9.6v Battery: Will it work?

[Roberto]

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I was shifting through some old junk yesterday, and came across a 9.6v rechargeable battery pack. If I plug it into the $50 robot, will it fry it, or will the 5v regulator stop it from frying.(By the way, this is my first robot  )

Help is appreciated,

Roberto

[superchiku]

its ok.. it will work but you will need regulators both for the control board and the servo

use 7805 for your control board and 7806 for regulating 6 volt for your servo, oh yes since  the wasted voltage drop is around 3.6 volts, your power wastage will be high ...

[airman00]

regulating power to servos is a big no no   

[AndrewM]

regulating power to servos is a big no no   

Why do you say that?  A regulator that can handle the servo current draw during a stall, with a decent capacitor across the regulated power supply for surges should be fine.

[superchiku]

a 7806 can withstand 1.5 ma current load so i see no problem , just that you need to attach a heat sink to it

[AndrewM]

a 7806 can withstand 1.5 ma current

1.5ma is quite small, you sure it is not 1.5a?   

[pomprocker]

I was shifting through some old junk yesterday, and came across a 9.6v rechargeable battery pack.

Don't batteries expire?

[Roberto]

This one still works...

[ed1380]

get a switching regulator.

[airman00]

A regulator that can handle the servo current draw during a stall, with a decent capacitor across the regulated power supply for surges should be fine.

As admin said

Regulating Voltage to a Servo
As you should already know, servos have a voltage rating. Go above that voltage and your servo overheats and possibly fries. So suppose you have a 7.2V battery and you want to use a 5V regulator to power your servos, is that a good idea?

Short answer: No!

Longer answer . . . it will work, but its a huge waste of battery power.

So lets say you have your 7.2V regulated to 5V and the servos draw a total of 1.5A of current.

Wasted power is:
(7.2V-5V)*1.5A = 3.3W

Percentage wise, its
(7.2V-5V)/7.2V = 30.6%

Thats the battery energy percentage wasted to thermal heat - almost 1/3rd!!!

Speaking of heat, your voltage regulator probably has thermal shutdown, meaning that if it overheats it will throttle down current to your servos - meaning your servos will have lower torque and lower speed. If your voltage regulator doesn't have thermal shutdown, it will just fry instead (not a good thing).

But if you still really really need to regulate for servos, get a switching regulator (like ~83% efficiency on average).

[AndrewM]

Two servos through a regulator rated for 1.5a should not put the regulator into thermal shutdown (assuming it has a thermal threshold built in that is), as you are only pulling around 300ma per servo under normal use, 600ma at stall.  Granted if your servos stay at stall you might have problems, but if your servos are in a stall condition that long you have other problems anyway.  So thermal shutdown is not an issue.

Energy waste is a concern, but you waste energy whenever you put a regulator into a circuit, even for your mcu power.  In an ideal world a battery that is rated at 5V would stay at 5V from full charge to discharge, but seeing as that isn't the case, we dump in 6V or 7.2V packs to power the 5V mcu, wasting energy across a regulator.

At this level (meaning the hobby entry level robotics), you aren't concerned with the energy efficiency needed to get through a darpa grande challenge track on battery power alone, so you really only have one concern with running stuff of a regulator besides an mcu, and that is amperage rating.  Which in this case in not really a factor given the regulator mentioned by superchiku and it driving only two (I would assume) standard servos.

So that leaves me with one good reason to say "you can use a regulator to drop voltage to drive your servos."  If you have a batterypack laying around, and you want to keep the $50 robot a 50 dollar robot by not having to purchase batteries, but instead a $1.50 regulator then it seems like a good choice.

[superchiku]

yes its 1.5 A , power wastage was always a issue but what can one do , the servos can withstand max 8v actually but for safety it is rated at 6v so without the regulator , we can say the servo bye bye

[airman00]

only pulling around 300ma per servo under normal use, 600ma at stall. 

Servo stall is usually much higher than 600ma.

 And even if the thermal shutdown does not occur, the heatsink of the regulator gets very very hot , and that heat can do some damage to other components on your circuit board  , so make sure to add an extra heatsink to the existing one.

