Re: Hysteresis needed in 741 Op-Amp circuit

From: CF (form_at_web.site)
Date: 06/11/04 

Date: Fri, 11 Jun 2004 05:16:49 GMT

Wow again! This is great stuff Michael. I've pored over what you've written,
which is excellent, and I'm with you 100% on the need to retool the whole
circuit (ideally). I wish I'd started with a better basis. I don't know what
the designer was thinking, but perhaps he modified it from a heating circuit
where the comparator output would be low when the heater was inactive -- or
something like that.

I've attempted to draw my take on your suggestions and posted this at:
www.copperleife.com/craig/tech/thermo/mb.htm

The small changes in hysteresis you mention are not an issue at all, nor is
the non-linear nature of the thermistor seeing as I will be using it within a
very narrow band.

Being a rank novice, I have no idea what resistor to use to pull up the 311's
open collector. Also, my mathematically-challenged brain is hurting too much
to apply your formula to everything and work out a value for the hysteresis
resistor, but I'm thinking that I can whack a trimpot in there and experiment!
This is shameful, I know.

Please let me know if I've got it right or wrong otherwise, and any idea you
might have on the R4 value.

Thanks heaps,

CF

In article <slrncchaph.2tc.ballbach@wayreth.rten.net>, Michael Ballbach
<michaelballbach@sbcglobal.net> wrote:
>On 2004-06-10, CF <form@web.site> wrote:
>> I'm looking for a bit of advice on a 741-based thermostat circuit that
>> needs some hysteresis, or lag between "on" and "off".
>>
>> http://www.copperleife.com/craig/tech/thermo/
>
>Let me preface - I'm not an expert.
>
>I've just recently done something similar. What you want in this case is
>a bit of positive feedback around the comparator. As a quick aside, I
>would use an LM311 or similar actual comparator here, instead of the
>op-amp, though the op-amp is probably fine if it is rail-to-rail enough
>(I do not know if the 741 is). If you do switch, make sure you note that
>the output of the LM311 is open collector.
>
>Let's take a starting state of Vni > Vi. Since your thermistor has a
>negative time coefficient, its resistance gets smaller as the
>temperature gets higher. So, in the context of your divider, Vni
>approaches ground as the temperature approaches infinity. Thus, Vni > Vi
>(Vo = +Rail) means that the temperature is lower then the preset
>(transistor does not conduct). Vni < Vi (Vo = -Rail) means the
>temperature is higher then the preset (transistor conducts). This you
>know already, I say it for my own benefit.
>
>I suppose that sampling some of Vo for Vi (as suggested on your web
>page) would work if the feedback resistor was a lot higher then the Z
>looking into the inverting pin. In my limited experience though, I've
>only applied positive feedback, that is to say feedback to Vni, around
>comparators for the purpose of hysteresis. Feedback to Vi will have the
>op-amp behaving like an inverting amplifier, with gain set as usual,
>Rf/Ri. This will, at least over a small range, have the output slew
>instead of step.
>
>If it was me, I'd retool this circuit. Replace the PNP with an NPN, and
>put the preset on Vni and the thermistor on Vi. This would invert the
>output of the comparator (high when the temperature is too hot (Vi <
>Vni), low when too cold (Vi > Vni)). Replacing the PNP with an NPN
>compensates for this from the transistors perspective. Then, if you were
>to sample some of Vo (call it Vsamp) and apply it to Vni, you'd get
>hysteresis as I would expect it.
>
>Imagine Vo positive, that is to say, Vi < Vni, or too hot. This would
>cause Vni to be modified by Vsamp such that the Vni voltage would be
>driven higher, effectively decreasing the preset temperature by forcing
>the thermistor to cool past the turn on point. When Vo is off, Vni is
>normal, the hysteresis is removed.
>
>As to picking the value of the positive feedback resistor. Envision the
>applied feedback as a voltage divider made up of the feedback resistor
>and the preset voltage network, use superposition and sum the voltage
>from that divider with the preset divider. With the above configuration,
>you will probably end up with something like:
>
> V2*R2*R1 + V1*R2*(R3+R4)
> Vni = ------------------------
> R2*R1 + (R4+R3)(R2+R1)
>
>In the above, R1 and R2 make up the divider programming Vni, R3 is the
>positive feedback resistor, and R4 is (since this came from my project)
>the resistor pulling the LM311's open collector up. V1 is the supply
>voltage, V2 is the output voltage of the comparator (Generally V1 or 0).
>For the 311, R4 should be zero when V2 is zero, because R4 acts as a
>pull-up and isn't 'in the circuit' when the output of the LM311 is low.
>
>Note that the value of the hysteresis depends on the divider programming
>Vi. That means, that with an adjustable temperature threshold, the
>hysteresis will change with the temperature threshold, but not by much,
>since generally R3 will be >> then R1 || R2 (note your R1 and R2 is a
>bit more complicated with how you have your pot setup, but the point
>holds).
>
>Since thermistors are non-linear, the hysteresis will not be measurable
>in degrees, per se, and will not represent a constant temperature
>difference for different presets. If that's important, you may want to
>switch to a linear temperature sensor, like an LM335. That would give a
>more constant hysteresis in degrees over a larger temperature range.
>You'll need to work that out based on your project parameters.
>
>With hysteresis, I would consider the capacitor unnecessary. All it's
>doing is, as you note, smoothing out some of the comparator jitter, in a
>not very sophisticated way.
>
>At 12 volts and 1 Kohm, you're driving about 12 mA into the base of the
>transistor. With a minimum beta of say, oh, 50, that's more then
>necessary, yes. You could probably replace it with 10k or so to bring
>the drive closer to a milliamp, which should be plenty to run your 40mA
>load.
>
>Oh, if you replace the PNP with an NPN, make sure to put the relay
>between the supply and the collector, not the emitter and ground.
>
>Again... I'm not an expert.
>

Relevant Pages

  • Re: Hysteresis needed in 741 Op-Amp circuit
    ... a bit of positive feedback around the comparator. ... Let's take a starting state of Vni> Vi. ... approaches ground as the temperature approaches infinity. ... comparators for the purpose of hysteresis. ...
    (sci.electronics.basics)
  • Re: Current-sense AC over a threshold
    ... One thing to watch out for is the hysteresis in these devices. ... off again until the temperature drops to 20. ... adjust the hysteresis by selecting component values. ... but I reckon that the normally open switch that will switch on ...
    (sci.electronics.repair)
  • Re: Current-sense AC over a threshold
    ... One thing to watch out for is the hysteresis in these devices. ... off again until the temperature drops to 20. ... electronics 'cooking' temperature. ... your original power slave switch idea was the better option. ...
    (sci.electronics.repair)
  • Re: Temperature Relay Circuit Required
    ... you were not asking for the relay to close as the aquarium temperature ... If you decide you *do* want some hysteresis, ... some useful 'snap action' to the relay. ... Actually, for a circuit with hysteresis, I might drop the op-amp ...
    (sci.electronics.design)