8 replies to this topic

[NeonMika / Markus VV.]

Dear Netduino-Community, I just finnished school, and to be a real grad, you have to forget everything after your final exam No, just kidding, but i want to order a Netduino Plus in the next few days and I have a project idea in mind in which i have to power 2 servos. After reading through quite a lot of threads i often heard of things like "Use external powering" and "Dont use the Netduinos V5" etc. To do this faver to my Netduino, i wanted to power this 2 servos (needing 5V) from an 9V battery (or some other power source different to 5V). Still remebering Ohm's law and Kirchhoff, I though of something like this: Battery -> Resistor -> 2 Servos (parallel) -> Back to Battery. Am I right? Now my problem is: How many Ohm must my resistor have? I only know my Servos need 4,6 - 6 V... Thank you for your help, Markus

[Mario Vernari]

I want only say that: - the resistor is the worst way to regulate the power; - a 9V battery probably will have a short duration. Usually the servo have a double-bridge driver, PWM controlled, being able to regulate the power without having to choose a certain battery. Moreover, using the PWM way to regulate the power, the life of the battery is much longer: the energy waste is minimal. Anyway, there are several guys, much addicted than me about the servo usage. Cheers

[NeonMika / Markus VV.]

Thanks for your replie. Regarding the resistor: We just learned the electronic basics in physics, an i remembered something called "Voltage divider" and so i went to wikipedia and thought this could work... http://en.wikipedia....Voltage_divider Regarding PWM: I know that a servo has 3 wires: VCC - powering the servo, GND and the control-wire - handeld with PWM... I could connect the VCC to the V5 of the Netduino, GND to GND, and control the PWM with Pin 5... But I shouldn't use the V5, so i really would appreciate, if someone could help me to understand how to power the servo external. Greets, Markus

[Mario Vernari]

You are right about the Kirchhoff equations, as long the Ohm's law and all the rest. The resistor is a component which its goal is *only* to transform the electric energy to heating energy: nothing else. Unless you have to warm up your water for cooking the spaghetti, a resistor is wasting energy! Ohm stated that the voltage across a resistor is the current multiplied by the resistor value. So, the higher is the current, the higher will be the voltage drop. The voltage-divider works, but the resulting value keeps "stable" as long you don't suck current from it, otherwise the voltage changes. If your servo is running free the current will be low, but when it require a bit of torque, the required current will increase. That's the way the voltage of a servo won't be stable using a simple resistor. I don't know what is the typical usage of the servo: you should best choose several models, then buy the right one. You should first know what's the average/max current required for the servos. It's also true that you should avoid to connect loads requiring enough current to the Netduino supply. BUT...you may have another supply. For example, you may use a AA-battery pack (total 6V) to power the servos only. However, this depends on the servo specifications. Cheers

[NeonMika / Markus VV.]

I don't really like spaghetti, so i think a resistor is really not what i need Ok, changing current results in different voltage drop at the resistor sounds logical. And that the current will change between idle and moving is also clear. So nobody could answer my question "How many Ohm must my resitor have", because nobody knows the actual current that will flow... Just for example, lets say I have the following servo: http://www.produktin...05_Graupner.pdf There is nothing sayed about avg/max current. But your idea with the AA-battery pack is great... Probably one of the easiest things to do is to use a 4x battery pack, resulting in 6V, and just to connect the servo to this pack, right? No need to reduce the current etc. But, just because I'm interessed to learn as much as i can, how could I use 9V battery to power a consumer (for example my servo) that needs an input voltage of 6V? You said, it doesnt work with a simple resistor. But there has to be another way to get it working Thanks, Markus

[CW2]

In addition to what Mario said, you can use fixed voltage regulator (7805, 7806) or adjustable regulator (LM117) to power the servo. They require a little bit higher supply voltage (V_IN ≥ V_OUT + V_drop, V_drop = ~2 - 3V, see the datasheet), but they provide regulated output and additional features like short-circuit or thermal overload protection. There are also variants in smaller packages (with lower output current), lower dropout voltage etc.

[Stefan]

I don't really like spaghetti, so i think a resistor is really not what i need

For lowering the voltage, using resistors aren't the best choice, I have to agree with Mario about that. You asked for advise, now you get it!

CW2 is right that using voltage regulators are a better way. Also generate the remainder power in heat, but it's working is actually quite different.

Try this:
Put a resistor against a power source, what will happen with the current? It'll stay the same. Only the amps will get lower.
So you actually didn't control the voltage but the amperes.

Now do the same with a voltage regulator. You can guess the result

[NeonMika / Markus VV.]

... you can use fixed voltage regulator (7805, 7806) or adjustable regulator (LM117) to power the servo ...

Great! The last day I dug a little deeper and this is really a nice way. Thank you for the advice.

You asked for advise, now you get it!

That's why I really like this community! I rarely see unanswered topics here, you really try to help =)

Try this:
Put a resistor against a power source, what will happen with the current? It'll stay the same. Only the amps will get lower.
So you actually didn't control the voltage but the amperes.

Now do the same with a voltage regulator. You can guess the result

I'll ask: The voltage will drop?

Now I understand the useage of a resistor and know that there are things like voltage regualtors.

So there is one last question I have regarding servo control:
When I connect the servo to a battery pack and send the PWM signal from my Netduino, where do I have to link the GND? Back to the batteries + AND to the netduinos GND?

So it's 3.30am, have to go to bed now, good night

[Henry_Jones]

So there is one last question I have regarding servo control:
When I connect the servo to a battery pack and send the PWM signal from my Netduino, where do I have to link the GND? Back to the batteries + AND to the netduinos GND?

Yes, you have to connect GND from your servo power source and the GND from the netduino power source. Otherwise the signal sent to the servo has no valid reference.