dropping voltage activation circuit - Page 1
Forum section: electronics / beginners

author: J4e8a16n
author_rank: Regular Contributor
timestamp: December 26, 2015, 10:49:32 am
content: 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

author: Simon
author_rank: Unknown
timestamp: December 26, 2015, 01:11:53 pm
content: 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.

author: retrolefty
author_rank: Super Contributor
timestamp: December 26, 2015, 01:28:35 pm
content: 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.

author: Simon
author_rank: Unknown
timestamp: December 26, 2015, 01:32:36 pm
content: 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.

author: Seekonk
author_rank: Super Contributor
timestamp: December 26, 2015, 02:26:43 pm
content: Three pin LM431 and a relay.

author: J4e8a16n
author_rank: Regular Contributor
timestamp: December 26, 2015, 03:40:18 pm
content: Quote from: Simon on December 26, 2015, 01:11:53 pm"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

author: J4e8a16n
author_rank: Regular Contributor
timestamp: December 27, 2015, 08:55:03 pm
content: Quote from: J4e8a16n on December 26, 2015, 03:40:18 pm

author: Simon
author_rank: Unknown
timestamp: December 27, 2015, 09:19:35 pm
content: 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.

author: J4e8a16n
author_rank: Regular Contributor
timestamp: December 28, 2015, 11:25:43 am
content: Ok. Thank you. I will try that.JPDaviau

author: J4e8a16n
author_rank: Regular Contributor
timestamp: December 30, 2015, 01:02:24 am
content: 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

author: Richard Crowley
author_rank: Super Contributor
timestamp: December 30, 2015, 04:32:14 am
content: 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.

author: Simon
author_rank: Unknown
timestamp: December 30, 2015, 08:25:02 am
content: 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.

author: J4e8a16n
author_rank: Regular Contributor
timestamp: December 30, 2015, 11:30:42 am
content: 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

author: Simon
author_rank: Unknown
timestamp: December 30, 2015, 12:32:50 pm
content: 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

author: J4e8a16n
author_rank: Regular Contributor
timestamp: December 30, 2015, 01:02:09 pm
content: Thank you for your answer.This hobby is full complexities.JPD

author: Simon
author_rank: Unknown
timestamp: December 30, 2015, 01:03:22 pm
content: 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.

author: Simon
author_rank: Unknown
timestamp: December 30, 2015, 01:06:24 pm
content: 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.

author: Richard Crowley
author_rank: Super Contributor
timestamp: December 30, 2015, 02:13:05 pm
content: 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

author: Simon
author_rank: Unknown
timestamp: December 30, 2015, 02:25:49 pm
content: 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.

author: Simon
author_rank: Unknown
timestamp: December 30, 2015, 02:42:23 pm
content: most of your circuits overlook the fact that the comparator will not work with 10mV

author: Richard Crowley
author_rank: Super Contributor
timestamp: December 30, 2015, 03:21:00 pm
content: Quote from: Simon on December 30, 2015, 02:42:23 pmSo 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.)

author: Simon
author_rank: Unknown
timestamp: December 30, 2015, 03:54:30 pm
content: 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.

author: Richard Crowley
author_rank: Super Contributor
timestamp: December 30, 2015, 03:56:58 pm
content: Quote from: Simon on December 30, 2015, 03:54:30 pmAs 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?

author: Simon
author_rank: Unknown
timestamp: December 30, 2015, 03:58:05 pm
content: Yes I was thinking that too but I'm trying not to overly confuse a newcomer to electronics.

author: Richard Crowley
author_rank: Super Contributor
timestamp: December 30, 2015, 04:02:49 pm
content: Quote from: Simon on December 30, 2015, 03:58:05 pmGood point. But one of the things that I like about this forum is the balanced coverage of theoretical AND practical aspects of circuit design in the Real World.

author: Simon
author_rank: Unknown
timestamp: December 30, 2015, 04:06:16 pm
content: Well maybe the OP can tell us what solution he is leanig towards. I also expect that diode voltage drops are not guaranteed to be the same from diode to diode but that should not matter if this is a one off.

