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# connecting high current and low current devices to the same battery

Posts: 200
edited 2005-02-05 - 21:07:26
Are·there any negative effects in connecting·a device that draws 40,000mAh and a device that draws 50mAh·to the same battery?

·

• Posts: 6,351
edited 2005-02-01 - 07:40:39
While this is not an answer from experience, in theory if your battery is capable of supporting such power, no. But there are many other factors that figure into the equation that could change the answer.

Are you stating your figures correctly? Typically mAh referes to a battery capacity (1V, 1Ah battery has 3600 Joules of energy stored (1 Joule/sec = 1 Watt = 1Volt x 1Amp)), consumption is expressed in current (Amps) or power (Watts). The term mAh is 3.6Coulombs/sec^2 which without knowing the voltage does not provide enough information to know how much power is being consumed.

Sorry a bit of a ramble with a bunch of terms flown at you, its late, I'll try to rephrase/simplify after some rest.

Paul
• Posts: 187
edited 2005-02-01 - 14:23:42
Mixing high and low current draw items on a single power supply is common in robotics and causes lots of problems. In particular, the sudden current draw from motors and some infrared sensors can cause the microprocessor voltage to suddenly drop causing it to reset.

A common solution is to place a large capacitor on the power input to the microprocessor to "carry it" over the loss of voltage. The size of the capacitor depends on the current draw and duration of current draw by the high current component.

Another solution is to use a separate power source for voltage sensitive components, like a Basic Stamp.
• Posts: 1,563
edited 2005-02-01 - 14:55:31
If you're connecting motors on the same supply as your uP....are the caps big enough to sort out the back EMF?· Wouldn't you want a diode inline as well (inline to the uP).

Just wondering what most do?!

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·

Steve
"Inside each and every one of us is our one, true authentic swing. Something we was born with. Something that's ours and ours alone. Something that can't be learned... something that's got to be remembered."
• Posts: 6,432
edited 2005-02-01 - 15:58:55
I have used the diode capacitor method when I have plenty of Voltage overhead during a surge to supply a uP, but in extreme situations I have
used a MOSFET capacitor method. The difference is that with the MOSFET method you do not see a .6V diode drop. Sometimes this little amount
can make a BIG difference in keeping a uP alive during a surge with limited Voltage overhead.

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Beau Schwabe - Mask Designer III

National Semiconductor Corporation
(Communication Interface Division)
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Norcross,GA 30071
• Parallax Engineering Posts: 14,406
edited 2005-02-05 - 21:07:26
Remember too, depending on what's connected, these may not be relevant issues in his design.· I have had high-power and low-power devices running on the same supply before...When the high-power device isn't drawing in surges (Switching on/off) you don't have as much to worry about, especially if the battery/supply is already more than adequate for the higher-powered device.

On a related-note...I remember some years back a colleague of mine did a controller for a hospital designed to reduce their electric bill.· Apparently the electric bill was based on peak current provided.· They had these huge exhaust fans that would come on, I think like 4 to 6 of them all at once, drawing a huge amount of current for startup.· His controller turned them on one at a time reducing the overall sudden current draw on the system.

Later we used this conecept for a lighting system to reduce the cold-startup current draw.· Something to keep in mind for applicable projects.

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--==<{Chris}>==--

Post Edited (Chris Savage) : 2/5/2005 9:20:07 PM GMT