UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > General Vintage Technology > Components and Circuits

Notices

Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

Closed Thread
 
Thread Tools
Old 19th Mar 2019, 9:45 pm   #41
Martin G7MRV
Heptode
 
Join Date: May 2008
Location: Selby, North Yorkshire, UK.
Posts: 949
Default Re: 741 op-amp supply question

Well, tried it now with the other half, same result. Tried without the sensing par tof the circuit in case that was somehow affecting it, same result!

Totally stuck!
__________________
I got food in ma belly and a license for ma telly

My Blog - http://g7mrv.blogspot.com
Martin G7MRV is offline  
Old 19th Mar 2019, 10:51 pm   #42
MrBungle
Dekatron
 
MrBungle's Avatar
 
Join Date: Jun 2016
Location: London, UK.
Posts: 3,687
Default Re: 741 op-amp supply question

Think there's an error in your circuit. If you look at the LM358 datasheet, the opamp oscillator described has a virtual ground at the non-inverting input. Yours is missing a resistor from the non-inverting input to positive supply. Without that, the oscillator will settle at one extreme and go to sleep.

Click image for larger version

Name:	datasheet.png
Views:	49
Size:	19.7 KB
ID:	180226

Also to note with the LM358 the bias current is extremely high, making it somewhat more sensitive to touch etc as well. I'd swap the 4.7M resistor for a 470k resistor and the cap for a 10uF one.

If I was doing this I'd probably be super lazy and replace the whole thing with a really bottom end PIC10F with a hardware comparator, zener dropper and couple of sample resistors. They have a full whack current doing their thing of around 25uA and sleep current of nA and are the size of a SOT23 (your MMBT3904).

Option 2 is an LM311 (very low current comparator) and a good old 555 astable. You can control the reset pin of the 555 to turn the astable on and off.
MrBungle is offline  
Old 20th Mar 2019, 12:25 am   #43
Martin G7MRV
Heptode
 
Join Date: May 2008
Location: Selby, North Yorkshire, UK.
Posts: 949
Default Re: 741 op-amp supply question

Just nipped out to the workshop and tried adding an extra 100k to supply - yes, that was indeed the problem! Thankyou!

Can I ask which datasheet your using there? Ive been using the Texas Instruments and the ON Semi versions, and couldnt see anything about a virtual ground in either, in fact neither have that specific circuit in them! My design came from the generic astable circuit. Odd that it worked ok with the 741, but that saying, I was getting something like a 3:1 duty cycle and a time period of roughly 13seconds before, and now with the virtual ground working, the duty cycle is 50/50 over 6 seconds! So it does look like the 741 was struggling.

At least this means my pre-made PCBs are still usable! Just need to find a convenient way to add the extra resistor! Im still looking at well under a mA though not including the current for the optocouplers LED, which is a massive improvement on the 741s! (not an issue with a 4Ah battery)

One problem though, I rather liked the 3:1 duty cycle! It meant the warning signal was noticeable, but the time between meant it wasnt too disruptive. I wonder can the duty cycle be adjusted via those (now three) feedback resistors?
__________________
I got food in ma belly and a license for ma telly

My Blog - http://g7mrv.blogspot.com
Martin G7MRV is offline  
Old 20th Mar 2019, 12:50 am   #44
MrBungle
Dekatron
 
MrBungle's Avatar
 
Join Date: Jun 2016
Location: London, UK.
Posts: 3,687
Default Re: 741 op-amp supply question

Glad to hear it worked

I was using this one, page 17: http://www.ti.com/lit/ds/symlink/lm158-n.pdf

Not sure why the 741 worked! I hate the wretched things. Really I am only bitter because I have a whole tube of them here I bought on a whim and now am determined to use them all up!

You can change the duty cycle but it's not as easy as changing the bias resistors. Basically the thing works by charging and discharging the capacitor through R1 in the diagram above. When the output of the opamp hits ground it pulls current through R1 and discharges C1. When the output of the opamp inverts, it pushes current through R1 and charges up C1.

Sneaky trick here is to use different resistors when it's charging and discharging and you can do this with two steering diodes as per the following. If the Rc and Rd values are different by a reasonable ratio then it'll allow you to set the duty cycle. It's not particularly accurate as the thing is using an exponential charge circuit rather than an integrator and the LM358 doesn't got to the top of the supply voltage but it gives the right effect. Crude diagram (I've left out the other bits of the circuit):

Click image for larger version

Name:	DDE99B08-F2EF-46FC-87FE-48765A2E4F8D.jpg
Views:	44
Size:	28.5 KB
ID:	180231

Edit: also if you designed a PCB for it, it takes me at least 3-4 tries before it works so don't sweat doing another board run. I am using JLCPCB and it costs around £5 a go which is only a couple of beers these days. Just last week I screwed up the pads for an SMA connector pretty badly!

Edit 2, reading back through the thread: to the earlier complainers about the LM358's ability to distort, this is only prevalent if it has to drive a lower impedance load, via capacitive coupling. Not whether or not it comes from China (they all do!). If you keep it DC coupled, make sure it always sources or sinks current and isn't heavily loaded then the distortion goes away. It's a pretty good and cheap single supply opamp though. There are better ones now but not at that price!

Last edited by MrBungle; 20th Mar 2019 at 12:59 am.
MrBungle is offline  
Old 20th Mar 2019, 1:11 am   #45
Martin G7MRV
Heptode
 
Join Date: May 2008
Location: Selby, North Yorkshire, UK.
Posts: 949
Default Re: 741 op-amp supply question

I seem to have had a rather truncated copy of the TI datasheet by the looks of it! Since last replying I found a National one which has the circuits.

