Is this circuit safe for a J-fet Op-amp ? - Page 1
Forum section: electronics / projects

author: pedroteck
author_rank: Contributor
timestamp: January 17, 2015, 07:28:11 pm
content: I had the idea to make this circuit just to check out some Opamps i have ( tl061cn - lm741) .The output is supposed to go low when the (+) input is bellow (-) input voltage and go high when (+) equals (-) input .Some opamps are eather low or hi at the output other do just as the circuits says but i need to know if the circuit is safe though .Here is the schematic :

author: wiss
author_rank: Frequent Contributor
timestamp: January 17, 2015, 08:03:13 pm
content: Most opamps do not behave when input is close to either rail. You do need input rail-to-rail opamps for this circuit to work. Neither 061 nor 741 are that.

author: pedroteck
author_rank: Contributor
timestamp: January 17, 2015, 08:42:00 pm
content: Quote from: wiss on January 17, 2015, 08:03:13 pmOk man thanks , but at least is this circuit safe , even if it does not work ?

author: paulie
author_rank: Frequent Contributor
timestamp: January 17, 2015, 09:17:08 pm
content: No, it is not safe. At least 1500 deaths last year attributed to incorrect wiring of LM741. And that was just the USA hobby market.

author: wiss
author_rank: Frequent Contributor
timestamp: January 17, 2015, 09:40:08 pm
content: Quote from: paulie on January 17, 2015, 09:17:08 pmBut that is only for the chinese j-fet 741! The bipolar 741 would be safe...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 17, 2015, 09:44:01 pm
content: Thousands of electrical engineers go to sleep, crying, every night, knowing that the 741 is still in use. Many of them have families. Please, for only $0.30 a day, you can save them. Think of the children!(Insert picture of sad baby)Tim

author: ajb
author_rank: Super Contributor
timestamp: January 17, 2015, 09:50:59 pm
content: Some Op Amps have a pair of antiparallel diodes between the inputs to limit differential input voltage. If you were to use such an op amp this circuit will cause excessive current to flow through those diodes when the pot is at either end of its range. A resistor between the wiper of the pot and the non inverting input will prevent this.

author: paulie
author_rank: Frequent Contributor
timestamp: January 17, 2015, 09:55:05 pm
content: Quote from: wiss on January 17, 2015, 09:40:08 pmLOL. Good one. Except by definition there is no such thing as FET 741.It's unfortunate lonely kids or those with learning deficit abuse forums with dumb questions. IMO best thing you can do is nudge with a little sarcasm. Then there would be a lot fewer threads with zero replies and REAL noobs with REAL questions might get more help. In this case a quick ggogle for "lm741" shows 300k hits with half the diagrams intending to do what yours was probably intended to do. Except most of them are competent designs.BTW contrary to what the other guys say in this case nothing at all to do with R-R, input specs, or diode. I.E. if you forget the offset pins and swap neg and gnd your circuit will work fine and make an excellent step one opamp learning tool. Even better to put a pot on the other input too.That safety issue is just plain stupid.

author: pedroteck
author_rank: Contributor
timestamp: January 17, 2015, 10:38:27 pm
content: Quote from: paulie on January 17, 2015, 09:55:05 pmHi mate .Just to be clear i just want to know if the design i was using before has a chance to destroy the opamps .

author: c4757p
author_rank: Super Contributor
timestamp: January 17, 2015, 10:42:40 pm
content: Quote from: pedroteck on January 17, 2015, 10:38:27 pmAnd it's a perfectly valid question. Some op ampscanbe destroyed by using them as comparators, because of the diodes that ajb mentioned.

author: atferrari
author_rank: Frequent Contributor
timestamp: January 17, 2015, 11:07:48 pm
content: Quote from: paulie on January 17, 2015, 09:55:05 pmWhat is a REAL noob? Is the OP a dumb question? Is he abusing this forum? Sorry but I do not understand your post.I always had the feeling that when you know what to ask and how to ask it, probably you do not need to ask.Nothing is more painful that sarcasm coming from those you rely on.Nudge? Bah...Google for it? I can say that too.

