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1000 VA 230 VAC 50 Hz - How Much Inrush Current?
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dannybeckett:
Hi guys, made some interesting progress. I bizzarely found a complete soft-start circuit harvested from a microwave when rummaging through my old salvaged parts box! What are the chances ;D I noticed 3 problems with the circuit though:

* The current limit resistor value was very low (15 R), worried this would still cause trips
* If you powered it on and off quickly, the relay remained closed (no inrush limiting)
* If the limiting resistor somehow blew or failed, the shorting relay would still close (no inrush limiting)I bought a 40W halogen bulb (~90 R) and replaced the load, simulated the circuitry needed to mitigate the points above and then built an add-on board to the original soft start circuit I found. Tested and it works perfectly! Transformer gives me no inrush trouble at all now. Relay shuts down extremely quickly after loss of AC and won't engage if the halogen bulb has for some reason blown.

In parallel, I tried connecting mains to my arduino but it tripped my RCBO as live current found a path to earth via the cable shield. I've got a USB isolator on the way and so will resume exploring the phase-accurate switching method when that arrives.

But for now, see attached for the Tina-TI simulation, pics and graphs of the fancy inrush limiter as it stands today!
Kleinstein:
15 Ohms is a reasonable size for the resistor to limit the inrush current. The current may still exceed the 16 A current rating for a short time, but that is Ok with the fuses. In addition to the resistor the transformer also has some impedance.  To trip the fuse need significant higher current (like > 50 A) and the inrush current may well reach that level when the core is saturated.

If the relay is power from behind the current limiting resistor, than with a blown resistor nothing happens, as the relay would never close.

A very short power down phase is a possible problem, but the relays are usually reasonable fast in turning off. It is responsibility of the user to avoid such short turn off phases. This is a larger problem with the NTC type solution that is sometimes used with lower power. Here the cool down may need 10s of seconds.

The USB isolators may not all be safe for mains work. To be on the safe side, I would test the circuit first with a lower AC voltage from behind a transformer.
BravoV:
Alternative circuit, soft starter for inductive load which will turn on at the peak voltage, the snubbers (A or B) are optional.

dannybeckett:

--- Quote from: Kleinstein on March 12, 2022, 09:33:20 am ---The current may still exceed the 16 A current rating for a short time, but that is Ok with the fuses.

--- End quote ---
I'm using fast blow fuses and my ring main is on a 20A breaker


--- Quote from: Kleinstein on March 12, 2022, 09:33:20 am ---If the relay is power from behind the current limiting resistor, than with a blown resistor nothing happens, as the relay would never close.

--- End quote ---
If there's a sizeable load attached to the xformer and the resistor isn't blown, this arrangement prevents the relay from closing like it should due to vdrop developed


--- Quote from: Kleinstein on March 12, 2022, 09:33:20 am ---A very short power down phase is a possible problem, but the relays are usually reasonable fast in turning off. It is responsibility of the user to avoid such short turn off phases.

--- End quote ---
The user should not have to count to ten before they re-energise a device to prevent blowing fuses. My home experiences short duration blackouts (cost of rural living)


--- Quote from: Kleinstein on March 12, 2022, 09:33:20 am ---The USB isolators may not all be safe for mains work. To be on the safe side, I would test the circuit first with a lower AC voltage from behind a transformer.

--- End quote ---
Good idea. The mains is being fed to my arduino via a 1M resistor


--- Quote from: BravoV on March 12, 2022, 11:06:47 am ---Alternative circuit, soft starter for inductive load which will turn on at the peak voltage, the snubbers (A or B) are optional.

--- End quote ---
Very nice, I like the simplicity. Ty for sharing
Kleinstein:
Even a fast low fuse is OK with quite some current for a few short peaks. The peaks from saturating the core are usually short, like 1-5 ms once per mains period. Here the resistor is quite effective to limit the current. So with 120 V mains even 5 Ohms would be enough to limit the current enough. With a significant load on the secondary the 15 Ohms from the microwave may be too high.

With a transformer there is usually no need for a fast blow fuse.
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