which method should I use for C7 DIY LED window candles?

[Sam Seagate]

I recently took advantage of post-Christmas sales and purchased several
strings of warm white C7 size LED lights of 25 in each string. My
intention is to brighten the window candles we currently have.
Initially, when I designed the candles last year, I used a single light
from each string in place of the original candle bulb. I used a DC
"wall wort" 12 volt power supply to power each one with a dropping
resistor to reduce the current in each bulb to around the 17 mA level,
the same level I measured in the 120 V string before I cut the bulbs out
to use them. However, this left the bulbs too dim and the wife didn't
like the dimness. Then, a few days ago, since there's enough room
inside the bulb envelope, I doubled up two bulbs in series and adjusted
the current going in to be around 40 mA. Using the two series bulbs in
each candle envelope brightened the result considerably, but now I'm
concerned that there may be too much current flowing through them.
Although I didn't measure it, I believe the current in series bulbs
remains the same (40mA) but the voltage divides(?). This may be too
much for the bulbs.

Today, in an experiment, I decided to wire four bulbs in parallel and
set the input current at 10 mA. Measurements were 2.7 VDC @ 10 mA going
into the parallel combination. Four bulbs inside these small envelopes
begins to get difficult, but still not impossible.

My goal is to have the longest life possible out of these candles and
the maximum brightness which is the reason I've dropped the input
current and/or used more than a single bulb for each candle.

I have several questions:

1) If I use either the four bulbs in parallel driven at 10 mA and/or
the two bulbs in series at 40 mA input, is the resulting output
brightness going to be the same for the bulb combinations?

2) Which is the best method to use for preserving the longest LED life?

Thanks in advance,
Sam

[N_Cook]

2) Once you've decided on your current , add a polyswitch per path. To
make a bit more failsafe add some heat insulation around each polyswitch
and reduce the thickness of insulation if they falsely cutout. If you
don't want to go overboard with current limiting circuitry

[Jerry Peters]

For 4 leds in parallel you want to *increase* the current. Assuming
each led has an equal voltage drop, the current is divided among the
leds, so at 10ma total, each led is getting 2.5ma. I'd go with a
series configuration so you don't need a matched voltage. In a series
configuration each led gets the same current with the voltage divided
among them.

Since you have leds to spare, try increasing the current for a single
led until it fails, this gives you an approximate upper bound on the
current you can use.

> Thanks in advance,
> Sam

[Phil Allison]

Jerry Peters wrote:

>
> Since you have leds to spare, try increasing the current for a single
> led until it fails, this gives you an approximate upper bound on the
> current you can use.
>

** IME leds do not fail in any simple way when driven with excess current.

Years ago, I repaired Mesa Boogie guitar amp that was almost new. The owner had purchased it in the USA so it ran here from a 120V step down transformer. After a few months, it developed multiple strange faults: the volume level would suddenly drop and then return, tone settings varied all over the place in a similar way and the problem got worse the longer the amp was running.

Another tech decided it must be due to bad output valves an changed the lot - for no good reason. The fault remained and then it came to me.

After verifying the owners weird story, I removed the chassis and eventually spotted something odd. The changing behaviour of the amp corresponded with varying DC voltages on the leds that were part of the many Vactec opto-couplers in this model. The Vactecs controlled tone settings and channel switching.

The 5VDC supply to the leds was quite steady so it must be faulty leds - but all 20 of them! How ?

Due to a screen printing error on the PCB, resistor positions that should have been 470ohm were installed with 47ohm parts increasing the led current to 50mA or more. The leds tolerated this for a couple of hundred hours then began to fail by dropping light output intermittently. Some would go dark and then come back minutes later, when dark the full 5V was across the led terminals.

New leds and resistors fixed it completely.

Naturally, diagnosis and repair took a fair bit of time (though leds and resistors are cheap) the owner got a substantial bill which she was quite grumpy about.

