Frequently Asked Questions

1. What is the difference between a ServoStation and a BEC? And what is a BEC anyway?
2. Why does the ServoStation generate 6 volts for the servos and 5 volts for the receiver?
3. What happens if the input voltage goes below 8 volts?
4. Does the ServoStation have a low voltage cutout?
5. Is it better to use a 3S, 4S or 5S LiPo?
6. Can I use a receiver pack to run my ServoStation?
7. Can I use an A123 6S battery?
8. What are servo pulse amplifiers, and what are they for?
9. I just connected my ServoStation up to my battery backwards. Have I damaged anything?
10. I just connected my receiver and servos to my ServoStation, and all the servos went crazy. What is going on?
11. I can't work out which channels are which. Why aren't they numbered?
12. Does it matter which way around the receiver cables go?
13. I want to run one of my servos on 5 volts, but the rest on 6 volts. Can I do this?
14. Can I use a ServoStation in my RC Heli?
15. The manual for my heading lock gyro says it needs to run off 5 volts. Can I do this?
16. Can I use a ServoStation in my UAV?
17. I've disabled my BEC. Do I still have to remove pin 2 from my ESC's radio connector?
18. I live in Antarctica / the Simpson Desert. Will a ServoStation still work?
19. Can I program the ServoStation to output different voltages?
20. Can I use a ServoStation in my Petrol/Gas/Glow/Nitro/Jet/Rocket powered vehicle?
21. Will a ServoStation interfere with my radio receiver?
22. I have an opto-isolated ESC. How do I wire it up?
23. My ESC doesn't have a BEC. How do I wire it up?
24. How many servos can the ServoStation drive?
25. What happens when a servo jams?
26. I'm only using 4 channels. Is it ok to leave the spare receiver channel inputs on the ServoStation unconnected?
27. Can I use 2 ServoStations to get twice the current?
28. Once upon a time I had a BEC that somehow lost its ground connection and passed high voltage straight into my receiver and servos, destroying them all. Will a ServoStation do this too?
29. Where can I get some receiver cables?

1. What is the difference between a ServoStation and a BEC? And what is a BEC anyway?

A BEC is a Battery Eliminator Circuit. RC vehicles typically need 5 volts to power their radio receiver and servos. Unfortunately electric motors usually require batteries that have a much higher voltage than this. A BEC solves this problem by taking the main battery voltage and regulating it down to 5 volts. This avoids the need for a second battery to power the receiver and servos, hence the name.

The main advantage of a BEC is weight - batteries are an order of magnitude heavier than an equivalent BEC. Using a BEC also means one less battery to remember to charge.

The ServoStation achieves the best of both worlds: it is both efficient and low noise.

Most Electronic Speed Controllers (ESCs) come with an on-board BEC. These are linear regulators, which work by wasting the excess voltage as heat. They are only really good for small craft with small servos. The higher the main battery voltage, the more heat they must dissipate for a given output current, and the more likely they are to go into thermal shutdown. This results in immediate and total radio and servo failure, usually followed by a crash.

External BECs (also known as UBECs) are much better as they generally use switching regulators. Switching regulators are vastly more efficient than linear regulators, but this comes at the cost of increased electrical noise, which can interfere with radio reception. By contrast, linear regulators - although horribly inefficient - are very quiet electrically.

The ServoStation contains both a switching and a linear regulator. There is a heavy-duty 6 volt switching regulator that provides power for the servos, and a low-dropout 5 volt linear regulator that provides power for the radio receiver. The linear regulator steps the 6 volts down to 5, eliminating the noise without incurring any significant efficiency penalty. The ServoStation achieves the best of both worlds: it is both efficient and low noise.

2. Why does the ServoStation generate 6 volts for the servos and 5 volts for the receiver?

Servos are faster and have higher torque at 6 volts, which is important for extreme manoeuvres. Helis in particular, benefit from the improved servo performance possible at 6 volts. Running the receiver from a completely separate and slightly lower voltage has 2 key advantages:

• Servo transients are isolated from the receiver, and
• Electrical noise from the 6 volt switching regulator can be filtered out.

