Spehro Pefhany wrote:
>
> At 09:10 AM 5/7/01 -0500, you wrote:
>
> >Just that we still don't know the input voltage, which is a major factor.
> >He mentioned a dirty 18V, but I'd like to see at least a meter measurement,
> >if not scope.
>
> Ah, yes. If it was 18VAC, poorly regulated and full wave rectified, it might
> be more like 30V, leading to almost 2W dissipation. At 65'C/W and 25'C Ta,
> you'd need 1.9W to get to the 150'C Tj typical shutdown temperature.


30 volts is also beyond the maximum input voltage of some 5 volt
regulators.


>
> >With the power levels mentioned though, I'm beginning to suspect oscillation.


What if it is a TO-92 packaged 7805? Then the 75ma could cause a thermal
shutdown.


>
> I've never been able to get a 78M05 to oscillate, even without any load
> capacitor. I suppose with really, really bad layout it might be possible, but
> they are pretty sluggish devices.


He is using a wall wart with the filter capacitor at the end of a cable
away from the 7805. The manufacturers always say to put a bypass
capacitor at the input of the 7805 in this situation.


-- Rich


>
> Best regards,
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