# Power Issues

Hi, so I have a project I am doing for someone. To simplify things, I have two servos, four LED's/lasers, one potentiometer, and one itsy bitsy 32u4. I need to be able to run all these things at once. However, I am not sure how much power--current and voltage I need to do this. Does someone just know the answer, or is there an accurate way to figure out. I know the board can output 150mA at 5v, but how much would that be at 3.74v. Is there a way I can figure this out myself in the future?

Thanks everyone!

The regulator on the 32u4 won’t be used to power the servos, so you can eliminate them.
All the other stuff combined should come in well under 100mA if you design the circuitry properly.
Now you need to decide on your primary power source, and isolating the servo power spikes from the uC supply.
Typically RC setvos can draw up to 2A @ 5V under stall/full load conditions (they don’t want to run properly at 3.7V unless you have special low-voltage servos).

BC, the servos didn't have enough power when plugged into the itsy bitsy, they turned but weakly, and not all the way. When I put them on my UNO, they worked fine though. So you are saying they would work fine at 5v.

Current is what makes heat because it's what drives things while voltage -can- make current flow.

No kidding, when electricity flows through a wire loop (a coil is many loops) the field equations (Faraday/Lenz Law) has no term for voltage, it's about current. What voltage does do is push more or less current along.

Voltage limits are important and knowing voltage makes figuring out what current will flow, voltage is the pressure and there are components that will pop if V gets too high across some leads.

You can get around the controller current limits by having it use small (signal level) power to open/close transistors that let a lot more current flow to devices that need it. Power FETs are getting cheap if you shop around and buy more than a few. You can get driver chips and boards with drivers pretty cheap for small apps.

Your servos may not need more current than an Arduino pin can handle. In that case, 1 pin can run the servo. It's just not a good idea to source or sink > 25 mA through that I/O pin and no more than 200 mA through all used I/O pins (5V and GND are not I/O) total at least for the Uno and AFAIK all 328-chip Arduinos.

You can use a transistor to switch 12V power with less V doing the switching, 5V or 3.3V can do that. You can give 5V and likely 6V to the servo (check, it probably won't hurt the servo) that way while using your 3.3V controller.

When I put them on my UNO, they worked fine though.

Don't do that, you will eventually destroy the Uno. Use a separate 5 to 6V power supply capable of 1 Ampere per servo.

Don't forget to connect the grounds.

I think that there are toy servos that won't stress an Uno pin, at least with a proper resistor they won't. They're weak but can serve as dial needles on sim guages.

Okay, thanks for the replies! I guess I am still a little confused though. If my servo can be run from the pin, (someone said it could and someone said it couldn't) it's voltage specs are 4.8v-6v and its min and max current are respectively 5ma and 700mA. So if the board can only output 150mA obviously there's a problem there, since the servo can need a lot more. The battery I have can output 3.7v and 1.2A/hour. Is there a way I can bypass/negate whatever controller the board is using that limits the current? Or is it a better idea of doing the FET Driver like this one: https://tinyurl.com/ya8a8lxz Would that particular FET Driver be capable of driving both servos without using up all the board's current/power?

Is there a way I can bypass/negate whatever controller the board is using that limits the current?

Use a different boost converter. Pick one that is able to produce 1 Ampere per servo.

It is very unlikely that your battery can output 1.2A/hour. What you probably mean is that it has a CAPACITY of 1.2Ah. On its own that tells you almost nothing about the maximum current it can supply (I have a 1.2Ah lipo battery that can supply over 20A and there are others more powerful). Do you have a link to the actual battery?

Steve

Here's the link to the battery: https://tinyurl.com/yd5qhgty

Those batteries are intended for cameras and the like, and are not suitable for servos.

They have no charge/discharge protection. If you decide to use them, they probably won't work, and will likely be destroyed very rapidly.

I suggest to use a 6V NiMH battery pack instead. Much safer for charging and discharging than LiPo batteries.

Vulcan666: If my servo can be run from the pin, (someone said it could and someone said it couldn't)

Some can run from an Uno pin. Yours..... As for 3.3V, check the device specs.

it's voltage specs are 4.8v-6v and its min and max current are respectively 5ma and 700mA.

Uno pin can source 25mA continuous safely. You would put a resistor between the pin and the servo to prevent more current from being drawn from the pin.

So if the board can only output 150mA obviously there's a problem there, since the servo can need a lot more.

The servo can use more, which your project may or may not need.

Is there a way I can bypass/negate whatever controller the board is using that limits the current? Or is it a better idea of doing the FET Driver like this one: https://tinyurl.com/ya8a8lxz Would that particular FET Driver be capable of driving both servos without using up all the board's current/power?

You can run a power wire from what supply you have to a transistor (FETs are very efficient, BJTs are very cheap) that the controller turns on and off to let the power flow to the servo. The servo ground wire should be connected back to the power supply (does not have to be the same as board power but those grounds should be connected so that board ground = servo ground). This will let you use 5V or 6V to run the servo and 3.3V for the board.

The important part is that motive power can run outside of the controller while the controller can switch that on and off using small power.

You will need to spend time learning about transistors but for this there are schematics you can follow. Which transistors you get will determine what schematic but you can get plenty of help with that right here.

