Advice on NES-related hardware project

Advice on NES-related hardware project
by on (#147252)
Hi,

My apologies if this isn't the right place, and please pardon my very limited knowledge of electronics, which has probably regressed to worse than it was when they taught us this stuff in highschool. I'm working on a project to build a Raspberry Pi into an NES. One of my goals has been to preserve the external appearance of the NES as much as possible, such that you could look at the NES and not be able to tell it wasn't original. That extends to using the original RCA ports, power brick, and power socket, if possible. For that purpose, I want to use the original NES power/RF box, using the actual power supply, and cutting the original RCA jack leads to solder in the audio/video from the Pi. I've managed to desolder one (a Mitsumi one from an NES-CPU-04).

The box seems to be in working order. When I plug it in, with the five pins in the top right, and assuming the chassis is ground (it seems to be), I see ~13.6V on the first pin, 0.26V on the second pin, and 0V on the other three. If I short the first and second pin, that seems to turn it "on", and I get 5.05V on the third pin, with the first pin dropping to ~12.8V. I understand the NES runs on 5V, so that would seem to be right.

I want to power my Pi with this thing. For that purpose, I have a small switched power supply for the Pi, which takes an input of 6V to 14V, and converts it into the 5V that the Pi needs.

I'm not sure I trust the 5V from the NES, since the Pi is apparently sensitive for voltage, so powering it directly off the NES is probably not a good idea. I'd also like to wire up the NES power switch to do a soft shutdown/bootup on the Pi, since simply cutting the power can cause problems with the filesystem. This leads me to my actual questions:

1) What's the 13.6V from? I read somewhere it's the "unregulated" voltage. This is in the range of my power supply, can I use this pin (with pins 1/2 not shorted) to power the Pi? The Pi consumes at most 10W, but in practice it'll probably be half or even a quarter of that. If I put a 5W load on this pin, is it going to be a problem?

2) If I can't use the 13.6V pin, can I semi-permanently short the first two pins, leaving the thing "on" pretty much permanently, run the 5.05V through a step-up converter to get in the range of the Pi power supply, and use that? I'm wondering if there would be any ill-effects from leaving the NES power supply on permanently. It normally wouldn't have a load on it (when the Pi is off and not in use). I wouldn't want to wire the NES power switch to interrupt the actual power, because when I release the power switch I want the Pi to do a safe shutdown and not just get the power cut.


If option 1 is fine, then that would probably be my desired option, being simpler.
Re: Advice on NES-related hardware project
by on (#147306)
Option 1 should work, but I'd recommend to use a regulator that can handle at least 18V at its input just to be safe. The unregulated DC power is just the voltage across the input capacitor(s), just after the diode bridge, just like the second circuit or maybe the third of this page: http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/rectbr.html

If your regulator somehow loads too much the unregulated supply, put a bigger capacitor at the input of your switching regulator. Also, switching power supplies can generate a lot of (electrical) noise, check the impact of the working power supply on the NES (if it still works and you consider the option to be able to use the original NES).
Re: Advice on NES-related hardware project
by on (#147309)
The voltage coming into the system is AC (or can be DC of either polarity), and goes through a diode bridge to become DC. Then it gets routed through the Power button on the console, so when it's pressed, it shorts those 2 pins together like you noticed, and connects it to the 5V regulator.

Yeah, like Jarhmander said, you might need a regulator that can take a higher voltage, 18V is pretty reasonable. The transformer that comes with the NES says 9V, but that means it's rated to be at least 9V at the maximum rated current. At a lower current, the voltage will be significantly higher (like the 13.6V you're seeing).
Re: Advice on NES-related hardware project
by on (#147310)
But if it's 13.6v with no load, won't it be even lower once I put a load on it? As in, wouldn't 13.6v be the upper limits of what I might see?

I'm pretty sure the power supply can't handle 18v (being rated for 6-14v), but are you worried about that as a spike, or as a prolonged thing? The documentation mentions that going over 14v might cause it to overheat, and then other bad stuff starts happening by 17v. So a brief spike to 18v probably might not cause any issues, but if it's going to spend any significant time over 18v, maybe using the unregulated power from the NES isn't going to work. I could ask the guy who makes the power supply if a brief spike to 18v would cause problems.

Here's the diagram of my power supply, which isn't something I'm able to read beyond identifying what some of the components are: http://4.bp.blogspot.com/-jMsKLeb9kGE/U ... _Power.png

If I'd need a power supply capable of doing 18v for extended periods of time, it sounds like it's probably not an option for me... which leads me back to the other option, using the regulated 5V output and stepping it up to like 9v or something, before feeding it into the power supply. Would I have any problems leaving the NES power supply on permanently (with little or no load most of the time)?

EDIT: Here's a link to the documentation rather than just the diagram: http://www.geppettoelectronics.com/2014 ... guide.html

EDIT 2: The idea of using a step-up converter didn't work out so well. The voltage coming out of the PiPower seemed stable (my bargain bin multimeter never shows it vary at all from 4.97V), but the Pi always locks up or reboots during startup when the PiPower is fed by the step-up converter. I guess it's not producing enough current? Anyhow, I decided to just go with the unregulated NES voltage and risk it: the worst case, I think, is that I fry the PiPower, which isn't all that expensive. As a bonus, the 2015 power regulator in the PiPower seems significantly more efficient than the 1986 power regulator from the NES: the PiPower is barely warm to the touch without a heatsink, while running through the NES power regulator causes its heatsink to get uncomfortably hot.

The unregulated voltage coming out of the NES seems to top out at around 13.6V with no load, and drops down to ~10V if I try to get the Pi as loaded up as possible, so the minimum voltage seems to be more than high enough, and the max voltage might not have any headroom (or the safety margin that an 18V power supply would have), but it seems to work well enough.

Next step, replace alligator clips with solder joints!