Power issues in my circuit? Can anyone advise?

Other Hardware General Electronics
1

Hi, me again.

I was wondering if anyone would mind taking a look at my circuit. I'm fairly sure it's a power issue (whether is a supply issue or my circuit setup) but was hoping I could get some help from you veterans.

Attached is a screencap of the breadboard setup, hopefully it's laid out clearly enough. It takes a 4x AA battery supply (around 6.6volts when everything is fully charged) and runs it through a 7805 regulator to drop it to 5V. This is then run through some 100uF capacitors to smooth out the voltage and then sent to the second bus on the breadboard as 5V voltage. There is a LED hooked up so I know when it is powered on, and it's also sent to the chip and crycstal where the sketch is running. This is all essentially supposed to be a replacement for the arduino Uno board, as the long term goal is to have the atmega328 chip permanently in the finished project bit not the Uno board so I can use it for other projects with a repalcement chip.

The power supply is also run off to a 7806 regulator to drop it to 6V. This supply is again run through some 100uF capacitors and is what powers all the servos. The servo signal wires are connected to the chip.

I'm trying to power the servos and the atmega328 chip off one power source but am running into some trouble. As it stands, I can power 2 servos with no problem in this setup, a 3rd sometimes and a 4th just stops everything working all together. During testing the battery voltage dropped below 6v, so I bypassed the 6v regulator connecting the servos to the power supply directly and all 4 servos moved (though a little weak). This is why I'm assuming it's some sort of power or circuitry issue.

At the moment only 4 servos are connected which I'm struggling with, but the end goal is to have 6. Is my power source just completely inadequate or is there a problem with the circuit? If it's a power source issue, could anyone recommend one to power 6 servos and the chip (preferably one that uses standard batteries or is a wallwart my only option?)?

I understand the regulators will limit current to 1A, is a wallwart with a 3A switching regulator my only option to move 6 servos and provide enough current to them so they can lift a load (if you need to know, the servos are HXT 900)?

I've tried looking for some robot tutorials (as that's the only project I can think of that would use multiple servos and possibley standard batteries rather than specialized ones) but am trouble finding information as everyone wants to use kits and is simply saying "plug part A into part B", which is no use if you want to actually learn why you're using those components.

Really appreciate any help, even if it's just to direct me to a post or tutorial or anything that can help.

Thanks in advance to anyone who tries to help!

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2

Don't "regulate" the 6V for the servos (you haven't enough from the batteries to do that anyway.)

3

I agree with Runaway Pancake's advice. Also, a standard 7805 regulator is not suitable for dropping 6.6V to 5V. For such a small voltage drop, you need a low-dropout regulator.

4

Thanks guys, I figured it was something around that area causing the issue, I'll have a rethink about this area.

The 4 servos I have connected will all move if I don't use a 6v regulator, but they can't move much when there's a load on them.

My understanding is the higher the load, the more current a servo needs to move. If there's only 2 connected, then they can move with a load with no problem. I'm guessing this is to do with the amount of current it can draw from the 4 AA batteries?

Can anyone reccomend what I should use as a power source if I want to have 6 servos that will have a load on them (google suggests you should budget 1 amp per servo but that sounds pretty high to me)?

Ideally I would like it to be standard type batteries (AA, AAA, 9v batteries etc...) or is a wallwart the only way to produce the ampage needed for this?

Cheers

5

Alkaline cells (whether AA or any other size) are not very good at high currents. It sounds to me that you need to switch to lithium AA cells (which are expensive) or rechargeable NiMH AA cells (which have a lower voltage, so you will need 5 or 6 of them instead of 4).

6

A wall pack is great if it's not for a mobile/robot platform.
Servos have an initial surge, just as any motor. So, kicking all 4 on at once would be a kick in the shorts.
The more weight, the more work, the more current.

7.2V NiMH rechargeable packs (a pack and a charger) can be found at hobby/RC stores handily.

