Converter In-Rush Thermistors etc, UPDATE -4
UPDATE on 17 Nov, Update 2 on 22 Nov, Update 3 on 16 Jan 14, Update 4 on 19 Oct 14. --------------- In a recent thread there was mention of how you can wreck your gizmo (in my case a converter) ...
Forum Discussion
Salvo wrote:
Yes, we're only talking about the actual thermistor, not the adjacent material. It has a large time constant. Take a look here.
I'm not sure why you're pointing to a bead thermistor. They are used for measuring temperature, not for controlling current inrush.
I once designed and built a device that measured how fast an elevator was rising/falling. It used two tiny bead thermistors in a tube connected to a thermos bottle. As the bottle went up/down in the elevator, air would flow in/out of the tube to fill or exit the bottle. The bead thermistors were positioned in line so that one or the other would be cooled more than the one behind as air pressure change due to elevation change caused air to flow into the bottle (elevator down) or air to leave the bottle (elevator up). A differential amp with a bridge circuit allowed measurements of only a few feet per second of elevator speed. I could hold it near my feet, raise it to my head and read off the rate of elevation change. Those devices are amazing.
Encased thermistors have 2.5 s max time constant. ....
I've provided a link showing thermistor time constant is max 2.5s.....
You're still looking at time constant for the entire thermistor. That's not what's relevant. It's the time constant for heat to leave the semiconductor active region and flow into the bulk semiconductor material on either side thereof. We're talking micron size regions and heat flow distances. That has little or nothing to do with the quoted time constant of the bulk device. The time constant I'm referring to isn't specified in any ratings. It is factored into the Joule rating and the capacitance rating of the device.
The capacitance rating and the voltage set the Joules that the device is able to absorb. That energy appears as heat in the active region of the semiconductor junction. It will move from those hot spots in milliseconds. If the heat isn't allowed the millisecond(s) time it takes to diffuse away from that region, it is damaged and the device fails.
The device time constant that is specified in the ratings is different. That number is used to determine how long it takes before the device has heated to the low resistance value (or recovered to the high resistance value specified at zero power after power is removed.) That time constant has nothing to do with the surge ratings.
