Electrical/Electronics FAQs
by TigerhawkT3

1. Please explain volts, amps, watts, and C.
2. What are series and parallel?
3. What are "direct drive" and "regulated"?
4. What are "boost" and "buck"?
5. What is "linear regulation"?
6. Can I wire driver boards in series?
7. Can I wire driver boards in parallel?

1.
Q: Please explain volts, amps, watts, and C.
A: That's not a question, but okay. Volts are electrical potential, amps are electrical current, watts are total power equal to volts*amps, and C is electrical current as a function of battery capacity. Think of volts as the width of a pipe: In general, a wider pipe has more punch than a narrower one. Think of amps as the water flowing through a pipe: Some pipes can only handle little trickles of water, and others can handle lots of water pushing through with great force. Think of watts as a combination of volts (pipe width) and amps (flow of water): A large pipe with water flowing through really slowly has the same output as a small pipe with water blasting through it. This is why high-voltage applications are preferred over high-current applications, as a stream of water zooming at 200mph through a 1"-diameter pipe is much more dangerous and difficult to maintain than a calm, 3mph flow of water through a 4'-diameter pipe. As for C rates, that's just a function of current draw and battery capacity. Any power source discharged at a 1C rate will be depleted in 1 hour, any power source discharged at a .25C (or C/4) rate will be depleted in 4 hours, and so on. As an example, a 1.8Ah AA NiMH capable of an excellent 10C discharge rate can manage 1.8*10=18 amps.

For more about volts, amps, and watts, consider the following analogies (I like analogies). 1) A low-voltage source at low current is like a well-behaved kid sitting on the couch watching TV. 2) A high-voltage source at low current is like giving that kid way too much sugar and watching them make a huge display but not really break anything. (Also consider what happens when you see discharges of static electricity, which provide an exciting jolt but don't really do much.) 3) A low-voltage source at high current is like a strongman at a circus bend iron bars, or break a chain wrapped around his chest just by inhaling. 4) A high-voltage source at high current is like giving that strongman sugar, PowerBars, amphetamines, and a rifle. Situation 1 is like a little 1AA LED flashlight - low power, low worry. Situation 2 is like an LED string operating at mains voltage, which requires precautions and insulation, but low power, which is what allows it to save energy. Situation 3 is like running the parallel-wired P7 (see below for explanations of each term), which consumes more power and requires thicker wiring but doesn't pose much danger of sparks. Situation 4 is what you should be extremely cautious of. It's entirely possible to have a reasonably safe high-voltage, high-current system, like the mains power that everyone takes for granted, but those who work with it definitely have to know what they're doing. Like the juiced-up, armed strongman, it's not intrinsically bad (maybe he'll hold up a car while its owners change a tire, run a marathon, then put on an impressive display of tin-can plinking - who knows?), but if something goes wrong, there is a definite potential for calamity.

2.
Q: What are series and parallel?
A: Series connections have a device's positive terminal connected to the next device's negative terminal. This is what you get when you line up some ordinary C-cell alkalines (for example) end-to-end, like in a Maglite or other flashlight. This arrangment adds up the voltages of the cells. Such a battery neither handles more current nor contains more mAh capacity than a single cell. This is the opposite of a parallel configuration, which has positive terminals joining together and negative terminals joining together. An example is those 3AA>1D adapters where all three AA cells' positive terminals meet at the top, and all their negative terminals meet at the bottom. Such a configuration has the same voltage as a single cell, but can handle more current draw (or contains more capacity). For example, 1AA alk can push about 500mA at around 1.5V for about four hours. 2AA alks in series can push 500mA at around 3V for about four hours. 2AA alks in parallel can push 1000mA at around 1.5V for about four hours (or 500mA for eight hours, and so on).