You should be able to run the servos (6V) safely off the 4-cell pack through a diode.

Try running the rest using a 9V battery, however short-lived, into the 5V regulator to see how things go better.

7

Cheers dc42, really appreciate you getting back to me so quick.
Edit: You too, runaway pancake! You must have posted that just as I was making this reply!

5 or 6 AA batteries is something I can live with (especially if they're rechargeable) if it provides adequeate current for all of this :slight_smile: I was a little worried someone was going to say the only way is to have 4 batteries per servo or something!

How do you actually judge the max current you can draw from a AA battery? My understanding is the mAh rating on a battery (e.g. 2,700 mAh) is a rough indication of how long the battery would last if you drew that much current from it continuously (so the example would last 1 hour if you drew 2.7A continously or 30 mins if you drew 5.2A continuously).

The reason I ask is that if I connect 6 AA batteries in series, my understanding is the voltage may be 7.2v (where each battery is 1.2v), but the current remains the same as if there was 1 AA battery. If I connect them in parallel, then the volatge remains as 1.2v whilst the max current you can draw goes up by the number of batteries connected.

I'm slightly confused as to how I can draw enough of both the voltage and current from the batteries to power 6 servos, if they need 4.8-6v each + 1A each (in theory), if I connect these in series and 1 battery on it's own cannot provide enough current?

Sorry for sounding like such a total beginner, but electronics is all new to me I'm afraid. Software on the other hand, I'm fine with! I've been doing that for years!

Really appreciate the help, and thanks again for taking the time to respond to me.

8

Your reckoning of batteries is sound enough.
I avoid paralleling batteries, preferring bigger cells.
Are these just hobby servos (futaba type)? Those won't take 1A, except, maybe(?) all four in a surge-on, but still.
Try my previous suggestion, go with 2 servos, then add more.
You could make it so over-laden with batteries (capacity) that it can't move, too. :wink:

9

These are just some micro hobby servos (Hexatronic HXT 900) I'm hooking up. If I hook one up to a power supply (which there is no problem powering) and measure the current whilst holding the servo arm in place with my hand to simulate a very high load, it seems to use 750mA. Letting it run free, it was around 75-125mA depending on the speed I set it to move.

So with that, in theory 4.5A is needed if all servos are under very heavy loads and I also need the 4.8v-6v of power for them in general. It is extremely unlikely that all servos will be under such heavy loads, I would imagine at most 2 servos would be, but for very short periods of time (10 seconds or so), but I want to make sure I cater for this situation.

Is it possible to even draw this much current from 5 or 6 AA batetries whilst drawing 6V, as it does sound like a huge amount?

One thought is that the voltage regulator says it can only deliver 1A. Is that the problem? Would I need a regulator for every 2 servos so each can get at least 500mA (so for 6 servos I need three regulators)?

I gave a 9v battery a try and it only managed to power 3 out of the 4 servos when it connected to the circuit (so 2 voltage regulators are in the circuit I tested. One 5v regulator for the atmega328 chip, and one 6v regulator for the servos). However, I checked the 9v battery voltage with the multimeter and it only measured 8v, so the battery probably hasn't got enough life left in it.

The project is going to basically be similar to one of those robot arms, and I need 6 servos for all the various movement areas. As it's stationary, a wall pack is certainly an option, but I did want to see if batteries were viable as that would be the more preferred option, as I'm not sure if it will be near a wall socket.

If someone knows of a DIY tutorial for robot arms like this I probably would be able to learn from that also. The only problem I have is they all seem to use kits or hack store bought robotic arms instead of making you build one yourself completely from scratch with the electronic theory work. So they're all pretty much saying "just plug this into this", without an explaination of how the circuitry works or why you're making your circuit in that way :frowning:

Again, really appreciate your feedback. One day I will understand all of this!

