Confused Beginner

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Confused Beginner

Postby Guest » Fri Feb 02, 2007 9:42 am

Hi,

I'm a complete novice and I'm trying to build my first mecahnical television. I'm using a Peter Smith design (from the NBTVA website http://www.nbtv.wyenet.co.uk/beginners.htm )

However, I'm confused to tha choice of resistors used in the LED arrays. The circuit shows 2 rows of 3 LED's in parallel, each row with a 22R resistor in it. If the supply voltage is 12V, the LED's 2V 20mA I work out that the 2 resistors need to be 330R. Am I right or am I missing something?

Many Thanks
David
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Re: Confused Beginner

Postby ac7zl » Sun Feb 04, 2007 12:53 pm

David-

This is my take on the question you raise...

First, a minor correction: The specific diagram I think you are referring to does not show the the diodes in parallel, but rather, in series.

The voltage drop across each LED depends upon the device. If, for the sake of argument, each LED had a 2 volt drop across it, the total drop across a chain of three LEDs would be (3 x 2.0) = 6 volts. If we assume a supply of 12 volts, and a desire to limit the current to 20 mA, this means we need a series resistor in each chain (of three LEDs) on the order of ((12.0 - 6.0) / 0.020) = 300 ohms. So, as I see it, you are correct about the 330 ohm resistors.

On the other hand, you aren't simply connecting the array to a static 12 volt supply. Presumably, it will be driven by the the LED driver circuit, also on the web page. This changes the game a little bit.

Note that because there are resistors between the source of the mosfet and ground, the voltage appearing at the source terminal (of the mosfet) will be proportional to the current being fed through the LED array. By sampling this voltage and feeding it back to the opamp, the circuit essentially meters LED current as function of video voltage. Since the maximum video voltage is defined, you can limit max LED current by your choice of feedback resistors.

Stated more simply, it appears that the max LED array current is being scaled and limited by other parts of the circuitry than just the series resistors in the LED array itself.

Pete
AC7ZL

Anonymous wrote:Hi,

I'm a complete novice and I'm trying to build my first mecahnical television. I'm using a Peter Smith design (from the NBTVA website http://www.nbtv.wyenet.co.uk/beginners.htm )

However, I'm confused to tha choice of resistors used in the LED arrays. The circuit shows 2 rows of 3 LED's in parallel, each row with a 22R resistor in it. If the supply voltage is 12V, the LED's 2V 20mA I work out that the 2 resistors need to be 330R. Am I right or am I missing something?

Many Thanks
David
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Postby Radio_Dave » Mon Feb 05, 2007 12:54 am

Thanks for the reply :). I sort of understand what you mean, and it certainly looks like I'm using the wrong resisors!

I think the best thing to do is try the, recommended, 22R resistors and see what happens.

Anyhow... I've just tried it for the first time and it sort of works :shock: .

However there are a few problems. The first one I want to sort out is that it seems to be displaying a negative image :? Here's a link to the circuit that I've used http://bs.cyty.com/menschen/e-etzold/ar ... onitor.GIF any ideas?

Best Regards
David
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Postby Andrew Davie » Mon Feb 05, 2007 2:08 pm

Radio_Dave wrote:However there are a few problems. The first one I want to sort out is that it seems to be displaying a negative image :? Here's a link to the circuit that I've used http://bs.cyty.com/menschen/e-etzold/ar ... onitor.GIF any ideas?


Your link doesn't appear to work in IE, so I attach it here.

"Negative image" jogged my memory, and the following may be of use...

"When you're ready to use the LED driver, make sure that your CD player outputs inverted video on the left channel. If the CD player outputs positive video, either flip the input leads or use an inverter. If your don't know which type video your CD outputs, skip this step. When you begin watching pictures, you'll either see a positive or a negative image. If you see a negative image, you have a CD player with positive video. Flip the input leads for a positive (normal-looking) picture."

taken from http://www.hawestv.com/mtv_page/mtv_page.htm
Cheers
A
Attachments
NBTVmonitor.png
from http://bs.cyty.com/menschen/e-etzold/archiv/TV/mechanical/img/NBTVmonitor.GIF
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Postby Andrew Davie » Thu Feb 22, 2007 1:17 pm

This thread appears to have a continuation in http://www.vintage-radio.net/forum/show ... hp?t=13585
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