3.
Q: What are "direct drive" and "regulated"?
A: A direct drive (DD) light is one that has the battery directly connected to the bulb or LED. A regulated light has some sort of driver circuitry between the two. A DD setup is heavily affected by the battery size and type. In a regulated light, the circuitry will try to minimize the effects of the battery. The huge majority of incandescent lights are DD. They start out bright, then fade over time. The effect is greatest with alkalines, which don't do well in many situations. The effect is least noticable with Lithium-Ions, which maintain a steady voltage under relatively heavy loads. This is why traditional Maglites, which are DD by alkalines, start out bright for about half an hour, then quickly fade out and become dim for the next few hours until the battery gives up. One example of a regulated incan, which provides rock-steady output for the majority of the battery life, is Surefire's A2. In order to drive mostly similar LEDs with wildly different battery solutions, a regulation circuit allows steady output for as long as the battery has power. As an example, the Fenix E0 runs on a single AAA alkaline for eight hours with no decrease in output. If it were DD, it wouldn't light up at all, much less provide constant output. An appropriately DD LED flashlight would be one driven by button or coin cells at somewhere above the LED's Vf. This results in a long runtime with slowly decreasing output, determined by the battery's remaining power.

4.
Q: What are "boost" and "buck"?
A: Boost and buck circuits increase and decrease, respectively, the output voltage of a battery. This is used because of Vf requirements (discussed elsewhere in the Welcome Mat). Such a circuit will usually have battery + and - inputs as well as LED + and - outputs. The interesting thing about these circuits is that they can also be used to tweak battery current consumption, as a boost circuit will draw more current from the battery than is flowing at the output, and a buck circuit will draw less current from the battery than is flowing at the output. This generally means that boost circuits are hard on cells, while buck circuits are easier on them.

For example, 2AA NiMH powering an XR-E with a Vf of 3.7V at 700mA would require a boost circuit. If the circuit was 100% efficient (not actually achievable), the following equation would apply:

3.7V/2.4V*700mA= ~1080mA

This means that we can use a lower voltage source like 2.4V, but we will have to draw over 1A to produce the desired 700mA at the emitter.

For real-life circuits with efficiencies under 100%, simply divide the required battery current by the efficiency (expressed as a number between 0 and 1). For example, an 85% efficient boost circuit applied to the above situation would result in the following equation:

1080mA/0.9= ~1270mA

For buck circuits, the opposite situation applies. For example, powering a 5mm LED with a Vf of 3.4V at 20mA with a 90% efficient buck circuit on a 9V battery would result in the following equation:

3.4V/9V*20mA/.9= ~8.4mA

Keep in mind that these are simplified situations, with real flashlights being influenced by a number of limiting factors.

5.
Q: What is "linear regulation"?
A: LDOs (Low Dropout) and linear regulators, like the AMC7135, have Iin=Iout. The energy loss comes from reducing the Vin down to Vout without reducing Iin below Iout (which is what a buck converter would do). So, a Vin of Vf+Vdropout+Vdrop, with Vf being the LED's Vf at whatever current you chose, Vdropout being the regulator's minimum voltage drop (which is very small for LDOs like the 7135), and Vdrop being any additional dropped voltage, would waste (Vdropout+Vdrop)*I watts for an efficiency of Vf/Vin. As Vin falls due to sagging battery voltage, Vdrop will decrease, resulting in a more efficient regulator but, of course, reduced cell capacity. If Vin falls below Vf+Vdropout, the light will basically DD with as much voltage as it can muster, with Vout (no longer a fixed value, but determining the emitter's Vf, and therefore its If, as well as Iin from the battery) being equal to Vin-Vdropout. At that point, efficiency would be Vout/Vin or, equivalently, (Vin-Vdropout)/Vin.

6.
Q: Can I wire driver boards in series?
A: Except in certain cases, no. If, for example, you have a driver with a max speced voltage of xV, putting two of these drivers in series with a 2xV battery may fry your drivers.

7.
Q: Can I wire driver boards in parallel?
A: Yes, that's fine.