Provide more current to servos when they need it?

So besides switching from AAAs to AAs, is there any other way to smooth out the spikes in current draw when my servos activate?

I tried a 47uf capacitor, but that didn't do anything except stop my AtMega from freezing up occasionally, after activating them, presumably due to noise in the lines. (I placed the 47uf capacitor after the servos on the +5v and Gnd lines going to the rest of the circuit, and did the same with a 0.1uf capacitor.)

Would a larger capacitor do the job? The servos I'm guessing drawing around 200mA each when they activate, possibly up to 400mA when they reverse.

Or is an inductor needed here?

Or will neither of these provide the little extra bit of juice I need to avoid having my LEDs dim slightly when the servos activate?

Or will neither of these provide the little extra bit of juice I need to avoid having my LEDs dim slightly when the servos activate?

Probably a good answer. You don't menion how many AA batterys are used, and how they are wired. Generally speaking, typical AA batterys are a little low powered for servos unless the servos are very small. That being said, below is a setup I use to minimize the voltage disturbance to a servo control chip due to the operation of servos from the same power supply.

They're sub-micro servos, and there are two of them. They're powered with 4.8v supplied by a regulator connected to 6 AAA batteries providing 6v. The regulator also supplies power to the rest of the circuit, which draws maybe 50-100mA.

Power the rest of the circuit thru a low voltage drop (.1v) diode, and use a high value capacitor down stream of the diode to minimize the voltage disturbance to the circuit.

It's a little surprising to hear that AA batteries are underpowered for servos - they're what R/C modelllers have been using for decades

It's a little surprising to hear that AA batteries are underpowered for servos - they're what R/C modelllers have been using for decades

Yes, but R/C flight battery packs are almost always ni-cad or nimh rechargeable cells that are known for good peak current characteristics.

Lefty

Your numbers don't add up. 6 AAA batteries provides 7.2 to (over) 9V, depending on battery type. What battery type are you using, and is THAT changeable?)

As zoomkat implied, it may be a lot easier to use a capacitor to provide the 50-100mA of "the rest of the circuit" for the duration of servo movement than it is for it to provided the 200-400mA needed by the servos!

You REALLY REALLY want to ISOLATE the Servo Supply from the rest of the circuit. Your current solution will still let the servo loading and spiking to impact the stability of the pseudo 5V source you now have relying on voltage drop on diodes.

If it were me, I'd be dedicating a separate 7805 to the logic board and installing extra capacitors for assistance with spikes and dips.

West,

Sorry, you're right, the supply is currently 9v. The plan is to move it to a 4 AAA supply when I get such a battery pack.

You REALLY REALLY want to ISOLATE the Servo Supply from the rest of the circuit. Your current solution will still let the servo loading and spiking to impact the stability of the pseudo 5V source you now have relying on voltage drop on diodes.

Diodes? What diodes?

Are you talking about that schematic up there? That's not my circuit.

You REALLY REALLY want to ISOLATE the Servo Supply from the rest of the circuit.

Okay, but WHY do I want to do this? Because of these dips in current? Because they introduce noise to the voltage supply? Because they might blow stuff up?

In my circuit (not the one pictured above) I've got a 47uf and a 0.1uf capacitor after the servos to try to remove noise.

Do I need more than that? I know GrumpyMike suggests inductors after motors, but I've heard protection diodes and those sorts of things aren't really necessary with servos. And I haven't seen any schematics with servos that have inductors. If I do need an idncutor here, I'll have to go get one. But I don't know what size I need.

If it were me, I'd be dedicating a separate 7805 to the logic board and installing extra capacitors for assistance with spikes and dips.

I don't understand how that will help. Remember you're talking a newb here.

How will a voltage regulator solve my current supply problem? If the servos are sucking up all the current, isn't the voltage still 4.8v? How is a voltage regualtor gonna fix that?

Power the rest of the circuit thru a low voltage drop (.1v) diode, and use a high value capacitor down stream of the diode to minimize the voltage disturbance to the circuit.

