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Support Forum » Quick LCD question...

March 07, 2010
by mcai8sh4
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Alrighty people, just a quicky for you.

I've just received one of the new LCD displays (the 4 line one with the backlight) and before I started playing around, I thought I'd just check one thing.

I've also got a new programming header and that works wonderfully, and I've used the it to also power my little test circuits. I notice in the NK manual, that it states if you want to use the backlight feature, then you're better off using a 'walwart' (where does that name come from, we call it a transformer... but I digress) due to the power drain. Would it be ok to use the USB power with the backlight, or am I going to get bad things happening?

In short - is the backlight part of the LCD ok to run from the USB power?



March 07, 2010
by Rick_S
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The USB is designed to supply 100mA to 500mA Maximum. This current is defined as the unit load and can be requested by the USB device in 100mA increments to the max. If the higher current requests aren't made by the device (to my understanding anyway) the port is set to 100mA or 1 unit load.

The reason it is not normally recommended to connect the backlight to USB power is because it can draw a substantial amount of current on it's own and combined with the current draw of the USB to serial adapter + your test circuit, it may be enough to overload your port.

While typically the port would just shut down, there is always the remote chance that bad things could happen to your PC. That is why, while in some cases it may not be required, it is usually recommended to use external power such as a "Wall Wart" or battery through a regulator to provide power to your circuit when using the LCD backlight in your project.

Also, if you do opt for external power, keep the ground of your USB adapter connected to your circuits ground. Just disconnect the USB power line from your circuit. This will prevent any possibility of different ground potentials causing problems.

Off topic, I believe the AC adapter/transformers got the name "Wall Wart" because they look like a growth protruding from the wall :)

Hope that helped clear it up a bit.


March 07, 2010
by mcai8sh4
mcai8sh4's Avatar

Rick, thanks for that. Makes sense now.

I've got lovely blue sky at the minute, so there is no chance I'd see the backlight. I'll try it later from my other power source.

Also, I've connected the NO switch to the reset line and the 10K res as you explained to me a while back - lovely addition, makes life a lot easier.

Thanks for all the info you share with people here, I certainly appreciate your time and clear advise.

Right, back to playing with my new stuff.


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