7805 5V regulator trouble!

Dominick22 · 11th May 2006, 12:38 PM

So I bought the 7805 5V regulator for the digital voltage meter and wired it up.
After rectification, the voltage is about 27V
The 7805 says it operates up to 35VDC
I measured the voltage from the leads ouput and ground and it read 0!!

What am I doing wrong. I have the +output of the rectifier going into the 7805 input. Then the output of the 7805 goes to my digital volt meter. The volt meter and 7805 are grounded back to the rectifier.

I have to be doing something wrong, but I can't figure it!

Thanks guys,
Dominick

richie00boy · 11th May 2006, 01:03 PM

You need to load the 78/79 series regs a bit for it to work.

Dominick22 · 11th May 2006, 01:24 PM

Thanks for the reply, but I am still a very new amateur.

Can you please explain how to apply a "load"???

Thanks,

Dominick

richie00boy · 11th May 2006, 01:54 PM

Put a resistor, like maybe 1k, on the output.

Bit puzzled what you are doing with the volt meter though. What are you measuring with the meter? With a 5V supply it probably won't measure more about 4.5V so seems pretty useless to me?

Dominick22 · 11th May 2006, 02:21 PM

Richie00boy,

I tried the load idea, but I used an available 1.5k resistor and it did not change anything. The meters are brand new so I know it is not them.

Should I try a lower resistor?

Thanks,
Dominick

janneman · 11th May 2006, 02:26 PM

I know that some 7805's have different pinouts than others, depending on manufacturer. Has caught me once too.

Which pinout do you use? And can you post a drawing of your circuit?

Also, you need some capacitance at the output, see the data sheet, (you already have a cap on the input, right?) to avoid oscillation. Depending on the dig voltmeter, it may confuse eventual oscillation into zero volts.

Jan Didden

cpemma · 11th May 2006, 08:30 PM

Quote:

Originally posted by Dominick22
I tried the load idea, but I used an available 1.5k resistor and it did not change anything...Should I try a lower resistor?

The 7805 spec guarantees the voltage tolerance with loads 5mA to 1A; 5V/1k = 5mA, 1.5k gives 3.3mA. You should get a reading, but maybe not in the guaranteed 5V+/-0.25V range.

The resistor should be between the output pin and ground.

Bazukaz · 11th May 2006, 08:36 PM

I think you should have wired something wrong.recheck all connections , pinout , etc.

Regards,
Lukas.

Dominick22 · 11th May 2006, 08:57 PM

I sketched what I guess you could call a schematic of what I have set up that is not working.

I appreciate any feedback or suggestions!

Dominick

Bazukaz · 11th May 2006, 09:07 PM

Where is the filter cap , directly after rectifier between + and ground ?

11th May 2006, 12:38 PM	#1
Dominick22 diyAudio Member Join Date: Aug 2005	7805 5V regulator trouble! So I bought the 7805 5V regulator for the digital voltage meter and wired it up. After rectification, the voltage is about 27V The 7805 says it operates up to 35VDC I measured the voltage from the leads ouput and ground and it read 0!! What am I doing wrong. I have the +output of the rectifier going into the 7805 input. Then the output of the 7805 goes to my digital volt meter. The volt meter and 7805 are grounded back to the rectifier. I have to be doing something wrong, but I can't figure it! Thanks guys, Dominick __________________ A real amateur enthusiast!

11th May 2006, 01:03 PM	#2
richie00boy Did it Himself diyAudio Member Join Date: Nov 2003 Location: Gloucestershire, England, UK	You need to load the 78/79 series regs a bit for it to work. __________________ www.readresearch.co.uk my website for UK diy audio people - designs, PCBs, kits and more

11th May 2006, 01:24 PM	#3
Dominick22 diyAudio Member Join Date: Aug 2005	Thanks for the reply, but I am still a very new amateur. Can you please explain how to apply a "load"??? Thanks, Dominick __________________ A real amateur enthusiast!

11th May 2006, 01:54 PM	#4
richie00boy Did it Himself diyAudio Member Join Date: Nov 2003 Location: Gloucestershire, England, UK	Put a resistor, like maybe 1k, on the output. Bit puzzled what you are doing with the volt meter though. What are you measuring with the meter? With a 5V supply it probably won't measure more about 4.5V so seems pretty useless to me? __________________ www.readresearch.co.uk my website for UK diy audio people - designs, PCBs, kits and more

11th May 2006, 02:21 PM	#5
Dominick22 diyAudio Member Join Date: Aug 2005	Richie00boy, I tried the load idea, but I used an available 1.5k resistor and it did not change anything. The meters are brand new so I know it is not them. Should I try a lower resistor? Thanks, Dominick __________________ A real amateur enthusiast!


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11th May 2006, 02:26 PM	#6
janneman diyAudio Member Join Date: May 2002 Location: Where Germany, The Netherlands and Belgium meet Blog Entries: 6	I know that some 7805's have different pinouts than others, depending on manufacturer. Has caught me once too. Which pinout do you use? And can you post a drawing of your circuit? Also, you need some capacitance at the output, see the data sheet, (you already have a cap on the input, right?) to avoid oscillation. Depending on the dig voltmeter, it may confuse eventual oscillation into zero volts. Jan Didden __________________ /Another new issue: Linear Audio Volume 3!

11th May 2006, 08:36 PM	#8
Bazukaz diyAudio Member Join Date: Nov 2005 Location: Vilnius	I think you should have wired something wrong.recheck all connections , pinout , etc. Regards, Lukas.

11th May 2006, 08:57 PM	#9
Dominick22 diyAudio Member Join Date: Aug 2005	I sketched what I guess you could call a schematic of what I have set up that is not working. I appreciate any feedback or suggestions! Dominick __________________ A real amateur enthusiast!

11th May 2006, 09:07 PM	#10
Bazukaz diyAudio Member Join Date: Nov 2005 Location: Vilnius	Where is the filter cap , directly after rectifier between + and ground ?

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