So place the diode where, on the positive rail, or ground?

And what exactly constitutes a high value capacitor here? I know capacitors can be huge or tiny, and I have no idea what size would be appropriate in this instance.

The cap goes across the positive rail and ground I assume.

It's a little surprising to hear that AA batteries are underpowered for servos

I'm using AAAs here. But I know someone has built this same circuit powering two servos and he used a 9v battery. What I don't know is how he got away with that because my leds dim quite a bit when I try to use a 9v.

Power the rest of the circuit thru a low voltage drop (.1v) diode

I'm loooking on Mouser, and I can't find any diode near that. .3v was the lowest I could find. Most were at least .5v.

You want to look for a schottky diode for the low voltage drop for your setup. Now that you say that you are using a 9v supply, you can try the setup I posted, as all the parts are available at radio shack and the diodes used are the usual .7v voltage drop type. For a capacitor, you want to use one rated something like 15v, and the biggest uf rating that is reasonably available (this acts as a battery backup for the board).

http://en.wikipedia.org/wiki/Schottky_diode

I picked up a 4 AAA battery pack at Radio Shack, so I'm now running off 6v as stated originally with a 4.8v regulator.

I also grabbed an IN4001 diode: http://www.radioshack.com/product/index.jsp?productId=2036268

I meant to grab the IN4004 diode: http://www.radioshack.com/product/index.jsp?productId=2036270

Which GrumpyMike has in his motor tutorial, but apparently they had the bags mixed up. I think they're interchangable. I can't tell the difference between them, besides the 4004 being good to 400v and this one being good only to 50v. I don't know if that matters.

And I grabbed a 100uF capacitor. The 470uF capacitors they had were way too big for this project. I'm hoping 100uF is suitable.

Unfortunately, they didn't have any small inductors, so I couldn't get one of those. Can't be sticking a coil of wire an inch and a half long on this board.

Anyway, the project runs as well off the 4 AAAs as it did off the 6 AAAs. Which is to say, it runs, but the LEDs still dim when I activate the servos.

I'd still like more information about these changes being suggested. For example, how is adding a diode going to solve the issue of the batteries not being able to supply enough current?

You probably need to post a schematic of your wiring as I'm not sure just what you have done.

I’ve got 4 AAA batteries in a battery pack supplying 6v to a LM317 voltage regulator which is configured with two resistors to output 4.8v.

This is connected to the positive and negative rails on my breadboard.

The power lines for the two servos are attached to these rails.

After the servos, I have a 47uF capacitor and a 0.1uf capacitor bridging the two rails. The electrolytic is connected so the negative pole is on the negative rail and the positive pole is on the positive rail.

After that on the rails is everything else. The AMmega, a multiplexer connected to a dip switch, and a shift register for controlling the leds.

It’s fairly straightforward. I guess I can post a photo of the breadboard tomorrow. I don’t know what you expect to find though.

If you are running at ~6v, then directly supply the servos with the 6v (standard servos are usually rated for 4.8-6v) and just use the voltage regulator for the arduino power supply. Take the big capacitor and use across the +5v-ground on the arduino board. If you are feeling daring, you might could remove the voltage regulator and connect the arduino directly to the 6v thru one of your diodes. This would supply ~5.3v to the arduino, as well as prevent the stored power in the capacitors from back feeding the servos.

I tried running the servos off 6v, but they chattered, even when the circuit wasn't active. I'm guessing the batteries were either supplying a little more than 6v, or, as I have read, a lot of servos which work fine with 4.8v don't work so great with 6v, even if they should on paper.

Also, I'm not so sure dropping voltage for the Arduno using a diode is a good idea. It's just a gut feeling, but I suspect it won't stand up to a lot of current being put through it, and there's a possiblity I may replace the piezo with a speaker and drive it with a dac and an op amp.

Anyway, I've got those photos of the circuit ready and will be posting them shortly.

If you think that 6v is too much for the servos, then put one of your diodes between them and the battery to drop the voltage to the servos from ~6v to ~5.3v.