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I'm building a linear voltage regulator using the LM 741 (part of school project) but everytime I connect a load than draws more than 1A the output voltage drops down. I'm not sure why this happens and what can I do to solve this ?

This is the schematic I'm using

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    \$\begingroup\$ My crystal ball is broken, post a schematic. \$\endgroup\$ – Matt Young Nov 17 '16 at 2:23
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    \$\begingroup\$ What happens to the input voltage to the regulator when you try to draw more than 1 Amp? \$\endgroup\$ – Peter Bennett Nov 17 '16 at 2:25
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    \$\begingroup\$ @PeterBennett the input voltage drops down as well \$\endgroup\$ – Simeon R Nov 17 '16 at 4:28
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    \$\begingroup\$ @MichaelKaras I'm restricted to the 741 by project requirements \$\endgroup\$ – Simeon R Nov 17 '16 at 4:28
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    \$\begingroup\$ If the input voltage to the regulator drops when you try to draw more than 1 Amp, this is an indication that your raw power supply cannot supply the required current - your regulator may be working fine, but the raw supply doesn't give it enough voltage to work with. \$\endgroup\$ – Peter Bennett Nov 17 '16 at 4:35
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  1. The TIP31C gain is not very high. In the ST datasheet, some binned parts are guaranteed not to have gain (beta) greater than 50. Other bins are guaranteed not to have gain greater than 24.

    This means that your 741's output definitely has to produce at least 20 mA, and maybe as much as 42 mA to drive the TIP31 to deliver 1A.

    This is somewhat mitigated by the fact that the TIP31's beta increases as it heats up, but still you're likely trying to get 10's of mA out of the 741.

  2. The 741's short circuit output current is only 25 mA (based on the TI datasheet).

    And the output voltage range spec implies an output equivalent resistance of about 150 ohms. (Because the typical output swing drops by 1 V when the ouput current is 6.5 mA)

    So when sourcing 20 mA, the 741 output voltage is likely going to drop by about 3 V, giving a maximum output of about 11 V at this load.

    Your maximum regulator output is then one Vbe drop lower, or about 10.3 V, lower than the 11 V you were trying to get.

So it looks like you were just trying to get a bit too much out of these parts.

(Also consider Peter Bennett's comment and double check that your 15 V supply is not sagging with 1 A load)

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  • \$\begingroup\$ Thank you ! But what do you mean by 'binned parts' ? Also, will a darlington pair help me to drive loads that require more than 1A \$\endgroup\$ – Simeon R Nov 17 '16 at 5:12
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    \$\begingroup\$ Yes, a darlington should work here. The drop out voltage will be more, but I think still okay to get you 11 V output. You could also use a lower-power but higher-gain BJT for the base part of the pair and reduce the loading on your 741 even more. \$\endgroup\$ – The Photon Nov 17 '16 at 5:17
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    \$\begingroup\$ On the binning, ST says they sell TIP31C's in "Group R", "Group O" and "Group Y" with different gain specs, and with no way for you to order a specific group. \$\endgroup\$ – The Photon Nov 17 '16 at 5:19
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    \$\begingroup\$ @PeterBennett, well sure, but he said that's an imposed requirement. You'd also still be asking for a lot of output current close to the rail. "Rail to Rail" op-amps tend to get a lot less rail-to-rail when they're sourcing 20 mA. \$\endgroup\$ – The Photon Nov 17 '16 at 5:20
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    \$\begingroup\$ In addition you are generating your reference voltage from your supply output....you need to move R2 so it's connected to the 15V rail. \$\endgroup\$ – Jack Creasey Nov 17 '16 at 5:24
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Agree with what The Photon has said, and Jack Creasey's comment on R2 should be connected to 15V.

But since you are stuck with the 741 and not enough base drive voltage, replace the TIP31 with something like below.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Thank you rioraxe, but why the PNP - NPN configuration ? \$\endgroup\$ – Simeon R Nov 17 '16 at 7:23
  • \$\begingroup\$ The Photon suggested a darlington to get more gain, but a darlington has two Vbe drops in a row, that is why he said the dropout voltage would be more. This combination moves the Vbe of the PNP "upstream" and avoids adding it to the 741 base drive path. \$\endgroup\$ – rioraxe Nov 17 '16 at 7:31

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