8/02/2016 – A couple notes on power supplies

The power supplies in pinball machines are magical things that convert AC voltage into a DC voltage.  Without them, the wonderful toys that we enjoy tinkering with, would not work, and we would be forced to play bagatelle machines.  Not nearly as much fun.

Power supplies come in many different varieties, but the two that you see most frequently in a pinball machine is a toroidal transformer (linear unregulated power supplies), and switched mode power supplies (SMPS).  Toroidal transformers have been used in pinball machines since the beginning of time and just slightly before electricity was added for lighting.  (OK, I just made that last part up).  If you look in your pinball machine and you see a big hunk of metal in the bottom that looks like it weighs 20 pounds, you have a toroidal transformer.

A simple toroidal power supply works like this.  (Note:  This is going to be overly simplified)  It takes the AC voltage from the wall and runs it through a transformer.  That transformer converts from a high AC voltage to a lower AC voltage.  That lower AC voltage is converted to DC using a full bridge rectifier (makes positive and negative voltages all positive), and then a large cap to filter out the 60 Hz noise so a constant DC voltage is output.  The power that it can deliver is based on the number of windings, type of material of the toroid, etc.  The AC voltage is converted using magnetic waves or flux in the transformer.  If too much current is being drawn from the transformer, the output voltage droops.  (In power supply terminology, this would be a linear unregulated power supply).

Switched mode power supplies are being used in newer pinball machines.  They convert the AC voltage to high voltage DC using an itty bitty transformer, and then use buck/boost converters to convert the high voltage to the desired lower output voltage.  Because things are done at a high frequency, energy storage components can be much smaller which means a lot less weight.  They are also more efficient.  They also cost a lot less. A 48V 8.3A SMPS is going for about  $22 shipped on Ebay right now.  A 48V 10A supply goes for about $35 if you prefer.  The equivalent toroidal power supply such as a PS-5N48 goes for $107 and weighs 12 lbs.

You may ask why this matters to you.  One point is that the SMPS seem to always have active circuits to make sure that they aren’t shorted.  (Probably for safety reasons.)  If too much current is drawn they shut down.  If it goes from a relatively quiescent state to full on, they can detect that as a short.  When a solenoid fires, it goes from requiring no current to requiring max current in a very short time.  That can trick the power supply into thinking that it has a short and shut down.  To get around this problem, you need to add some bulk capacitors to reduce the instantaneous current draw of the power supply.  Either 8.2 mF (listed normally as 8200 uF), or 10 mF (listed normally as 0.010 F).  Here are the links for the Mouser capacitors.  (8200 uF, .010 F).  The capacitors run about $4 each in low quantities for the 63VDC ones.  These capacitors act as storage, so when there is an immediate draw of current, the capacitor can help provide that current, so that there is a lower current draw from the power supply itself.

Ahh, but these capacitors come with a terrible curse.  At startup, when they are charging, they look like a very low resistance across the power supply, and because of this, they draw a ton of current.  There are many ways to fix that problem.  One is simple but wasteful and cheap, while the others are more elegant.  The last one requires a lot more components and if you want that you should buy somebody elses power filter board.  (The right way would be to add the appropriate sized inductor to limit the current, but inductors are expensive, so we will ignore that option at the moment).

1. Easiest way is to add a current limiting resistor before the capacitor.  The main issue with doing this is that you just converted your precious power supply current to heat using the resistor, and now the power supply can’t supply as much current to anything in the system because this resistor is in series with all the solenoids.  It is a solution, and I have used it when I needed to get something working quickly, but it isn’t ideal.
2. Add a NTC (negative temperature coefficient) thermistor.  If you look at any SMPS you will see these guys.  They look like a big old circular caps that you used to see back in the 80s.  These start at a high resistance when cold, and then after current goes through them to heat them up, they drop to a much lower resistance.  The  Mouser part number is 995-SG26, but there are many others out there.   This one starts out at 5 Ohm when cold, and drops to .022 Ohms as it heats up.  That seems to be the sweet spot because using a 48V power supply, you get 48V/5Ohms = 9.6A and the NTC and handle 12A.
3. Last option would be to add a resistor that is switched out after startup time.  This would involve a good number of components (a couple caps and a couple of resistors to turn on/off the switch or MOSFET).  The MOSFET that I’m current using (FQP13N06L) has an RDS(on) of .110 Ohms (typical).  Since the NTC thermistor has a lower resistance at steady state, it seems to make a lot more sense.
4. Inductor, to limit the initial current, but as mentioned before, to get an appropriately sized inductor it is too expensive.

Well that’s about it.  Joe, if you still read this blog and still work on Blue October, my suggestion is to switch out the current limiting resistor for an NTC thermistor.  That should give your flippers a little more snap.