[Chris55000]

Hi!

NTC Thermistors sold these days are characterised by:–

1) Nominal room–temperature resistance at 25°C;

2) The B value – this is basically the exponent of the power law that relates the temperature variation with resistance. If you buy new ones from a reputable maker like Epcos or Vishay, there'll be a datasheet which has the approximate value of B given – by the way B I think is dimensionless, i.e., just a number without a unit specified.

Now to the OP's post – it was very common German and Continental practice to denote thermistors with an oblong box and a double–headed arrow through it, hence the OP mistaking it for a potentiometer! This is why, once again, the traditional resistor with a black spot next to it used by BSI originally was far preferable, and is the one I use myself!

Regarding replacement of these in exsisting equipment, if the maker provided a type no. in his parts list or diagram, you might be able to find out what the B value was and select a modern replacement, but if no type number was quoted, then one of the same cold resistance and approximately the same size should be OK.

3) Power rating in milliwatts – again, because an NTC varies with temperature, it's difficult to predict the exact dissipation in many circuits, but since most small NTC thermistors are used for low–current bias stabilisation or temp. compensation I don't think the rated power dissipation would be something to worry about – some datasheets do specify maximum initial current curves for each size and type but it's not universal practice!

Chris Williams