Guru
Joined: 21/06/2018
Location: AustraliaPosts: 1206
| Posted: 08:12am 16 Aug 2024 |
It was time to actually see what peak currents are involved in a 3.5kW toroid when it is dropped across the mains asynchronously. I used the new second stage switcher to provide the degauss before closing the relay that bypasses the soft start in rush current NTC limiter, connecting the toroid directly to the mains.
The purpose of this is that the mains will be provided by the inverter stage1 and I thought it prudent to confirm what peak currents to expect, so the correct value of inrush NTC resistor can be chosen with something other than an educated guess. I also wanted to see the current waveform for a few successive cycles as the current peaks taper off as we transition away from saturation.
I could only find 5R NTC inrush limiters in my parts bin so I fitted them in the two locations giving me 10R of limiting. I will provide some CRO pictures of the current peaks I encountered in my next post but I have run out of time atm. The 10R resistors allowed me to capture approx 25A peaks worst case so it would appear that when we find saturation at switch on, the transformer is a very very low impedance as expected.
The current peak is a one off event for that half cycle and then quickly reduces to around 20% on the second half cycle and after about 500m Seconds has settled to its low running value.