3. What happens if the input voltage goes below 8 volts?

8 volts is the lowest input voltage at which the ServoStation is guaranteed to meet all of its performance specifications. The ServoStation will keep operating if the input voltage falls below 8 volts, but with correspondingly reduced output voltages.

As the graphs show, the output voltages cannot exceed the input voltage. The decline in output with a failing battery is quite graceful: the ServoStation retains full current capability, and output voltages are maintained within 1 to 2 volts of the input, depending on the load.

Once the input voltage falls to 5 volts, the output voltages fall to ground. Any aircraft still aloft will probably follow suit.

4. Does the ServoStation have a low voltage cutout?

Not deliberately, no, but like most electronic devices if the supply voltage goes low enough, it will stop working (see above).

Low voltage cutouts exist to prevent batteries from becoming too depleted. This is important because some types of batteries - notably Lithium Polymers (LiPos) - can suffer permanent damage if discharged too far. Low voltage cutouts are normally implemented by ESCs and work by cutting power to the motor when the battery voltage falls to a predetermined level. At this point the radio receiver and servos should remain operational so that landing can be attempted.

Although low voltage cutouts are generally a good idea, they are not always appropriate. Helis, for example, tend to respond poorly to sudden loss of power to the main rotor. An alternative strategy is required, which is getting a little beyond the scope of this document.

The ServoStation is guaranteed to work down to 8 volts, so if a low voltage cutout is programmed into your ESC it should be set to more than this. If you value your LiPos, the cutout voltage should be at least 3 volts per cell.

5. Is it better to use a 3S, 4S or 5S LiPo?

It makes very little difference to the ServoStation - use whatever is most appropriate for your electric motor and ESC. The graph shows that the ServoStation's switching power supply draws less current from higher input voltages for any given load, but its efficiency remains largely the same.

Please note that the graph is correct: the ServoStation generates more current than it consumes.

Consider the example of a ServoStation powered by a 12 volt battery. If the servos draw 5 Amps from the ServoStation, then the ServoStation will draw 2.7 Amps from the battery.

6. Can I use a receiver pack to run my ServoStation?

Yes, absolutely. Any DC power source between 8 and 22 volts is fine.

7. Can I use an A123 6S battery?

Yes, an appropriately charged A123 6S battery is fine.

A 6S LiPo, however, is not. The nominal terminal voltage of a 6S LiPo is 22.2 volts, which exceeds the ServoStation's recommended input voltage. Furthermore, a fully charged 6S LiPo can go above 25 volts, which exceeds the ServoStation's absolute maximum input voltage rating.

8. What are servo pulse amplifiers, and what are they for?

Servo pulse amplifiers are variously known as "servo boosters", "servo pulse boosters", and even "servo boost amplifiers". Whatever you like to call them, the ServoStation has 8 of them and each one consists of:

• A level detector to reject electrical noise,
• A limiting amplifier to square up the received signal, and
• A balanced, high-current, push-pull output stage.

The TTL input thresholds allow the direct connection of 3.3V, 3.0V and even 2.7V logic, avoiding the need for any additional level conversion circuitry. The low-impedance output stages allow the ServoStation to drive long cable runs out to distant servos, or multiple servos per output.

9. I just connected my ServoStation up to my battery backwards. Have I damaged anything?

Your receiver and servos are also fine; they've been protected by the ServoStation.

No, it's fine. Your receiver and servos are also fine; they've been protected by the ServoStation. Just connect it up the right way and you're good to go.

10. I just connected my receiver and servos to my ServoStation, and all the servos went crazy. What is going on?

In the absence of a valid transmitter signal, PPM (Pulse Position Modulation) receivers will decode background RF noise into full amplitude digital nonsense, and mindlessly output this to the servos. The servos will attempt to comply with these random instructions, resulting in violent and sometimes destructive (to your aircraft) movements.

A ServoStation won't stop garbage from reaching your servos; in fact it makes the problem worse by providing 6 volts and plenty of current for maximum destructive power.

