Simple resistor / electronics question

[Dave_S]

This is doing my fcuking nut in :mad:

I'm trying to build this (http://www.idiots.org.uk/vga_rgb_scart/index.html).

I've triple checked all the wiring and used a multimeter on every pin to make sure it's going where it needs to go. I'm not getting a picture at all on the TV. I can only assume this is because it doesn't know it's supposed to be displaying RGB instead of Composite.

The thing thats doing my nut in is the resistor on the 5V line from the PC. Now, you can't measure voltage across a resistor can you, it just comes out the same as the input. I want to know where they got that resistor value from.

I'm supplying 5v and want about 2v of it. Now to calculate what resistor to use I'd need to know the current right?

R = (Vs-Vr) / I

would that be the current the TV would be drawing?? I'm not sure if a 100R is correct or not. If the TV isn't getting between 1v and 3v it's not going to go into RGB mode.

HEEEEELP! :(

Dave

[John Bennett]

I think the 100 Ohm resistor is in series with the input impedance of the blanking pin. If that was say 50 ohm, then there would be about 1.6V between the pin and 0V. I've ran arcade boards on a SCART TV and I just stuck 5V on the blanking pin (no resistor) and it worked fine, so I'd just trust the circuit (the 100 ohm resistor will limit the current to 50mA if something goes wrong).

You've got to drive the graphics card at an extremely low refresh rate to do this: 15kHz horizontal - I'm suprised the Radeon allows it - the oldest VGA monitors (640x480, 60Hz) run at 32kHz.

Edit: You'll probably have to set the TV to RGB mode as I don't think they always detect an RGB input properly.

[Dave_S]

Thanks for the reply John thats excellent :)

This definately works as others are using it ok :(

I can't tell my tv theres an RGB signal on that input, thats what the 1-3v is for, to tell it to switch to RGB :(

Oh well, more playing around with Powerstrip I guess :)

thanks.

Dave

[Big_ReD]

I dont fully under stand the document or how to work it. But if you want 2 volts from 5 you could set up a potential divider with say a 300 and 200 ohm in series, then take the voltage from the 200 ohm resistor which should be about 2v. where your current will be 5/500 = 0.01A through the series network. This will ten drop depending on the resistance of what its connected to. I think :wack:
sorry if this is no help, thought id give it a go.

[Dave_S]

Thanks for the suggestion, the more the merrier :thumbs: :)

Dave

[ACECROSS]

I made up one of these cables a few months ago, and it does work.

I think the TV looks for a 0.3v signal and the PC outputs 5v,so you insert a 1Kohm resistor between pin 20 on the scart and pin 13 on the VGA


I followed an article in a magazine that I have. Can probably get a copy to you if that would help. (Getting a new scanner tomorrow so need stuff to scan :D ).

This diagram was differnet to the one on that site...

http://img53.photobucket.com/albums/v161/Acecross/DSCF0216.jpg


IIRC I ended up using information from both that site and this article to get it to work in the end.

[Dave_S]

Sorted!!!!!! :D

The image is absolutely bloody amazing :eek: soooo much better than Svideo.

The cable was perfect, no problems there, it was the sync settings in Powerstrip that were causing the problems :)

Dave

[ACECROSS]

Good glad it is all working now :)