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If I use a switch load and an digital oscilloscope to test my linear power supply response, then which type of trigger should I use, if I want to display on CH1 the PSU output and on CH2 the output of the switch load? Should I use DC trigger on CH2 (switch load) and AC coupling on CH1 to show the response of the PSU under test?

This is the schematic of the switch load: https://ibb.co/dMtCBLh enter image description here

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  • \$\begingroup\$ Details of the load switch might be needed. \$\endgroup\$
    – Andy aka
    Dec 21 '20 at 11:40
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    \$\begingroup\$ Why don't you just try and see what works best. That's what I generally do. After gaining some experience you will learn what works best and use that setting from the start. If you have the logic level signal that switches the load on and off, I'd use that for triggering. \$\endgroup\$ Dec 21 '20 at 11:41
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    \$\begingroup\$ Consider using the 555 output signal as the trigger input to the scope. \$\endgroup\$ Dec 21 '20 at 12:18
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NE555 is capable of above 100mA output current in push-pull, so you don't need a bipolar transistor to drive your MOSFET switch. You can use the NE555 output directly. With a push pull driver the FET should switch fast and clean in both directions which means the first µs of your regulator transient response will really be the transient response, and not an artifact of slow FET switching.

You can put a resistor in series with the FET drain to set the load current that will be switched.

Then,

  • one scope channel in AC mode on the output of the power supply

  • the other channel on the FET drain, or on the NE555 output

A more useful setup would be to put the resistor in the FET source (if voltage allows) or to use a PMOS with a resistor to ground. This allows measuring exactly the current pulse on the resistor, to make sure it has a flat top, which is important if you want to know you're really measuring the transient response of the device under test, and not the flatness of the test pulse.

I've had problems with this setup before, as there was a tiny bit of crosstalk between scope channels. The regulator transient response was really tiny, so I had to boost scope gain to visualize it, and it turned out the scope crosstalk was enough to screw up the measurement. To test this, move the scope probe from regulator output to nearest ground. If you measure ground, you should see a flat line. If you don't see a flat line, the measurement setup must be corrected. Also make sure the ground of both scope probes is connected at the same spot. If it is not, some of the current may flow in the scope cable shields, and that will add some error voltage on top of your measurements.

If you have crosstalk problems, then don't use the second channel to scope the test pulse, use the trigger input instead.

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