Author Topic: power supply design decisions  (Read 1233 times)

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Offline szhangTopic starter

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power supply design decisions
« on: October 22, 2008, 02:36:24 AM »
I am still the in prototype process for my robot, and I haven't decided exactly on the hardware.  So currently I have a 12V DC motor, and I got a 12V battery pack to power it.  Though it might be possible for me to make the robot a bit smaller, and switching to a 6V motor, though I still have the 12V battery pack.  So, what I am wondering is do you all think it would be a good idea (or just any reason not to?), in terms of power dissipation and stuff, to run motors with a 12V battery pack regulated to 6V. 

This does have an advantage in that I can maintain a pretty consistent voltage to the motor (no voltage drop during high load).  I found a pretty good switching regulator http://focus.ti.com/docs/prod/folders/print/ptn78020w.html which has a listed efficiency of ~96% with a current out of 6A.  I would need to redesign my motor driver, though that won't be too bad.

Offline ArcMan

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Re: power supply design decisions
« Reply #1 on: October 22, 2008, 07:44:50 AM »
IMO, the battery pack that's meant to drive your motors should be built for the voltage of the motors - no regulation.  Generally, the motors are the highest current draw on a wheeled robot, so don't waste money or battery energy regulating the motor voltage - just build (or buy) the correct battery pack.

Offline Admin

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Re: power supply design decisions
« Reply #2 on: October 23, 2008, 11:04:25 PM »
You never want to regulate voltage to a motor:
http://www.societyofrobots.com/actuators_servos.shtml#regulate

Just buy a lower voltage battery - the weight savings will be an added bonus :D

Looking at the datasheet, your efficiency will be under 90%. That chip looks like a good find, although it's somewhat large at 1.5" x .9". . .

Offline szhangTopic starter

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Re: power supply design decisions
« Reply #3 on: October 24, 2008, 01:16:20 AM »
At 6V, it'll take less than 3A to hit the 90% mark, and I can see my combined current draw to be higher than 3A.

Normally I would agree not to regulate something like a motor, but I think I can afford the 10-20% energy loss (since I have double the energy capacity).  Also, I really don't want the robot to slow down as the battery voltage drops, which is another reason I'd like a regulated power supply.  The weight won't be a problem, size might be, so I still don't know about that.

I was surprised to see TI making that chip considering it is more a completely module with a PCB and everything.  They even gave me a couple of samples (they are about $10-15 a pop otherwise).  Though these things come with non-.1" spacing, which makes prototyping bit of a pain.

 


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