## LCD Module Backlight Power

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I have this LCD module: http://www.lumex.com/pdf/LCM-S01601DSR-page2.pdf. It lists the voltage for the backlight as 4.6V max and current as 200mA max. Does this mean I can't connect this directly to a 5V supply? Do I need to add a current limiting resistor like a discrete LED? I want to use a transistor controlled by a ATMEGA to turn on and off the backlight, but I'm not sure if I need to add resistor.

Yes, you need to add the series resistor.

If you want to vary the backlight, then use a FET switch in the ground lead of the backlight and include the on-resistance of the FET as part of the total series resistance. For example, if 4.3V gives 200ma, then you would use a total resistance of (5V-4.3V)/.2A = 3.5 ohms. If the FET on resistance is 1 ohm, you would use a series resistor of 2.5 ohms.

All that said, if they say 200ma ABSOLUTE MAX, then that is NOT the average current. Simply do not exceed 200ma at any time. So, you need to verify that you are under BOTH the 4.3V limit and the 0.2A limit.

Jim

Until Black Lives Matter, we do not have "All Lives Matter"!

Quote:
Do I need to add a current limiting resistor like a discrete LED?
Some have resistors already on the pcb BUT ALWAYS CHECK. You should be able to see if a resistor is fitted.

John Samperi

Ampertronics Pty. Ltd.

https://www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

Thanks. It wasn't clear.

Typical values are 4.2V and 150mA. So, that gives (5.0V-4.2V)/0.150A = 5.33 ohms. So, maybe this is a safer value?

Close. (If I did this correctly...)

If you have 5V to start with, drop 4.2 across the Backlight LED(S), you have 0.8 V left.

That 0.8 V is expended across BOTH the in-series resistor, and the FET used to turn the BL On & Off.

If the Fet has a 1 ohm resistance, as suggested by Jim, then the 5.33 ohms is actually the total of the series resistor + the 1 ohm from the FET. So the series resistor is 5.33 - 1 = 4.33 ohms.

Pick a nice value, we are actually splitting hairs on this one.

Note, also, that a 4 ohm resistor with 0.15 A will dissipate about 0.1 Watts. (Vr = I* R = (0.15A) * (4 ohms) = 0.6 V. (So, 0.6 V across the resistor, 0.2 V acorss the FET). P=VI = (0.6V) * (0.15A) = 0.09 W ~ 0.1W.

A 1/8 W resistor would get quite warm, a 1/4 W or 1/2 W resistor would be much larger physically, but would also dissipate the heat better.

JC

Actually, I was thinking of using a bipolar transistor like a 2N4401 (with VCEsat=0.4V) or a ZTX450 (with a VCEsat=0.25V), but a MOSFET might be better. Can anyone recommend one?

Something doesn't look right here. Why would an LED backlight on a 1x16 LCD module consume that much power? And the Vf is in the 4 volt range which would only make sense with a blue LED. It seems there must be be some driver circuitry already on the module or the datasheet in wrong.

Not necessarily; many LED backlights are in series-parallel connection.

I tend to use multiple series resistors to limit the dissipation in each resistor.

Rick_L wrote:
Something doesn't look right here. Why would an LED backlight on a 1x16 LCD module consume that much power? And the Vf is in the 4 volt range which would only make sense with a blue LED. It seems there must be be some driver circuitry already on the module or the datasheet in wrong.

It does seem like a lot of power for some simple backlight composed of LEDs.

devils4ever wrote:

It does seem like a lot of power for some simple backlight composed of LEDs.

No it really is exceptionally low in current consumption. There are for example LCD displays with red backlights that go up to 360mA.

- Jani

Jepael wrote:

No it really is exceptionally low in current consumption. There are for example LCD displays with red backlights that go up to 360mA.

I wouldn't dispute that, though I would like to see one. However, bear in mind that the only reason I question the data sheet is because this is a simple 1x16 character module. Most of these I've seen use only one or two standard type LEDs for backlighting (though some use EL type lighting). I have several of the LED type and the "typical" current consumption for the backlight is 20 mA. Personally, I have no idea why anyone would even use a small LCD display that required 150 mA to light.

Rick_L wrote:
Jepael wrote:

No it really is exceptionally low in current consumption. There are for example LCD displays with red backlights that go up to 360mA.

I wouldn't dispute that, though I would like to see one. However, bear in mind that the only reason I question the data sheet is because this is a simple 1x16 character module. Most of these I've seen use only one or two standard type LEDs for backlighting (though some use EL type lighting). I have several of the LED type and the "typical" current consumption for the backlight is 20 mA. Personally, I have no idea why anyone would even use a small LCD display that required 150 mA to light.

Because this is AC line powered. Power consumption is not an issue.

But the power you dissipate in components IS an issue!

If you use a 100mw rated resistor and dissipate 200mW in it, the board will get toasty color and the resistor will fail much earlier than it should. Thus, be aware!

Jim

Until Black Lives Matter, we do not have "All Lives Matter"!

Rick_L wrote:
Something doesn't look right here. Why would an LED backlight on a 1x16 LCD module consume that much power? And the Vf is in the 4 volt range which would only make sense with a blue LED. It seems there must be be some driver circuitry already on the module or the datasheet in wrong.

The last LCD module I used in a project (2x16) had a green LED backlight specified as:

2xLED in series, 18 groups parallel, for a total of 36 green leds, would make it 4V drop, and 180 mA with 10mA leds.

So doesn't sound too far off.