Converter In-Rush Thermistors etc, UPDATE -4

BFL13 wrote:

DryCamper11 wrote:
.............. You then plug in the assumed inrush current into a formula for minimum R.

Choosing an R above the minimum is OK. Below the minimum, you risk failure at turn on.

DryCamper11 was going from vague memory there (18 Nov), but now I can tell what he was talking about, wrt those two lesson plans, etc.

This relates to my current puzzle over the meaning of "minimum."

He and Ken seem to contradict the lesson plans where with 5.7 calculated they went with a 5 and with 1.2 calculated, they went with a 1. Ken says with 3.4 to go higher and use the 5.

I found in an earlier post that in series, the two 2Rs would be 4R, so that is answered (which is above 3.4 if that comes into play)

Getting this "minimum" definition clarified would tell me whether, with a 3.4 calculation I should use a 4 or 5 or whether I could use a 2. ( If going with Ken's low size amps where that 3.4 came from)

Can anybody help with that definition of "minimum" business? Thanks.

As to two 2R's = One 4R. Maybe, maybe not. Yes, they will have similar cold resistance values. But you also need to insure each 2R can handle the required inrush energy (joules) and and max power disipation. E.g. will get hot enough, but not to hot or to cold at max current.(Kinda like the soup in Goldilocks and the 3 bears). It needs to get hot enough to be low resistance but always be below max power limit.

I didn't go back to the lesson plan, but I suspect when they say "minimum" they are referring to the minimum resistance that will insure the inrush current is below the maximum limit. Therefore, you find a minimum resistance, then find a part that is at least that minumum resistance when cold at lowest operating temperature and lowest value per spec'd resistance tolerance. It can be higher, but not lower. Then determine if that part can handle the joules and power. Keep iterating till you find a part that meets all the requirements.

Remember, the initial resistance of these devices can have a significant variation, look at the tolerance for the initial resistance and determine it's lowest resistance per the tolerance, not the "nominal" resistance. And if your going to be boondocking at high ambient temps plug that into the equation as well. Low temps don't hurt you, resistance goes up. High initial ambient may, initial resistance goes down.

So, as a "hypothetical" example, suppose the calculated "nominal" R value is 5.2 ohms. After doing some checking you find that the values available that meet inrush spec's, power spec's. etc are 4.9ohms, 5.3 ohms and 6 ohms, each with a 10% resistance tolerance

OK, 4.9 is out, to low, below thee 5.2 minimum. 5.3 can range from 4.8 to 5.9 ohms, still to low. 6 ohms can range from 5.4 to 6.6 ohms. Looks like 6 is the one to go with. Although I'd verify that the 6 ohms at high ambient doesn't drop below 5.2.