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I have a simple power supply circuit that needs to drive a 5V load and another 12V load, and it's behaving very strangely.
I've used a LM7805 voltage regulator with the input pin bridged to ground with a 0.33 uF capacitor and the output pin bridged to ground with a 0.10 uF capacitor. The ground pin of course goes to ground. A 5V 0.4W (80mA) micro-fan is connected between the output pin and ground. 12V from a battery goes to the input pin. This is all as shown in typical LM78xx datasheets (e.g., Figure 1 on page 7 of https://www.sparkfun.com/datasheets/Components/LM7805.pdf, or Fig 6 on page 18 of http://www.fairchildsemi.com/ds/LM/LM7805.pdf). I also put a switch between the 12V source and the circuit.
[edit: added circuit diagram below as suggested]
When configured as described above, it works perfectly. However…
(1) When I add another line, this one running from the 12V source to a 12V 160mA micro-pump, and from the pump to ground, the 5V fan no longer works. The LM7805 also gets much, much hotter, very quickly. The regulator output is ~ +0.5V relative to ground in this condition.
(2) When I switch off the latter circuit, the 5V fan very briefly spins. I initially guessed this was because of some charge stored in the capacitor attached to the output pin. However, it still happens if I remove either or both capacitors.
(3) If I start the circuit with the 12V load disconnected, wait a second for the fan to get up to speed, then connect the 12V load, everything is fine.
Same results with whether the DC source is a battery or a plug-in DC power supply.
Can anybody help?
Best Answer
is there a heat sink on the regulator? use a vice grip if not. double check your grounds. replace the regulator. check the power source. use am amp probe and a scope. inspect the pump for odd components or something you might not expect.