Because the amp was purchased in the USA, there was no warranty cover here in Australia. The tech who unnecessarily replaced the output valves worked for the warranty agents here in Sydney and she had paid a big bill for that work too.

Her bargain priced Boogie turned out to be no bargain at all and she had to carry around a 6kg step-down tranny too.


.... Phil

[mike]

The brightness of each bulb is proportional to the current thru
that bulb no matter what the configuration.
Your numbers represent a 4:1 (or 16:1 if that's 10mA total for all 4)
difference in current thru each bulb
with a 2:1 difference in the number of bulbs. So, the answer is "no"
but the relationships aren't entirely linear.

>
> 2) Which is the best method to use for preserving the longest LED life?
>
> Thanks in advance,
> Sam

The hardest part of any project is setting the objectives/specs.

My favorite system architect, Beyonce, said it best:
"If you want it, put a ring on it."
You need to tie down the objective.
"Longest Possible" is not quantifiable and a useless constraint.

Sounds like the real objective is, make the wife happy.

Since we know zero about the LEDs, I'd assume that 17mA is a
good number for reasonable life. 40mA is probably too much.

First, decide if you have 12V. If it's a regulated wall wart, you will.
If it's one salvaged from a 20 year old screwdriver, it won't.
Adjust the resistor to whatever it takes to get the current right.

See if there's anything you can do with tinfoil and LED positioning
to aim the light where the wife wants it.

Beyond that, start adding lights at 20 mA until the wife is happy
or you run out of space. With 12V, you can put up to three LEDs in series
with appropriate dropping resistor to set the 20mA current.
At four, I'd put two sets of two, with a separate dropping
resistor for each series string.
For maximum life, you probably don't want to connect LEDs directly
in parallel.

Depending on the size of the lights you bought, you might find
that the 27 LED's from a Harbor Freight free flashlight are smaller
and can be packed in more tightly. Or you can buy unpackaged LED's
dirt cheap.
Just because you have Xmas lights doesn't mean that it's an optimum
means to achieve your objective. Xmas lights are designed for low
cost. They run for a few hundred hours and nobody worries much
about whether they run at all next year. If you want high quality
candles, it's better to start with high quality components.

Remember that, for most electronic stuff, reducing the temperature
by 10-degrees C doubles the life. And that's the CHIP temperature,
not the ROOM temperature. Chip temperature is proportional to current.
If you pack stuff too tight
and raise the temperature, you're defeating your purpose.
While you can certainly buy single LED's that are bright enough,
they're also hotter and will likely require some heat sinking.
And the resistors add heat too.
There is no free lunch.

If that's not enough brightness, you have to make a tradeoff.
Shortening the life by increasing the current increases the wife's
happiness.
Anybody who has the slightest indecision about which way to go
has never been married.

The lights probably came with sockets. Use the sockets and
just replace them when they burn out.
Spin it like a politician. Every time the light burns out,
you're a hero for fixing it. Maybe you want them to fail every
28 days...be sure to get the phase right. ;-)

Bottom line is that it's not about the technology.
Your tradeoffs are about the benefits of a happy wife.

[Sam Seagate]

Thanks for the tip! A "polyswitch" wasn't something I was familiar with
until I looked it up. It looks like a fuse that resets itself once a
fault is corrected, is that right?

The configuration I've gone with is four LED's for each C7 bulb. The
LED's are driven in series by a 12 V power supply with current set at 10
mA.

I have a current resistor set for each four LED candle since each one
drew a different current amount with the same resistor. All set for 10
mA.

The question is, where do I place the polyswitch and how do I determine
what amperage/ voltage I need? For two rooms, I have a 12 V supply
powering two candles, with each candle set as above (3V @ 10mA) and in
another room, I have four candles powered by a 12 V supply. Ideally,
I'd like to place the polyswitch at the power supply output before it
branches off to either the two or four candle arrangement.

Any tips here would be helpful.

