Temperature Sensing - Page 1
Forum section: electronics / beginners

author: xoom
author_rank: Regular Contributor
timestamp: April 01, 2018, 10:36:54 am
content: Hi, im trying to understand how this design works of this temperature measurement.. i reverse engineered schematic from it to attempt to fix it as it now shows error that there is no sensor connected.. but sensor is connected and opamp gives output to ADC of ATMega8.. but what purpose is of those Mosfets..? is it like checking some kind of reference?Also there is 270Ohm divider across 5V rail that "eats" 18mA current .. and tiny SO8 78L05 voltage regulator is always hot .. is it bad design or what)Shematic:Product is:

author: picandmix
author_rank: Frequent Contributor
timestamp: April 01, 2018, 03:43:52 pm
content: Hi,The op amp is receiving set voltages from the two voltage dividers, but the positive is receiving additional variable voltage from the NTC.Looks like that can be switched in or out by the mosfet and then one of the other two mosfets switch on and apply their set voltage, eg frost stat or night temp.You want to measure the output voltage at point ADC and see what the voltage is when each mosfet is turned on, one at a time.Assuming they are true logic level mosfets, " from MCU I/O" then you can apply +5v to their gate via a 1k approx resistor..If you ca,n check that the MCU is providing a voltage to the mosfet gate.You need to show the 2700ohm divider , and how hot is the 7805, hold you finger on it for how many seconds ? just pcb copper for the heatsink ?

author: xoom
author_rank: Regular Contributor
timestamp: April 02, 2018, 10:58:10 am
content: Those mosfets are logic level.. they are connected without resistors..that divider is +5V - 200ohm - | - 75ohm - GND.. heatsink is only copper.. and its already brownish from other sidebut i guess its something wrong with Atmega.. (i seen there was MOV exploded) but it was replaced before me.. (with wrong part (capacitor) ).. tryed to power up desoldered atmega so see how much current it takes.. and i think it was bit excesive.. (12mA) nothing connected to it.. datasheet say 3.6mA in active mode.. BTW.. still fixed that temeperature sensing..and it was strange fix.. i was connected it to lab power supply and was decreasing voltage until it brown-out.. then increased slowly but 7seg display was not showing anything.. then reconnect power and it works (showing temperature instead of "E 1")have no explaination how it fixes like that) but it works now.. only problem chat excesive current draw.. ~120mA with 2 relays powered.. ant that small transformes is rated only for 120mA.. will see how long it works

author: picandmix
author_rank: Frequent Contributor
timestamp: April 02, 2018, 04:56:33 pm
content: Hi,Afraid its a bit hard to follow as you know what everything is but we don't.First the 78L05 only outputs 100ma max so it its running at 120ma you have a serious short /problem and the regulator probably almost fried.What you say but is unknown to us -Your pot divider was 2000 /75 in the first post but now its 200/75, so which is it, plus, key, what is the divider feeding ?Your diagram shows the op amp running on 12V , is that so, or just part of your schematic package.The relays, whats powering them 5v or 12v and what type are they / what is their current needs ?A full diagram would be helpful....

author: xoom
author_rank: Regular Contributor
timestamp: April 02, 2018, 06:04:28 pm
content: Quote5V side "eats" like 70mA.. 120mA is total current on 12V rail.QuoteIn first post it also say 200Ohmmaybe missread as it O letter close to 0(zero)divider schematic i draw in picture its only go there..QuoteYes AMP is running on on 12V (unregulated) directly from rectifier (that same goes to 5V regulator) and 5V feeds atmega and display + what is shown in schematic..Relays is driven with 12V..QuoteYeah but i dont have itbut there is nothing specialtransformer and rectifier that feeds 7805,LM258, Relays..Sorry for unclear postsit just mystery now how it started working just by manipulating with external voltage..

author: xoom
author_rank: Regular Contributor
timestamp: April 02, 2018, 06:23:37 pm
content: sorry for Paint made schematics but maybe now more clear

author: picandmix
author_rank: Frequent Contributor
timestamp: April 02, 2018, 08:07:07 pm
content: Hi,Sorry, my bad, eyesightthough I'm not in to op amps, those values look wrong 200/75.You say its been repaired before, suspect the wrong parts put in ?Think I would try replacing them with just 10k/3k3 like the other input, cannot harm things and that should bring the current down.That might then give you chance to check the mosfets one at a time while monitoring to op amp output.Note that you need to either have +5v or 0v on the mosfets gates, no not leave them floating.

author: Seekonk
author_rank: Super Contributor
timestamp: April 02, 2018, 08:12:33 pm
content: I've seen designs that use that heat to allow electronics to work at low temperatures and reduce condensation. One design actually placed a resistor over the chip with silicone to allow operation in sub zero temps. This looks like it could be installed in a cold room.

author: xoom
author_rank: Regular Contributor
timestamp: April 03, 2018, 08:24:24 am
content: Quote from: Seekonk on April 02, 2018, 08:12:33 pmhmm.. that makes some sense.. its for milk mixing and cooling.. its installed in farm in high humidity environment... but its not very close to Atmega.. its closer to opamp..

author: mikerj
author_rank: Super Contributor
timestamp: April 03, 2018, 01:04:18 pm
content: I suspect the OP has made a mistake with the positioning of the 100k/16k resistors. As drawn the circuit has very low gain since the series combination of 100k+16k+Thermistor are driving into the ~2.5k impedance of the 10k||3.3k divider. This gives a tiny voltage output from the opamp; over a 200k to 100 ohm range of the thermistor (likely far more than would be seen in practice) the output only swings from around 200mV to 500mV.