Well, although I only needed a few DIL LM358s for testing on the breadboard, and the rest as SOIC for the actual PCBs, it was cheaper to buy 50 of the bleeders than to get just a few! So like you with the 741s, it looks like i'll have to dream up some jobs for them! I can only think that the ancient design of the 741 allowed it to work when it really shouldnt have done.

I also used JLCPCB for this run of 10, came out at I think £6.40 of which £4.80 was shipping! Ive sort of already decided that if I do another run i'd change up from 0603 to 0805 parts, as without a proper SMT workstation im finding 0603 to be pretty much on the limit of my capabilities! If I do a redesign for 0805 and including the extra resistor, then adding in the steering diodes wouldnt be too much of a trouble. Im also thinking that the devices ability to sink current might allow me to do away with the switching transistor? Worth a play I think!
__________________
I got food in ma belly and a license for ma telly

My Blog - http://g7mrv.blogspot.com
Martin G7MRV is offline  
Old 20th Mar 2019, 1:24 am   #46
MrBungle
Dekatron
 
MrBungle's Avatar
 
Join Date: Jun 2016
Location: London, UK.
Posts: 3,687
Default Re: 741 op-amp supply question

I'm trying to build a pong game with my left over opamps. Figured I'd need about 25 for it so worth a punt. I've managed to get it to bounce a ball on the scope screen so far with 8 of them (2 triangle oscillators, quadrature oscillator and two summing amps). Needed a break from amateur radio for a few weeks! I'll post the design on here when I'm done with it

I can handle 0603 just about but it's not much fun even if it is a little cheaper so I standardised on 0805 as well.

Not sure about the current sink / source capability. Depends on the opto/LED. I would breadboard it first.
MrBungle is offline  
Old 20th Mar 2019, 1:43 pm   #47
Martin G7MRV
Heptode
 
Join Date: May 2008
Location: Selby, North Yorkshire, UK.
Posts: 949
Default Re: 741 op-amp supply question

That sounds an interesting project! I have the plans for the PW Teletennis somewhere - lots of i think 7400 series DIL logic chips!

Ive tried out the current sinking capability and it seems to be working well, although I did of course have to swap the inv/non-inv inputs over to get the output polarity right. Ive yet to see how this affects the current the circuit draws. In this configuration the optocouplers LED is reverse biased whenever the supply voltage is above the warning threshold and so the LED remains off.

I also looked at your suggestion to up the cap to 10uF and lower the timing resistor to 470k, which works nicely on the breadboard. I looked into this as I ordered the 4M7 resistors and they cost more than all the others together! But this has led me to a worrying discovery - my 1uF caps are only rated 16V! At 24V supply, the voltage on the cap is right on the rating, or maybe a fraction higher! Not good at all. And the boards I have are far too small to add a bigger cap unless I use standard parts secured underneath! Bit of a silly cockup that I should have known better! So im looking now at the possibility of a stay of execution for the circuit using a potential divider to limit the supply voltage!
__________________
I got food in ma belly and a license for ma telly

My Blog - http://g7mrv.blogspot.com
Martin G7MRV is offline  
Old 20th Mar 2019, 2:17 pm   #48
Martin G7MRV
Heptode
 
Join Date: May 2008
Location: Selby, North Yorkshire, UK.
Posts: 949
Default Re: 741 op-amp supply question

Well, the current sink works but is much less efficient than the switching transistor! Nearly 4mA when 'on' against the transistor versions 1.5mA, and 2mA when off against 460uA! Think i'll stick to the transistor!

A quick and dirty test of a 10k/22k potential divider on the supply input sent it nuts! I think because it effectively put the 22k in parallel with the divider for the detector input! I might try it with a diode to isolate them, but its adding more parts each time that I didnt really want.


Anyway, the suns out so I think i'll leave it for now and go for a walk!
__________________
I got food in ma belly and a license for ma telly

My Blog - http://g7mrv.blogspot.com
Martin G7MRV is offline  
Old 20th Mar 2019, 2:31 pm   #49
MrBungle
Dekatron
 
MrBungle's Avatar
 
Join Date: Jun 2016
Location: London, UK.
Posts: 3,687
Default Re: 741 op-amp supply question

If you have a scope, check the peak voltage across the cap. It may still be in limit with 28V across it. Also look at the MLCC 0805 sized caps for low accuracy timing - they are tiny and go up to about 47uF these days. For example: https://uk.rs-online.com/web/p/ceram...itors/1726851/

Transistor is usually a good idea with these things. It's running switching mode whereas the opamp is running linear. If you used an LM311 that is a possibility as well as that's a dedicated comparator and the output has an open collector output.
MrBungle is offline  
Old 20th Mar 2019, 8:31 pm   #50
Martin G7MRV
Heptode
 
Join Date: May 2008
Location: Selby, North Yorkshire, UK.
Posts: 949
Default Re: 741 op-amp supply question

Ive got one of the standard batteries on a discharge/charge cycle at the moment, so I can measure exactly what its maximum fully charged voltage is, i'll then feed that exact voltage to the circuit and measure the peak voltage one the capacitor. It might 'just' get away with it on the standard NiCd batteries. There are 33v Lithium batteries out there, but ive never yet heard of anyone using one!

Those 0805 ceramics look good, high capacity and 35v rated! I'll consider those if I decide to make the mkII version!
__________________
I got food in ma belly and a license for ma telly

My Blog - http://g7mrv.blogspot.com
Martin G7MRV is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 6:21 pm.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.