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Old 1st Mar 2019, 6:51 pm   #1
Martin G7MRV
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Default 741 op-amp supply question

Yes, I realise I should know this - but ive always had an odd mental blindspot for op-amps!

Im looking at using a 741 in a very simple battery monitor circuit. However, the battery is nominally 24v and potentially up to about 28v.

The datasheets of course all state the absolute maximum ratings for supply as for dual rail, so in this case +/- 22V. But - how does that equate to a single supply rail, rather than a dual rail supply? Am I still limited to 22v max or can I make use of the whole range, which in the case of the 741 would be potentially 44v?

I know there are higher voltage devices available, but, well like I suspect most experimenters ive more 741s than I have a use for!
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Old 1st Mar 2019, 7:03 pm   #2
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Default Re: 741 op-amp supply question

There is no connection for the midpoint of the supply on the 741. Yes you can tie inputs to said midpoint (conventionally called 'ground') but you don't have to.

Therefore you can have up to 44V between V+ and V-. The 741 can't know if you're calling that +22V and -22V or +44V and 0V

BUT

There will be restrictions on how near the inputs can go to the supply rails. I am pretty sure the 741 does not allow the inputs to be equal to the supply rails (unlike some other op-amps). If your circuit keeps the inputs about midway between the supply rails then it should be fine.
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Old 1st Mar 2019, 7:44 pm   #3
Martin G7MRV
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Default Re: 741 op-amp supply question

The circuit in question is this simple little beastie here http://www.reuk.co.uk/wordpress/elec...dicator-lm741/ but modified for a nominal 24v supply (my apologies for the poor loading of the website)

It looks then like a 741 will be ok, i'll be setting the thing up to switch at about 20v. The only difference is instead of an LED, I will be using an optocoupler and switching a tone circuit thats already present in the equipment I want to monitor the battery of.

I think I do have some LM358s which are single supply up to 32v, but i'll try a 741 first

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Old 1st Mar 2019, 8:04 pm   #4
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Default Re: 741 op-amp supply question

Personally I'd use a comparator IC (LM311, half an LM393, etc) for such a circuit, but...

I think I'd up the zener diode to perhaps a 12V one to keep the inputs around half the supply voltage when using a 741 here.
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Old 1st Mar 2019, 9:09 pm   #5
Martin G7MRV
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Default Re: 741 op-amp supply question

Yes, ive selected a 12v zener to try it out on the breadboard, but ive a good selection of zeners so can easily play with that and see which is best

The 741 choice is really due to having lots of them available to hand to try it with,
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Old 1st Mar 2019, 9:31 pm   #6
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Default Re: 741 op-amp supply question

The 12v zener will prob have a much worse tempco than the 5v6. Prob around 0.05% per degree C or say 0.12v for a 20 degree change. May well not matter but best to be aware.
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Old 1st Mar 2019, 9:47 pm   #7
Martin G7MRV
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Default Re: 741 op-amp supply question

I dont think the temp co is of much concern in this case - its to be used to give an indication that a vintage military PRC344 radio's battery is low, thats all,

Ive just breadboarded it and its works quite well (well, it did once I moved all the connections off of pin 5 and onto pin 4 where they should have been! )

Very compact size and very low component count are key here as theres very little room to play with.

I now have to work out where I can switch a suitable warning indication within the radio. Im limited really to pinching the tone oscillator used for remote user calling I think.
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Old 1st Mar 2019, 11:11 pm   #8
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Default Re: 741 op-amp supply question

If the battery is up to 28V it will be fine! That's even less than the traditional +15V/-15V rails. Just as others have said!

If you have bread boarded it and it's fine, then use with confidence.

The 741 has never been my op-amp of choice. Also, op-amps don't make particularly good comparators, just as comparators don't make good op-amps. There are so many others, so much better, pin-compatible, even. The 741 can't operate with its inputs too near either supply rail, and it can't pull its output to either supply rail, either. Though between these limits it's fairly well-behaved, and if you don't want fast response, and you've got a boxful, then go ahead and have fun!
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Old 1st Mar 2019, 11:46 pm   #9
Martin G7MRV
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Default Re: 741 op-amp supply question

Yes tested up to 28v and worked nicely. Set to switch at just under 20v. With the supply >20v the output is about 2v lower than supply, so around 18v up to 26v, and with supply <20v the output pin drops to 2v. This makes sense from the data that the 741 can only get to within a couple of volts of its supply rails.

