Really elementary, basic circuit design question.

Hey guys. I must start out by saying that the knowledge you all posses here is lightyears beyond what I have, however, by lurking for so long I have been able to learn some things which I'm really appreciative of.

In my current setup, I have an SNES which is connected to a SCART to HDMI scaler and then into an HDMI switcher which finally goes into my AV Receiver (which just passes through everything to my TV).

As it is currently set up, the SCART to HDMI scaler is always putting out a signal to the switcher so when I turn on my SNES it won't automatically switch. I have temporarily fixed this by wiring in a simple on/off switch on the power line for the scaler, but I'd like to come up with a more automatic method until I replace it all by a proper scaler/scanlines/etc.

The SCART to HDMI scaler takes a +5V DC at 1000 mA power supply. I was thinking I could use an NPN Transistor and tap into the +5V DC output from the SNES multi-out so that when I turn on the SNES, it will automatically feed the scaler the +5V DC 1000 amps it needs.

Is this possible to do? I'm trying to figure out what specs I would need for the transistor and resistors, but I don't know what the amperage is coming from the SNES +5V DC line. Is it a constant amperage or does it change over time?

Also, for the wiring, I understand that the +5V from the SNES would pass through a resistor to the Base of the NPN Transistor, the emitter would be connected to the scaler and the collector to the power supply?

I've got a ton of resistors/diodes/transistors/capacitors I grabbed from a dead AV receiver that I could use, but just want to have the "experts" here advice. Many thanks!

Because you want to switch the +5V side, what you want is a "high side switch".

Here's an article about how to use them.

Many thanks! That actually makes quite a bit of sense to me. I'll work on drawing up a schematic to verify that I have understood it right. Thanks for helping out this newb.

If you really don't want to think, use something like an Infineon BTS432E2XK.
Datasheet


5 legs, looks kind of like a voltage regulator (in a TO-220 style package, through hole). 4.5V minimum working voltage.
Readily available from Mouser.

Will take a look at that too. Thanks. Though I still may go through a more complicated method simply for the sake of learning and having that experience.

Okay, after some researching I am now looking at the Texas Instruments TPS2022 chip for use as the 'switcher' that I'm thinking of. The only concern is that this chip is an Active Low Enable. My goal for the circuit is to make it so that when I turn on my SNES and the A/V Out sends the +5V DC to the circuit, it will then allow the +5V DC @1A from the power supply to flow to the scaler and turn it on at the same time.

So to do this, wouldn't I need an Active High in order to ensure the scaler gets no power until the SNES is turned on?

I also want to make sure I don't accidentally feed 1.0 amps of current to my SNES via the +5V line and fry the console.

For the circuit, I would have the SNES +5V DC coming in to Pin 4 (EN), the +5V DC from the power supply coming in to Pin 2(Vin), Ground from the Power Supply connected to Pin 1(Gnd) and also to the ground from the scaler on the power supply cord, and the +5V out from the TPS2022 to the scaler via Pin 8(Vout). (With all necessary capacitors/resistors as per the datasheet).

Sorry if I'm being dense or asking stupid questions, but I'm still learning here. Heh.

This IC appears to basically be a pMOSFET and a current sense resistor in one package. So, yeah, you'll have the same problem as "ordinary" pMOSFET-based design.

(In fact, if you're not planning on using the overcurrent pin, you might just look for a power pMOSFET instead, e.g. DMP3099)

Both power supplies are floating, so it's really unlikely that you could damage things in that way ... but if you want to be careful/paranoid, you could use an BJT-output optoisolator (4N26, e.g.) in lieu of a plain pulldown BJT in the schematics on the Hackaday article.

The only stupid thing is persisting in ignorance

Alright, I think I have this figured out. For what I'm doing, a Texas Instrument TPS22990 Load Switch will be the simplest way. I can connect the +5V power supply to the Voltage In and Voltage BIAS pins, and can use the SNES +5V to turn the switch "on" which will allow current to flow through to the Voltage Out pin which I will connect to the scaler.

The ground would connect to the load switch and the scaler, and when I turn on my SNES the scaler will also turn on achieving what I want. Hopefully I've figured this out right, but if not, the part is super cheap. Just need to figure out how to solder to it. May need to find a similar device but in a larger form factor. But for now, at least I know what I'm looking for.

Just wanted to give an update on my project. Went to Digikey and bought a bunch of parts including a 10k Ohm resistor, 1 uF bypass cap, 0.1 uF bypass cap, 120 uF electrolytic cap, and an AP22802 load switch. Of course silly me didn't check the size of the load switch and had quite the surprise when the teency little parts arrived. Glad i bought 5 of them as I messed up the first one trying to solder it. Moved to a finer point iron tip and got a good magnifying glass to see what i was doing. Heh.

Anyway, got it all connected by using the suggested circuit diagram in the AP22802 datasheet and it works! This is so awesome being able to turn on my SNES and the scaler automatically turns on and the switcher automatically switches. When I turn the SNES off, the scaler turns off and regular TV comes back on.

There's still the issue about the 4:3 image being stretched out to 16:9, but I also have parts for a tiny charge pump circuit to try and see if ~10V on Pin 8 of the SCART connector will handle it. No big deal if it doesn't.