A couple of years ago I got \$10 worth of IRLZ44N's on eBay, 60 of them at 14.5 cents ea. They can switch > 10A, more than enough for my needs. For just a few (like 10) they were about 35 cents ea and some sellers wanted close to \$1 for 1, it pays to shop around! You may find better deals, you may find other FETs (field effect transistor) you like more. The IRLZ44N (and all that end with N instead of P) is best used to drain power from a circuit to ground (the power is always there but can't flow until the path to ground is open through the FET) and that's what affects the circuit you use.

Heck, I have relays that control with 3.3V or 5V and can switch 10A of house power (120VAC here), those are cheap ones too. The thing about relays is that they can't take being switched on-off-on-off quickly for long, it will make them hot. Transistors can be switched very quickly though with FETs only full ON and full OFF or they will get hot.

I hope that the background will help you get into this easier.

And last tip; look into DC-DC converters. Cheap ones can output 1.5A continuously. You have loads of options.

Well relay drivers aren't going to work for me nor different batteries as due to the nature of the project it won't work, (not enough space) as I would have to reprint a 3D box I have already made and take everything out which would be a giant mess :( Guess I should have come here sooner! However, thanks for the tip about the batteries, though again, changing them would kind of wreck the project. So I guess in the future I will know this and definitely incorporate relays(as needed) into my project, check data sheets, and depending on the project do the right rechargeable batteries. So I guess what I really need to know is if there is a micro answer to my problem. If I used a single FET, not a board) it would fit. But I dont' know how to do that, as the battery powers the board, I would have a problem since I need a higher voltage to be the "switcher".

Again, thanxs for the help everyone!

A DC-DC boost converter can raise voltage efficiently. Boost type raises, Buck type lowers. Used to be < \$3 for 3A max from China, might still yet. A good US source will charge less than 3x but get it to you in a few days. The things are small but I dunno how big the board you have is.

Relays are much bigger than FET + resistor (to the controller pin to keep flow to the gate from draining too much current), are inefficient (energy to switch is huge compared) and slow switching. I have them for switching mains power because other means are nowhere near as cheap! IMO code for relays needs timers to keep from switching any one more than about once a second.

I think that a solid resistor on lines going to the servo should help keep the battery from draining too quick but best check with the members who know more about those batteries and hardware in general.

A lesson you should take from this is don’t make a box until you have a working final circuit. Use something else or spread the thing across a desk or table area. An Altoid tin or some other product packaging may do for prototype enclosure if you need one to test mobility or like.

Okay, thanks for the advice. That's a good point about not making the box :) Would this work for my 3.7v battery: https://tinyurl.com/yamovnfe and give it the power of a 3.0 usb port on my laptop? Because my laptop can run the whole thing just fine, as the ports have more power (enough) than the battery. The dc-dc converter only changes the voltage though, wouldn't you want more current? And what would you recommend the resistance of the resistor to the battery be?

Thanks.

This is an adjustable boost converter with a 35V max output. It can output lower V as long as the input is lower than that.

This one boosts 1 to 4 volts up to 5V and cost less than \$2 shipped (figure 3 weeks from China). https://www.dx.com/p/usb-dc-1-5v-to-dc-5v-voltage-step-up-boost-module-green-143571#.W6xA0K3gVj8

"The dc-dc converter only changes the voltage though, wouldn't you want more current? "

The battery supplies the current until it drops below 1V. To get 1A out of the converter takes a bit (10% or less) more than output A / input A from the battery. Your servo maxes at 750mA? Will it do what you need with less?

" And what would you recommend the resistance of the resistor to the battery be?"

I'd put the resistor between the boost converter and the servo (after some thought) and use Ohm's Law to find the resistance.

Amps = Volts / Ohms

rearrange and I am guessing at 5V and 700mA max...

Ohms = Volts / Amps = 5V / .7A = 7 ohms.

If I didn't need 700mA to drive the servo, maybe a 10 Ohm resistor would do.

You can put resistors in parallel to get a smaller resistance than either. https://www.electronics-tutorials.ws/resistor/res_4.html

Resistors mostly come in standard values which is why they get paired up at times. http://www.resistorguide.com/resistor-values/

Still, check with jremington and others about using resistance to limit battery draw just in case.

Thanks, a lot, I think I've got figured out what I need to do to get this project back online, (and a few helpful guidlines for the future)! I think the DC 3~34V to DC 4~35V Adjustable Boost Converter Charger Module would be better for my project, and the servo's won't need to have the max current, so it should be fine. However, I need quick shipping, 1 day at best and 2 at worst, so I'll have to go the Amazon route.

This should be the same thing right? https://tinyurl.com/y6u7tmcy

As for the resistors, thanks, I will check your work with someone else, but thanks for the estimate!

Thanks again! :)

the servo’s won’t need to have the max current

Servos momentarily draw the maximum (stall) current every time they start moving.

They draw what they can get, right? That's what the limiting resistor is for same as pins with leds or FET gates?

Bad idea to limit stall current of a servo with a resistor. Servo power is for the motor and for the electronics inside the servo. If you mess with servo power, then the position feedback could get flaky too, resulting in jitter. Leo..