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I am trying to modify a circuit Circuit schematic that requires three 9v batteries. Two to supply +9v/-9v for an lm358N opamp, the other +9v is to drive a DMM display.

I have dozens of wall warts of just about every voltage and amperage available. Can I simply use two 7809's for the two +9v and one 7909 for the -9v all off of one input ps?

I haven't dealt with electronic circuits for well over 30 years. I'm not afraid to search if someone can gently lead me in the right direction.

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  • \$\begingroup\$ The LM386 is not an op amp, it's a rather crummy audio amp. \$\endgroup\$
    – Leon Heller
    Commented Dec 15, 2015 at 21:04
  • \$\begingroup\$ How much current do you need for the DMM display? \$\endgroup\$
    – EM Fields
    Commented Dec 15, 2015 at 21:34
  • \$\begingroup\$ You can use4 multiple 7809s on a single 12 volt wall-wart, but the negative terminals of all the 7809s will be connected together, so you can't make a -9 volts supply from one if the others are for +9. Some DMMs can't have either of their supply terminals connected to the voltage being measured, so that may also limit your options. \$\endgroup\$
    – Peter Bennett
    Commented Dec 15, 2015 at 22:28
  • \$\begingroup\$ perhaps a link to the schematic of what I'm trying to build will help. LPM \$\endgroup\$
    – GregMa
    Commented Dec 16, 2015 at 14:45
  • \$\begingroup\$ The DMM doesn't list current, all that I can find is: Supply Voltage: 3.5-30V. Red Wiring: Power Supply + (3.5-30V). Black Wiring: Power Supply -, Measure – Yellow Wiring: Measure + (0-33.000V). \$\endgroup\$
    – GregMa
    Commented Dec 16, 2015 at 14:48

2 Answers 2

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Yes, you can use two DC-output wall warts together to make a + and - supply.

Connect the - output of wall wart A to the + output of wall wart B. That point then becomes your ground (0V reference for everything else). The + output of wall wart A will then be the + supply, and the - output of wall wart B the - supply.

If these are 9 V regulated wall warts, then you don't need the 7809 and 7909 regulators. That would be simpler and the recommended arrangement.

If you do use a regulator, not that the negative one needs to be a negative regulator, not a 7809 as you say. A 7909 would work, and is the negative equivalent of the positive 7809. The unregulated voltages from the wall warts would also need to be a few volts past (higher for positive, lower for negative) what the regulators need. The 78xx and 79xx series require a few volts headroom. See the datasheet.

There is no need to use a separate supply for the DMM display. It should be able to run from the same +9 V supply as the amplifier.

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  • \$\begingroup\$ Just make sure before you do this that the GND on the DC jack doesn't connect to the earth pin on the socket as otherwise this won't work at all (you'll short out one supply). And yes, I have seen some supplies where this is the case. \$\endgroup\$
    – Tom Carpenter
    Commented Dec 16, 2015 at 0:55
  • \$\begingroup\$ How do you sleep nights? \$\endgroup\$
    – EM Fields
    Commented Dec 16, 2015 at 9:04
  • \$\begingroup\$ EMFi: Huh? I really can't figure out what you think is irresponsible in what I said. I just looked at your answer, and you said mostly the same thing a different way, except for not needing regulators at all if the wall warts are 9 V regulated to begin with. If you have a problem with that, say so. I still think that's a valid way to proceed, in fact the simplest thing for the OP to do if he can get 9 V regulated wall warts. \$\endgroup\$
    – Olin Lathrop
    Commented Dec 16, 2015 at 11:49
  • \$\begingroup\$ It would be much easier to use two warts, but I am trying to only have one external connection to AC. Perhaps this is undoable? \$\endgroup\$
    – GregMa
    Commented Dec 16, 2015 at 14:47
  • \$\begingroup\$ @Greg: You can buy a dual output +-9V supply, but two 9 V wall warts and a outlet strip will be cheaper. \$\endgroup\$
    – Olin Lathrop
    Commented Dec 16, 2015 at 15:53
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If the current into the DMM display isn't too great, and it doesn't have to be isolated, you'll be able to get away with using two wall warts and two regulators; a 7809 for the +9 volts and a 7909 for the -9 volts.

Your wall warts should put out at least 12 volts in order to provide headroom for the regulators, and it would be a good idea to use reservoir capacitors on the inputs of the regulators.

Like this:

enter image description here

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