Homebrew Pinball #3, Part 9

Cross posted from the original Pinside thread, this is one of many posts regarding my third homebrew pinball machine, creatively nicknamed 'P3'

Started wiring all my drop target banks at my workbench. Each will have a connector for easy removal. Not going to bother with that on every mech, since most are pretty simple and either serviceable from the playfield or unlikely to need service, but drop targets and pop bumpers are another story.

Realized I'd made a big mistake during initial planning... Most of my banks are williams or gottlieb, which is fine, but the 5 bank on the right is a bally, which I chose because they have thicker targets, which will make shooting past them into the shooter lane smoother, and they had 'memory', so I could knock them down with code if needed to make the shooter lane shot easier. Problem is, Bally used 50V coils, while gottlieb and williams used 25V, and I'm using a Gottlieb transformer. A quick test revealed that, of course, the giant 50V reset coil on this bank doesn't even move when you apply 25V to it. I dug through my spare mechs box, but the only other 5 bank I had wouldn't fit due to space issues with the flipper mech immediately above it.

So I needed to get creative. How do I get 50V when my transformer only puts out 25V? With a voltage doubler! Here was my simple test setup to see if this would even work:

Basically, creative insertion of two big capacitors in the middle of a half bridge rectifier allows each to be charged by half the AC circuit to 25V. But they're wired in series, so that combines to 50V (technically, I guess it's -25V and 25V compared to the rest of the voltages in the system). Those TO-220 transistor looking things are actually just really big diodes, I think 10A each? I overspecced this thing for sure, since I couldn't find exact details on what tolerances were needed. And it worked! Bank resets just fine. You can't energize anything continuously with it since the caps will empty pretty quick, but it also recharges within a second, which is good enough for resetting one drop target bank. I got a new PCB made for this, with even more fuses (again, wasn't sure where to fuse, so I just fused everything), and a bleeder resistor so the 50V (actually, it reads as 80V for whatever reason) doesn't sit around after the machine is powered off

The issue with this of course is that, since it's operating off its own rectifier with a weird half-ground system, I can't control it via the MOSFETs on my driver boards, since they don't share a common ground. For the reset, this isn't a big deal; I can just add a relay in to control it. But I also have the 5 knock down coils. I don't want to have to wire up 6 relays for this bank. Luckily, since the knock down coils are so small, I find that they can be moved via 25V. But not enough to actually knock the target down. So I added a 4700uF cap onto my 25VDC rectifier, similar to what gottlieb did on games like Black Hole to give the kickers more juice. That gives enough strength for the knock down coils to operate 90% of the time, and I can just code them to try again if they don't detect the switch closing to handle the other 10%.

Posted Sunday, September 13, 2020
at 12:46 PM

Tags: Blog Post, Pinball, Project, P3,

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