Proper USB power mod for the Megadrive 2

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gameblabla
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Proper USB power mod for the Megadrive 2

Post by gameblabla » Fri Feb 28, 2020 1:30 am

Hello guys, long time no see !
So after seeing a few people doing a USB mod to their NES, i had the idea of doing the same to the Megadrive.
Of course as you may know, the 7805 regulator outputs 5V, just like the USB standard, hence why i decided to apply the same mod to it.

I managed to make it work and also reduced the noise by adding a few caps to the 5V line (one 0.1uf cap, another 1uf and the last one 10uf).

Image
(It looks really nasty, i will clean it up and shorten the wires after my issue is solved)

Basically the 5V line goes to one of the power switch pins first then goes directly to the output pin that was formerly used by the 7805.

Image

So far i loved it, the power consumption is much improved and there's less noise & inteference visible on the screen with the French Sega SCART cable.

...That is until i realized one minor issue with my Mega CD. (so far the Mega CD has not been touched)
It powers on just fine but when you power it off (via the power switch), the Megadrive powers off but the Mega CD is still powered on and still plays the CDDA track !
I have no idea how it is supposed to know how to be turned off or anything and looking at the schematics did not enlighten me either.

I'm sure there's something wrong with my mod, so is there a better way of doing the mod ? (without using the power button pins for example)

Thanks

EDIT: I fixed it, it was kind of dumb. See my posts below.
Last edited by gameblabla on Sat Mar 14, 2020 10:26 pm, edited 1 time in total.

cero
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Re: Proper USB power mod for the Megadrive 2

Post by cero » Fri Feb 28, 2020 8:25 am

Beyond the cd issue, I don't think usb gives enough power? The original PSU was something like 10V 1.2A = 12W, while usb only gives 5V 0.5A = 2.5W. Maybe you're connecting to a standalone usb charger rated for 2.5A, using usb charging cables (not regular data cables!), but that's a slightly rare setup. Computers won't give that much even with charging cables, let alone TVs or routers that you might be plugging into.

gameblabla
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Re: Proper USB power mod for the Megadrive 2

Post by gameblabla » Fri Feb 28, 2020 12:16 pm

cero wrote:
Fri Feb 28, 2020 8:25 am
Beyond the cd issue, I don't think usb gives enough power? The original PSU was something like 10V 1.2A = 12W, while usb only gives 5V 0.5A = 2.5W. Maybe you're connecting to a standalone usb charger rated for 2.5A, using usb charging cables (not regular data cables!), but that's a slightly rare setup. Computers won't give that much even with charging cables, let alone TVs or routers that you might be plugging into.
Well first of all, the original 7805 voltage regulator isn't used : i removed it and completely bypass it.
I measured the power consumption before doing the USB mod and power consumption on Sonic 1 was 4.3 watts and 5.8 watts on Virtua racing.

After i did the USB mod, power consumption dropped to 1.6 watts for Sonic 1 and 2.1 watts for Virtua Racing, which is well below the USB 1.1 standard which says that only 500ma may be supplied to the 5V line.
Also in reality, many USB power supplies and computers go above that. My computer USB port can provide as much as 14 watts from its port (i tested it with a USB tester) and most of my 5V1A USB wall plug can supply 3.5 watts stable enough.
Of course if you use it with a 5V2A charger, those concerns are gone and you can go beyond that.

When i plugged my USB powered Megadrive to a computer, power consumption drops to 0.9 watts for Virtua Racing.
I have yet to know the reason why but suffice to say that lack of power isn't the reason.
As far as i know, the only reason the Megadrive consumes so much power is due to the 7805 voltage regulator being inefficient : only up to 60% efficient. Meaning 40% is lost and spread as heat, hence the need of a heatsink...

So again, i strongly doubt this is a "lack of power" issue. I also tested the voltage on my PC USB port and it's a stable 5.0 volts. Some of my USB chargers go below that (as low as 4.75 volts) but it still works reliably.

EDIT: Forget to say it but my motherboard is a VA1 PAL revision. Also i am using a properly shielded USB cable with ferrite beads. (type B, the kind you find on printers)

Miquel
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Re: Proper USB power mod for the Megadrive 2

Post by Miquel » Wed Mar 04, 2020 6:15 pm

Usb requires to a least to power 500 mA, but there is no upper limit. But the point of this project, I think, is to reuse old usb chargers; I don’t see the scenario were data cables will be useful.
Last edited by Miquel on Wed Mar 18, 2020 8:41 pm, edited 1 time in total.
Help. Spanish TVs are brain washing people to be hostile to me.

gameblabla
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Re: Proper USB power mod for the Megadrive 2

Post by gameblabla » Sat Mar 07, 2020 3:02 am

So i redid my mod, shortened the wires and also tried the other pads and i still could not make it work properly.
Unfortunately still the same darn issue :
When you manually turn it off, the Sega CD stays on but when you unplug the USB cable from your megadrive,
the Sega CD will power itself off.

I really feel like there's something else i am missing : i am directly wiring it up to the power switch pins.
Again, is there a better way ?
Miquel wrote:But the point of this project, I think, is to reuse old usb chargers; I don’t see the scenario were data cables will be useful.
Yes and being able to use it on a solar generator without having to use the inverter.
So instead of :
DC 12V -> AC 220V (20% lost if not more) -> DC 9V -> DC 5V (40% lost)

You'll have :
DC 12V (or 3.3V for USB chargers) -> DC 5V

I had some people tell me that i should suck it up and just use the inverter instead. Really (lol)
Miquel wrote:
Wed Mar 04, 2020 6:15 pm
I don’t see the scenario were data cables will be useful.
Because for the most part they only sell data cables. You're right that a power cable type B should fare better and have less noise/interference but as far as i know, i couldn't find good ones. (at least that were type B)

gameblabla
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Re: Proper USB power mod for the Megadrive 2

Post by gameblabla » Sat Mar 14, 2020 10:25 pm

So i found my issue... my wiring was not done properly and required a few modifications.
I wired up the input line to the output line, wired up the 5v line to the proper switch pin this time (the other one always gets enabled for some reasons) and used one of the other pin which turns out to be required for Virtua Racing to work.

Now when i power off my Megadrive, the Mega CD will also power itself off and i can also play Virtua Racing properly.

Image

So yeah i don't regret doing this mod. I would recommend you do this if you have a soldering station and want
to decrease the power consumption of your Megadrive (or your 7805 regulator died), decreasing the electrical noise (if you have a good USB power supply of course) and so on.
Personally i did so i could power it off a solar generator or in a car or other similar things...

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