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LM7805 voltage regulator output 4 volts? Answered

I have a LM7805 voltage regulator hooked up to a saltwater battery that outputs 16 volts, and the output is either slightly above or below 4. When I wire it to a 9v battery, it outputs a steady 4.98, but still will not power a USB port


To measure the internal resistance:
Find a potentiometer. . Connect the pot across the supply, connect the wiper of the pot to one end of the track.

Measure the voltage across the (variable resistor), and adjust the pot so that the voltage across the pot reads half the open circuit voltage. The internal resistance = the resistance of the pot.
It'll be pretty high !


5 years ago

The usual cause is insufficient voltage. It is possible that you are measuring the unloaded voltage of your first battery and that it drops to <6 or so when loaded.

Well i tried to fry an LED with 9v of them hooked in series, it lit but not very bright. What would that mean?

I'm unfamiliar with this salt battery.

A 9v powering a single LED, no other components? If the LED was blue/white the forward voltage drop and higher internal resistance of a 9v (theorizing here...) might have protected it. Or if it were NIR it might have had the current consuming capability to survive it.

Not bright indicates insufficient current, or a weak LED (they are rated at a specific brightness). Insufficient current comes from resistance.

I will check the resistance.

Either my multimeter is messed up, or I dont know how to read it. I know that my skin has a resistance, so I get a reading when i touch it to myself, but when I touch it to the wires and not touch the clips at all it reads 1 . and i have no idea what that means

Erg, I'm sorry I went to bed right after this (1am local). Thats not how to use the meter, I hope it is ok.

Come to think of it, I'm not sure how to check the internal resistance of a battery with a standard multimeter.

That means the ohm_meter part is out of range consider it (the meter) is in electric pain.

You must never hook up the ohm_meter to a powered active circuit.

What you have is a micro-power source, and you can't waste any of the energy with a simple 7805 device. If you're serious, you need a switching power regulator, which will deliver >80% of the energy in your source, as opposed to <20%. That way, you'd only need 100 cells......

Problem is a salt water battery provides extremely low current. You need a lot of them to get enough current to light an LED. Even though your getting up to 16 volts from the home made battery the current output isn't enough to light an LED. You may only be getting 0.02 watts from your setup when the LED needs around 0.04 watts.


5 years ago

Your saltwater battery is not able to supply your electric current needs.
Are you sure of your wiring ?


Yes I have done made two and I am sure this one is wired correctly. I tried to double the current of each cell by wiring 6 in parallel instead of 3 and the current is still super low

What you describe double the load makes my point.

Try no load output,
The 9v is able to supply more current.

I figured it out and to get enough current to match a 9v battery, I would need 300 cups in parallel