7805 5V regulator trouble!

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Dominick22

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Joined 2005
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
 
jan.didden

jan.didden

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Joined 2002
linearaudio.net
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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
 
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cpemma

Member
Joined 2003
cpemma.co.uk
Dominick22 said:
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?
Click to expand...

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

Bazukaz

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Joined 2005
Remember , the output voltage at filter cap will be 1.4 times the RMS voltage of your transformer ! So , if you have , lets say , 10V AC transformer , you'll get about 14V after rectification(probably a bit less , a volt or so would drop on rectifier diodes).

By the way , 4.4 volt is a bit too big tolerance error , how large cap did you put before regulator ? measure the voltage before the regulator.
 
anatech

anatech

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Hi Dominick22,
Earlier specs for the 7805 is 25VDC max. The higher voltage units were 35V max.

You should use a high voltage rated 15V regulator first (or discrete transistor type), then on to your 5 V reg. Don't forget decoupling caps in between regulators and the output. 0.47uF would do, 10 uF max on the output.

-Chris
 
Bazukaz

Bazukaz

Member
Joined 2005
anatech said:
Hi Dominick22,
Earlier specs for the 7805 is 25VDC max. The higher voltage units were 35V max.

You should use a high voltage rated 15V regulator first (or discrete transistor type), then on to your 5 V reg. Don't forget decoupling caps in between regulators and the output. 0.47uF would do, 10 uF max on the output.

-Chris
Click to expand...

i'd sugest chaning transformer or buying a regulator that can stand a full voltage drop (LM317,LM317HV(60volt version).Dos your trafo have only one output ?
 
anatech

anatech

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Joined 2004
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Hi Bazukaz,
I agree. Get the input voltage down first. The heat will be your next big problem. Side step these issues by reducing the voltage to reasonable values. 5V regs normally are run around 9 VDC input. You may still need a heatsink depending on the current draw.
Earlier specs for the 7805 is 25VDC max. The higher voltage units were 35V max.
Click to expand...
What I meant was that 7812 and up were 35V max.

-Chris
 
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