LM258D question

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#1
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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.

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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

Atmel Studio 7.0 on Windows 10

Xplained/Pro/Mini Boards mostly

 

 

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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 ... ?

 

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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
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Thank you all.

 

Michael.

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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

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

 

Michael.

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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. 

 

'This forum helps those who help themselves.'

 

pragmatic  adjective dealing with things sensibly and realistically in a way that is based on practical rather than theoretical consideration.

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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

Last Edited: Fri. Sep 22, 2017 - 01:02 PM
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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

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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.

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Yes - that's the way I read it.

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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.

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