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Ed.D
16th February 2015, 16:59
Hey everyone,

If I have a 5.03V DC signal coming out of a scart connection on pin 16, what value resistors do I need to reduce it to: 4.3VDC

Already had a some advice on reducing to 3.0VDC, which is 270 Ohms (thank you Steve and TC) but I have a DVD player that works over scart for my old TV that outputs 4.3V. This is why I'm asking.

Would the 5V output normally be too high to activate RGB switching? This is all Greek to me sorry! :unsure:


Thank you,
Ed.

johnim
16th February 2015, 17:03
http://www.gtsparkplugs.com/Dropping_Resistor_Calc.html

http://www.bcae1.com/ohmslaw.htm (http://www.gtsparkplugs.com/Dropping_Resistor_Calc.html)

Ed.D
16th February 2015, 17:24
Thanks John, trouble is I don't know the current draw so Ohms law won't work.


D'oh I can get the current from the 270ohm example. I'm being thick today.


Sorry. :oops:

- - - Updated - - -

Nope still struggling. If I say I=V/R

I = 5 / 270 = 0.0185A

If I use that first link and enter those values of 5V, 3V and 0.0185A I get 111ohms which isn't 270ohms. Even more confused!


Thanks again.

johnim
16th February 2015, 17:25
http://www.rapidtables.com/calc/electric/Voltage_Divider_Calculator.htm

rootboy
16th February 2015, 17:45
Do you have room on your board for this?

http://www.circuitstoday.com/few-lm317-voltage-regulator-circuits

Otherwise, hook a 1k linear pot in series with your load. And starting from max resistance, crank it back down until you get the voltage that you want.

bebek
16th February 2015, 17:51
how about diode in series, anode to 5V, on cathode you will have 4.3 (if you lucky :) )

Ed.D
16th February 2015, 18:27
Thanks guys, I just tried a 270ohm resistor and it didn't have any effect, no noticeable reduction in voltage. I'll try a couple more things; my old 8833mk2 and the GBS8220. I don't really want to start getting involved in building circuits just to get a scart cable working. I appreciate the suggestions though :)

roy_bates
16th February 2015, 18:59
so,wont it switch with a rgb scart lead? from what machine?

to switch to rgb on scart i always thought it was 3 volts to switch to rgb.on pin 16


there is something else you need to take into account mate...are you taking the readings on the scart plug without load?
as:not plugged into the tv?

bebek
16th February 2015, 19:45
Just try any small diode ...

Ed.D
16th February 2015, 20:39
Hi Roy, it's a Neo Geo MVS and you're correct I am testing the voltage with no load. The scart cables work ok with my DVD player. The pin 16 voltage on the DVD player is 4.3 volts no load and on the MVS it's 5.03 volts no load.

Here's a photo of what I'm getting on the TV screen.

EDIT: Just to add, I tried adding in a 270ohm resistor on the scart plug but no joy.


http://webed.pwp.blueyonder.co.uk/Projects/MVS/neogeo.jpg

roy_bates
16th February 2015, 20:56
try to get a reading from the pins when there plugged into the scart mate,just remove the cover on the scart plug and plug it in..

you should be getting 12 volts on pin 8(if present) and about 3 volts or a little more on pin 16

or try the primary scart socket first,some arnt rgb compliant on the second socket.

Ed.D
16th February 2015, 21:35
Woohoo, Sorted! ...and a good tonic for a bad back!

Earlier today I cleaned all the card edge connectors and double checked connections. What I didn't think to check were the scart pins. I've just cleaned them with IPA but didn't expect anything but reconnected and got a lovely picture, all 36 inches of SD wonderfulness. I can get on with the project now.

I did check the voltage when connected Roy. it's 3.88V on pin 16. There was no 12V on pin 8.

Thanks you once again for your help guys :thumbsup:

Ed.


http://webed.pwp.blueyonder.co.uk/Projects/MVS/MVSgood.jpg

johnim
16th February 2015, 21:54
a clean saves the day :)

roy_bates
16th February 2015, 21:56
lovely:)

rootboy
17th February 2015, 00:06
how about diode in series, anode to 5V, on cathode you will have 4.3 (if you lucky :) )

Duh! I should have thought of that... Great suggestion, but it looks like it's fixed.

sir_beaker
17th February 2015, 00:15
Fantastic news :thumbsup: