Rocket Ignition Power Supply

Previously, I introduced a remote rocket ignition system and discussed the benefits. Now, we are going to look at the first of three schematics that make up the system.

Model rocket igniters can take multiple amps to ignite. Many people use eight to ten AA NiMH cells as a power source. If you’re a little more serious, a battery from an uninterruptable power supply, motorcycle, or car are popular choices.

Unfortunately, most digital circuits can’t operate on voltages above 5 V, and the aforementioned power sources are around 12 V. So, the first schematic in the rocket igniter system is the power supply regulator.

Rocket ignition system schematic part 1 Power Supply

Rocket ignition system schematic part 1: Power Supply

B1 Battery. Although a wide range of voltages are supported, somewhere around 12 V to 14 V is optimal.

D1 This diode prevents damaging current from flowing if the battery is connected in reverse. The choice of a 1N5819 (40V) Schottky diode is to reduce the amount of voltage lost when the battery is connected properly.

F1 RXEF110 is a 2.2 amp self-resetting circuit breaker to prevent too much current from flowing in the correct direction. So, if there is a short circuit (detached wire or nut contacting the back of the board), this will become very resistant (limiting current) until the short is removed.

SW1 Power switch. Unlike other rocket ignition systems, this switch does not carry the current that passes through the igniter. Therefore, this switch can be smaller and less expensive. Total circuit current averages less than 100 mA normally, with a peak around 500 mA for up to one second during ignition. This higher current is due to all the LEDs being lit and the relay coil being energized.

C1 680 µF 35 WV bulk capacitor (such as Panasonic EEU-FR1V681) to supply power when the battery voltage drops due to the high current usage of the igniter. Because this capacitor is after the diode (D1), the capacitor’s power cannot flow backwards, and thus is reserved for this circuit. Because this capacitor is before the voltage regulator (VR1), it can store more power because it is at the battery voltage (say 12 V) rather than the post-regulation voltage (5 V).

C2 10 µF 35 WV faster speed, but lower capacity capacitor for providing power at higher frequencies.

VR1 This five-volt regulator outputs a steady voltage from a variable higher voltage input (the battery). A steady voltage maintains the brightness of the LEDs and prevents too high of a voltage from affecting the relay coil and microcontroller.

C3 10 µF output capacitor (5V) for smoothing the voltage during changes of higher or lower circuit consumption.

R1 Current-limiting resistor to prevent too much power from damaging LED1.

LED1 Power indicator.

Linear or Switching Regulator?

The classic 7805 linear regular is inexpensive ($0.50) and readily available. The downside is that it wastes the voltage above 5 V as heat.

A fancier type of voltage regulator is called a switching regulator. This quickly turns an inductor coil on and off so that the average voltage is the desired output (5 V). Capacitors and inductors provide smoothing. Most of the extra voltage is efficiently converted, rather than lost as heat. This reduces the load on the battery, so the battery lasts longer.

7805 versus 7805SR and 78SR-5/2-C

Left: 7805 linear voltage regulator, Middle: 7805SR switching voltage regulator, Right: 78SR-5/2-C higher-capacity switching voltage regulator

In the past, if you wanted a switching regulator, you had to design the circuit and buy a bunch of components. However, now you can purchase pre-built circuits that have the same pinout as the 7805. There are some differences in current capacity, drop out voltage, noise, and so on, but the switching regulators are compatible with most applications.

The Murata 7805SR runs about $10 and provides 500 mA. The Murata 78SR-5/2-C is $12 and provides 2000 mA, but is larger. Both are parts from Murata’s DC-to-DC converter line.

Voltage Drop

Battery voltage is not a fixed value. The voltage starts out highest when the battery is fresh, and declines as the battery charge is depleted. If you allow a battery to rest for a short period after heavy usage, the voltage will go back up slightly due to gradual chemical reactions. However, over longer periods, the voltage will slowly decline due to internal discharge.

There is a more significant effect that occurs in high-current applications, such as launching a rocket. During heavy discharge, the battery voltage drops dramatically until the heavy usage stops. The amount of drop depends on the chemical type and capacity of the battery.

You don’t have to worry too much about voltage drop when using a purely mechanical ignition system (pair of switches in series with a battery). If the igniter doesn’t fire, the battery is exhausted and should be swapped out. However, with a digital system, the battery voltage may drop too low to continue running the electronics even if the battery still had enough charge to otherwise ignite the igniter.

Allow me to demonstrate.

Using a plain 7805 voltage regulator, notice how the 5 V output drops for a short time during ignition. It drops further down and stays there longer when the base input voltage is 9V than when it is 10V.

Regulated voltage drop for 7805 with different input voltages

Regulated voltage drop for 7805 with different input voltages and a 680µF capacitor

The microcontroller may reset when the voltage gets that low, which may prevent ignition.

The switching regulator performed better. It doesn’t show similar effects until the base input is around 7.5 V.

Regulated voltage drop for 7805SR with 680uF capacitor

Regulated voltage drop for 7805SR with 680µF capacitor

Your experience will vary based on the types of igniter (current load), battery capacity, and temperature (cold slows battery chemical reactions). The point is that a digital controller sharing the igniter power supply requires a power source with a higher voltage and capacity than does a mechanical controller.

Capacitors can temporarily boost the local power supply. I wondered whether it would be better to put the capacitor before the voltage regulator (where it could charge to full battery voltage) or after (5 V ready). As you can see, storing full input power is superior in this case, as the input voltage is the constraint -- not the throughput of the regulator.

Effect of input versus output capacitor placement on 7805 voltage regulator

Effect of input versus output capacitor placement on 7805 voltage regulator

Don’t let any of this information panic or dissuade you. The point is: use a 12 V battery with enough capacity and include a capacitor on the power supply board so that enough voltage is available to keep the digital circuitry running during launch. You want the ignition system to stop working when the battery can no longer supply enough power to the igniter, as opposed to a chip.

Now let’s look at the schematic for the relay driver.