dropping voltage activation circuit

[J4e8a16n]

Hi,

It is easy to bias a transistor that activates a relay at    x   volt.  How can this be done when a voltage drop at  .05 volt?


JPD

[Simon]

I think he is saying he wants the transistor to turn the relay off wehn the voltage being sensed goes low. This requires an inverter, a second transistor can be used.

[retrolefty]

Any application that requires that amount of precision/accuracy of the trip point is going to require a more complex solution. An op-amp comparator using a reference input voltage ( fixed or adjustable) and the voltage being monitored to determine when to turn the relay on or off could work. A small microcontroller would be a good choice.

 

[Simon]

Well I'm not sure what the OP actually means so might be helpful for him to clarify. A comparator is probably a good choice with a boost transistor if required.

[Seekonk]

Three pin LM431 and a relay.

[J4e8a16n]

I think he is saying he wants the transistor to turn the relay off wehn the voltage being sensed goes low. This requires an inverter, a second transistor can be used.

"he wants the transistor to turn the relay ON  wehn the voltage being sensed goes low"   ;o)

I am sorry if I have been unclear.

JPD

[J4e8a16n]

"he wants the transistor to turn the relay ON  wehn the voltage being sensed goes low"   

Hi,
I am sorry if I have been unclear.
Is it possible?
JPD

[Simon]

the easiest way is a comparator, as it will give you very accurate control over the switch voltage, you can use it either way around.

[J4e8a16n]

Ok.  Thank you. I will try that.


JPDaviau

[J4e8a16n]

Hi,

Here is my trial.

The comparator does not stay off at start.

I  joined the asc file and his companion in case  you want to try it.

JPDaviau

[Richard Crowley]

It seems pretty unlikely that a 311 could possibly pull down against a 1 ohm resistor.
And then when that is fixed, measuring against such an extraordinary low reference voltage (12 microvolts???, really??) seems very unlikely. 

The original post asked for detecting a drop of 50mV.  But the schematic shows that the reference is 0.012mV, so there seems to be a fundamental problem here with scaling or units or something.

The reference voltage is so close to the "- rail" I don't think an antique device like 311 could even operate that close to the rail.

And it also isn't clear whether we are trying to detect an ABSOLUTE voltage drop, or a RELATIVE drop, and over what time period?  This is why we typically use hysteresis here in the Real World.

Perhaps revealing the larger picture here would help us understand the project parameters.

[Simon]

I think there are some fundamental misunderstandings here. I think the OP wants to use a Hi Low 5V signal to turn the relay on and off. So we need to understand what low actually is and what high is too for that matter. Lets assume the threshold is at around 2.5V. So anyrhing over 2.5V is hi and anything under 2.5V is low.

So now you can set your threshold at 2V because your unput must go UNDER the threshold in order to activate the circuit. In digital systems a "low" output can be anything up to 1V if it comes from an IC's output.

[J4e8a16n]

Ok.

The LM35D needs  3.7 volts for optimal result.  This temperature sensor, délivers  .01 V  for 1° celcius. At 1° celcius a heater should start (relay) to warm up the freezing pipe.

I understand that for the comparator to work I would have to amplify the signal  with an opamp before sending it to the comparator  which will make the relay ON (closed).


I wish this is clearer.

JPD

[Simon]

the other thing you can do is use a split supply so that you can power the comparator from a more negtive voltage than the sensor, this will bring the 0.01V up away from the negative supply of the comparator. But then it mist be just as easy to use a a comparator.

You can get other sensors that have their 0C offset say at 2.5V out of 5V so that you have better range.

1C is a bit late to heat a pipe before it freezes, go for 3C

[J4e8a16n]

Thank you for your answer.

This hobby is  full complexities. 

JPD

[Simon]

Yes many things in electronics are not as they appear at first. We are after all just playing with physics at a higher level and nothing comes for free.

[Simon]

another thing you can do is raise the ground of the temperature sensor. if it only uses a few uA a decent low value volotage divider might do it or you can use an opamp as a virtual ground generator.

[Richard Crowley]

There are perhaps hundreds of example circuits and application notes for LM35 out on the internet. Here is just the first example Google returned when I did an IMAGE search for "LM35 circuit"...


http://www.polarsys.ca/docs/electronics/lm35.html

[Simon]

this seems to have become a problem of how to get an average comparator to work near ground, or how to move the ground of a device outputting a voltage close to ground.

[Richard Crowley]

[Simon]

most of your circuits overlook the fact that the comparator will not work with 10mV

[Richard Crowley]

most of your circuits overlook the fact that the comparator will not work with 10mV

So offset the output with a few diodes (or a zener) in the ground leg of the LM35.
This isn't rocket surgery.

(PS: They aren't "my" circuits.)

[Simon]

Yes I know they are not your circuits, I'm just saying they don't help the actual problem.

Putting diodes in series with the ground of the sensor is of little help. Diodes are temperature sensitive and can infact be used as temperature sensors due to the variation in their voltage drop over a temperature range.

Either the negative of the comparator needs to be below ground or the ground of the sensor needs lifting.

the datasheet itself of the LM35D suggests that for negative tempratures the output needs a pull down resistor to a negative voltage to allow for negative voltage swings for a negative temperature.

[Richard Crowley]

Putting diodes in series with the ground of the sensor is of little help. Diodes are temperature sensitive and can infact be used as temperature sensors due to the variation in their voltage drop over a temperature range.

As a practical matter, the problem at hand is detecting near-freezing temperature in order to turn on a protective heating system.  If the offset circuit IS temperature sensitive, it all comes out in the wash during set-point "calibration".  So as an esoteric argument I agree with you.  However, as a practical argument, so what?

[Simon]

Yes I was thinking that too but I'm trying not to overly confuse a newcomer to electronics.