author: Simon
author_rank: Unknown
timestamp: December 30, 2015, 04:08:14 pm
content: I diodes are used to raise the ground a "load" resistor needs connecting to it as the voltage on it will also change with the current going through it which at the low current the sensor uses can also be unpredictable. So say 5mA is passed through it the voltage drop will be mosre stable than with a few hundred uA and a varying temperature.

author: J4e8a16n
author_rank: Regular Contributor
timestamp: December 30, 2015, 04:28:09 pm
content: Quote from: Simon on December 30, 2015, 04:06:16 pmThe OP (if it is me) is leaning, if he can't succeed in a virtual ground, towards a diode.JPD

author: alsetalokin4017
author_rank: Super Contributor
timestamp: December 30, 2015, 04:50:01 pm
content: From the Texas Instruments data sheet for the LM35:Discuss.

author: Richard Crowley
author_rank: Super Contributor
timestamp: December 30, 2015, 05:03:29 pm
content: Quote from: alsetalokin4017 on December 30, 2015, 04:50:01 pmYes, either of those schemes would seem appropriate for @J4e8a16n's application.(OP = Original Poster = J4e8a16n)

author: Richard Crowley
author_rank: Super Contributor
timestamp: December 30, 2015, 05:06:23 pm
content: Here is a case of the Celsius scale being less practical than Farenheit. At least for measurement around the freezing point of water. Unless you have a more complex split/dual power supply (like +/- 5V, etc) working with signals very close to zero/ground becomes rather problematic.

author: poorchava
author_rank: Super Contributor
timestamp: December 30, 2015, 05:14:21 pm
content: I would use a cheap RRIO opamp like MCP6002 to amplify the voltage x20 or so and then use the other half of the opamp as a comparator (with some hysteresis added) with TL431 as reference.Sent from my HTC One M8s using Tapatalk.

author: Simon
author_rank: Unknown
timestamp: December 30, 2015, 05:27:01 pm
content: Use 1 or two diodes and connect a resistor from the diodes to Vcc such that 5-10mA flow, don't use a schotky diode, you will of course need a thermometer to calibrate the setup and it may not be repeatable on other identical units you build.As i said before waiting until 1 degrees before heating may be too late as you have to consider that your heater may take time to overcome the temperature drop and if it's not powerful enough the water will freeze anyway.In air conditioning we don't like the cold heat exchanger to freeze as it stops the air flowing, but we don't wait for the cooler to get to 1C we cut it off at 3C

author: J4e8a16n
author_rank: Regular Contributor
timestamp: December 30, 2015, 08:22:46 pm
content: I will take youre advices.JPD

author: J4e8a16n
author_rank: Regular Contributor
timestamp: January 05, 2016, 04:05:03 pm
content: Hi,In LTSpice the comparator lm311 does not switch from 0 to 6 volts.Thanks for following.JPD

author: Simon
author_rank: Unknown
timestamp: January 05, 2016, 05:42:40 pm
content: The negative of U2 need tonnecting to the negative supply (real ground) and the negative of the sensor to the virtual ground. Of course the potential devider will also need taking to the negative supply (real ground) as you are so close to the virtual ground with your desired trigger point youmight as well connect two resistors of equal value, one from pthe potitive rail and one to the negative rail and then put a trimmer in between them of cuitable value so that the wiper can select the trigger point and have a good amount of resolution in the area that interests you.

author: Richard Crowley
author_rank: Super Contributor
timestamp: January 05, 2016, 07:09:58 pm
content: Quote from: J4e8a16n on January 05, 2016, 04:05:03 pmPerhaps you missed this....QuoteIf R6 is really 1 ohm. Then the output can never go low. R6 is simply wrong.

author: Simon
author_rank: Unknown
timestamp: January 05, 2016, 07:14:13 pm
content: ah yes you probably need a transistor, that way you don't try to use a tiny pull up resistor that the comparator can't cope with.

author: J4e8a16n
author_rank: Regular Contributor
timestamp: January 05, 2016, 08:23:46 pm
content: Like that?

