7805 regs under low load conditions

Hi Bruce, It sounds like the 7805 is a dud. I'd remove the offending regulator, pop in another and see what happens. (I haven't seen this before except with one of the low-power low-dropout 5V regs which required a 10uF cap on the output) ... Steve

"bruce varley"

** Please explain.

Glossing over a significant point like this is very suspicious.

...... Phil

there should be a cap on the input as well, 10uF should prevent instability

Equipment will be running outdoors, ambient temperature up to 40C. In fact, all the testing so far has been inside, so no temperature extremes have been encountered. At no time have any of the devices involved been above moderately warm to the touch.

Sorry, I should have elaborated.

I was going to suggest that too.

Bob

It seems that you have tried different devices and get the same results, so unless the manufacturer is dodgy and all devices from them are prone to failure, or there is some other explanation. Have you tried devices by different manufacturers? Also, without knowing the input/output configuration for the 7805 it is difficult to speculate. Try the old Vout - Vin protection diode trick and see if this solves the problem.

"bruce varley"

** Input side oscillations.

....... Phil

Ed and Bob might have both hinted that loop instability might be the culprit resulting in oscillation, which is usually compensated using

10uF at input and output.

National Semi have this paper on linear regs

Sounds like the ground pin may have become disconnected. Replace the regulator. Since it's happened with 2 regulators so far, perhaps you've struck a bad batch, which would be extremely unusual. I'd get some regs from another manufacturer.

Since you're using very low current, have you considered using a switching regulator instead of the 7805? Or are you using the '05 as a voltage reference for an analog to digital converter?

BTW, how is the ground terminal of the regulator connected to the rest of the circuit? Is it located on a seperate board and makes connection through terminal pins of a connector, or a similar arrangement, or is the regulator soldered directly to the board where the micro is located?

Sorry Bruce, I didn't read your post properly and didn't realise that you'd already tried swapping the reg. Steve

There aren't any there in this case. Steady as a rock.

Do you have a bypass cap (like around 0.33uF ceramic as suggested by the manufacturers) between ground and the input pin, nice & close to the regulator? If not, it'd be worth putting one there then seeing if you still have this problem.

Bob

7805s are fine with small loads & are /very/ hard to kill. It sounds to me like your regulator is oscillating or losing its ground connection.

To fix oscillation: Solder 100nF ceramic monobloc capacitors (the small yellow rectangular kind used for digital bypass caps) directly onto the legs of the 7805. One cap from pin 1 to pin 2 (input to ground) & a second from pin 3 to pin 2 (output to ground).

If the 7805 loses its ground connection, its output will 'float' up to almost the *full value of the unregulated input*, & will act almost as if pin 1 is shorted to pin 3. Make sure that your 7805 has its tag or middle pin *solidly* connected to both input & output grounds.

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