Vout too high on 2 out of 3 MCP9701 temperature sensors - Page 1
Forum section: electronics / beginners

author: pyroesp
author_rank: Regular Contributor
timestamp: February 16, 2020, 02:24:04 am
content: I bought three MCP9701 temperature sensors in SOT23-3 package made by microchip to use for doing some PWM controlled fan speed.The analog output of the temp sensor is the following equation:Vout = Tc x Ta + V0Tc = Temp Coeff = 19.5mV/°CTa = Ambient tempV0 = Vout at 0°C = 400mVSo for an ambient temperature of around 25°C I should get a Vout of around 800mV.The first one I tried gave an output of around 2.3V when powered at5V.The 2nd one has an output of 2.9V, also powered from 5V.I'm not sure what's going...I've soldered these onto adapter boards so that I can put them on a breadboard for testing.I did not put my soldering iron on the pins longer than I usually do, which isn't long, so I didn't fry them. Soldering these went without issues.I've tried adding a pull-down but that didn't change much to the output.It's a 3 pin chip, I checked and rechecked the connections.There is no circuit, I just connected GND and Vdd and measured the output with a multimeter and scope.I cannot believe Murphy got me twice in a row with 2 dead temperature sensors out of the tape.I haven't tried the last one yet.Hoping someone can help.

author: DrG
author_rank: Super Contributor
timestamp: February 17, 2020, 03:59:37 am
content: Quote from: pyroesp on February 16, 2020, 02:24:04 amWish I had some magic bullet for ya, but I don't. I looked through the datasheet and there is no reasonable explanation for your results that I can see...other than:1. The chips are bad or not what you think they are. Were they purchased from a reputable source? Do they have markings consistent with Microchip's markings as per the data sheet?2. There  a connection problem. You checked, I know. Maybe check while assuming nothing at all. Including whether the carrier board is wired as you thing, your breadboard is not messed up, your supply really is 5V - all that stuff.There is no way that I can see for a Vo of 2.3V unless your temp is ~97 C [(2300-400) / 19.5]. No amount of error or need for calibration or self-heating could give you that in my view.I'm curious how this works out so please let us know.edit: does Vo change at all if you hit it with a hair dryer or heat gun or otherwise raise the temperature?

author: pyroesp
author_rank: Regular Contributor
timestamp: February 17, 2020, 10:07:23 am
content: 1. I bought then from digikey a few months ago. I didn't really pay that much attention to the markings but from what I remember it did look like they are correct.2. I will check the adapter boards again, but I've use the same one for an attiny10 which works fine. Supply is coming straight from the AVR programmer I have (USBasp), so it's the 5VDC from the USB on my PC.I wish there was an internal diagram to see if I can measure something in diode or ohms mode from one pin to another but there isn't one.3. The first one I tested that gave 2.3VDC did increase a bit when I heated it up with the soldering iron for half a sec, but I'll try again after work. I did not test the 2nd one.I might just solder the last one with my hot air station and see what that one is measuring.If the last one doesn't work I might just do my own temp sensor with a 1N4148.

author: pyroesp
author_rank: Regular Contributor
timestamp: February 26, 2020, 06:26:01 pm
content: Sorry for the delay, I've finally managed to test this again and I got them all to work.I've soldered the 3rd one on an adapter board and it behaved similarly.My 5V supply is straight from my PC USB. Measuring the Vpp on the USB voltage on my old 54645D it seems like there's 1Vpp on there, which seems like a lot.I can't check with my Rigol DS1052E as it's currently disassembled, so I don't know if it's truly that bad or if the 54645D is out of spec... probably both.Anyways, to make it work I just needed to add the smallest amount of capacitance over the MCP9701 Vdd and GND. The lowest I had on hand was a 220nF, but I tried with a 220uF first.What I'm not sure is why it doesn't work without the cap. The datasheet shows it working from 3V to 5.5V.Noise is obviously bad but I would expect the thing to work, albeit with lots of noise on its output?PS: The typical application in the datasheet doesn't show a capacitor either.

author: Zero999
author_rank: Super Contributor
timestamp: February 26, 2020, 07:06:13 pm
content: I'm not surprised it didn't work, without the decoupling capacitor. Proper decoupling is essential and is mentioned on the data sheet.Quotehttp://ww1.microchip.com/downloads/en/DeviceDoc/20001942G.pdf

author: pyroesp
author_rank: Regular Contributor
timestamp: February 26, 2020, 08:05:08 pm
content: I always seem to miss something in the datasheet ...Thanks for pointing the chapter out Zero999

author: Zero999
author_rank: Super Contributor
timestamp: February 27, 2020, 10:01:16 pm
content: Quote from: pyroesp on February 26, 2020, 08:05:08 pmGood supply decoupling is required for most circuits. There are occasions when it isn't necessary, but as a general rule, put a 100nF capacitor across the supply rails of every IC and in places where there's going to be rapid switching, such as next to a transistor switching a load, such as a relay or buzzer.https://en.wikipedia.org/wiki/Decoupling_capacitorhttp://www.capacitorguide.com/coupling-and-decoupling/https://learn.sparkfun.com/tutorials/capacitors/application-examples