5 volt regulator not getting desired output

Question

I have been trying to assemble a simple 5V regulator circuit using the L7805CV and have had no luck at all getting the output to 5V. I have tried many circuits and multiple L7805CV regulators, thinking that maybe it was the regulator itself that wasn't doing its job.

So far the closest I've come to 5V is 6V, which won't work for what I'm trying to power.

First, I tried the simple circuit with a 10uF cap from input to ground pin and 1uF cap from ground pin to output, and was getting 3.8V output with an input of 9V. I was getting 11V out with a 12V input.

I tried another one that was similar but with a 1K resistor connected to an LED on the output, which gave me 6V. I thought maybe the capacitors were the cause so I tried going with 100uF from input to ground and a 10uF from ground to output and another 100nF on the output for filtering and still no luck. I basically played with cap values all the way up to 2200uF and didn't get anything close to what I needed. I either got really low (about 3V) or really high (close to 12V) on the output and also tried different input values of 6V, 9V, 12V, and even 18V and nothing worked.

I used a heat-sink on the l7805CV and am stumped how this is happening. I eventually went back to one of the first designs that had the output of 6V and tried using a diode on the output for its voltage drop and was getting almost the exact reading on the output (actually a few mV higher) and am stumped at this point. After a while I got bored and tried out a few different transistors I had around to see how they would affect the circuit and was surprised to find almost exact results.

What am I doing wrong? How can I successfully achieve 5V on my output?

Answer 1

The usual reason for an unexpectedly high output voltage on a properly wired (as in pinout correct) 7805 regulator is a flakey ground connection. An open on the GND pin will cause the output to rise to a volt of two of the input. 100 ohms resistance will cause the output to rise by about half a volt.

Solderless breadboards are prime offenders, and this can expose the circuitry connected to the regulator to damaging voltage, so it's best to do that part with solder.

Answer 2

Here is a L7805CV datasheet.

Note that on page 7, for the 5V version, Vout is specified with loads of 5 mA to 1A and there is a footnote that says "a Minimum load current for regulation is 5 mA".

To meet this minimum current requirement a load of no more than
R = V/I = 5V/5mA = 1 K is required.

That said, "usually" a 7805 will "more or less just work" if connected correctly. Caps as shown help but usually are not essential to make it work. Even Vout is liable to be "about right" without a load. (These older style regulators power themselves between Vin and ground so load current is less important.)(The LM317 family power themselves from Vin to Vout so a minimum Iout is needed for correct operation).

SO

1. Try it with a 5mA or higher load. (1 K or lower).

2. Look at the data sheet(s) and ENSURE that you have the pinout correct. ||

Answer 3

From looking at the internal schematic and example test circuits, I'm pretty sure it works with input at 6V min with no load and 7V min at 1A . 7.5 V is ideal. , but 9V drops too much power across regulator.

It must use a good heatsink to keep cool. It will shutdown when too hot. If it is too hot to hold, its too hot. Expect the heatsink to be at least 1 sqin per watt. For a rectified input, the input cap must be large enough to ensure the Vin min does not sag below minimum so you must measure AC pp and DC voltage. My guess 7.5Vdc avg in with 1Vpp ripple @ 1A needs C=80ms/5 ohms = 1.6mF not 100uF.

If you raise Vin more than Vmin, it must dissipate the drop voltage x load current, so 9V is not good.

Recheck your connections and shorting the input will damage the part, so reverse power diodes are suggested between in and out.

There is no need to put series diodes on output, but if you only have 9V, use diodes on input to run cooler to reduce the drop power across series regulator to just above Vmin at 1A.