Bulbs replaced With Leds, about resistors?

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kuer

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#1
I know how to calculate the resistance value of a resistor for one single LED light. Currently I need to replace all 6 glass bulbs ( 12v 100ma ) with LED lights, They are supposed to be in series circuit, not sure how to determine EACH Led's resistance value? can you tell the the calculation formula?

Thanks.
 
#4
kuer said:
Thank you for your quick answer. seems not very difficult to deal with.
Does the 470R means 470 ohm? and what is the wattage of it?
Click to expand...

Are you sure that the six 12V lamps are all in series? That would mean a 72V power supply.
Then the 1.5V LEDs x 6 drop 9V, so there's 63V left for the single series resistor.
If we have 10mA current through the LEDs, then 63V/0.01A = 6.3k, use 6.2k.
Then the resistor's power is 63V x 0.01A = 0.63W, so use a 1W, 6.2k resistor.

If the six bulbs are actually in parallel (more likely) then for EACH LED (also all in parallel)
you have for the series resistance, (12V-1.5V)/0.01A =1050 ohms (use 1k)
and for the power rating, (12V-1.5V) x 0.01A = 0.1W, use 1/4W.

Are you sure that this is 12V DC, and not 12V AC? If it is AC, you must add a series 1N4005 diode
between the LED cathodes (all connected together) to ground. You don't want reverse voltage
on an LED. The diode could also be on the common connection of the six 1k resistors instead.

You can't put the LEDs in series because the available voltage isn't large enough.
You can't use just one resistor for all six LEDs because they won't share current equally.
 
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#6
The proper way to calculate required resistance is to subtract the forward voltage of all your leds from your supply voltage, then calculate the required resistance to pass your desired current at that voltage.

12V - (6 x 1.8) = 1.2V = not really enough voltage so you should do 2 strings of 3 instead.

470R = 470 ohms
 
#7
nigelwright7557
rayma
jwilhelm

Thank you.

you all right. please read the following review about it. should be 2 groups for two meters and 3 bulbs in series each group.

Sony TA-N77ES Review

Here is part of the review on the amp.

" While every American electronics engineer knows never to connect incandescent bulbs in series, Sony did exactly this: connecting three 12V 100mA bulbs in series, connected to 36V DC. This sounds fine in theory, but is never done in practice because any variation between bulbs will divide the voltage unequally, so one or more bulbs will be fed with more than 12V, greatly shortening their life. Worse, as soon as the first bulb goes, all the bulbs for that meter go dark at the same time. Sony also made the mistake of running 12V bulbs at 12V, not a lower voltage which would have let them run much longer. "

The meter bulbs are all blew out, need to replace. while I want to use LED lights for long time use.

Thanks.
 
#8
kuer said:
should be 2 groups for two meters and 3 bulbs in series each group.
Click to expand...

Then use two parallel strings, each of a resistor and 3 series LEDs.
For each of the two strings, (36-4.5)/0.01 = 3150 ohms (use 3.3k)
and for the power, (36-4.5) x 0.01 = 0.315W (use 1/2W or 1W).
Using 10mA LED current is usually plenty. Note that other LED colors than
red will have different voltage drops, but the result won't change too much.
 
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#12
rayma said:
Then use two parallel strings, each of a resistor and 3 series LEDs.
For each of the two strings, (36-4.5)/0.01 = 3150 ohms (use 3.3k)
and for the power, (36-4.5) x 0.01 = 0.315W (use 1/2W or 1W).
Using 10mA LED current is usually plenty. Note that other LED colors than
red will have different voltage drops, but the result won't change too much.
Click to expand...

Ok, so I may get two 3k-4k ohm resistors and 6 pieces of 1.5v 10ma LED locally, thanks.

btw, what about the brightness of the LEDs by comparation to the original bulbs?
 
#13
kuer said:
Ok, so I may get two 3k-4k ohm resistors and 6 pieces of 1.5v 10ma LED locally, thanks.

btw, what about the brightness of the LEDs by comparation to the original bulbs?
Click to expand...

Look for wider angle LEDs and you may need to mount them back a little farther than the bulb as they will be spotty. The colour usually dictates the forward voltage and brightness. You may need to adjust the current a bit to improve their brightness.
 
#15
kuer said:
Ok, so I may get two 3k-4k ohm resistors and 6 pieces of 1.5v 10ma LED locally,
what about the brightness of the LEDs by comparation to the original bulbs?
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That will likely be less. Also, the light doesn't spread out evenly. You can get white diffused LEDs.
There are also high brightness types, but they need more current.
 
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#18
rayma said:
That will likely be less. Also, the light doesn't spread out evenly. You can get white diffused LEDs.
There are also high brightness types, but they need more current.
Click to expand...

That's right. I went to a local store to talk to a tech about it, He has same opinion, the sony meter area is too big, leds may not be good enough for even brightness. I haven't tried them out yet.

But the store has glass 14v 80ma bulbs, the original is 12v 100ma. so I think 14v 80ma is perfect for replacement. what do you think?

Thanks.
 
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