What is hysteresis in a microcircuitry context?

Factual Questions
1

My motherboard came with a monitoring program for things like temperature and fan rpm’s. You can choose to be notified when the cpu/system is above a certain temperature (the overheat temperature, oddly enough!), or you can set the value for the hysteresis temperature. The alarm goes off when the system is BELOW this temperature.

I did some searching and only found general definitions of hysteresis, that is, the effects of forces on a system lagging behind the actual forces. My questions are:

1.) What is hysteresis in the context of microprocessor/mobo
2.) Why is it bad to be below the hysteresis temperature? Does permanent damage occur?

2

It doesn’t have anything to do with the circuitry per se, but hysteresis is important when controlling/monitoring temperature.

Think of it this way - let’s say you want your temperature to be 70 degrees, and you want an alarm to go off if it goes higher or lower. Without hysteresis, minute fluctuations would be constantly triggering alarms, and they’d drive you nuts.

Hysteresis is built into your thermostat in your house for the same reason. Let’s say you set your thermostat to 70 degrees. If you didn’t have hysteresis, your furnace would kick in at 69.9 degrees, and go off again at 70.1. In other words, your furnace would be constantly kicking on and off, which would add a lot of wear, and the constant cycling would drive you nuts.

So, hysteresis is built into the system, both positive and negative. So you want the temperature to be 70 degrees on average. The furnace goes on when the temp goes to maybe 68, and stays on until the temperature reaches 72. The average temperature will still hover around 70 degrees, but the furnace won’t kick in and out nearly as often.

The same principle applies to your motherboard temp alarms.

3

So basically they could have called it the “underheat temperature” and meant the same thing? My overheat temp is 60C, and hysteresis temp is 40C. If I understand correctly, the ideal temperature is somewhere between those, and the extent of the hysteresis is how large a range those predefined temps form around the ideal temp. So are there bad things which happen to these types of electronics below 40C?

4

A Hysteresis Loop.

Hysteresis is a behaviour that can be applied to many forces, as your definition makes clear. There is no one “hysteresis temperature”, as far as I know. I don’t know why you would want to receive an alarm when temperature dropped below 40°C (104°F). I would think you’d be more interested to know when the temp went above 40°C, especially where electronics are concerned.

I am most familiar with explaining hysteresis as applied to magnetism, so maybe Sam will step back in and make reference to the hysteresis loop I linked to, as applied to thermal energies, and assign values to the two axes & points of intersection.

5

There is nothing bad that happens to your cpu if the temp drops below 40 deg C. In fact, if you can get it closer to 30 deg C it will probably last a little bit longer. The one thing you want to avoid though is “thermal cycling” (having the temperature go up and down and up and down, etc). Computers are made of many different materials, and they all expand and contract at different rates. These stresses caused by varying temperatures can eventually cause faults (for example, the tiny little wires that connect the silicon to the pins in the chip can eventually lift up off of their pads and disconnect, and then the chip is broke).

This is what leads to the age old “leave it on all the time” or “turn it off when not using it” debate about computers. If you leave your computer on all the time, then there is one set of effects trying to kill it (friction on moving parts like fans and disk drive motors, heat, transistors constantly switching on and off, etc). If you turn it off when you are using it, then there’s a different set of effects trying to kill it (thermal cycling being the most prominant). Oh, and by the way, even if you never turn the thing on, there’s an aging effect inside the silicon trying to kill it. You can’t win.

I personally don’t like to have any chip in my entire system running at more than 40 deg C. Coincidentally, the human pain threshold for heat is right around 40 to 45 deg C, so you don’t need any fancy measuring device. If you touch it and it makes you say “ouch” then it’s too hot and needs more cooling (what I like to call the highly scientific “ouch” test). There’s a trade off between cost and long term reliability, and since most people replace their computers every few years, it doesn’t make much sense for the manufacturers to design them for a 10 year life. Hence, many CPUs run a little hot. A rough rule of thumb is that every 10 deg C above room temperature cuts the expected life of the device in half (which is notoriously inaccurate on the low and high ends of the temperature range, so don’t worry much about the change from 30 to 40 deg C). Above 70 things go bad very quickly, which is why most computer alarms are set around there.

Find out what temperature your CPU tends to run at. If it’s more than 50 I would recommend getting some better cooling for it. Once you know what temp it runs at normally, you can set the alarms much closer to that temp.

As for the hysteresis temperature, I don’t have a system that has this feature so I can’t say for certain, but what I would expect is that the temperature alarm turns on when the temperature gets above the high temperature, but the alarm doesn’t turn off again until it gets below the low temperature. This is what Sam Stone was getting at, you don’t want it to be constantly turning on and off and on and off as the temperature fluctuates around the high mark, you want to build some hysteresis (delay) into when the alarm shuts off.