Hybrid PF/9V Systems

Following up on my previous article introducing LEGO’s 9V system and their Power Functions (PF) system, I’m going to go a little more in depth about building hybrid systems that utilize both PF battery packs and 9V train track. I’ve developed and iterated through several different systems that combine the best of both and have come up with several easy to implement systems.  Anyone with a few dollars, a volt meter and a soldering iron can hack together one of these hybrids in a matter of hours.

First off, there are a few words to know: voltage, diode, bridge rectifier. For this guide, it’s not so important to know the minutiae of how these things work, but more so to simply know what they do. Voltage is simply a measurement of potential between two points. 6 AA alkaline batteries will have a voltage of about 9V. 6 NiMh AA batteries will have a voltage of around 8.4V. A 2 cell LiPo pack (like Lego’s 8878) will have a voltage of around 7.2V. This will be important later on.

A diode is a simple electrical component that only allows voltage (and thus, current) to flow in one direction: from high to low. Since we are going to be powering our track in addition to providing battery power, we want the track power to be used if it is present, and the batteries only to be used if the track

Notice the band on the left.

power disappears. This is simple to do with a single diode as long as the track voltage is slightly higher than the battery voltage. For example, if the track voltage is 9 volts, and the battery voltage provided by the LiPo pack and is 7.2 volts, power from the LiPo will not flow, since the 9 volts provided by the track is greater than 7.2. On the other hand, if the track connection is lost, the LiPo’s 7.2V will be higher than the 0V track and thus the batteries will take over. This will happen instantly and seamlessly. An appropriate diode to use is the 1N5820. With such a diode, the track voltage can be as high as 24V.

Shamelessly stolen from Philiohome

Installing the diode will require butchering a PF extension cable by splicing the diode in on the 9V line of the cable, with the line pointing away from the battery pack. The line around the circumference of the diode is the indicator for the direction that current can flow. Installing it pointing away from the battery will allow current to flow only if the voltage on the other side is lower than that of the battery pack.

Because your locomotive needs some sort of motor controller,
whether it be the PF IR controller, the SBrick, or any other, you’re going to need one more electrical component to prevent against reverse polarity death. Namely, a bridge rectifier. A bridge rectifier is an arrangement of several diodes in a bridge. They are configured such that regardless of the polarity of the incoming voltage, they always output positive voltage. Luckily, you don’t have to buy a bunch of diodes and solder them all together, you can simply buy a bridge rectifier like the 2W06. One of these is necessary because the polarity of your incoming track voltage may not always be consistent. If you pick up your locomotive and turn it to face the other direction, you will have just reversed the voltage. Electronics and batteries are delicate; applying the opposite voltage of what the designer intended never has positive consequences.

The two ambiguous leads on your bridge rectifier should be soldered on to your power pickup leads. They are marked by a tilde (~). The positive and minus leads should be attached to the positive and minus on the incoming track power.

With a diode in front of the battery pack, and your incoming track voltage rectified, there’s nothing stopping you from running your locomotive now on a test loop of 9V track combined with PF track. Freedom! But wait….

Our first problem that we’ll run in to in our example 9V track power and 7.2V battery might be obvious to some of you. Motor speed is directly proportional to voltage: more voltage = more speed. If our locomotive is chugging along using track power at 1/2 throttle, the voltage being supplied to the motor is 4.5V. Suddenly we lose track power and switch over to battery. No problem, however, the motor will still be at half throttle, but instead of half of 9V, it’ll be half of 7.2V, which is 3.6V. It might not sound like a lot, but suddenly dropping from 4.5V to 3.6V is going to be immediately apparent. And furthermore, it will be problematic when the locomotive catches track power again, the motor voltage will jump back to 4.5V, possibly causing the magnets to decouple between cars.

This can be fixed with a simple DC-DC regulator. There are many models available on eBay for very cheap and they work very well. For our application, we want one that can not only lower incoming voltage, but also increase (AKA buck/boost). I recommend this one. As a bonus, it is exactly 3 studs wide. Simply solder the wires for incoming track power and battery output to the positive and negative terminals of the denoted input pads, adjust the output voltage of the module to the desired setting (9V, unless you’re feeling adventurous), and connect the output of the module to your motor controller input. Not only will this make the transition from track power to battery power seamless, but your locomotive will no longer slow down when the batteries are running low or when it is far away from the track power hookups.

There you have it, for a few dollars and some tinkering with a soldering iron, you have a hybrid locomotive that can run on your home PF layout, on your clubs 9V layout, or on some combination thereof. But Andrew, how do we get power from the track to the electronics? That’s another article…

2 thoughts on “Hybrid PF/9V Systems”

  1. Great article. It’s really great to see you lay out some of the work you’ve been doing. At first, moving to non-stock electronics seemed like a bridge too far to me, or at least something beyond me, but you’ve done a lot to bring me around to it.

    1. It kind of happened gradually. First I did a LiPo that charged from 9v track using the normal train controller. Then I switched to diode protected AA battery packs /w PF. Next I added voltage regulators. Then I added bluetooth, etc. Got all the way to where I am now, and it’s hard to remember how I got here.

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