Thanks,
Sam

[mike]

The reason for that was mentioned earlier. 12V is not enough voltage to
reliably run 4 in series.
Two sets of two in series with two resistors
will be far more stable over temperature and with variations in led
characteristics. The cost is twice the energy consumption.

You might find that 3 in series at 15mA gives you more light and only
50% energy penalty. It's always something...

>
> The question is, where do I place the polyswitch and how do I determine
> what amperage/ voltage I need? For two rooms, I have a 12 V supply
> powering two candles, with each candle set as above (3V @ 10mA) and in
> another room, I have four candles powered by a 12 V supply. Ideally,
> I'd like to place the polyswitch at the power supply output before it
> branches off to either the two or four candle arrangement.
>
> Any tips here would be helpful.
>

On the surface, it seems silly to replace incandescents with LED, then
use an incandescent as a current limit.
BUT
Incandescent lamps make excellent positive-temperature-coefficient
resistors. You just use a 12V lamp. Pick the lamp current so that,
when you short the output, the lamp doesn't overload the power supply.
Can get them at any auto store and they last forever in this mode.

The polyswitch has a hockey-stick resistance curve and is clearly
a superior device for this application.
But you might already have a bulb in a drawer somewhere.
I just measured a 1458 lamp. It drops 0.44V at 40mA
so you'll need slightly smaller series resistors...or just not
worry about it. Probably won't work well with 4 LEDs in series.
Higher current incandescent will have lower voltage drop at 40mA

You will still want to use two sets of two series LEDs/candle.
There will be some interaction between candles if you plug/unplug one.

And you get a "short indicator" for free.


Life is a tradeoff...or two...
> Thanks,
> Sam

[Sam Seagate]

I wasn't clear on your reason. So far, so good. The 12 V input divides
down to 3V per bulb in series with each holding at the 10 mA level.

> Two sets of two in series with two resistors
> will be far more stable over temperature and with variations in led
> characteristics. The cost is twice the energy consumption.
>
> You might find that 3 in series at 15mA gives you more light and only
> 50% energy penalty. It's always something...
>>
>> The question is, where do I place the polyswitch and how do I determine
>> what amperage/ voltage I need? For two rooms, I have a 12 V supply
>> powering two candles, with each candle set as above (3V @ 10mA) and in
>> another room, I have four candles powered by a 12 V supply. Ideally,
>> I'd like to place the polyswitch at the power supply output before it
>> branches off to either the two or four candle arrangement.
>>
>> Any tips here would be helpful.
>>
> On the surface, it seems silly to replace incandescents with LED, then
> use an incandescent as a current limit.
> BUT
> Incandescent lamps make excellent positive-temperature-coefficient
> resistors. You just use a 12V lamp. Pick the lamp current so that,
> when you short the output, the lamp doesn't overload the power supply.
> Can get them at any auto store and they last forever in this mode.

I like the lamp idea and actually forgot about it. I used to use such a
set up with a high voltage circuit. Been so long ago that I forgot
about it. I'll have to see how much total current the circuits use and
then try out a bulb. I don't see any reason I couldn't place the bulb
at the power supply point before it branches off to the 2-4 candles.
Would certainly be a lot easier than trying to wire one up per candle.
Thanks for the idea here!

> The polyswitch has a hockey-stick resistance curve and is clearly
> a superior device for this application.
> But you might already have a bulb in a drawer somewhere.
> I just measured a 1458 lamp. It drops 0.44V at 40mA
> so you'll need slightly smaller series resistors...or just not
> worry about it. Probably won't work well with 4 LEDs in series.
> Higher current incandescent will have lower voltage drop at 40mA
>
> You will still want to use two sets of two series LEDs/candle.

So you're saying I would have been better off with two sets of two LED's
in series per bulb. A series-parallel combination, but that would have
required more resistors to set the current correctly per set.