author: wiss
author_rank: Frequent Contributor
timestamp: January 17, 2015, 11:17:33 pm
content: Thread is not in beginner, wrong subforum...Hell I made these mistakes 25 years ago when I was a lonely noob (internet? seriously?).A 741 is cheap enough, hell, get 50 and and burn away!

author: Mechatrommer
author_rank: Super Contributor
timestamp: January 18, 2015, 12:20:18 am
content: Making an opamp as comparator will not destroy it. What destroy it is when you exceed the limit or not following rules as stated in the datasheet. So please read the datasheet.

author: jaxbird
author_rank: Frequent Contributor
timestamp: January 18, 2015, 06:47:37 pm
content: I was told for the first time more than 20 years ago to not use 741s as there were better and equally cheap/cheaper alternatives available, can't believe anyone still using them.Anyway, sure OP has already been told (haven't read all replies) but a resistor between pot and opamp input would be a really good idea. Something like 1k to 10k depending on voltage applied. Then have fun playing comparator with your opamps. I do believe the outputs are current limited/short circuit protected as in high output impedance.

author: Zero999
author_rank: Super Contributor
timestamp: January 18, 2015, 06:51:53 pm
content: Look at the datasheets for the uA741 and the TL061.http://www.ti.com/lit/ds/symlink/ua741.pdfhttp://www.ti.com/lit/ds/symlink/tl064.pdfThat should tell you that the ICs can be used as comparators, although they'll perform poorly.A better option is the LM311 or LM393 which are designed for the purpose.

author: pedroteck
author_rank: Contributor
timestamp: January 18, 2015, 09:31:06 pm
content: Quote from: jaxbird on January 18, 2015, 06:47:37 pmThanks for all the replies .I must admit i have no idea how OpAmps work , i should dedicate some time to learn about them .As for the test circuit i have made this circuit and i have checked all the OpAmps i got .

author: wiss
author_rank: Frequent Contributor
timestamp: January 18, 2015, 09:38:54 pm
content: Some opamps can have diodes anti-parallel between the inputs, check "max differential input voltage" in the datasheet. In this circuit you can get more than 0.6 V between +in and -in.

author: paulie
author_rank: Frequent Contributor
timestamp: January 18, 2015, 09:49:09 pm
content: Quote from: pedroteck on January 18, 2015, 09:31:06 pmWhat part numbers? And how did they work out for you?LOL. This thread is hysterically amusing. Every response has virtually nothing to do with the LM741 originally mentioned. Just a confusing mishmash of unrelated issues. Except of course the one from me at the beginning which handed the solution on a silver platter.And yes, it might be a good idea to learn how these work. Specially from places like Wikipedia that start at kindergarten level. Only drawback less opportunity to chat.

author: pedroteck
author_rank: Contributor
timestamp: January 18, 2015, 09:59:25 pm
content: Quote from: wiss on January 18, 2015, 09:38:54 pmIn this circuit i got a max of 8 v on positive input and a max of 6v on the negative input letting 2v for the led .Quote from: paulie on January 18, 2015, 09:49:09 pmLOL ,not only the lm741 but also tl061cn.Read well

author: wiss
author_rank: Frequent Contributor
timestamp: January 18, 2015, 10:05:10 pm
content: No! If the opamp is working, the difference will be small ( Vos in datasheet), if the difference is that large the opamp is way overdriven.

author: pedroteck
author_rank: Contributor
timestamp: January 18, 2015, 10:24:27 pm
content: Quote from: wiss on January 18, 2015, 10:05:10 pmReally weird . But the led light is varying just as the circuit is supposed to do .

author: Mechatrommer
author_rank: Super Contributor
timestamp: January 18, 2015, 10:57:34 pm
content: Quote from: pedroteck on January 18, 2015, 10:24:27 pm"varying"? Or "blinking"? Because fast blinking can give the "varying" effect. Yes one is not suppose to config opamp as comparator.the normal way is with feedback hence maintaining -ve and +ve input equal.otherwise if no feedback loop,it will be overriden yes,but if the max diff inp v is not exceeded as in dsheet,we can assume it will not be destroyed and function as is (poor comparator).

