Power loss backup cap causing power supply restarts

[keyz182]

As suggested here https://docs.duet3d.com/en/User_manual/Tuning/Resume I've been looking at adding a 10,000uF capacitor a cross the 24v line to make sure there's ample power left to do what's needed.

I've moved to a mains powered bed, and moved down to a 150W 24v power supply, as that's all I need. The issue is that it appears the supply can't handle the inrush current (I think that's the right term) of the big cap, and keeps restarting every second or so.

What's the best way to limit the inrush current? NTC seems to come up in searches, but not sure how to appropriately pick one for the job. Any other ways?

[dc42]

@keyz182 you could connect a Schottky diode (rated at 5A or more) and power resistor in parallel, between the positive terminal of the capacitor and the positive terminal of the PSU. Diode cathode to the PSU terminal. Then the capacitor will charge through the resistor, and discharge through the diode when power is lost.

[keyz182]

@dc42 Would something like 10Ω, 100W work for the power resistor? Digging out the old ohms law (if I'm using it correctly, a bit rusty) says that would allow 2.4A, and hit 56 or so watts?

E.g. would these work?

• https://uk.farnell.com/on-semiconductor/sb530/rectifier-schottky-5a-30v-do-201ad/dp/2454184
• https://uk.farnell.com/arcol/hs100-10r-j/resistor-wirewound-10-ohm-5-axial/dp/2478154

[dc42]

@keyz182 said in Power loss backup cap causing power supply restarts:

@dc42 Would something like 10Ω, 100W work for the power resistor? Digging out the old ohms law (if I'm using it correctly, a bit rusty) says that would allow 2.4A, and hit 56 or so watts?

E.g. would these work?

• https://uk.farnell.com/on-semiconductor/sb530/rectifier-schottky-5a-30v-do-201ad/dp/2454184
• https://uk.farnell.com/arcol/hs100-10r-j/resistor-wirewound-10-ohm-5-axial/dp/2478154

Rectifier: yes.

Resistor: Yes, but it's overkill. The energy stored in the capacitor will be about 6mJ and the same amount will be dissipated in the resistor during charging. Once charged there will be negligible power dissipation in the resistor. So a 25W resistor should be more than adequate, e.g. https://uk.farnell.com/arcol/hs25-10r-j/resistor-wirewound-10-ohm-5-axial/dp/2478168.

As you don't need a particularly fast charge time, you could use a higher value resistor. For example, 47 ohms would still charge the capacitor to more than 85% in 1 second and 98% in 2 seconds. Then a 10W resistor would be adequate, and you might even get away with 5W. Or you could use 100 ohms and 3W.

[keyz182]

@dc42 fantastic, thanks!