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 Forum index » DIY Hardware and Software
LM358 not working as expected
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NuttyMonk



Joined: Jun 30, 2020
Posts: 57
Location: UK

PostPosted: Mon May 03, 2021 9:42 am    Post subject: LM358 not working as expected Reply with quote  Mark this post and the followings unread

Hi all,

i am trying to use the LM358 as a comparator using a 5V single-sided supply and i expected to get 5V out on the upper part of the comparison but i'm only get about 3.8V. The reason i got them was due to a suggestion here on EM. I expected them to be able to go up to the supply rail voltage but that isn't happening.

Can anyone advise on an opamp which will do what i want?

When u search on a local electronics supplier
https://uk.rs-online.com/
one of the stats that you can search with in their opamps section is "Rail-To-Rail", yes or no. Is this the stat i should be paying attention to when looking for an opamp to go to the full rail voltage for comparators?

Cheers

NM
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dk



Joined: Feb 12, 2019
Posts: 108
Location: Europe

PostPosted: Thu May 06, 2021 11:38 pm    Post subject: Reply with quote  Mark this post and the followings unread

If you won't be using the chip for anything else, why not use a comparator chip? Something in the LM339 family perhaps?
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NuttyMonk



Joined: Jun 30, 2020
Posts: 57
Location: UK

PostPosted: Fri May 07, 2021 7:20 am    Post subject: Reply with quote  Mark this post and the followings unread

Yeah, i've been looking at comparators since i posted this. They seem simple enough to use. I've ordered some LM311's to try out since i see them a lot on schematics.

Thanks

NM
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PHOBoS



Joined: Jan 14, 2010
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PostPosted: Fri May 07, 2021 9:13 am    Post subject: Re: LM358 not working as expected Reply with quote  Mark this post and the followings unread

In this case a comparator would indeed be useful, or you could place a logic gate after the opamp if you happen to have an unused one in the circuit.

NuttyMonk wrote:
i am trying to use the LM358 as a comparator using a 5V single-sided supply and i expected to get 5V out on the upper part of the comparison but i'm only get about 3.8V. The reason i got them was due to a suggestion here on EM. I expected them to be able to go up to the supply rail voltage but that isn't happening.

LM358 is useful for single supplies as its output can get pretty close to 0V but as you already found out it doesn't go up to the positive rail.
Fo that you would indeed need a rail-to-rail amp though those don't always work perfect either. Something like a CA3140 could work
but there are probably better options available.

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