Stretching a power source too far?

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ShocKWavE
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Posted by ShocKWavE: Sun. Jun 11, 2006 - 08:19 PM
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I believe I am trying to stretch my power source too far, but I wanted to check with the more informed before strapping another unwanted battery to my project(s).

I am attempting to power a Mega88, a TI SN754410 Quad Half-H Driver, 2 IR detectors and 2 IR (LED) Emitters, and 2 small Mabuchi DC motors (which I am dumping 9v into) off of a single 9v battery.

The H Driver takes 5v for circuitry on VCC1 with a seperate VCC2 to power the motors. I tried using a 7805 to feed 5V to the circuit and LEDS and then fed a straight 9V to the VCC2 from the same source.

The result is an erratic circuit with obvious problems. When I connect a seperate 9v battery to VCC2, it works fine.

My problem in this case is weight. The motors are not geared and require the full 9V to operate well in my application. I am wondering if there is a way to fix this situation without the extra battery, or if I am just asking too much of a single one?

Thanks.

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AVR Microcontrollers, megaAVR and tinyAVR
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MegaUSBFreak
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Posted by MegaUSBFreak: Sun. Jun 11, 2006 - 08:40 PM
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Hi,

i don´t see a problem using the 9V battery for the driver IC and the %V regulator at the same time.

How much current?

Do you have a schematic for us?

Where exactely is your problem?

Klaus
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Look at: www.megausb.de (German)
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Bronze Marktheusch
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Posted by theusch: Sun. Jun 11, 2006 - 08:48 PM
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Quote:

My problem in this case is weight.

Then why are you starting with 9V batteries, which have really crappy power density?

Lee

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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ScottKroeger
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Posted by ScottKroeger: Sun. Jun 11, 2006 - 08:52 PM
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Here's a datasheet for a typical 9V alkaline.

http://data.energizer.com/PDFs/5...

You'll see that they don't like to work very hard. Your circuit malfunctions with only one 9V battery because your load pulls the terminal voltage so low the 7805 goes out of regulation. It needs about 6.5-7V to operate properly, and it doesn't take much load to yank a 9V alkaline below that. When you add the 2nd battery, the first no longer had to supply current to the motors and everything works, even if the 2nd battery sags.

You'd be better off with an LDO regulator, two batteries, or 2-3 cell LiPo stack if you can provide for proper charging.

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Gwen
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Posted by Gwen: Sun. Jun 11, 2006 - 09:20 PM
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I use tiny Li-ion camera batteries in some projects...they are cheap
if bought from a Hong Kong seller on ebay and they have a lot of oomph!.
And they can be recharged a lot of times.

Are you using the low-voltage version of the mega88?
Perhaps a small 1000uf cap placed somewhere in the circuit?
It could give you a little ballast to carry the circuit through a
short current spike...maybe the motors draw a lot when they
start up.

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ShocKWavE
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Posted by ShocKWavE: Sun. Jun 11, 2006 - 09:21 PM
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Thanks. I was working on putting up a schematic, but Scott's post seems to sum it up.

Lee, I appreciate your input, but a solution would do me much better than a question. I am not a battery expert. (1) 9v didn't pose a weight issue, but (2) possibly does (which I am testing now).

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ShocKWavE
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Posted by ShocKWavE: Sun. Jun 11, 2006 - 09:25 PM
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Gwen wrote:
I use tiny Li-ion camera batteries in some projects...they are cheap
if bought from a Hong Kong seller on ebay and they have a lot of oomph!.
And they can be recharged a lot of times.

Are you using the low-voltage version of the mega88?
Perhaps a small 1000uf cap placed somewhere in the circuit?
It could give you a little ballast to carry the circuit through a
short current spike...maybe the motors draw a lot when they
start up.

A smaller, lighter, and still relatively cheap battery to run the circuit would be nice, but there is of course the issue of life. I am not using the low-voltage 88, although that wouldn't have hurt. Still, the h-driver wants its 5v to operate. I am definetely experimenting with caps. The motors must remain reversible though.

