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schematic

simulate this circuit – Schematic created using CircuitLab

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?

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  • \$\begingroup\$ A circuit diagram explaining the circuit would be better than your description. You can make it in circuit lab press the mini schematic in the toolbar of the editor :). \$\endgroup\$
    – Dean
    Commented Jan 23, 2014 at 4:59
  • \$\begingroup\$ Sounds like a ground problem, do you have all the grounds of your different power supplies connected to eachother? Battery (-) to the middle pin of your 7805? I assume you have but I can't think of anything else... D. \$\endgroup\$
    – Xilinx
    Commented Jan 23, 2014 at 7:13
  • \$\begingroup\$ May be noise between circuits. MUCH larger input cap located right at 78-5 is recommended. Say 100 uF - 1000uF range. Pump MAY be loading the battery at startup. \$\endgroup\$
    – Russell McMahon ♦
    Commented Jan 23, 2014 at 7:17
  • \$\begingroup\$ Dean: thanks very much, I didn't know about the circuit lab thing! \$\endgroup\$
    – son voltage
    Commented Jan 24, 2014 at 2:00
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    \$\begingroup\$ (2) As an inductive load maybe your pump, when turned off, still have some charge and so it acts like a generator supplying the 7805 \$\endgroup\$
    – Pedro Quadros
    Commented Feb 24, 2015 at 19:50

2 Answers 2

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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.

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This is most probably a problem with high frequency oscillations. Place a 100uF (or bigger) electrolytic capacitor before the 330nF capacitor at the input of the 7805.

Also make sure that the distance between the small capacitors and the input and output of the 7805 is really small! Not more than 1cm is best!

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  • \$\begingroup\$ The input cap has nothing to do with oscillation. That is the output cap. A bigger input cap is still a good idea. \$\endgroup\$
    – Matt Young
    Commented Jan 24, 2014 at 13:53
  • \$\begingroup\$ @MattYoung since the oscillations start then a load on the primary side as added we should also look for the problem there. \$\endgroup\$
    – kruemi
    Commented Feb 4, 2014 at 14:15
  • \$\begingroup\$ @Matt - I've had 7805s oscillate when there was a long wire (2 feet) for the input power and inadequate decoupling at the 7805 input. I am sceptical it is the problem here though. \$\endgroup\$
    – Kevin White
    Commented Jun 25, 2015 at 18:34

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