10

Power just the Arduino-related part of the circuit with the 9V-to-7805.
Power the servos, separately, with the 6V pack. 6V is pushing the envelope, but I think you'll be OK.
[Remember, connect the Grounds together...] OK?
I think a hand is too much. No servo of the hobby sort is going to push your hand out of the way no matter what battery it's connected to.
Some of this stuff you just have to experience out like.

You could get a big 12V sealed lead-acid (SLA) battery for big-time current.

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11

Quality NiMH AAs, like Sanyo Eneloops, can handle a 4.5A discharge and then some:

If you use "regular" (non-Precharged) NiMH performance will not be as good as "regular" cells have a higher internal resistance.

12

Bobu:
How do you actually judge the max current you can draw from a AA battery? My understanding is the mAh rating on a battery (e.g. 2,700 mAh) is a rough indication of how long the battery would last if you drew that much current from it continuously (so the example would last 1 hour if you drew 2.7A continously or 30 mins if you drew 5.2A continuously).

Look at battery datasheets, for example the ones at http://data.energizer.com/. These show you the voltage over time at various discharge currents.

13

Thanks again everyone for the info.

I wasn't really trying to get the servo to push my hand out the way, just simulate a high enough load to see how much current 1 servo will attempt to draw. That way I could just estimate the maximum current the 6 servos may attempt to draw on very high loads.

I was trying to avoid having 2 power sources (i.e. a 9v battery for the atmega328 and 4x AA batteries for the servos), but if this is the ONLY way to do this I may have to bite the bullet on this and do just that, or possibly go for a wall wart to replace the batteries all together. A wall wart does slightly worry me just because I'll be dealling with the mains electricity, but if that gets me what I need instead of having a large number of batteries in this, I may have to consider this route.

If I had a 5v 2.25A wall wart, I take it this would be fine to power everything (in this situation I guess I wouldn't need any regulators/capacitors either as the wall wart is converting everything to 5v regulated DC for me)?

Here's a link to one (but it seems pretty pricey to me, so I'll see if I can find one on eBay or any electronics around the house that I don't use anymore) http://www.maplin.co.uk/ac-dc-fixed-voltage-switched-mode-power-supplies-48484

I'll definitely take a look into some rechargeable NiMH batteries for this, as it will save costs in the long run if they're rechargeable.

Thanks for pointing out datasheets for batteries. It's good to see that they can produce 2A of current, as short lived as it may be (30 mins I think before the voltage starts really dropping according to the energizer AA NiMH sheet).

I figured the power side of things would be the easy part getting into electronics, apparantly not!

14

Bobu:
I was trying to avoid having 2 power sources (i.e. a 9v battery for the atmega328 and 4x AA batteries for the servos), but if this is the ONLY way to do this I may have to bite the bullet on this and do just that, or possibly go for a wall wart to replace the batteries all together.

Just thought that'd help you get past your current logjam so you could get on with development.

Bobu:
...instead of having a large number of batteries in this,...

A 4-cell servo pack and a 9V battery isn't unwieldy.

Bobu:
If I had a 5v 2.25A wall wart, I take it this would be fine to power everything (in this situation I guess I wouldn't need any regulators/capacitors either as the wall wart is converting everything to 5v regulated DC for me)?

I can't make that guaranty.

Bobu:
I figured the power side of things would be the easy part getting into electronics, apparantly not!

It's easy to take a lot for granted, but many factors get overlooked, everything adds up, before you know it "power" turns into "high power".

15

Just wanted to close this post off and say thank you to everyone for their help. I believe that the battery power issue is now resolved and I'll go with the 2 battery source approach so many of you recommended (9v for the atmega328 chip and 6v battery pack for the servos that consists of 4x AA batteries or 6x AA if they're rechargeable 1.2v batteries & I'll add a regulator).

I am thinking of changing the power supply now to a wall wart and add a switching regulator rather than using all these batteries, but I think I'll start a new post on that as that's a different topic from the original post :slight_smile:

Thanks once again for everyones help !