Replaced the 7805 in my NES with this one from DigiKey https://www.digikey.com/en/products/det ... 5CV/585989 its an STMicroElectronics 78s05, figuring it would be a good idea if I plan to use Everdrives, 8bitdo Controllers and Power the NESRGB from it.
To my surprise it created a wavy horizonal line in the lower half the screen. The noise seemed random, sometimes it wouldn't happen. Going back to the original regulator made the problem go away. I'm no EE by any means and I don't HAVE to replace it, but I'm kind of wondering what exactly is going on here? Is the performance of the older part superior?
78s05 Created Video Noise in NESRGB?
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SamIAm MkII
- Posts: 20
- Joined: Sat Feb 13, 2021 3:54 am
Re: 78s05 Created Video Noise in NESRGB?
I'm no EE either, but here are a couple of thoughts.
The recommendation of the 78s05 has always seemed strange to me. I suspect that what happened was that at some point, someone replaced a 7805 with a 78s05 and saw a problem go away, and assumed that it was the 78s05's higher current limit that made the difference when in fact it was merely that the original 7805 was faulty.
Most game consoles that use a 7805 are only drawing in the region of 400mA. Even with power-hungry add-ons, the danger of exceeding the 7805's 1A limit is low.
The real bottleneck to worry about isn't the current limit but the thermal limit. 7805 and 78s05 regulators will both produce the same amount of heat in a given situation: (input voltage - 5) x current. For example, a system with a 9V power supply and 400mA draw is going to generate 1.6W of heat at the regulator, and that alone is enough to get it quite hot. Most consoles seem to put some sort of heat sink on the regulator, which helps a great deal, but an unregulated power supply that's closer to 10V and a substantial increase in power draw from external devices can get things pretty toasty.
The thing to do if you're worried about thermal limits is probably first just to check and see if the 7805's heat sink gets worryingly hot while the system is running. If it doesn't, don't sweat it. But if it does, you basically have three choices:
1. Improve the heatsink, which may or may not be practical.
2. Use a lower voltage power supply. 7.5V regulated supplies are common, but beware, that's basically right at the limit for minimum input voltage. 8V is more ideal.
3. Use a switching regulator that generates almost no heat, but may introduce high frequency noise because of the way it works.
Basically, I'd recommend #2 if your system gets hot enough that you actually need it.
I can only speculate about the noise you see with the 78s05, but it seems possible to me that compromises were made in the 78s05's design to give it that higher current rating. If the ripple rejection and/or load transient response is worse, that would do it. But it is strange that you don't see the noise constantly. Did it at least appear consistently at certain places when you're playing?
The recommendation of the 78s05 has always seemed strange to me. I suspect that what happened was that at some point, someone replaced a 7805 with a 78s05 and saw a problem go away, and assumed that it was the 78s05's higher current limit that made the difference when in fact it was merely that the original 7805 was faulty.
Most game consoles that use a 7805 are only drawing in the region of 400mA. Even with power-hungry add-ons, the danger of exceeding the 7805's 1A limit is low.
The real bottleneck to worry about isn't the current limit but the thermal limit. 7805 and 78s05 regulators will both produce the same amount of heat in a given situation: (input voltage - 5) x current. For example, a system with a 9V power supply and 400mA draw is going to generate 1.6W of heat at the regulator, and that alone is enough to get it quite hot. Most consoles seem to put some sort of heat sink on the regulator, which helps a great deal, but an unregulated power supply that's closer to 10V and a substantial increase in power draw from external devices can get things pretty toasty.
The thing to do if you're worried about thermal limits is probably first just to check and see if the 7805's heat sink gets worryingly hot while the system is running. If it doesn't, don't sweat it. But if it does, you basically have three choices:
1. Improve the heatsink, which may or may not be practical.
2. Use a lower voltage power supply. 7.5V regulated supplies are common, but beware, that's basically right at the limit for minimum input voltage. 8V is more ideal.
3. Use a switching regulator that generates almost no heat, but may introduce high frequency noise because of the way it works.
Basically, I'd recommend #2 if your system gets hot enough that you actually need it.
I can only speculate about the noise you see with the 78s05, but it seems possible to me that compromises were made in the 78s05's design to give it that higher current rating. If the ripple rejection and/or load transient response is worse, that would do it. But it is strange that you don't see the noise constantly. Did it at least appear consistently at certain places when you're playing?