LCD back light

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Natalie Wilkie

New Member
I have a quick question about a typical lcd.

Pins 15 & 16 are the back light connections to 5v & gnd.

My question is can I connect the 5v directly or should a use say a 1k resistor?

Thanks
Nat
 
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manuka

Senior Member
I've no idea what "pins 15 & 16" connect too- there may well be further supply massaging included already! How about some technical specs. for this display!? In general, as each regular white LED runs on 3.6-4V & draws 30mA, a dropper R will of course be needed on 5V. To be on the safe side suggest you try ~1k series & monitor current , & see how bright the display looks.
 
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moxhamj

New Member
I use a 1k resistor and it works fine. Some sort of resistor is needed and it could be 330 ohm if you want it a bit brighter.
 
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manuka

Senior Member
That's better! Those specs. indicate Pin 15 (Anode) for LED back light is rated at 5V DC, & 0V at Pin 16.
 
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lanternfish

Senior Member
Forgot to mention. If you have a good magnifier, check if there are any smd resistors on the tracks that lead to the LEDs. If they exist you should be able to read their value. Then use Ohms law to get an external resistor value for (say) 10mA therough the LED's. Allow a voltage drop of about 2.0 - 2.6V for the diodes.

Cheers
 
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Dippy

Moderator
I'd follow Stan's advice. Try a 1k0 and measure the current and don't go over 30mA.
Then try lower values.
In fact you may find that 10mA is bright enough.
I would suspect you'll need much lower values, but start high and measure. Better dim than popped !

Typical naff Data Sheet doesn't give backlight Vdrop or max current.
(Unless there are more pages to this Data Sheet missing?)
With it just saying "5V" that implies that the limiting resistors are on-board, but I wouldn't be too "Sure" about that :)
 
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kevrus

New Member
As others have said, I would be inclined to add a series resistor, measure the current and check the brightness of the backlight. Good illumination is achieved at quite low currents.
You need to ensure that a resistor is fitted on board first before applying 5v direct. A 1p resistor is a lot cheaper than replacing the display.
 
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Andrew Cowan

Senior Member
Most backlit LCDs I have used have a space for a limiting resistor on board - sometimes it is there, sometimes there is a jumper lead. You should be able to see if there is an onboard resistor, and then measure it.

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hippy

hippy

Ex-Staff (retired)
Even if the backlight doesn't need a resistor it can be beneficial to use one. Backlights can draw considerable current ( often many times more than the rest of the circuit ) and be overwhelmingly bright. Put in a low value resistor and you can reduce current considerably and give a much more comfortable display.
 
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moxhamj

New Member
Mpep, the text in that link is a bit ambiguous:

- LCD Type: STN Blue

LED Backlight White

- Driving Voltage: single +5V supply

- Viewing Direction: 6 o'clock

Quoted verbatim with the - missing on one line, and it could mean the driving voltage refers to the led but I read it that the driving voltage of 5V is the supply for the module itself.

I've got 4 CP/M boards running next to me at the moment with 20x4 LCD displays on each and they all have 1k resistors on the backlight and the brightness is perfect in a room lit with a 60w globe.
 
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fizzit

Member
Dippy said:
I'd follow Stan's advice. Try a 1k0 and measure the current and don't go over 30mA.
Then try lower values.
In fact you may find that 10mA is bright enough.
I would suspect you'll need much lower values, but start high and measure. Better dim than popped !

Typical naff Data Sheet doesn't give backlight Vdrop or max current.
(Unless there are more pages to this Data Sheet missing?)
With it just saying "5V" that implies that the limiting resistors are on-board, but I wouldn't be too "Sure" about that :)
Click to expand...
I believe a one kilo-ohm resistor limits the power to 5 ma at 5 volts and if the LED gets the voltage it wants it would probably end up at about 1 ma...
 
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Dippy

Moderator
Basically;
I = (Vsupply-VfLED) / R

Surely if there are any ambiguities or 'not sures' then the obvious way is to start high-ish res value and work down whilst measuring the current and observing the light output.

Even if there is a limiting resistor on-board the current/light may be unsuitable.
So difficult...
 
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MPep

Senior Member
@Dr Acula,

Yes you may be right there. I read that the following specs were for the backlight LED, but obviously that cannot be correct when reading the 6o'clock statement.

I too would play on the side of caution and add a resistor, just in case.
 
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Thomas Henry

New Member
Hello all,

This is my first message, and moreover I'm only one week into using the Picaxe, so I hope I'm posting the right way! I'm definitely a novice.

Anyway, with regard to the LED backlight on an LCD, I think the previous responses have been way too conservative. How about 4.7 ohms (!) instead?

Here's the deal, if in fact there was only one LED in a backlight, then a current limiting resistor of 330 ohms would be good. However after some digging on the Internet today, I discovered that there are actually 20 LEDs in a backlight (at least in the common Seiko L1682 type that I'm using). See the following:

www.glyn.de/data/glyn/media/doc/Manual%20Japan.pdf

They are arranged in two groups of ten. Ten LEDs are in parallel connected in series with another group of ten LEDs in parallel. According to the spec sheet, the entire bundle drops 4.1V, and needs to stay below 200mA. (That's 10mA times 20 LEDs).

To be conservative, I went with 100mA. This works out to a 4.7 ohm series resistor. I monitored both the current through the LED array and voltage across the current limiting resistor and observed 110mA at 0.5V. The power dissipation is well below a quarter watt.

Finally, the display is actually quite bright at 110mA, so even though the spec sheet says one could go double that, I don't see any reason to.

Hope this helps,

Thomas Henry
 
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westaust55

Moderator
Welcome to the PICAXE forum.

Yes, a much lower resistor may well be adequate in many cases.

It is hard to provide a fixed and simple rule for all cases.

I have a number of LCD modules with backlight and these all have an onboard 8 Ohm current limiting resistors but the manual indicates that even my particular type/make is available with and without the currently limiting resistors. The board has a number of jumper pads that can be cut or bridged depending upon options utilised.

Like the LCD you have, mine also have an array as two rows of 12 LED’s for a total of 24. The datasheet indicates max LED current as 180mA.

I used a 100 Ohm multi-turn potentiometer for further adjustment and can wind the resistance up to have up to a further ~50 Ohms in series before backlight operation cannot be perceived.
 
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manie

Senior Member
I usually start off with the "golden" value 470R 1/4W. Sometimes the brightness is a bit high still and then I'll try a 680R etc untill the brightness is OK to taste. A limiting resistor can save the display's backlight and does no harm. I will fit one just to be "Sure"...
 
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Thomas Henry

New Member
Hi again,

It would seem, then, that some backlights consist of much less than 20 LEDs ganged together if resistors above 100 ohms or so are being used. I'll have to keep my eyes open for that.

I'd be curious to hear of any such displays, how many LEDs they contain and how brightly they shine with fewer elements.

Thomas Henry
 
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