In some cases, the voltage from the battery pack is not sufficient for desired operation, and it’s not possible to create a battery pack that will fit into the locomotive that is capable of producing enough voltage. In a case like this, the pack must either be placed in a permanently-attached car, or a step-up regulator be installed.
A step-up regulator, also known as a boost regulator, is a special DC voltage regulator with an output voltage that is greater than the DC supply voltage. At its simplest, the circuit is a switch-mode power supply with a regulated output. It is important to note that a step-up regulator is not the same thing as a voltage multiplier or voltage doubler, both of which us an AC input.
There are a few things to be aware of when selecting a step-up regulator. Modern step-up regulators are quite efficient when used correctly. Several factors affect efficiency, including the ratio of input voltage to output voltage, and current draw.
Because our LiPo batteries are capable of producing much more than their rated current in mA, it is important to know the maximum current that the motor can draw. For instance, a 30C, 240mAh LiPo pack is capable of producing 7.2Amps for a short period of time. The step-up regulator in the example is only capable of handling a 1.4A input current. If the input current exceeds the rated current, the regulator will fail.
Typically, the worst case scenario for current consumption in a model railroad application is when the locomotive is stalled, so it’s important to measure the stalled current of the motor at the desired running voltage. For most modern locos in smaller scales (including On30), this won’t be a problem, but if you’re in a position that requires a step-up regulator, it would be prudent to perform a stall test.
Another factor in step-up regulator efficiency is the ratio of input voltage to output-voltage. Simply, the closer the input-voltage is to the output-voltage, the more efficiently the regulator will operate.
In the case of the 9V Pololu U3V12F9 step-up regulator, maximum efficiency is achieved with a 7.2V input voltage. The lower the input voltage, the harder the circuit must work harder to achieve the 9V output. At the typical current draws of On30 model trains, efficiency is still quite high.
The math to calculate the operational current range of the step-up regulator is fairly straight forward:
input current * (input voltage/output voltage) * efficiency
Using the 9V step up regulator and a single LiPo cell as an example we get:
Full charge: 1.4A * (4.1v/9v) * 0.80 = 0.51A or 510mA
Approaching depletion: 1.4A * (3.1v/9v) *0.80 = .38A or 380mA
Comparing these calculations with the stall current chart, we can see that while the single cell step up solution may work fine under full-charge conditions, if we happen to stall at full speed when the battery voltage is nearly depleted, we run a real risk of exceeding the specifications of the step-up regulator, which will cause it to overheat and either shut down until it cools, or possibly fail altogether.
If we instead are able to use two LiPo cells for a 7.4v pack, the numbers look much better:
Full charge: 1.4A * (8.2v/9v) * 0.80 = 1.12A
Approaching depletion: 1.4A * (6.2v/9v) *0.80 = 0.77A or 770mA
One final consideration is dealing with inrush voltage. Depending on the installation, it is possible that a voltage spike of nearly double the rated input is possible. The longer the power leads, the larger the possible induced voltage spike. These spikes can be suppressed using a 33uF electrolytic capacitor across the supply voltage. If the battery and the step-up regulator are very close together, the capacitor may not be necessary.