From: Justin Richards <justin.richards@gmail.com>
To: Microcontroller discussion list - Public. <piclist@mit.edu>
Sender: "piclist-bounces@mit.edu" <piclist-bounces@mit.edu>
Date: Sat, 12 Sep 2020 01:57:14 -0700
Subject: [EE]: OpAmp Max differential input voltage GL358
Thread-Topic: [EE]: OpAmp Max differential input voltage GL358
Thread-Index: AdaI5BBF27bGm4zDSryXjqT8pYkMhg==
Message-ID:
 <CAKMvr86vaQr_rq8hPMw0UtY5nrdp-dM_oHYEwj1R9HPh9ZNE0w@mail.gmail.com>
Reply-To: Microcontroller discussion list - Public. <piclist@mit.edu>
Accept-Language: en-US
X-MS-Exchange-Organization-SenderIdResult: Pass
X-MS-Exchange-Organization-PRD: mit.edu
X-MS-TNEF-Correlator:
received-spf: Pass (protection.outlook.com: domain of gmail.com designates
	209.85.216.45 as permitted sender) receiver=protection.outlook.com;
	client-ip=209.85.216.45; helo=mail-pj1-f45.google.com;
dkim-signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com;
 s=20161025;	h=mime-version:from:date:message-id:subject:to;
	bh=lUJLHRZM1ts1FK5ro3org2HuWqmjympQdsx8kQdaWWs=;
	b=Q4FQHCqCWkW/LLLfZNi/nNX/UVPQXfY7hQjwEZ1Kh14OHHl6GZInuMEsFNSYCPWNuq
	97WlBtko4Dq7PqIkyrgbIbCXp2bSA6yVAqGCz5muHcyIRlmMrJnMkFkg6pe1kL+IRanC
	962UZXL07TxhsyXNwGpENEQGPkd1grRhA4Epa0mjTm2z93roxcMZKTKbrnPbQdl7xNAR
	S2JbrqBarrMpVOQum0thhEinHM7uY94KADzFVFIrOw7teOi9oH21aANlTcN7ObtnfFJd
	JFalEZjToOAzOcdrZ7Vsf/1MTE49HW6Lg2flV5wgW82GP1BHNGDEUw9j6Y1YUvHgK4ae	DJ3g==
authentication-results: spf=pass (sender IP is 209.85.216.45)
	smtp.mailfrom=gmail.com; mit.edu; dkim=pass (signature was verified)
	header.d=gmail.com; mit.edu;	dmarc=pass action=none header.from=gmail.com; 
errors-to: piclist-bounces@mit.edu
list-id: "Microcontroller discussion list - Public." <piclist.mit.edu>
list-post: <mailto:piclist@mit.edu>
x-beenthere: piclist@mit.edu
x-mailman-version: 2.1.6
x-received: by 2002:a17:90b:707:: with SMTP id
 s7mr3264760pjz.25.1599901049342;	Sat, 12 Sep 2020 01:57:29 -0700 (PDT)
x-topics: [EE]
x-content-filtered-by: Mailman/MimeDel 2.1.6
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: quoted-printable
MIME-Version: 1.0

Experimenting with using a ESP8266 as a soldering iron PID controller
driving the ADC with the output of an opamp.

The GL358 opamp datasheet specifies max differential input voltage as 32V.

I would like to know if that is as it sounds and I can force the inputs to
have a max of 32v across them with a single supply to the Opamp of +5V.  I
am still learning about opamps and thought the inputs were to be considered
as a virtual ground and they will effectively be at the same potential but
here i would be brute forcing a voltage across them and concerned it may be
damaging and bad practice.

The GL358 (was in the junk box) is configured as a dual amp to amplify the
thermocouple (integral part of soldering tip) output of approx 2mV - 8mV
(~30oC - 400oC)  .to approx 40mV - 1500mV.

One side of the thermocouple is grounded and connected via a resistor to
the inverting input while the other side is connected via another
resistor to the non-inverting input.

I plan to measure temperature then apply 24V across the thermocouple. which
will drive 24V across the two 1k ohm opamp input resistors.

As a caution I plan to have back to back diodes at the inputs to limit the
voltage across them to approx 0.7v.

But back to the original question, are the protection diodes needed given a
maximum differential voltage of 32 volts or is that only if the supply is
also at 32V.

I found a design that disconnects the thermocouple from the opamp during
heating but not sure if that is actually required and obviously simplifies
the design if not.

Regards
Justin
--=20
http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.