The solution is to always switch your transmitter on before turning your receiver on, and vice versa when turning things off. If this is not possible, then disconnecting your receiver from your ServoStation will also prevent garbage from getting through to the servos.

IPD (Intelligent Pulse Decoding) PPM and PCM (Pulse Code Modulation) systems suffer less from this problem (having traded some response time for improved noise immunity), but it is still good practice to have the transmitter on whenever a receiver is powered up.

11. I can't work out which channels are which. Why aren't they numbered?

They're not numbered because they're all exactly the same. You can allocate channels any way you like. The servo outputs lie directly opposite their corresponding receiver inputs.

12. Does it matter which way around the receiver cables go?

No, the two ends are identical.

13. I want to run one of my servos on 5 volts, but the rest on 6 volts. Can I do this?

Yes, absolutely. Just plug the servo you want to run on 5 volts directly into your receiver. Bear in mind that the total current drawn from the 5 volt supply must be less than 1 Amp.

14. Can I use a ServoStation in my RC Heli?

Of course. The ServoStation is perfect for medium to large helis.

15. The manual for my heading lock gyro says it needs to run off 5 volts. Can I do this?

Yes, of course. Just plug your gyro directly into your receiver, usually on channel 4 (and sometimes 5 as well). You now have the choice of either:

• Plugging your tail rotor servo directly into your gyro, which will run the servo on 5 volts;

Or, if the servo tolerates 6 volts you can:

• Route the signal through the ServoStation as shown in the diagram. This will run the tail rotor servo on 6 volts, but the gyro will be still be running at 5 volts.

You can have your cake.

16. Can I use a ServoStation in my UAV?

Yes, the ServoStation was designed with UAVs in mind.

The ServoStation is specified to operate over the full industrial temperature range (-40 to +85°C).

Stabilised aircraft place a heavy and continuous burden on their power supplies because of the constant corrections needed to maintain vehicle attitude. With its 5 Amp continuous / 7.5 Amp peak current rating, the ServoStation has plenty of reserve to cope with these demands.

The switching power supply in the ServoStation has a specified efficiency of 90% when delivering 1 to 5 Amps to the load, but will achieve more like 95% at the business end of the load spectrum. The datasheet includes typical efficiency curves covering all possible combinations of supply voltage and load current. Higher efficiency means lower device temperatures, higher reliability, and of course longer battery life.

UAVs can be electrically hostile environments. The ServoStation buffers all of the servo control signals and provides balanced low impedance push-pull drivers to maximise signal integrity over long cable runs. It also has a low-noise 5 volt rail to maximise receiver sensitivity.

UAVs often have demanding environmental requirements. The ServoStation is specified to operate over the full industrial temperature range (-40 to +85°C).

17. I've disabled my BEC. Do I still have to remove pin 2 from my ESC's radio connector?

It is the smart thing to do. Even if your BEC is disabled in software, one day it may lose its mind and start up again. Here's a step-by-step guide on how to remove the pin.

18. I live in Antarctica / the Simpson Desert. Will a ServoStation still work?

Probably not in Antarctica if it's winter, but otherwise yes. The ServoStation is rated to work from -40 to +85° Celsius (-40 to +185° Fahrenheit). Some care needs to be exercised around zero degrees, as damp air and cold surfaces can lead to condensation, and liquid water and electricity do not mix.

19. Can I program the ServoStation to output different voltages?

No. The outputs are fixed at 5 volts for the receiver and 6 volts for the servos. You must ensure that your servos are rated for 6 volt operation, and if not, either don't use a ServoStation, or if the total current drawn from the 5 volt supply will be less than 1 Amp you can simply plug the offending servos directly into your receiver (which runs on 5 volts).

20. Can I use a ServoStation in my Petrol/Gas/Glow/Nitro/Jet/Rocket powered vehicle?

Of course, although ServoStations don't tolerate fuel internally very well.

21. Will a ServoStation interfere with my radio receiver?

Highly unlikely, although it is always a good idea to mount your ServoStation at least 5cm (2 inches) away from your radio receiver, and not to run the antenna wire too close.

Always perform a range check after making any changes to your aircraft.

