Heatsink temp approximations ?!

[cmjones01]

Originally Posted by G0HZU_JMR

It's a well known method of making a cheap sensor and I think it's worth experimenting with the B-E junction of a transistor rather than a regular diode like the 1N4148. I think you could start by biasing the diode junction from 5V via a large resistor to set it at a light (but essentially constant) current and then just read Vbe.

I've played a bit with such temperature sensors. The Tektronix 7D13 multimeter plugin uses one for its temperature range, as does its Soviet cousin the Я4С-97. What I've found when replacing the sensor transistor is that the scale (mv/deg C) remains nearly the same, but the offset (voltage for a fixed temperature, say 0 deg C) varies, so adjusting the electronics for the correct reading at 0 deg C (which is easy to make using melting ice) is all that's required.

Chris

[Al (astral highway)]

Originally Posted by G0HZU_JMR

At work we often use the LM50B or LM50C temperature sensor and these are cheap and are pre calibrated up to +125degC. It will also give a higher gradient/slope so it will be easier to use with a basic comparator and a couple of LEDs. Not as much fun as trying to calibrate a cheapo homebrew PN junction sensor but they do work well.

Thanks for the alternative option, Jeremy. I'm going to try both. I quite like the mindfulness involved in your suggested approach using the BE junction of a bipolar transistor...

Cheers!

Al

[Al (astral highway)]

Update, for those interested.

I bought a laser temperature gauge and used it to probe around various parts of my circuit that were of potential concern to me.

I needn't have worried at all. After ten minutes, from a starting temperature of 23.0 degrees, I had the following readings:

3.3R Metal oxide resistor in gate circuit of a large IGBT: 54 degrees C

Heatsink with blacked-out masking tape on IGBT gate drivers: 49.5 degrees C (only one IGBT currently being driven, both drivers active, complimentary pair).

Body of gate driver 1: 48 degrees C

Body of gate driver 2: 43 degrees C.

So I have quite a lot of headroom to drive the IGBT brick that will be used in the final design. These measurements are with no fan. Even a small fan with PWM turning slowly will add extra safety.

Before I took the measurements, I thought the MOX resistors would burn out, but they're miles and miles from being in any danger.

Anyway, thought this could be of interest to some.

I'm still inclined to build the sensor that Jeremy describes and advocates, and embody it in the final design.