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
BFL13 wrote:
Ok, but given they all run steady state at the same temperature, then is that where the Joules spec comes in? Which can take the heat better-- a 200J or a 300J one? Or is it that dissipation figure that matters? The 020 and 025 have the same dissipation constants and the same thermal time constants.
What is that black stuff anyway and is bigger, fatter, more black stuff better for heat tolerance? The 020 and the 025 have the same diameter but the 025 is 6.3 vs 6.0 thick for the 020, so the 025 is fatter.
The 020 is 206C at mx current while the 025 is 236C at max current. But they have different max currents 20 and 25. The converter's current is say 13a at full bore. 13 is less of 25 than it is of 20 but it might be at the same temp as suggested, which would be less of 236 than of 206.
BFL, it takes 4.184 joule of heat energy, or one calorie, to raise the temperature of a 1 gram of water from 0 degree C to 1 degree C which is the heat to joule relationship.
Power is the rate at which energy is transferred or consumed or delivered and has units of joule/second.
The minimum joule rating of the thermister must be larger than the energy storage capacity of the capacitor or it will fail.
If we look at the worse case current for the 5 ohm thermister, it would allow approximately 170 volts peak divided by 5 ohms or 34 amps of initial current to flow.
This equates to an instantaneous power of 5.78 kW.
If this instantaneous rate only exists for 1 ms, then the instantaneous energy would be 5.78 joules.
If it exists for 1 second, then it would be 5.78 kJ.
If we look at the worse case current for the 2 ohm thermister, it would allow approximately 170 volts peak divided by 2 ohms or 85 amps of initial current to flow.
This equates to an instantaneous power of 14.45 kW.
If this instantaneous rate only exists for 1 ms, then the instantaneous energy would be 14.45 joules.
If it exists for 1 second, then it would be 14.48 kJ.
So you can see, the smaller 2 ohm part starts off at an energy disadvantage even though it has a 100 joule higher initial capacity...
EDIT: Oh, and this should help explain why the hot restarts are bad since the resistance value is much lower when hot.
