Low wattage light bulb G4 size

[UK-Corlett]

I needed a G4 size light bulb to fit in my new light fitting.
http://www.timage.eu/Productimage.aspx?id=938

I want to use a small 7Ah lead acid battery. But at 10 to 20 watts I would only get 9 to 10 hrs use. There are LED based bulbs which are very energy efficient.

http://www.ultraleds.co.uk/default.php?cPath=74
http://www.ultraleds.co.uk/default.php?cPath=51

But I could not find a G4 size for "end on" use which would fit so I made one and here it is.

It work very well gives as much light as a 10 watt bulb for just 1 watt. The life expectancy is 100,000hrs and will be damaged less by vibration and rough use.

*** my IE7 shows no picture here !!! ****
*** but my Opera does *****
*** however the pic is in my album****

[Chuck Craven]

UK-Corlett

If you replace the 10-ohm 5watt resistor with a 25-ohm 2watt resistor, you can run the LED on 12v. The DC-to-DC converter will make the led less efficient, as it takes energy to make the converter work. By using a 25-ohm resistor you will current limit the led and not loose any efficiency.

Chuck

[UK-Corlett]

Possibly but my electronics man says DC/DC is the way to go. The thing is that the LED is drawing .350A @ 3.5V =1.25Watts

If I have understood.
The LED drops 3.5V which means the resistor has drop 8.5V.
If 0.35A goes through the circuit then the resistor has to sink
W=AxV . w=0.35x8.5 =3watts.

So at a pinch yes you could. But I was told that a DC/DC convert was much more efficient than just turning the excess power into heat.

However if the LED takes 1.25watt and the resistor takes 3watts thats 4.25 watts. There is little point in going to all this effort with LEDs if you are only going to save 50% power over a standard incandesent 10watt bulb.

I shall ask the man who knows tomorrow. And get back. If this whole LED thing is worth persuing we need to thrash this out.

Clive

PS. I am a mechanical engineer, the finer points of the circuit need specialist help.

[Chuck Craven]

Well the DC to DC converter is 75% efficient. So it really doesn’t make much difference both ways will eat up 4 watts of power. I did tell you wrong the 25-ohm resistor needs to be 5 watts not 2 watts. The only savings is in the cost; the DC/DC converter verses the 25-ohm resistor. A LM317 voltage regulator could be use to drive the LED with out a resistor in series. The LM317 still will not be more than 80% efficient and it generates heat also.

Chuck

[UK-Corlett]

Hi Chuck, thanks for your challenge. I appreciate someone's help to get my head around this.

I have had a chat with my "man who knows"

You wrote "Well the DC to DC converter is 75% efficient."

Is this not the key? it only burns 25% of what it regulates as it is a switching device, so its off for a lot of the time. A resistor and a the LM317 (For anyone else reading this, a conventional regulator) are linear devices and burn 100% of what they regulate.

Just putting a limiting restore in series with an LED is the standard way but this LED draws a lot of current 350mA as opposed to 20mA for a standard one.

I have a DC/DC here, so I will LASH up a test and give you the actual vales I measure. Its a 10w 12V-5V 79%eff one but it will prove concept.

Clive

PS I used a 5watt resistor because I had one. Probably the most common selection perimeter for and desecrate device.

PPS If I am correct and DC/DC is the way to go. There is a TMR1210 which gives 3.3V out. You would not need the limiting resistor.

PPPS I only offered the 12V circuit as an option for other users, as I have a 10W 12V-5V DC/DC supply in my little power box. Its main use is to recharge mobiles phones, mp3 players etc. I have only now pressed it into service to drive the lighting.

[UK-Corlett]

Hi Chuck

I did the experiment.
Bench PSU set to 12.8V. A 10w 12V-5V DC/DC converter.
Providing 5V to my LED-G4 bulb with a 10ohm series resistor.

Measured.
0.139A through the 12V circuit and 0.190A through the 5V.
using W=AxV
I get 1.67w drawn from the 12V PSU or would be Battery
and 0.95w drawn from the 5V LED-G4 bulb plus resistor
and measured 3V across the LED.

Conclusion.
The 10 ohm resitor drops a lttle too much voltage as the LED can run at 3.5V, however 3V is still very bright.
The LED-G4 bulb draws the 1w I was looking for when I set out on this project. But droping from 12V to 5V, costs me an extra 0.7w.
So my LED-G4 bulb 1.67w will operate 6x longer than a conventional 10w incandesent bulb.

Probably worth the effort over and incadesent.

Now your point. Should you just use a resistor and forget about the DC/DC?
In the above case its close. Your resistor has to drop 9V@ 0.190A=2.71w
+ 1w through the LED 2.71w. So 2.71 verse 1.67.
However I have had a play with the LED and its not running at its rated out put. If I increase the voltage by 0.5v its doubles the current and gets very very bright. At this its drawing 350mA, a full watt across the LED.

Then your resistor would have to drop [email protected]=3.15w + LED =4.15W



MY MUM SAY'S "you pays your money, you makes your choice"

Clive

[Chuck Craven]

Yep the LED and the 25 ohm series resistor pulls 4.48 watts.

I am a little surprised that the DC/DC converter is only drawing 1.77 watts on the 12v side. But that may be do to you are not running the output at full power.
If you put two LED’s in parallel and run them of the DC/DC converter you will draw close to 4 watts of the 12 source. But losing .77 watts to the converter is doing real good.

My LED lights are using 12 LED’s in series parallel with a 1 ohm current limiter resistor and the total wattage is 1.15 watts off the 12 source. But the total light output is not as bright as the one you are using. The one nice thing about using 12 LED’s is, I bought 100 of them for $3.00, At 3 cense each I can make lots of cheep lights. The LED’s are super bright and draw 30 mA at 3.3v.

1.77 watts is real good in replacing that 10 watt heat lamp.
Good gong!

Chuck

[UK-Corlett]

Does'nt It make you sick

You argue your point, you experiment, make, justify, sort out all the problems and when it's all over you find out what you should have done in the first palce.

Data sheet and build instructions
http://www.maxim-ic.com/appnotes.cfm/ap ... umber/3532



This circuit works with 3V to 12V and has a built in dimmer and is supper efficient. Dirves the same super bright LED mine did.


Sorry Chuck, we argued about the the wrong thing.

Clive

[Chuck Craven]

I just looked up the speck on the MAX chip, with your LED, that circuit will draw 4 watts off the 12.8volt battery. Your DC/DC converter is more efficient.
That chip is a liner current and voltage regulator device, just a little better than the LM317. With all the parts needed for that circuit your DC/DC converter is even cheaper.

I don’t think we are arguing “Gust having a good tech chat”… It’s fun to do things like this now and then!! Show one can be civil and talk electricity…
Like they say there is more than one-way to power an LED, or is that skin a cat?


Chuck

[Chuck Craven]

OOPS was looking at the wrong chip. That chip is a chopper regulator. That’s way the low current draw. The only problem with this chip if it is not built right it can interfere with radio/TV receiving equipment near the circuit. If built on a perforated proto board it could take out radios and TV up to 1000’ in weak signal arias, like country campgrounds…
It will still cost a lot.

Chuck