Trying to check some 7805 voltage regulators. I do not have a DC power supply and do not want to test with the circuit hooked up to mains. That leaves me and my multi-meter.


This seems reasonable to me, and both of my 7805s have failed step 4. Before I go out and buy two new ones I was hoping that I could get some confirmation of this method.

Thank you

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    \$\begingroup\$ Have you got a 9V battery? If so, connect the battery + to VIN, the - to GND, and then measure the voltage between VOUT and GND. \$\endgroup\$ – Tom Carpenter Jul 19 '16 at 16:20
  • \$\begingroup\$ @TomCarpenter oh wow yea I didn't think of that. Thanks! \$\endgroup\$ – Coberra Jul 19 '16 at 16:36
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    \$\begingroup\$ Testing a 78xx 'hooked up to mains' would be rather interesting. \$\endgroup\$ – Vladimir Cravero Jul 19 '16 at 17:45

No that is NOT a proper or useful way to test 3-terminal regulators. The technique is valid for SOME (but not all) kinds of transistors. But it is completely unsuitable for an integrated circuit like a 3-terminal regulator. It will yield unpredictable (and most likely meaningless and misleading) results.

If you don't have a DC power supply, why do you even need to test 3-terminal regulators? For that matter why do you even need 3-terminal regulators at all? Absent the context it is not clear what you are trying to do here?

You can use batteries as a DC source. Remember that you typically need 3V HIGHER input voltage than the regulator puts out. It would also be prudent to put a LOAD on the output of the regulator. At least something around 500 to 1000 ohms or so.

  • \$\begingroup\$ The TI (National) data sheet says the minimum input voltage is 7.5V for LM7805 To be sure there are more modern products with lower dropout voltages, but the 79xx and 79xx are not "low dropout" products. \$\endgroup\$ – Richard Crowley Jul 19 '16 at 16:43
  • \$\begingroup\$ Hey Richard! You were helping me before on another forum. Still trying to fix my Turntable. This is where I am at if you are curious. Trying to determine if I have some blown voltage regulators and that is why nothing is coming on! electronics.stackexchange.com/questions/246569/… \$\endgroup\$ – Coberra Jul 19 '16 at 16:46
  • \$\begingroup\$ Right,I missed that part.So that translates into 0.5V unwanted dissipation? \$\endgroup\$ – Daniel Tork Jul 19 '16 at 16:50
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    \$\begingroup\$ @Daniel Tork No, the ENTIRE DIFFERENCE between the input voltage and the output voltage is "unwanted dissipation". But older, analog designs needed a larger voltage differential to work with. More modern switching (and even linear) regulators are more efficient. \$\endgroup\$ – Richard Crowley Jul 19 '16 at 16:54
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    \$\begingroup\$ The MAXIMUM voltage drop is 30V. The absolute maximum input voltage for the 78xx regulators is 35V. Which means that the 5V 7805 is the worst case if you have 35V going in and 5V coming out. \$\endgroup\$ – Richard Crowley Jul 19 '16 at 17:12

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