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Gwen
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Posted by Gwen: Sun. Jun 11, 2006 - 09:29 PM
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I'm just thinking that the low v version of the m88 may make your circuit work ok as it is.
When the motor starts or reverses it may be pulling the V so low that the m88 is gasping
for breath :)

A small cap at the power pin of the m88 along with a diode to prevent the motors from
pulling cirrent from this cap might keep 5v at the m88 long enough to ride out a momentary
loss of regulated v from the regulator?

ps.. also a cap at the H bridge chip set up the same way might help

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ShocKWavE
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Posted by ShocKWavE: Sun. Jun 11, 2006 - 09:32 PM
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I'll experiment with it, thank you. I'll try to get my hands on a few low votage 88's to see the difference...unfortunately it's not an (immediate) option right now.

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ShocKWavE
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Posted by ShocKWavE: Sun. Jun 11, 2006 - 09:39 PM
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For reference. I appologize for the lined paper and scrappiness. I am also still fuzzy on how the circuitry between the h-driver pins and the motors should be.

Last Edited: Mon. Jun 12, 2006 - 02:42 AM
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Gwen
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Posted by Gwen: Sun. Jun 11, 2006 - 10:02 PM
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I'm curious...what exactly is this little project??
Some kind of a little bot maybe :)

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ShocKWavE
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Posted by ShocKWavE: Sun. Jun 11, 2006 - 10:08 PM
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yep...at least the beginning of one :)

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Bronze Marktheusch
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Posted by theusch: Sun. Jun 11, 2006 - 11:27 PM
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Quote:

Lee, I appreciate your input, but a solution would do me much better than a question. I am not a battery expert. (1) 9v didn't pose a weight issue, but (2) possibly does (which I am testing now).

Common sense, given the low-power-density fact. If a 9V battery has low power density, and there isn't enough oomph with a single 9V battery, and 2 9V batteries are too heavy, then use a battery that has a higher power density, and more oomph can be had with the same weight.

I'm no battery expert, either. But If weight is my consideration, I'd be looking at grams-per-mAH or some similar criteria. Beyond that it depends on what you budget can afford, whether you want rechargeable, etc.

Lee

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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ShocKWavE
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Posted by ShocKWavE: Sun. Jun 11, 2006 - 11:53 PM
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Thank you for your input.

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Bronze Markbobgardner
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Posted by bobgardner: Mon. Jun 12, 2006 - 12:02 AM
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You can measure the motor stall current... now you know exactly how many amps your power supply needs to put out. I vote for 4 AA cells.

Imagecraft compiler user

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pykedgew
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Posted by pykedgew: Mon. Jun 12, 2006 - 12:04 AM
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You could use a seperate LDO or 7805 regulator just for the M88 chip.

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ShocKWavE
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Posted by ShocKWavE: Mon. Jun 12, 2006 - 12:49 AM
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Thanks for all the input. By going through it all I think I found a solution. It seems that with either 4 AA or a 9v, I just needed to add a capacitor from the 7805 +5v out to GND. I have the circuit stripped down a bit right now, but I'll be sure to check back in when I complete it (or break it again :) ).

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mneary
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Posted by mneary: Mon. Jun 12, 2006 - 04:16 AM
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The 7805 has a fairly high standby current; not a big deal when it's alongside motors, but it becomes very interesting if the circuit spends a lot of time in standby. If this concerns you, look at the LM2936.

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Guillem Planisi
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Posted by Guillem Planisi: Mon. Jun 12, 2006 - 09:42 AM
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My 2 cents for a little bot: Li-Pol batteries, only one cell, since they are the higest density energy storage system in the market. One stepup to power the high voltage side of the sn7554410, and another to power all the control circuitry at 5V. Perhaps the Mega88 could run directly from the battery, but then the interface to the SN754410 at 5V can be a problem (it shouldn't since inputs are TTL thus a 3V3 is a clear Hi level). Also the Mega88 can control the pwm for the step-up and all the charging.

Another option is to use a cypress PSoC to step up to 5V (it has an step up controller embedded) and also to step up to 9 to 12 V for the motors with low resources involved, better regulation, and nearly all chip flash and ram ready for other uses, like motor control and ADC (up to 14 bits resolution though).

In fact, a two uC solution would be my approach for little bots, PSoC for small tasks, like controll all power management, and ATmega as the main brain for the system.

Guillem.

Guillem.
"Common sense is the least common of the senses" Anonymous.

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Bronze Marktheusch
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Posted by theusch: Mon. Jun 12, 2006 - 01:06 PM
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I'm not into bots, but it would seem that the remote-control airplane people would have power/weight as a prime criterion. You might want to look at those battery packs.

Lee

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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