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Problem about using 7805 regulator with 9V battery vs power supply

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You need to place decoupling capacitors across pins 1-2 and 2-3 and drive an actual load with it before it will regulate the voltage.
100-220uF and 100nF capacitors on both sides is usually fine for decoupling.
Read the datasheet for the 78xx series regulators and you will understand what's going on...

rgds
 
i have tried to adding cap. on both sides, but the result is the same. is the function of the cap.s is to lower noise?
i cant get 5V with batteries, but power supply does, it leads a question that what are difference between power supply and batteries?
 
What kind of 9V battery? What's the current draw of the circuit?

A 9V NiMH (rechargeable) is only 7.2V it's not enough to reliably run a 7805, you could try a low dropout regulator. Also most little 9V batteries are pretty weak and won't last long with a linear regulator.
 
Why isn't the schematic a drawing file (PNG file type) and why isn't it posted here? I can't see it over at Google when it is a DOC file type.
 
What kind of 9V battery? What's the current draw of the circuit?

A 9V NiMH (rechargeable) is only 7.2V it's not enough to reliably run a 7805, you could try a low dropout regulator. Also most little 9V batteries are pretty weak and won't last long with a linear regulator.

alkaline batteries.

do you mean the voltage of 9V batteries is "weak". i even series them to 20V , but the output is 10V:mad:

i am going to start a bluetooth module(5V,8-35mA). batteries not work but power supply in lab. does, so confusing.
 
You 9V battery must have a high ESR, try changing brands or a fresh battery. Either way it won't last very long on a 7805 with a Bluetooth device attached.
 
i have tried to adding cap. on both sides, but the result is the same. is the function of the cap.s is to lower noise?

The function of the caps is to make it work, it's VERY likely to be unstable without them, and you should never leave them out. The fact that it's too high a voltage means it's unstable.

With the correct caps 7805's work perfectly off 9V batteries, but the batteries don't last that long because the 7805 isn't a low-dropout device.
 
The function of the caps is to make it work, it's VERY likely to be unstable without them, and you should never leave them out. The fact that it's too high a voltage means it's unstable.

With the correct caps 7805's work perfectly off 9V batteries, but the batteries don't last that long because the 7805 isn't a low-dropout device.
thanks, i have added caps into the circuit but the output voltage is the same.
 
The datasheet shows the very important capacitors. They must be connected very close to the regulator with very short leads. they prevent the IC from oscillating.

I use the Go advanced button to upload my schematic here instead of at some other site that might not keep it or might not work.
 

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