Update 2/3/20:
Over the weekend I started the upgrade on my MTH Great Northern S-2 from PS3 to a Loksound L v5. This is a beautiful loco, and has been on my list for this for a while. As a result, I now have a set of PS3 electronics to play with, so naturally I was curious to follow up on this idea.
I was careful when removing the original electronics to retain everything in a usable configuration for bench testing. I made a modified harness to connect the tender board and the boiler board without needing the wireless drawbar, etc. For simplicity, I also eliminated the 3/2 switch and associated wiring, so the board is just powered directly via it's PCB inputs. I left the motor and tach wiring intact on the chassis, so that I could confirm the actual motor operation as part of the test.
Initial poking around and powered tests with a scope and benchtop power supply proved promising, so I built up a modified version of my O scale keepalive and connected it to the main PS3 board. Power up, let things charge a bit, and...
It works. After letting the cap charge for a few minutes, I pulled the plug for several seconds and then reconnected with no impact on the loco operation. It turns out the PS3 electronics maintains normal operation for almost ten seconds before stopping the loco, given sufficient charge. The actual runtime pulling a train will be less, of course, as the motor is drawing very little current in this test configuration. But with a big enough capacitor, you could achieve run times up to that limit if you wanted. The keepalive is connected to the PS3 board with just two wires, so installation is pretty easy and non-invasive. I'll write up details on the keepalive and hookup to the decoder when I get a chance.
Here's a video of the test setup. The scope is showing the DCC input. The yellow voltmeter shows the capacitor voltage, which you can see charge and discharge when it the track is plugged and unplugged. The orange voltmeter shows the PS3 decoder bus voltage, which runs about 12.3V while powered, and about 11V while operating off the keepalive. The voltage considerations for the boost regulator described in my keepalive post apply here as well, which is why the keepalive voltage is slightly lower than the powered voltage.
Some caveats worth noting:
- I run DCC, so that's what I tested with. I expect DCS would behave similarly, as the protocols are similar, but that is just a guess.
- I tested with conventional A/C as well, and it did not work. The motor stopped when I removed power, despite the keepalive maintaining the decoder bus voltage at 11V. I suspect the processor interprets the loss of track power as a stop/reverse command and acts accordingly.
- Bench test only of course, and only on one PS3 board.