[mike]

I'm saying that if you start with 12.0 volts and drop .44V across the
bulb and need 3V per LED...you don't have it. The purpose of the
resistor is to normalize the current given variances among the LEDs.
With 4 in series, the resistor gets so small that it doesn't normalize
very well.

Sure, the candles may light, but you'll get a different intensity
per candle and the resistance of the bulb will create interaction
between the candles.

I'm suggesting that if you
put two LEDs in series with a resistor.
put two of those series combinations in parallel in each candle
put the candles in parallel
put one bulb at the power source
You'll be at the sweet spot of low cost, long life,
fault tolerant, happy wife and just works.

In reality, you can probably tweak it to work with 4 in series.

What I'm trying to teach you, and people reading this next year,
is the importance of system design where the performance is largely
independent
of the parameter variations of the devices. And single
point failures will not set anything on fire.
And if you built a thousand
of them, they'd all perform the same. And if you substituted
a red LED, it'd still work the same. And if you put it outside
in below freezing temperatures, it'd still work the same.
And if your power supply was 11V or 13V, it'd still work the same.
And the margins are so wide that you'd never have to test any of that.
It will just work. The obvious tradeoff is that it takes
twice the energy to run it.

We haven't discussed your wall wart. If it's electronically regulated,
it's likely current limited to a level that protects itself and won't supply
enough current to set your house on fire. If so, forget the
bulb/polyswitch and let the wart take care of the current limit.

If you wanna discuss more "clever" designs, we can do that.
If you want a bullet-proof design that anybody could build
using random LEDs without
any test equipment or exotic devices, this is my recommendation.

Are we having fun yet?

[Sam Seagate]

For me, this was getting too complicated for the short time I have off
from work, and time available to complete this project. Tried the bulb
in series first, but the one I found-- 12 V @ 1.4 W-- pulled down the
current by 1/3 so it dimmed the bulbs too much. Other bulbs to try are
too expensive to keep purchasing, so I just went with 100 mA fuses.
Problem solved.

Thanks for the suggestions though, may come in handy for similar
projects in the future.

[hrho...@sbcglobal.net]

I would put the leds in series. Don't worry about the lifetime, it far exceeds a few hundred holiday seasons. The leds should be good for 50,000 hours if you don't run them close to their max rated current. 10 hours per day x 30 days x many many years to get close to 50,000 hours lifetime.

[mog...@hotmail.com]

Can you blame people for wanting to just buy new?

[Sam Seagate]

Update as of May 26, 2015: Well, using 4 of the warm white C7 LED bulbs
in series @ 10 mA current should have done the trick, but my resulting
lights have started burning out one by one. Lost two so far and another
is flickering. I can't figure out why, current is low and they didn't
seem to generate much heat to speak of. Also, when I was just using a
single bulb @10mA, none of the candles burned out for over 3 years and
even then when I decided to brighten them by using 4 in series, none of
the original singles had burned out. Any idea why the four in series
are slowly failing with same current and voltage as the single lamps
were? I'm disappointed in this for sure.

Sam

[Sjouke Burry]

On 26.05.15 18:44, Sam Seagate wrote:
> Update as of May 26, 2015: Well, using 4 of the warm white C7 LED bulbs
> in series @ 10 mA current should have done the trick, but my resulting
> lights have started burning out one by one. Lost two so far and another
> is flickering. I can't figure out why, current is low and they didn't
> seem to generate much heat to speak of. Also, when I was just using a
> single bulb @10mA, none of the candles burned out for over 3 years and
> even then when I decided to brighten them by using 4 in series, none of
> the original singles had burned out. Any idea why the four in series
> are slowly failing with same current and voltage as the single lamps
> were? I'm disappointed in this for sure.
>
> Sam

Part of the cooling comes from the mounting wires, and
if the leds are close together, they also get hotter.
That is bad.
Also a real current source instead of a (small??) resistor
might increase survival, because voltage drop for each
led is rather dependent on temp/current.