This means that a 2k7 series resistor and a PC817 optocoupler (all stuff I already have in abundance) between supply and pin 6 of the 741 can neatly control the 2kHz CALL oscillator in the radio, so when the battery is too low, you get an annoying 2kHz tone in the audio gear!

I'll measure the preset resistor and replace that with a fixed potential divider, and built dead bug style with some heat shrink over it it should fit into the tiny amount of spare space in the PRC344
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Old 2nd Mar 2019, 8:27 am   #10
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Default Re: 741 op-amp supply question

Quote:
Originally Posted by TonyDuell View Post
Personally I'd use a comparator IC (LM311, half an LM393, etc) for such a circuit, but...

.
I wouldn't.

Although it is a comparator function that the circuit performs, actual copmarators are more problematic in the there can be multiple transitions of the output at the switching point due to noise. So they invariably need hysteresis added (positive feedback), though obviously if the output is nothing more than to drive an LED its probably neither here nor there.

If the comparator output goes off to logic circuits, for example, then it can be a nightmare with a real comparator without hysteresis added.

The 741 has a low slew rate (by modern standards) and makes for a great trouble free "comparator" that is virtually fool proof where the slew rate is not important and it has intrinsic RF immunity too.
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Old 2nd Mar 2019, 9:05 am   #11
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Default Re: 741 op-amp supply question

Quote:
Originally Posted by kalee20 View Post

The 741 has never been my op-amp of choice. Also, op-amps don't make particularly good comparators, just as comparators don't make good op-amps.
If you look at the LM311 though, it has an open collector output. An OP amp output stage can pull the output both ways, so it can be more convenient to use an OP amp than a comparator in some cases, depending on the circuitry the output is connected to.

I agree that the 741 by modern standard has poor features, won't swing rail to rail , very low slew rate, higher input currents and offset voltages. Oddly though, some of these "poor" features can be exactly what you need. It is a bit like power germanium transistors with a low ft. In some circuits they excel, without HF instability issues, such as the Royer DC to DC power converter.

Years ago working with some circuitry in a super noisy environment (in a car) I found that a 741 made a much better comparator than a 311, because of its apparent deficiencies. So it really, it does depend on what you plan to use the device for, and the environment it is in as to which one is "better"
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Old 2nd Mar 2019, 1:22 pm   #12
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Default Re: 741 op-amp supply question

Treat the 741 as the default starting point for thinking about opamps.

It was the first really popular one. Others came before, but had inconveniences that the 741 brushed away. The LM301A was its contemporary, maybe a little better in some respects but the external compensation capacitor was both an inconvenience and an opportunity for better optimisation.

You had to design things so that inputs and outputs stayed comfortably within the rails, but the little opamp got on with it and did the job.

It was a bit noisy and it started to run out of gain at higher audio frequencies but you could still do a lot with it.

It has been bettered in every single parameter by newer designs, but not in all by any one part, and often improvements in one direction are at the price of worsening others.

So for opamp circuits, think first of the 741, then consider if you need better performance in any area, and then ask yourself what characteristics you're prepared to let get worse to pay for it.

Horowitz and Hill is a good opamp cookbook and has useful comparison tables bridging many makes and types.

David
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Old 2nd Mar 2019, 1:33 pm   #13
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Default Re: 741 op-amp supply question

Quote:
Originally Posted by Martin G7MRV View Post
Im looking at using a 741 in a very simple battery monitor circuit. However, the battery is nominally 24v and potentially up to about 28v.
I've attached a small circuit you might like made with 3 op amps that were left over in a device that ran off a 9V battery. (just ignore the 12V shunt zener and don't use it). The IC is 3/4 of a TLC274.You can modify it for other voltages and use other op amps.

The led is on with power, and when the battery drops to 6V the LED flashes.
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Old 2nd Mar 2019, 8:27 pm   #14
Martin G7MRV
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Default Re: 741 op-amp supply question

Thanks Argus, thats interesting. My simple 741 circuit is now built and installed in my PRC344, and it works nicely. But - It is pretty much an all or nothing indication! Once the battery drops to 20v, the radios internal 2kHz Call tone oscillator is enabled, and this tone pretty much is all you can then hear!

Im already considering a refinement, to use another op-amp as a low frequency pulse generator, such that instead of a constant tone, the tone is pulsed on briefly every few seconds. Far less intrusive but still sufficient to indicate that the battery is too low.

