Re: iMac 24 PSU LITEON PA-3241-02A1 250W
The "startup capacitors" ("startup and run" is maybe a better term) are just power supply filtering capacitors for the power supply of the PWM controller circuit (usually an IC)
Before the PSU starts running, these capacitor(s) are (usually) charged though high-value resistors that feed from the primary filter capacitors (the 200v or 400v one(s)) The capacitor has enough energy stored to power the PWM controller as it begins to start oscillating and switching the current through the primary winding of the transformer.
Once the controller is running, the PWM controller gets most of its power now from a secondary winding on the transformer which is rectified by a diode and filtered with the capacitor(s) in question.
If the capacitor is failing, it can't hold enough power to keep the PWM controller running and the PSU shuts down or ends up in a power cycling condition.
See the example in the schematic attached. Pin 7 is the supply pin of the PWM IC and R2 supplies the initial power. Once the supply is running, the power comes through D1 and D2.
The "startup capacitors" ("startup and run" is maybe a better term) are just power supply filtering capacitors for the power supply of the PWM controller circuit (usually an IC)
Before the PSU starts running, these capacitor(s) are (usually) charged though high-value resistors that feed from the primary filter capacitors (the 200v or 400v one(s)) The capacitor has enough energy stored to power the PWM controller as it begins to start oscillating and switching the current through the primary winding of the transformer.
Once the controller is running, the PWM controller gets most of its power now from a secondary winding on the transformer which is rectified by a diode and filtered with the capacitor(s) in question.
If the capacitor is failing, it can't hold enough power to keep the PWM controller running and the PSU shuts down or ends up in a power cycling condition.
See the example in the schematic attached. Pin 7 is the supply pin of the PWM IC and R2 supplies the initial power. Once the supply is running, the power comes through D1 and D2.
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