author: Simon
author_rank: Unknown
timestamp: January 05, 2016, 08:34:22 pm
content: The transistor is wrong, connect a 4.7K resistor from Vcc to the op amp output, connect an NPN transistor, not a PNP, base to comparator output, emitter to negative collector to the relay, the other end of the relay to Vcc

author: J4e8a16n
author_rank: Regular Contributor
timestamp: January 05, 2016, 09:23:21 pm
content: Does not go to zero.

author: Richard Crowley
author_rank: Super Contributor
timestamp: January 05, 2016, 09:33:43 pm
content: What is "U3" for?What does "U4" do? It appears to be connected to nothing.Where are the other two pins from U1 (ground and power)?The reference voltage for your comparator (U2) is ~ 5.8V.It seems highly unlikely that you will ever get 5.8V out of the LM35 (U1)So we would not expect the comparator (U2) to ever change state.Not clear what R1 is for?I would be looking at the output from U2 directly before attempting to add any downstream circuit elements.

author: Simon
author_rank: Unknown
timestamp: January 05, 2016, 09:42:10 pm
content: The best thing is to make this on a breadboard for real

author: J4e8a16n
author_rank: Regular Contributor
timestamp: January 05, 2016, 10:09:20 pm
content: They did not have a model with + and - but the lm35 will produce the voltage from .temp -55 to +85 degrees or so.U4 works. It will provide 3.7 volts (in the real circuit) for the lm35.I thought U2 would switch from 0 to Vcc when the + input would be lower than the - input. It is to say 30 mV. The plus input does go lower than the minus input.

author: Richard Crowley
author_rank: Super Contributor
timestamp: January 05, 2016, 10:56:53 pm
content: U4 will output 6.00V with the circuit you show. How do you get "3.7V"?If V5 is assumed to be providing exactly 12.00V, then R2 and R8 exactly divide that in half to make 6.00VWe don't know exactly how you are using the 6V output from U4?We have no clue how you have either ground or power for the LM35 connected because nothing is shown in the diagram.I still don't know what U3 is for, or what U4 is for either?If V5 is assumed to be providing exactly 12.00V, then the voltage divider R4/R5 is creating 5.81VUnless the output from the LM35 can go above and below 5.81V, U2 will never change state.I don't see how the LM35 can possibly go anywhere near 5.81V.But then I don't know where ground and power are connected.

author: J4e8a16n
author_rank: Regular Contributor
timestamp: January 06, 2016, 12:00:33 am
content: The dotted lines shows (in the previous post.) where the lm35 will be connected in a real circuit. The resistor is not shown.The LM35 requires 4mA. If ther is a resistor between it and Vcc it will make a voltage divider. Be it 5.8 or 6volts does not matter.In the image nothing say that the comparator inputs has to be higher than the output. It can be two sources of voltage, no? One to 7, 8 volts and the ouput can be connected to 12 volts going to the same ground as the 7, 8 volts? No?I will do a real circuit....JPD

author: Richard Crowley
author_rank: Super Contributor
timestamp: January 06, 2016, 02:55:35 am
content: Quote from: J4e8a16n on January 06, 2016, 12:00:33 amSorry, a terrible idea. A couple of diodes in series is a much more sensible and reliable solution.QuoteNo That is not the way to use the LM35 (or any other IC, for that matter)!Did you miss Figure. 18 in Reply # 30? If you want to offset the output of the LM35 up into a usable range you put something to offset the output UP from zero/ground. Putting anything in series with the supply pin does the OPPOSITE of what you need!QuoteSorry, I don't know what that means?QuoteThe comparator simply compares the two inputs. It switches one way or the other depending on whether one pin is higher than the other or vice-versa.If the + input sees 6V, then the comparator will turn OFF when the - input goes higher than 6V, and it will turn ON when the - input goes lower than 6V.Connect the power input of the LM35 directly to the power bus (12V or whatever).Then connect the ground pin of the LM35 through a couple of diodes to ground (as shown in Fig. 18 in Reply #30)The diodes will OFFSET the LM35 voltage UP to a more manageable signal level of around 1.4V at zero degrees Celsius.Then use a pot (like your "U3") to adjust the setpoint/threshold voltage into the + pin of the comparator.You can set the output of the pot at around 1.5V so that your compartor switches the relay ON just above freezing.