What is Hysteresis ????

[Paul Hubrich]

The VIA Hardware Monitor that came with my new Abit Mobo includes data for "CPU Hysteresis Temperture" and "System Hysteresis" along with the usual temperatures, fan speeds, etc. (Yes, I know "temperture" is an incorrect spelling, but that's the way the hardware monitor spells it)

What exactly do these numbers mean???

[Ultima]

I think that is meant with the critical points of cpu and mobo temps and fan speeds.

Pim

[daveleau]

Here's a link to maybe explain it...
http://www.lassp.cornell.edu/sethna/hysteresis/WhatIsHysteresis.html

Dave

[ScaryBinary]

Uh...that's when a woman has her uterus removed, right?

Scary http://members.iquest.net/~stonerville/sb02.gif Binary

[Fingers]

Hysteresis can generally be thought of as "lag time". The term can be applied to temperature, mechanics, and magnetism among other things.

[tonym]

Dr. Scary,

That's HYSTERECTOMY!

Paul,

The simplest explanation of hysetersis can be used using a controlled cooling fan in a system. You want the fan to turn on when the internal temperature gets above a preset point.

Now depending upon a number of factors, how big the fan is (CFM), how quickly you can cool down the case air, the desired accuracy of your set point, you may need to add a "dead zone", a hysteresis to your set point to insure that false triggerings or sporadic noise are rejected, or that you give your cooling fan a rest.

The way this is done is that you will have a positive going (heating) turn on threshold (Th) before the fan will turn on. You desire that the fan doesn't stay on almost constantly or turn on-off-on-off-etc. at a single threshold point, so you add a negative going (cooling) threshold (which is less than Th) that you have to cross before the fan will turn off (Tl). The hysteresis is defined as: Thl = Th-Tl. You can make the range as narrow or as wide as you desire, depending upon your performance goals:

___ ___
Th----------/ \ / \
/| \ /
/ | \ /
/ | \ /
/ | \ /
/ | \ /
/ | \ /
Tl-----/----------------|\__/
/ | |
/ | |
Temp ___/ | |
| |
_________ __
On | | |
Fan Control | | |
| | |
Off ______________| |___________|

[Edit: If you edit my message and look in the little window, poof the waveforms will appear!][2nd edit: and if the picture was right, that would be OKAY!]

The hysetersis above creates a "duty cycle" that the fan must behave within. It is obvious that as Th and Tl get closer together, the fan will stay on longer (when Th = Tl you will have 100% duty cycle), but the temperature will have less deviation. With the proper Thl width, you could have the fan work at 50% duty cycle (half the time).

Hysteresis in a control circuit is useful to reject noise in a system. Imagine if there was a small thermal noise, dT, that was present in the system. When you got closer to the non-hysteretic threshold you might trip at Th - dT below OR above the threshold. In the example of the cooling fan, this isn't too much of a problem. But if you're a trying to monitor a voltage in your PSU, then this noise will give you false power fault signals and create a nuisance. Hysteresis is used in the "power good" circuitry in a PSU to prevent this from happening.


Hope this helps...


Tony

[This message has been edited by tonym (edited 07-28-2000).]

[This message has been edited by tonym (edited 07-28-2000).]