Due to the incredibly limited space inside this radio, i'd need to use SMT for any such more complex circuit.

My prototype using the 741, along with a short video of it under test, can be seen here on my blog, if anyone is at all interested https://g7mrv.blogspot.com/2019/03/c...ery-alarm.html
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Old 2nd Mar 2019, 11:32 pm   #15
Martin G7MRV
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Default Re: 741 op-amp supply question

Ive now breadboarded a simple op-amp multivibrator (741, R=4M7, C=270nF, Vref resistors both 10k), which gives me a pulsed warning indication.

But the two 741s and associated parts, even though these amount to just 8-9 extra components, are now far too big to fit the available space.

I have little experience of selecting op-amps or comparators. Can anyone suggest a suitable dual device that would replace both 741s, and is readily and cheaply available in an SMD package? Im thinking perhaps the LM358, which can be had in SM very cheaply?

I have one DIL LM393 in stock, would this work as a replacement for the two 741s on the breadboard, just for test purposes?
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Old 2nd Mar 2019, 11:35 pm   #16
Phil G4SPZ
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Default Re: 741 op-amp supply question

Coincidentally, I've just used a 741 as the detector in a circuit with a Wheatstone bridge, designed to detect changes in a low resistance circuit as a form of track circuit detector on a preserved railway. It works extremely well and does the job perfectly.
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Old 3rd Mar 2019, 1:08 am   #17
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Default Re: 741 op-amp supply question

Here is a little secret , but not really. The mil spec 741 is a JM38510/10101.

I have a bit of a love affair with these (like 2N3055's). Ive a attached a picture of a small pcb I made with the 10101. You will see I put it in a socket with gold pins (I can barely bring myself to solder the pins on these lovely parts). Also, if you look closely, you might see that this tiny pcb has "plated through holes", even though single sided. That was done with the kit from RS.

Also, when I traced out the schematic of my Conrac Avionics video monitor (A super mil spec machine and the best monitor of all time) much to my delight I found two of these mil spec 741's in the vertical deflection stage (circuit attached). They run off +/-15V. It is the only monitor or TV like device I have with 741's in the vertical deflection stages. Of course they are perfect for the task at those low frequencies and totally deaf to radio frequencies. Also notice that the vertical oscillator is a UJT and the circuit has outstanding frame deflection linearity due to a combination of voltage and current feedback.
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Old 3rd Mar 2019, 9:35 am   #18
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Default Re: 741 op-amp supply question

Hi Martin, it is possible to get 2, 741's in an 8 DIL package, I think it is an LM 258 or equivalent.
Extend your circuit without going bigger!!

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Old 3rd Mar 2019, 11:51 am   #19
Martin G7MRV
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Default Re: 741 op-amp supply question

Just pulled up the datasheet for the LM258, and its part of the LM358 series, so looks like the 358 will do for my needs. These also have the advantage of being dirt cheap from china - £2 will get me 40 devices, 20 DIL and 20 8-SOIC, so long as im happy to wait a few weeks to complete the project! That will also bump up my stock of GP op-amps a bit!

If you have a look at the third photo down on my blog here https://g7mrv.blogspot.com/2019/02/p...-progress.html you'll see what im up against! Ive managed to fit the single 741 DIL circuit into a space wedged between connections, but its far from ideal, and other than that the only spare space is about 3/4" square! Anything even slightly more complex needs to go SMT just to fit in!

(If you read the blog, you might therefore be surprised that im planning to fit a whole microcontroller system in there! But, the space for that is coming from the removal of a module not required (Tx control) and i'll need ALL the space that gives to shoehorn the SMT MCU circuit in!)

I'll get on and order up some LM358s. These will actually cost £0, since while waiting for my son to finish a karate lesson at the sports centre, i found £2 left in the lockers.... [Edit - went mad and spent an extra 47p - got 50 of each!! That will probably see me out!)
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Old 3rd Mar 2019, 12:23 pm   #20
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Default Re: 741 op-amp supply question

Quote:
Originally Posted by Martin G7MRV View Post

I'll get on and order up some LM358s.
These are a good utility OP amp cheap Jelly Bean as H&H might call them. But I have had a lot of trouble with these, many out of the far east have significant cross over distortion. It is as though their internal output stage is incorrectly biased. It won't matter at all for an indicator circuit, but watch out later if you pull one out of your junk box and try to use it for an audio application.

A TLC272 is a very good dual op amp a LinCmos type, very well behaved and easy to use.

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