Help with Mosfets!

Hi Everyone,

I’m currently working on a project, based this board for converting RGB Scart to VGA :

So this is working great to a big 20inch CRT computer monitor, I’ve using RGB out of a Sega Megadrive.

As this board is powered all the time (no power switch, it’s designed to go internally into arcade machines), the CRT monitor doesn’t sleep - when the RGB Scart source to the converter isn’t on, it displays “no signal”

I was thinking I could control the 5V 2a power supply to the converter, using 1 of the 2 pin choices. The source on the scart will always be RGB :

From the wiki :

Pin 8, the switch signal pin, carries a DC voltage from the source that indicates the type of video present.
0 V–2 V means no signal, or internal bypass
4.5 V–7 V (nominal 6 V) means a widescreen (16:9) signal
9.5 V–12 V (nominal 12 V) means a normal (4:3) signal

Pin 16, the blanking signal pin, carries a signal from the source that indicates that the signal is either RGB or composite.
0 V–0.4 V means composite.
1 V–3 V (nominal 1 V) means RGB only.

I’m thinking Pin 16 would be the way to go here? Would I be able to use a mosfet to switch the 5V supply to the converter? I’m also unsure about what components to buy, I am able to solder neatly and build circuits though.

Thanks for any help!

Hey Brian,

I’m a little unsure what it is you are trying to do. Are you turning the CRT off or switching the state of the converter?

Switch off the converter - this will cut the output and the monitor will sleep

Hi Brian,

Could you please include a wiring diagram of how all this goes together? A fritzing diagram would be a nice clean method, or even just draw it by hand and upload it?


The DC power supply is normally powering the GBS-8200 board. I was wondering if I could use a MOSFET or something to sit in the middle, so when A >0V, B gets passed through to C and powers the GBS-8200

Hey Brian,

Yes, you can use a mosfet like this. You will need to choose an N-channel MOSFET. You will want the voltage you are sensing connected to the Gate pin of the Mosfet through a voltage divider, the Supply to supply and the Drain to the load.

Thanks, I’ll give that a go :slight_smile:

Is it safe to assume the mosfet won’t need a heatsink?


It will depend on the MOSFET though I would say up to about 50% of rated current you won’t need one.

Hi Clinton, I’ve got some parts to play around with, I’m not sure about what to do with the ground from the 5V supply, any tips?

I just don’t see the 5V and GND coming in from the PSU, and where the 5V and GND to power the video board comes out.

With any electrical circuits all of the grounds should be connected together connect together, unless you have a good reason to be working isolated. You want to connect sense Voltage to where you have labeled A, the +ive of the 5V supply to 5V supply and the Load to C and the grnd of the 5V supply to the gnd of the GBS as you would if the MOSFET was not there.

1 Like