12v to 7v For LCD Screen

[Xperiment]

HI Guys,

Some of you may have seen the LCD screen I am fitting to the front of my project Here.

I need some help with powering the screen. It takes a 7.5v @ 3.0A supply according to the mains adapter that came with the screen, but I really don't want another plug to get my feet caught up in.

So can I reduce my 12v rail from PSU with a resistor of a voltage regulator to give me that much needed 7.5V. If so what would be the best way to do it, I have a bit of experience with electronics but its quite basic.

Any help much appreciated, thanks

[.Maleficus.]

I'm sure you could find some way to modify this tutorial and make it work. It would run at 7v, but that would be enough to power the display fine.

http://www.techpowerup.com/articles/other/137

[xmastree]

I need some help with powering the screen. It takes a 7.5v @ 3.0A

I wouldn't use a resistor for that. That's only good for static loads, i.e. loads which don't change. Your screen will take a maximum of 3A, but that will vary in use. Especially if it goes into standby.

So, you need a voltage regulator to give the right voltage at any current.

The simplest form is just a zener diode, but for 3A you need something beefier. A zener diode and a transistor.

Here is a brief description of what you need. Their example drops 20V down to 14.3V. For your 7.5V, you will need a 8.2V diode. IIRC they're available in that voltage, which is handy. Where it says 'unregulated DC voltage input', that's where you connect the 12V. The regulated output will be your 7.5V for the monitor.

The transistor will be dissipating the extra power. In this case, at 3A, it'll be 3*4.5, or 13.5W. Bear that in mind when selecting one, and it'll need a heatsink. Don't just bolt it to the case unless you use an insulating mounting kit. The mounting surface is usually the collector and will be at 12V.

I'm sure you could find some way to modify this tutorial and make it work. It would run at 7v, but that would be enough to power the display fine.

http://www.techpowerup.com/articles/other/137

Nooooo!!! Don't do that!

Why? Well, whilst it's fine to do that with fans which don't have any other connections, the monitor will have a 0V line which will connect to the 0V in your computer. Connecting it from 5-12 will connect the monitor's 0V to your computer's 5V. So it will short circuit your conputer's 5V when you plug it in.

I have a bit of experience with electronics but its quite basic.

Let's see if we can put that to good use, and maybe expand it a little.

[.Maleficus.]

Hehe, in that case, listen to xmastree. He knows WAAAY more than I do.

[Xperiment]

Thanks xmastree - The solution you have given does seem to be exactly what I am looking for. I must admit I have never used a transistor before but after a bit of ready I understand 'basically' what it does.

I will have to pop over to maplins to buy some bits and play about with the circuit on some stripboard. on my shopping list will be:

8.2v zener diode (I assume This one will do the job, code AY71N
Transistor (Again I assume This one)
Resistors (I have no idea what rating to get, any ideas)

As you can see the extent of my knowledge is indeed very basic. If you could point me in the direction of the correct order codes it would be much appreciated. The extent of my knowledge doesnt really go much further than LED's and switches

[xmastree]

8.2v zener diode (I assume This one will do the job, code AY71N

That one only seems to be available in 12 or 15V. I don't think you'll need a 5W one, but I'll be able to work it out later, once we've selected the transistor.

Transistor (Again I assume This one)

Not beefy enough, you need one which can handle 3A
Something like a BD243C, (N91AH) would be appropriate.

Resistors (I have no idea what rating to get, any ideas)

What resistor you need depends on the transistor, as does the wattage of the diode.

The transistor does most of the power work, the zener diode just controls it.

Now that I know you'll be shopping at Maplin, leave it with me and I'll take a look for you.

[Xperiment]

Now that I know you'll be shopping at Maplin, leave it with me and I'll take a look for you.

That would be great, once I know what I need I can get all the bits and start playing about, and hopefully learn something at the same time.

[xmastree]

I've changed my mind about the transistor. That one I suggested has a gain of only 20, meaning that for 3A we'd need a higher power zener diode, and they only come in 12 or 15V. Instead, we'll use a darlington. This is basically two transistors in one package. It works in almost the same way, but gives a much higher gain. It also has a higher base-emitter voltage, so we need to adjust the zener diode slightly to compensate.

Let's use a TIP120 instead

At 3A the gain is 1000, so for 3A out we need a mere 3mA in. Just to be on the safe side, we'll make it 5mA. This doesn't mean we'll get 5A out, only what the monitor requires.
The voltage across this resistor will be 3.2V, for 5mA we need 640 ohms. 620 is probably the nearest value. 1/4W will be fine.

For the diode, we no longer want 8.2, we now need more like 8.8 since the darlington has a higher base-emitter voltage. 8.8V zeners don't exist, but you can still use the 8.2 and put another diode in series with it to make a further 0.6V.
So, 8.2V zener, QH12N with a 1N4148 QL80B) in series with it.

Confused yet?


You'll need some sort of heatsink, that transistor will be dissipating up to 13.5W. You can bolt it to the case, but use a mounting kit, with an insulating washer. Those used to come with insulating washers for the mounting screw too. No good fitting the insulator then shorting it out with a screw!

Have you given any thought as to where you'll mount it yet? I'd be very tempted to put it inside the PSU. It'll be very small, so that should be possible.
Build it on the bench first, and test it with a suitable load before plugging in your monitor... Something like a 12V bulb, maybe 21W.

[Xperiment]

Wow you sure know your stuff. . . .

I will get those parts next week and, as you suggested, connect it up on my work bench for testing.

As for mounting, i've opened the PSU to check for room but there isn't even a 10mm square of space in there. So my plan would be to mount it in a small project box with a heatsink on the transistor and a small opening to allow a bit of air flow. Then I can mount this inside the front panel of the case next to the screen.

Gotta say a huge thank-you for the effort you've gone to finding the parts and explaining it in such a way that I can understand and follow, its very much appreciated.

[xmastree]

See how we 'make' the 8.8V zener? 8.2 volts across the zener, plus the 0.6V forward voltage of the standard diode.

The b-e voltage of the darlington will be about 1.2V, so the output will be about 7.6V.

Rather than mounting it in a box with a heatsink, I would be tempted to bolt it to the case (remember the insulator) and fit a box upside-down over it, without the lid. Just use the lid as a template to drill holes in the case, and fasten the box to the case.

Use a JP42V or JM16S for the cables in and out.