author: dannyf
author_rank: Super Contributor
timestamp: January 18, 2015, 11:07:00 pm
content: QuoteOnly in the extremes.

author: pedroteck
author_rank: Contributor
timestamp: January 19, 2015, 01:41:04 am
content: Quote from: Mechatrommer on January 18, 2015, 10:57:34 pminteresting , but how could this led be blinking ?

author: Mechatrommer
author_rank: Super Contributor
timestamp: January 19, 2015, 05:40:02 am
content: When comparator kepp turning on n off at fast rate.i dont look at ur circuit,just questioning possibility,you should check it with ur scope.

author: pedroteck
author_rank: Contributor
timestamp: January 19, 2015, 12:47:59 pm
content: Quote from: Mechatrommer on January 19, 2015, 05:40:02 amThere is noswitcher in this circuit. Qnd its current controle design for a led.

author: Alex30
author_rank: Regular Contributor
timestamp: January 19, 2015, 01:19:45 pm
content: Seriously guys the smart alec comments don't help anyone. This thread should belong in the beginners section but being told your question is stupid can be really off putting for newbies. Not all of us have a university degree in electrical engineering or any engineering experience at all you know.OP, what exactly is it you want to check out? Yes a comparator is something you can make with an opamp but as others have said, not all opamps should be used for this because they will oscillate. If you want to learn about opamps try circuits specifically for opamp applications, such as differential amplifiers or negative feedback gain etc. If you have a scope and a cheap function gen (even a couple of dollars on ebay can get you a cheap function generator) try hooking up the opamp to provide gain on various signals and have a go at using the opamp as an integrator etc. As you learn have a go at understanding the calculations and opamp properties such as output swing.Most opamps don't like it if your input voltage is close or higher than the positive input rail, also do not make your input any less than the negative rail. For example if you have the negative rail at ground, do not provide the opamp with -2V etc. Some are better than others, but all this stuff should be in the datasheet if you hunt for it.Have fun!

author: dannyf
author_rank: Super Contributor
timestamp: January 19, 2015, 01:32:26 pm
content: QuoteBy adjusting the pot, you can control the circuit through the led, turning it on and off -> ie. blinking.What OP had created is a voltage-controlled current source.

author: paulie
author_rank: Frequent Contributor
timestamp: January 19, 2015, 10:56:29 pm
content: Quote from: Alex30 on January 19, 2015, 01:19:45 pmThen IMO they are in the wrong hobby and no business on a forum.We will have to agree to disagree on this. My observations indicate tough love usually proves more effective than coddling. In fact IIRC the communists during Maos reign demonstrated gentle coaxing with electro-reinforcement most effective of all.

author: kg4arn
author_rank: Unknown
timestamp: January 20, 2015, 06:30:25 am
content: "LOL. Good one. Except by definition there is no such thing as FET 741."Actually, I have some of these. From the datasheet: "The LF13741 is a 741 with BI-FET input followers..."

author: Alex30
author_rank: Regular Contributor
timestamp: January 20, 2015, 10:57:27 am
content: Quote from: paulie on January 19, 2015, 10:56:29 pmBrilliant hahaha. Can't argue with hard evidence haha

author: pedroteck
author_rank: Contributor
timestamp: January 20, 2015, 11:26:08 pm
content: Quote from: Alex30 on January 19, 2015, 01:19:45 pmAll i wanted to check is weather the opamps i have are working or not .

author: rob77
author_rank: Super Contributor
timestamp: January 20, 2015, 11:33:10 pm
content: btw.. what's wrong with 741 ?a 2+ decades old metal canned version of 741 is very nice decorative item !

author: wiss
author_rank: Frequent Contributor
timestamp: January 21, 2015, 08:50:05 am
content: Quote from: pedroteck on January 20, 2015, 11:26:08 pmapply +- supply (10 - 15 V or so, depending), 0 on +in, -in to out.measure output voltage, should be within Vos of +in.