We have this really strange problem with a regulator on a circuit we have.

Usually it outputs a good 3.3V , from an input source of 5V. When we consume more current from the 5v source (not the REG output) , we get a voltage drop from 3.3V to 2.8V on the regulator output itself.

This happens only when we pass the 1.5A from the source itself.

The 5v source is capable of 4A , and it happens when we consume about 1.6A from the source. The source stay at 5V when the REG drops.

Please check the REG schematics here (c14, and c15 are in place) :

enter image description here


  1. the regulator output is consuming less then 100mA.
  2. I found out that the transformer source is indeed 4A output, but it says 0.68A maximum input - is that changes things ? how is that possible ? we replaced a few of them, non worked.
  • \$\begingroup\$ Post the datasheet of the exact regulator in use! \$\endgroup\$
    – MAM
    Commented Jan 20, 2017 at 8:50
  • \$\begingroup\$ 1117 does not support that much current and is limiting the output for you to save itself from burning up. Feel it with your finger! \$\endgroup\$
    – winny
    Commented Jan 20, 2017 at 8:52
  • 1
    \$\begingroup\$ It is unclear to me how much current you're drawing from this regulator's output. \$\endgroup\$ Commented Jan 20, 2017 at 8:54
  • 1
    \$\begingroup\$ draw the complete schematic, including the ground connection, and where the 1.6A you are drawing is returning to ground. Look at your ground wiring carefully, try to estimate where it's good, and where it's a skinny or long wire, and show those details. More details about the power supply are required, so specify which one you are using (if commercial) or draw a schematic of it (if home-built). It's a good little puzzle, but you have more info than we do, and we don't have a enough to guess what the answer is. \$\endgroup\$
    – Neil_UK
    Commented Jan 20, 2017 at 9:23
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    \$\begingroup\$ If you have an oscilloscope, use it on the input and output of the regulator. If you don't, try a multimeter on AC volts range. If it reads 0 with stable voltages, observe what happens when the anomaly occurs (if it doesn't read 0, forget it). \$\endgroup\$ Commented Jan 20, 2017 at 13:33

2 Answers 2


When the LM1117 is providing 100 mA load current, the maximum drop-out voltage can be as high as 1.2 volts. This means that the input voltage to the regulator needs to be at least 4.5 volts (all the time) or you will see a lowering of the output voltage.

If the 5 volts is in fact (say) 4.7 volts and drops just a little bit under load this could explain the problem. Alternatively, the 5 volts may look like a solid voltage but whatever external current it is providing may occur in regular bursts and there may be an instantaneous voltage that drops below 4.5 volts that your meter cannot see.

You may need to use an oscilloscope to see this.

Check the input voltage to the LM1117 rather than the output voltage from whatever generates the 5 volts because there could be additional volt drop you haven't factored-in.


You have a lot of conflicting statements there so it is hard to tell what's going on. In general, you want to make sure that the source can really supply the rated amp at the rated voltage, the regulator can pass that much voltage, the regulator isn't into some shutdown mode (thermal shut down for example), the regulator has sufficient voltage drops to work with, and you are measuring it correctly, to name a few.


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