[Conscripted]

yes its 1.5 A , power wastage was always a issue but what can one do , the servos can withstand max 8v actually but for safety it is rated at 6v so without the regulator , we can say the servo bye bye

If the Vmax for the servo is 8vdc couldn't just put an appropriate sized resistor on the power line to drop a volt or two?

[airman00]

you could but that is incredibly inefficient

[superchiku]

well inefficient because 9.6 v rated battery is not 9.6 v always , it will be 10.XX something fully charged and 7.XX something completely discharged so adding a resistor means power supply will never be a constant 6v it will fluctuate according to the potential of the battery

[ed1380]

Two servos through a regulator rated for 1.5a should not put the regulator into thermal shutdown (assuming it has a thermal threshold built in that is), as you are only pulling around 300ma per servo under normal use, 600ma at stall.  Granted if your servos stay at stall you might have problems, but if your servos are in a stall condition that long you have other problems anyway.  So thermal shutdown is not an issue.

Energy waste is a concern, but you waste energy whenever you put a regulator into a circuit, even for your mcu power.  In an ideal world a battery that is rated at 5V would stay at 5V from full charge to discharge, but seeing as that isn't the case, we dump in 6V or 7.2V packs to power the 5V mcu, wasting energy across a regulator.

At this level (meaning the hobby entry level robotics), you aren't concerned with the energy efficiency needed to get through a darpa grande challenge track on battery power alone, so you really only have one concern with running stuff of a regulator besides an mcu, and that is amperage rating.  Which in this case in not really a factor given the regulator mentioned by superchiku and it driving only two (I would assume) standard servos.

So that leaves me with one good reason to say "you can use a regulator to drop voltage to drive your servos."  If you have a batterypack laying around, and you want to keep the $50 robot a 50 dollar robot by not having to purchase batteries, but instead a $1.50 regulator then it seems like a good choice.

thats why i said use a switching regulator. very little is wasted to heat. no matter what imput voltage

[airman00]

thats why i said use a switching regulator. very little is wasted to heat. no matter what imput voltage

right , thats the best thing to do

also you spelled input wrong , lol    no M in input

[ed1380]

exhaustion works wonders 
i know thats spelled wrong too

[AndrewM]

No disagreement on the switching regulator front.  But while we are on the subject, do you know of any good low current switching regulators (meaning will stay active at 5ma draw or so)?

[ed1380]

IDK any specific ones but search around on allegro

[superchiku]

switching regulators will be hard to find.. i hope digikey has it.

[cooldog]

switching regulators will be hard to find.. i hope digikey has it.

they should

[ed1380]

samples. duh
what i got http://www.allegromicro.com/en/Products/Part_Numbers/980502/
just dont order what you dont need

[AndrewM]

After a little peaking and poking around, I found this bad boy which would be pretty good for servos:  http://www.national.com/ds/LM/LM2596.pdf

3A rating, and the only switching regulator that hits 80% efficiency rating in the 5V range that I could find (12V I was seeing 90%).  I put in for a sample from National Semiconductor.  Seeing as weight will be a definite issue limiting the number of batteries I can use, I thought I would give it a go in one of the hoverbots to see how it works out.

[superchiku]

did national give u the samples withour a comapny email???

i send a sample request to national but they didnt give any tracking details??

[AndrewM]

My company has had an account with National, and several others, for quite a while and I have never had a problem getting samples.  Texas Instruments is the only company that has ever gotten wishy-washy with me only because I refuse to deal with their local rep.  I'm just not a people person I guess   

I don't know about National's policies on samples for individual users though.

[superchiku]

national doesnt ship free for single users , its not worth paying 60 $ for shipping a 2$ chip

[airman00]

national doesnt ship free for single users , its not worth paying 60 $ for shipping a 2$ chip

lol


many companies manufacture the same or similar parts

for example the 7805 regulator is manufactured by multiple companies

[superchiku]

but some parts are not i can ask TI for some switching regulator samples but i have ordered so many samples from them that i feel ashamed of asking for samples from them....