dkshema
rrMaster
Location: Cedar Rapids, IA
My Posts This: Topic Forum | Yup, no matter HOW they do it, a BEC and a regulator do the same job. They simply provide a different voltage on their output than what is supplied on the input. And their job is to try to provide a somewhat constant voltage independent of load.
The "battery eliminator circuit" regulates the higher LiPo (or multiple Nicad/NiMh) supplied input voltage down to a voltage that won't blow up your radio components so that you can power your electronics off the same battery as the motor. That way you don't need a separate battery pack to run the electronics. It eliminates the flight pack battery. In most speed controls, the regulator produces a five volt output for the electronics. BUT -- since a linear regulator is cheap, requires only a couple of capacitors (maybe a couple resistors) to work, they are used to make the "BEC" circuit. There are two drawbacks to using a linear regulator, however. The first is that in most cases, the the typical "3-terminal, low dropout voltage, linear regulator" just doesn't handle large amounts of current UNLESS you mount it on a HUGE heat sink. The second drawback is that for the amount of space most speed control folks allocate to their BEC, the physical devices available can't handle more than an amp and a half (again, heat sinked very well).
A linear regulator is not a very efficient device. Think of it as nothing more than a resistor. If you pass current through a resistor, you develop a voltage across it (Mr. Ohm says that the voltage that is developed across a resistor is equal to the amount of current in amps multiplied by the resistance in ohms -- E=I*R). With a bit more handwaving, the POWER dissipated by that resistor is equal to the current going THROUGH the resistor, multiplied by the voltage developed across it -- P = V * I).
Looking at a typical 5 volt, 3 amp, three terminal low dropout voltage linear regulator hooked up to a 3-cell LiPo (that at full charge is about 12.3 volts) the regulator has 12.3 - 5 volts "dropped" across it, 7.3 volts in all with a fully charged LiPo. Assume that you're averaging an amp of current while flying that all digital servo'd Trex. 7.3 volts multiplied by that 1 amp means the regulator has to dump 7.3 watts of power on average without smoking. If you were to look at the data sheet for that little regulator device, you will find that you need a pretty darn good sized hunk of finned aluminum (or several square inches of copper clad circuit card material to dissipate that heat. Then, try to do that on a hot day in mid-summer, with the speed control tucked away inside a canopy where there is not much air flow. The regulator has a built-in thermal protection circuit. If it gets too hot (the internal junction temperature is usually spec'ed at 150 degrees C), it shuts down till things cool off. Then it wakes up, and if it gets too hot, shuts down again. Dissipating 7 watts of energy, it takes very little time for a poorly cooled linear regulator to shut down from overtemperature operation.
The large amount of voltage "dropped" across the linear regulator is why the "BEC" on most speed controls is limited to three cell LiPo operation. Add a fourth cell, the input voltage goes up to about 16 volts, and with an output voltage of 5 volts, the regulator now has 11 volts across its input/output terminals, and at that average one amp load, it now has to dissipate 11 watts. You could use it to brand cattle with at that point.
A linear regulator is very inefficient. Inefficiency in this case shows up as lots of HEAT.
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The solution to the heat problem is to use a "switching regulator". These are neat, in that they use a little bit of power from the battery to run, but can be made upwards of 90 percent efficient. The switching regulator uses some cool properties of transformers to do its thing. In its simplest form the input voltage from the LiPo is hooked to one end of an coil (inductor) -- the "primary" winding of a transformer. The other end of the coil is hooked to a transistor that is switched on and off at a very high frequency. The secondary winding of the transformer (a second coil or inductor that is wound around the same "core" that the primary was wound on) outputs a different voltage than is seen on the input side. The output voltage is determined by the ratio of the number of turns of wire that make up the primary winding, and the number of turns of wire that make up the secondary winding.
In ideal terms, if you had a transformer with one turn of wire making up its primary winding, and five turns of wire making up its secondary, the output voltage would be five times the input voltage.
And if you had 5 turns of wire on the primary, and one turn of wire on the secondary, the output voltage would be 1/5 that of the input voltage.
Transformers only work with alternating current -- that's why the switching regulator has to "chop" (switch on and off) the incoming battery voltage to work.
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Ever wonder how older cars (before electronic ignition) got that 15,000 volts to fire your spark plug from that the 12v power system in the car? The "ignition coil" is nothing more than a transformer with a few turns of wire on the input, and a LOT of turns of wire on the output. Coils of wire have a neat property -- if you run current through them, they don't like to see that current suddenly stop flowing. Hook one end of your igntion coil to the 12V battery, and use a switch to open and close the circuit to ground (the "points" in that car's ignition system). As long as current is flowing through the primary, you really don't see any voltage on the secondary winding (the spark side). BUT -- open that switch (distributor cam opens the points) and the transformer becomes VERY unhappy. It tries to maintain the current by momentarily jacking up the voltage.
The voltage across an inductor is equal to the size (inductance) of the coil mulitplied by a number that tells you how fast the current changes (amount of current divided by length of time). If the current stops instantly, time goes to zero, and if you remember anything from math, big numbers divided by very small numbers (very close to zero) become REALLY REALLY big numbers.
Open that switch, the voltage on the primary side of the ignition coil skyrockets momentarily. The huge number of secondary turns of wire increases THAT voltage spike several thousand times and voila -- the spark plug gets a 15,000 volt jolt. Not a lot of current, but a lot of voltage.
The switching regulator uses those principles of transformers to do its thing -- the fact that inductors don't like to see their current suddenly stop, and the fact that you can make different voltages on the output than you have on the input simply by playing with how many turns of wire you have.
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The switching regulator is neat in that you can play with the frequency at which you switch the voltage, the magnetics design (transformer), the way you switch the voltage, and with some planning, you can limit the losses and get a highly efficient, SMALL, lightweight regulator that has a wide input voltage range, a very well regulated output voltage, and runs cool as a cucumber.
Since you can put more than one secondary winding on a transformer (and can play games with how you wind the secondary) you can generate a whole bunch of different voltages on the output with a single input voltage.
That would allow you to make a switching regulator with a 5 volt output for your receiver and gyro, for example, and a 6 (or other voltage) output for your servos. Pretty cool. Play your cards right, you could also generate that 100 volts needed to make your electroluminescent glow-wires work for night flying. All off your single battery.
All that neat stuff comes at a cost -- size, weight, and $$$.
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Linear regulators are CHEAP, very inefficient, but work pretty well if you heat sink them properly, or limit the amount of current you pass through them. The heat sink can be HUGE.
Switching regulators are more expensive, but can run circles around the linear regulator performance-wise. They have their own problems, though -- they can generate unwanted RF noise, may require a minimum load to maintain (regulate) the output voltage properly, and if something goes wrong in the input voltage switching circuit, they die suddenly and catastrophically.
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In the end, both approaches do nothing more than create a different voltage on their output than you give them on their input. And both can be used to "eliminate" the second battery pack that would be needed to run your electronics. They REGULATE voltages, and they ELIMINATE that second battery pack.
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* Making the World a Better Place -- One Helicopter at a time! *
Dave |
I don't think a regulator could handle this voltage. Maybe some one can explain this better 
+ 12 for the stock NiMh.
