Anyone familiar with Inrush Current Calculation for Thermistor Selection?

[Naysayer]

I'm still grappling with PSU issues but making progress (>300w carrier from 4/5w drive).
I installed a different NOS pair of 300a's (1 from a can).

I'm finding that Parallel NTC Thermistors do not heat evenly. Presently using four 20D-20's for a total of 5 ohm at room temp. One pair gets quite hot. So as I tweak PO, the burn smell frightens me into racing into shut-down to reinspect to prevent PCB damage. Rinse and repeat,

So, I need new Thermistor/s for Soft Start module.
I had a brief fantasy of equalizing resistors but the 20D-20's are only good for 6A each.

That brought me to the online ICL calculators but I don't see how "Input Inrush Current" is determined.
One website states it is 2-3 times the Steady State Current.. That site was about heavy motors not a small amp.
Is really it that simple, a rough estimate?

Edit: Continued tweaks blew the 10A fuse today. Eldico's 2x 4cx300 amp (1957) used a 15A plus a 5A (1957) so I feel safe with this besides, I don't have an 11,12 or 13A fuse. Online calculator yielding resistance figures a bit too high. Higher currents should require lower resistance, I get that. Still working on it.
neil, NYC

 

[Naysayer]

Found 25A Thermistors for Swimming pool pumps.
I'm going to try using a single one of these instead.

Amazon product ASIN B071HX7XNY

 

[brandon7861]

I havent watched it yet because my gf is sleeping and it apparently plays audio, but I found this.

https://www.digikey.com/en/ptm/a/ametherm/choosing-ntc-thermistor-for-inrush-current-limiting-capacitive-applications/tutorial

and these:

https://www.vishay.com/docs/33001/seltherm.pdf

How Do You Choose The Right Type Of NTC Thermistor To Limit Inrush Current For Capacitive Applications?

Matt Cuhadar from the Ametherm team answers a reader question about choosing the right type of NTC Thermistor to limit inrush current for capacitive applications.

www.electronicdesign.com

 

[Naysayer]

Closed up the amp first time by affixing bottom cover. Reduced internal fan noise a bit. 5 Rivnuts & unit up off table by 3/4". Air flow moves. Added padding cap to input variable. Here a tweak, there a tweak, everywhere a tweak tweak. Thing weighs 53 pounds

 

[nomadradio]

Always treated the inrush thermistors as the normally-open side of a soft-start relay. They are in the circuit for less than a second before the relay closes and shorts around them.

73

 

[Naysayer]

True dat for fine gear. I'll take another look at the soft start board. Hold the chuckles now, but I got it from AliExpress so anything is possible here.

I'm waiting for the larger 20A Thermistors. Presently using 4 6A thermistors in Para. after the original 2 smaller ones fried. It was all I had on-hand to keep moving forward. (I know Para Therms bad idea but it's temporary until I get right parts). Relay on SS board says 30A but we'll see.

This morning I had too much Cap on both the Tune and Load Air Variables. I had room to lower the Load Cap (had 2 sections strapped) but the Tune V Cap was at it's lowest. I have a good substitute part which I planned to install. After unstrapping the Load Cap sections, I tried amp again and now the Tune cap is perfect, PO way up and my 14A fuse went. That's progress here, a bit primitive but progress nevertheless. My fuse stock includes 15A so I can continue to go up but I'm going to wait on the new Thermistors so I don't burn up my AliExpress PCB special. Nervous as I approach the limits of 110vac.

 

[Naysayer]

After some thought on your point about typical operation of SS boards where the relay takes current after startup, the Thermistors on the AliExpress SS module stay warm during operation and in circuit. Apparrently, it's a light duty SS board that keeps Thermistors in circuit (?). My plan to replace the small Thermistors (original ones burned up) with 20A Thermistors (made for swimming pool motors) is my strategy. On par with amateur "Homebrew" it seems.

 

[Naysayer]

It's a $22 Soft-Start board. https://www.ebay.com/itm/1650394110...d=link&campid=5336136228&toolid=20001&mkevt=1

More of switched-relay with soft-start function entirely dependent on Thermistors. Board designed for 220. I replaced the original transformer with a 110-12v off-board unit. It a big improvement over a large toggle for the Mains. It's good I removed it because one of the Thermistor's I rigged in parallel already cracked open. Waiting on a 20A Thermistor Monday.
Will change when find something suitable.
Thanks Nomad for pointing that out, I entirely missed what was going on there.
neil, nyc

 

[Naysayer]

I get it now, I have to add another board with the relay switchover function. Thanks again Nomad. I needed a few days to understand what you were telling me.