I want to power 2x PTC heaters mounted in parallel with a 24VDC power supply (this one). If possible I would like to avoid using a current-limiting system to handle the inrush currents. The specs of each heater is:

  • 24VDC
  • Inrush current: 8A
  • Stable current: 0.2A

For 2x heaters I'm then expecting:

  • Inrush current: 16A
  • Stable current: 0.4A

This PSU is rated for 24VDC/10A. The specs sheet says:

Inrush current[A] - ACIN 230V - 40 typ (I0=100%)(at cold start Ta=25 degrees C). The value is primary surge. The current of input surge to a built-in EMI/EMC Filter (0.2ms or less) is excluded.

I believe "40 typ" means 40 * current rating = 40 * 10A = 400A? This PSU could then handle a 400A inrush current from the load? If yes, for how long?

Is my reasoning correct? Thanks for any hint

  • \$\begingroup\$ AC inrush of 240V*40A = 9600Wp with a DC inrush of 384 W and a steady-state load of 9.6W with a $100 power supply , that's a 100x start surge. for a 60 Ohm load when hot. What's wrong with this picture? Can you design a better solution or are you in a rush. ? pun intended ;) \$\endgroup\$ Dec 4, 2021 at 4:50

1 Answer 1


That's a case of confirmation bias.

Actually, it's talking about the power supply's own inrush current that it may demand from the AC side. The "40 typ." indicates that on 230V AC supply, it may draw as much as 40A to charge its own internal capacitors, and so they better provision AC-side power accordingly. (which isn't a problem from a utility supply, any common 15A breaker will tolerate a 40A overload for probably 20 seconds.)

It's no help to you, I'm afraid.

If you will have silicon switching the heaters, you could always PWM them to limit inrush current, same as a bench power supply. I.E. Drive them "constant-current" of say 2A up to target voltage, then constant-voltage beyond that. That strategy could actually reduce the size of the power supply you need.


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