22. I have an opto-isolated ESC. How do I wire it up?

It's even easier than wiring up a "normal" ESC. Opto-isolated ESCs expect to be provided with power, and that's exactly what the ServoStation will do. Don't remove the middle pin from the ESC's radio connector, just plug it straight into the appropriate servo connector on the ServoStation. If your ESC cannot tolerate 6 volts, no problem, just plug it directly into your receiver and it will get 5 volts instead. Bear in mind that the total current drawn from the 5 volt supply must be less than 1 Amp.

23. My ESC doesn't have a BEC. How do I wire it up?

It's exactly the same as for an opto-isolated ESC.

24. How many servos can the ServoStation drive?

There is no substitute for testing; if in doubt, do the experiment.

It depends on how much current they draw, and how large a margin of safety you want. A design that attempts to cater for the simultaneous stalling of all servos will be so heavy that it probably won't fly. On the other hand, a design that presumes only one servo will move at any given time won't be flying for long.

Servo manufacturers tend to be very coy about revealing how much current their servos actually draw. Volz are to be commended as one of the few manufacturers that provide adequate performance data. For example, a Volz DA 20-06-2505 can draw up to 0.6 Amps continuously or 0.9 Amps for short periods of time. A ServoStation can therefore drive 8 x Volz DA 20-06-2505 servos comfortably (a total of 4.8 Amps if all 8 servos are operating continuously), leaving 200mA to spare for the receiver.

The table below is a (very conservative) guide to the number of Volz servos that can be driven by a ServoStation:

Servo model number	Max continuous servo current	Max short-term servo current	Number of servos
Volz DA 20-06-2503	0.8A	2.4A	5 to 6
Volz DA 20-06-2505	0.6A	0.9A	7 to 8
Volz DA 22-06-2603	0.8A	2.4A	5 to 6
Volz DA 22-06-2605	0.6A	0.9A	7 to 8
Volz DA 22-06-4106	1.6A	2.1A	2 to 3

If less than 100% duty-cycle can be guaranteed, then the number of servos may be increased. There is no substitute for testing; if in doubt, do the experiment.

25. What happens when a servo jams?

The ServoStation is quite conservatively rated and chances are that even with a jammed servo drawing several Amps, the 6 volt rail will remain stable. If something has gone horribly wrong and there is an absolute short-circuit to ground, then things become more interesting. The ServoStation will reach its current limit, which at 25°C is about 8 Amps. If the fault doesn't clear with 8 Amps running through it then things are pretty grim. The ServoStation will try plan B and start hiccupping. This involves briefly shutting down the 6 volt regulator and then restarting it. Hiccupping will continue until the short-circuit clears.

26. I'm only using 4 channels. Is it ok to leave the spare receiver channel inputs on the ServoStation unconnected?

Yes. All of the receiver channel inputs have an internal pull-down resistor, which keeps them stable when left unconnected.

27. Can I use 2 ServoStations to get twice the current?

Yes, although some care needs to be exercised. You must ensure that the outputs of different power supplies - even those at nominally the same voltage - do not come into contact with each other. If this can be arranged then you can use as many ServoStations as you like.

28. Once upon a time I had a BEC that somehow lost its ground connection and passed high voltage straight into my receiver and servos, destroying them all. Will a ServoStation do this too?

A ServoStation which is deprived of its ground connection passes no current, so no. Here's a long-winded explanation of why some BECs do this.

29. Where can I get some receiver cables?

Here's what a cursory internet search turned up:

Although all of these cables look ok, we haven't actually tried any of them. If you accidentally end up with connectors that don't have bevelled edges and therefore don't quite fit, don't panic. You can easily bevel the edges, Chris Hansen shows you how.

And you can always make your own cables - Hansen Hobbies sell very reasonably priced cable, connectors, crimps and crimping tools, which we have tried and thoroughly recommend.

As you might have noticed, there is a diversity of opinion when it comes to specifying the gender of servo connectors. The RC community believes the plastic housing is the gender-determining element, and therefore calls these cables male-male. The rest of the world uses the actual mating parts - the metal contacts - hence female-female.

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