real quick, thus my initial suggestion of swapping out for all warm white LEDs but I understand if you have the fixture already with the bulbs installed it's a hassle...

15. ### Raul

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Sep 25, 2012
QUOTE=KrisBlueNZ;1495294]Connect the bulb to the power supply, and insert the meter, set to CURRENT range, in the circuit. [...] use the 10A range on your meter,[...] try the next range down, which is often 200 mA, if the current you measure on the 10A range is less than 0.2A.[/quote]

OK thanks. Using a low voltage PS I have hanging around I am getting 60mA on average. you are spot on CocaCola.

I’m thinking a Switched wall- plug in PS is the better course I can go up to 1 amp which should drive as many as 16 incandescent bulbs that can draw 60mA each. I’ll only have maybe 6 or 8 of them plus the 8 LEDs . I say switched because I’ve read they generate less heat.
I’m a tad shy about going higher than 1 amp. Hell I would have preferred to stick at 500mA
I’ve got
80mA in LEDs
480mA in Incandescant bulbs
Yah I need a bigger PS.
Is headroom an issue? Should I look to have X more mA than the anticipated load as a buffer to prevent the PS from overheating?

I've learned in this project.
My multi meter skills were limited to continuity and line voltage + polarity checking.

16. ### KrisBlueNZSadly passed away in 2015

8,393
1,271
Nov 28, 2011
If your maximum load is 560 mA I would think a 1A switching supply would be a good choice.

With incandescent bulbs you need to ensure that the power supply will start up when the bulbs are all cold. They draw significantly more current when they're cold than when they're hot, but I don't know exactly how much more. Does anyone know the typical ratio of cold current to hot current?

A 1A power supply should start up with all bulbs switched on, as long as the cold current is no more than twice the hot current. In your application, I guess the bulbs are switched separately, and most won't be switched on at startup. So there will be even less of a problem.

17. ### duke37

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Jan 9, 2011
A 8V 0.15A (1.2W) bulb measures 10ohm on my analog meter which rises to 18ohm in a few seconds as the filament warms.
The running resistance would be about 53ohm so resistance ratio equals 5.3.

18. ### Raul

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Sep 25, 2012
You have a doll house too?
It's amazing what a guy will do for a little girl
"You wanna a doll house? Oh maybe about a gozillion hours OK baby no prob"
Oh man did I not know what I was getting into.

This one is stick built with miniature 1/12th scale 2x4s and 2x6s and 2x12s all milled in my shop, we made the windows. also made the roofing shingles and clapboards and interior trim molding stairs - the works - all in my woodshop. It's been a project. We do the assembly together and a little of the milling together too. I'm trying to teach her a a few woodwork skills in the bargain.

At first I was testing these large long 12V incandescent bulbs that I bought for background lighting. They don't get bright enough so I switched to LEDS in the application I was going to use them. The long incandescent bulbs run about 60mA. Since I posted that I've tested a few of the commercially made fixtures which tend to clock in at 10mA. They are really small. But I did not try to run them over a period to check resistance over time.

19. ### duke37

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Jan 9, 2011
I do not have a doll's house, I use these bulbs for the dial lights in valve (tube) radios.
Using a 8V bulb on the 6.3V supply means that the bulb gives less light but lasts forever.

The variation of resistance with temperature can be used to stabilise the output voltage of a Wein bridge oscillator.

It is good that you are teaching a girl woodwork. I have a neice who has fitted two kitchens. I measure several times and then cut wrong, she measures once and then cuts right.

20. ### CocaCola

3,635
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Apr 7, 2012
You should invent a board stretcher... Because whoever invents that will certainly be a millionaire overnight