Electronics Q; 7805 running hot with no load?


F

fast_ian

Member
Hi,

Long story short:

~16Vdc comes in [Either way]
- Part of this is fed to a bridge rectifier's AC input such that it outputs about the same V (but now always one way).
- This goes to a 7805 (there's a resistor in the +ve leg that's turned the shrink tubing brown! But I can't remember R)
- The 7805 has the "standard" heatsink *and* ~10sq" of aluminum sheet - This is "warm/hot". [Heat conductive goop was used during assembly btw.)
- There is *no* load on the 5v output ckt!

Anyone know why this thing is running so warm without a load?

Cheers,
Ian
 
78xx regulators can tollerate a lot of misuse in circuits (bad design application) so when one plays up it can cause a lot of head scratching. While you can often get away with a lot, there are some guidelines to follow (and if you've ever had production runs of thousands you soon learn not to take short cuts).

78xx reguators are not guaranteed to be stable with no load and if they oscillate then they get hot. You probably won't notice if measuring with a multimeter, but can sometimes be seen as a lower than expected voltage (e.g. 4.5V). Datasheets for the devices don't give characteristics with load currents less than 5mA (varies with manufacturer). Connecting a 1k resistor across the Output and Common pins would meet the requirements if the actual load is removed. The commonly used power-on LED (and it's resistor) fulfils this purpose too.

Also there are minimum input and output decoupling capacitor requirements. Your description of the circuit mentions an input dropping resistor. In that case a 100nF capacitor across the input pins is required. The output capacitance requirements are about the same. Both should be wired within an inch-or-so of the device.

I hope that helps........
 
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Thankyou Rodders! [And welcome to the forums btw - Good to have you around!]

Wright2626, not so much! :) J/K - I know it's "eating" 11v, and that takes some power, but I was very surprised that it got as hot as it does without anything connected to the output - If it was driving something, OK, but with no load I was "perplexed".

Rodders - I guess pretty much any ol' 100nF caps across the input and output legs would do the trick? The 5V, along with appropriate current limiting resistors was destined for two pairs of IR LED's & detectors, but I think I'll add another regular LED as suggested for "power on" as well - I was never sure if the IR's were "on" anyway.

Thanks again - I'll get 'er modified "soon" and report if it cools down a little....

Cheers,
Ian
 
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