I'm a touch surprised that no one has suggested that the 7805 might be
oscillating in the MHz region...
I would 'scope the +5V rail making sure that all input filtering is off...
78xx regulators were notorious for singing if they coukd - hence the essential
100n input output caps mounted
I too was going to suggest the switchmode replacement approach.
Your heat will go away from the regulator, completely.
You can get one shipped to you for less than $9.
Click here:
Traco regulator
http://www.aliexpress.com/item/Traco-module-TSR-1-2465-TSR1-2465/1942822301.html
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You
Have you thought about replacing the 7805 with a switch mode regulator?
More expensive than cheap-as-chips 7805 but wyyy more efficient. That
means they will run cold and you shouldn't need to worry about the ambient
temperature in your case. Two to look at, both of which are pin-compatible
Something I want to note about how I used a heatsink; I epoxied it to the
tops of the cases of M1 U1; traditionally you'd bolt it to the back of
the component but at the time I couldn't figure out a better way, and I had
heatsink epoxy, so I just plastered it to the tops of the two components.
Kiran,
If you get your scope running, check the output of U1 (7805 regulator) and look
for a very flat DC. If you see any type of sawtooth or superimposed
oscillation, replace C1. You can use a any higher value than 10uF as long as
it physically fits and has a high enough voltage rating.
Just looked at the schematic. There are no suppression capacitors on the 7805.
That is, IMHO, a major flaw.
I would, as soon as possible, add those two 100n caps on the legs of the 7805.
Nick
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Kiran,
If you get your scope running, check the output of U1 (7805 regulator) and look
for a very flat DC. If you see any type of sawtooth or superimposed
oscillation, replace C1. You can use a any higher value than 10uF as long as
it physically fits and has a high enough voltage rating.
