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Author Topic: 7805 Voltage regulator produces 3.7v where feed is 8.nvolts?? help  (Read 2957 times)
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Hello All

I wonder if I am missing something.

I am trying to produce3 5volts from a 8.something voltage source.

I have tried 3 7805 regulators (2 different types) and they all produce 3.7 volts??

These are 1A Voltage regulators if that makes a difference. Also from 2 different suppliers

Am I missing something?

(pins in order as they appear on the regulator)

Pin 1 in input @ 8volts
pin 3 is common ground
Pin 2 is output @ 3.7volts and I expect 5 volts

I had capacitors etc as recommended by the datasheet and removed them in case they were dodgy.

I have no load on either the supply side nor the output side.

Can anyone advise?

For the questions asked - I am sure it is a 7805 - as per my earlier links. My multimeter is accurate.

With my dodgy power source I was measuring 8.1 volts on the input side and 3.4volts on the output when I had the red LED and a 150Ohm resistor attached.
« Last Edit: May 18, 2011, 02:18:49 pm by AgeingHippy » Logged

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Best Guess:

You have a wall-wart supply that says 8 Volts.   I bet it puts out is closer to 6 volts... which is not enough input voltage for the 7805 to work correctly.
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It should work....betcha there's something simple afoot. Can you give us the exact part numbers you used?

And I don't know if you made a typo but you have pins 2 and 3 swapped. Pin 2 should be ground, pin 3 should be the output.

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My supply voltage is what I have directly measured it as (a repurposed ghetto blaster power supply) using my multimeter. It initially read 11.something but I attached an LED & resistor in parallel to indicate power on ... don't know if that caused it to drop the voltage to 8.something but I am sure it should not. ??

I tried the MC7805

and 2 of these 7805CV

Just a thought - the difference seems to be about 5v between input and output. Could it be that I have mixed up the input and output pins?
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From the datasheet:

Pin 1. Input
     2. Ground
     3. Output

Your connection:

Quote
Pin 1 in input @ 8volts
pin 3 is common ground
Pin 2 is output @ 3.7volts and I expect 5 volts

Notice anything wrong?

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As others are pointing out, there is a potential issue with your pinout.  The two datasheets for the parts you link to are slightly different.

The "7805CV" lists the pin out like you do in your post, which is leading people to suggest something there is wrong.  Here's the key:  how they label as pin 1, pin 2, and pin 3 is different how the industry typically labels a 7805 (or TO-220).   Their pin function is in the correct order, but the numbers are nonsense.  (They made the middle pin 3, instead of 2 even though the function is still GROUND.)

Use the pinout and pictures of the "MC7805" which shows the pin out people expect to see.

As for the 3.7V are you placing any kind of load on the regulator or measuring it open?  You might put a 100K resistor on the output to see if it stabilizes.
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Hi James

I have indeed numbered my pins as defined for the 7805CV.

But regardless of pin numbering, the middle pin is GND, the first pin (reading from left to right) is INPUT and last pin is output.

I am measuring the regulator open. I will try pytting a load on it (100k resistor and maybe an LED) and see if it stabilises as you suggest.

This will have to wait until tonight as I am at work right now. Will give feedback.

As an aside, what happens if you swap your input and output pins?

Cheers
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You have decoupling capacitors too?  It may oscillate without them.
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The datasheet shows that output voltage is only specified when the output current is >5mA. So with no load, I don't think you can determine if it is regulating or not.

Quote
As an aside, what happens if you swap your input and output pins?
Well if you have spare parts, you can always try it and see.  :-D
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As an aside, what happens if you swap your input and output pins?

It has been my experience that miswiring the 1a 7805 chips can make them very hot, start to smell, and burn the finger tips. Amazingly when rewired correctly it still worked normally.
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Probably because they have high-temperature shutdown - or you have good luck.  Silicon devices survive till about 175C, fingertips suffer from about 50C.  Heat damage can be degradation rather than outright failure - basically the doped areas of silicon start to diffuse their dopant atoms around and drift out-of-spec - it may still work but it might not work well...

Basical rule with electronic circuits is check all the wiring _twice_ - once as you go and once before powering up - saves a lot of money!
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But regardless of pin numbering, the middle pin is GND, the first pin (reading from left to right) is INPUT and last pin is output.
This is ambiguous unless you state where the tab is and whether you are looking at the top or the bottom.  That's why most data sheets use a diagram to identify the pins.

Don
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Still not working smiley-sad

I placed a 100kOhm resistor across the (expected) 5volt side and measured 3.4volts.

I placed a red resistor (apparently pulls 20mA) and a 150 Ohm resistor across the (expected) 5volt side and measured 3.4volts.

Flipping the regulator so input was attached to the output pin and output to the input pin resulted in a no load voltage of roughly 7 volts on each side.

With the 100kOhm resistor the output dropped to about 6.4 volts.

Surely it is not that difficult to get 5volts from a 5v regulator??

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1.  Can you post a picture of your setup?

2.  When you have the resistor/load applied to the output and wired properly, what does your input voltage measure?  (Now we are testing the loaded voltage of your input supply.)

3. "I placed a red resistor (apparently pulls 20mA) and a 150 Ohm resistor across the (expected) 5volt side "
I think you mean red LED.  smiley  The LED will draw whatever current it can, once the forward bias voltage is applied.  If it is a 2V forward bias, then there is 3V on the resistor which means 20mA flows through the resistor which means 20mA flows through the resistor.  Well that all holds true when you have 5V... smiley-wink  Make sure you connected the LED in the correct way.

For the record, the 150ohm resistor alone as a load would be a good test load.

3.  Are you certain this a 7805?
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Since nothing else seems to be working, how about a "duh" check - have you verified that your multimeter is accurate?  Maybe check the voltage on a fresh 9V battery just to make sure it's not a meter problem?
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