LM258D question

Go To Last Post
13 posts / 0 new
Author
Message
#1
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Dear freaks,

 

I am using an LM258D opamp connected as a comparator, beeing supplied by single voltage 5.1Vdc. Would you be able to tell me what it's gonna be happen if the input (+) voltage is higher than the opamp supply? For ecample what will happen if the input voltage (+) will be at 30Vdc ?

 

 

Michael.

User of:
IAR Embedded Workbench C/C++ Compiler
Altium Designer

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

According to the following, as long IN(-) is between Vcc and GND, IN(+) can go to VccMAX, so +30V should be OK:

Greg Muth

Portland, OR, US

Xplained/Pro/Mini Boards mostly

 

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

icarus1 wrote:
LM258D 

What does the 'D' signify?

 

I'm not finding any datasheets which mention a 'D' suffix

 

Greg_Muth wrote:
can go to VccMAX, so +30V should be OK

The absolute max is +32V - so you could be cutting it a bit fine ... ?

 

Top Tips:

  1. How to properly post source code - see: https://www.avrfreaks.net/comment... - also how to properly include images/pictures
  2. "Garbage" characters on a serial terminal are (almost?) invariably due to wrong baud rate - see: https://learn.sparkfun.com/tutorials/serial-communication
  3. Wrong baud rate is usually due to not running at the speed you thought; check by blinking a LED to see if you get the speed you expected
  4. Difference between a crystal, and a crystal oscillatorhttps://www.avrfreaks.net/comment...
  5. When your question is resolved, mark the solution: https://www.avrfreaks.net/comment...
  6. Beginner's "Getting Started" tips: https://www.avrfreaks.net/comment...
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Andy, if my memory is correct, D is dip plastic. The first digit 2 signifies industrial. 1 is military and 3 commercial.

The OP can use a resistor divider to lower the voltage to the input of the opamp.

Last Edited: Fri. Sep 22, 2017 - 08:15 AM
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Thank you all.

 

Michael.

User of:
IAR Embedded Workbench C/C++ Compiler
Altium Designer

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

From what I read in the data sheet you must not exceed Vcc at your inputs so if Vcc is 5.1 then you cannot put 30vdc into an input. I am guessing there might be a protection diode at play here.

I've Vcc is 30v then you can apply 30v to the inputs. But if Vcc is 5.1 then you cannot

Jim

If you want a career with a known path - become an undertaker. Dead people don't sue! - Kartman

Please Read: Code-of-Conduct

Atmel Studio6.2/AS7, DipTrace, Quartus, MPLAB user

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

After Jim's reply I am completely confused !!! Where is the truth ?

 

Michael.

User of:
IAR Embedded Workbench C/C++ Compiler
Altium Designer

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

icarus1 wrote:

Where is the truth ?

 

From the datasheet...

 

Quote:

(4) The input common-mode voltage of either input signal voltage should not be allowed to go negative by more than 0.3 V (at 25°C). The upper end of the common-mode voltage range is V+ -1.5 V (at 25°C), but either or both inputs can go to 32 V without damage (26 V for LM2904), independent of the magnitude of V+.

 

So, for normal operation the input must not exceed (+ve pin - 1.5V) but no harm will happen if it goes to 32V.

 

But 30V is very close to 32V which is the absolute maximum. 

 

#1 This forum helps those that help themselves

#2 All grounds are not created equal

#3 How have you proved that your chip is running at xxMHz?

#4 "If you think you need floating point to solve the problem then you don't understand the problem. If you really do need floating point then you have a problem you do not understand." - Heater's ex-boss

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

jgmdesign wrote:
From what I read in the data sheet you must not exceed Vcc at your inputs

That's for correct op-amp operation.

 

The datasheet extract in #2 shows that you can go beyond that - up to 32V - without damage.

 

Since the OP is just using it as a comparator, that should be fine.

 

EDIT

 

So long as he also observes the other provisos in that extract

Top Tips:

  1. How to properly post source code - see: https://www.avrfreaks.net/comment... - also how to properly include images/pictures
  2. "Garbage" characters on a serial terminal are (almost?) invariably due to wrong baud rate - see: https://learn.sparkfun.com/tutorials/serial-communication
  3. Wrong baud rate is usually due to not running at the speed you thought; check by blinking a LED to see if you get the speed you expected
  4. Difference between a crystal, and a crystal oscillatorhttps://www.avrfreaks.net/comment...
  5. When your question is resolved, mark the solution: https://www.avrfreaks.net/comment...
  6. Beginner's "Getting Started" tips: https://www.avrfreaks.net/comment...
Last Edited: Fri. Sep 22, 2017 - 01:02 PM
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Yeah, but...

 

Is the op-amp going to function correctly,  as a comparator, with the 30 V input?

I don't think that is guaranteed.

 

So, are we talking about some transient voltage spikes, or about a steady state input at 30 V with which the op-amp is expected to correctly function as a comparator?

 

More info is needed.

 

One might consider clamping the 30 V to 5 V with a resistor and zener, for example, as part of the front end analog signal processing, prior to feeding that signal into the op-amp.

 

JC

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

From my interpretation of the datasheet, as long as one of the inputs is within valid range (-0.3 to VCC - 1.5V) then the output will at least have the correct phase, so it should work OK as comparator.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Yes - that's the way I read it.

Top Tips:

  1. How to properly post source code - see: https://www.avrfreaks.net/comment... - also how to properly include images/pictures
  2. "Garbage" characters on a serial terminal are (almost?) invariably due to wrong baud rate - see: https://learn.sparkfun.com/tutorials/serial-communication
  3. Wrong baud rate is usually due to not running at the speed you thought; check by blinking a LED to see if you get the speed you expected
  4. Difference between a crystal, and a crystal oscillatorhttps://www.avrfreaks.net/comment...
  5. When your question is resolved, mark the solution: https://www.avrfreaks.net/comment...
  6. Beginner's "Getting Started" tips: https://www.avrfreaks.net/comment...
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

The + input is a steady voltage of 30Vdc. I am not talking about spikes.

 

In any case I am going to put a voltage divisor before the input. In any case...

 

 

Michael.

User of:
IAR Embedded Workbench C/C++ Compiler
Altium Designer