I have things to do, so I procrastinated a bit by measuring the dropout of one of my LM317 supplies, with the output trimmed to 10.60 V at 15.00 V in, and then dropping the input step by step. The regulator gave up at around 12.25 V (so a 1.65 V drop):
This was with a light load (~12 mA), and a divider consisting of 100 ohm + 2k trimmer (larger trimmers would exceed their power rating). I measured the resistances in circuit after trimming the voltage, and got 100.5 and 753.0; 100.5×10.6/(100.5+753.0) = 1.248 V so checks out.
Need to get things done now, but may do some breadboard procrastination later today
yes I am also with you on this! the reason I dropped the 10r in this was because it ruined the dropout margin with the lm317, so I figured, it wouldn’t hurt to add a ferrite bead. hence wanting more headroom to return to 10r
very nice! yeah its just asking too much from the poor thing. I must confess I held onto the lm317 for wayyy too long on this design. I think it was the reluctance to go surface mount. hence hoping the lm1084 will be a nice drop in solution without much modification (less parts means more doodles, remember that class in school right??)
Hey all, just following up on Midimuso related stuff - here is some text copy pasted from an email I received from Robert at Midimuso. It answers some of our questions in this thread:
"I put the 500mA as a kind of maximum rating as I knew people would realise that Port D can feed 6 LEDs and the LM324’s can feed another 12 LEDs - and they will still produce the right ouput voltage as they are voltage followers.
Eventually, SOMEONE was going to do that - and people have!
So 18 * 25mA is 450mA
So I put 500mA to be safe.
I had no idea people would Euro / Kosmo rack it.
Obviously, the current draw is much smaller in practice.
The LM317 low drop-out alternative suggested is a good idea and has been used in known designs.
I’ll measure the current used by a CV-12 board with 10k loads on all outputs tomorrow."
Hope this is helpful to someone! I sent him links to this thread so he may pop up at some point.
-Wes
may do some breadboard procrastination later today
I tested with a bit more load, and as expected the drop goes up as the load increases (e.g. at around 200 mA you need a bit over 13 V in iirc). The lowest observed drops are simply the collector-emitter drop across a saturated driver transistor, and there’s absolutely no regulation going on.
So my main takeaway is that the midimuso schematics have never worked (at least not for 12 V supplies), but the circuit uses very little current so the designer (inadvertently?) ended up relying on the saturation dropout, and the regulated 12 V being good enough.
I think a TL431 might be exactly what’s needed here.
wow fredrick you have been doing your homework. yes it would seem so, but nevertheless it has worked for him, me and others, playing in tune and happy. ok well I dont want to put it out in its current state. so ill get one of those aswell. which actually works out a little bit less obtrusive as its only a change of a few tracks.
thanks for looking fredrick I have not had a chance to pull a Midimuso out of the rack since we last spoke
That’s the reference voltage accuracy, isn’t it? You’d still need to trim it. But not sure if 1.5 V headroom is enough; I meant to do a quick experiment but when I went looking for TL431s I only found a stash of LM317s. Almost so someone’s trying to tell me something.
(TI’s datasheet points to TL431LI as an improved option, btw, but at some point you’d be hitting the point where builders won’t know what exactly to buy since it all sounds roughly the same, so trying out the LMs sounds like a good idea.)
(Also, did you have to smash up This Old Tony’s nice tile or whatever that was that he had worked so hard on? Guess you could send him a PCB to make up for that.)
(EDIT: Ah, just saw you’ve posted it as well. He posted a YouTube story about your shenanigans yesterday, so saw it there first.)
I may be misinterpreting the information but a drift of 1 percent on a reference voltage that is 10 octaves worth of notes is a drift of a semitone or so. I must be honest with voltage references with such a small headroom im personally in unchartered territory and should have solved this issue much earlier than I have got to it based on the practical experiences I have had with the circuit but like you say trusting on the saturation of the lm317 is not a great idea, especially if someone wants to take the module out of 1 rack and put it into another rack with a slightly different output, and need to recalibrate it all again.
I have all of the aforementioned solutions on order and should be turning up early this week to come to some sort of conclusion.
haha oh great! yeah Colin furze arranged that pass the hammer was really nice of him! I threw the hammer without looking where it was going so yes through my own negligence his beautiful thing he was making has bitten the dust! poor dude.
I may also be misinterpreting, but afaik it’s how close any given component is to the 2.500 2.495 V ideal voltage out of the box, not how it behaves when running (that’s more temperature drift etc). This matters if you’re doing large scale manufacture and are willing to pay a bit extra to avoid having to manually trim your gear, but unless I’m missing something we’re not quite at those volumes here
(I mean, it is labelled as a *precision* reference and there’s even an example that shows to use it to make an LM317 more precise in the datasheet, which wouldn’t make that much sense it it was worse than the other regulators )
Still not sure about headroom, though, so needs testing. Looked a bit more and found some AMS1117s (LM1117 variants), but still no TL431s.
ok I understand yes it makes sense. after looking at the numbers. the temperature drift is much more workable!!. well I have them on the way too! and to be honest if the standard TL431 functions remotely well. thats gunna be the winner, purely down to the fact its widely available in a To92 enclosure!!
regarding the 1222 yeah not had a problem with the 4040 when I swapped to a polystyrene timing cap that completely solved any drift. but ok thats fair enoug
Found them (last place I looked, as always ), and some quick tests with low load (~5 mA) looks promising; you can dip quite a bit below 12 V before the shunt calls it a day.
This choice of component values worked for me down to ~11.2 V.
The numbers next to the trimmer are the measured values after dialing in 10.60 V, normalized to 10k and rounded. The numbers check out: theoretical Vout = 2.495×(1+7650/2350)−2uA×7650 = 10.602 V.
Note that the correct value is close to 25% of the potentiometer range, so a production version could use a ~5k resistor between 10.6 V and REF, and a 5k trimmer to 0 V.
Bonus round: one drawback with a shunt regulator is that the circuit consumes the same power whether the load uses it or not; to maintain the voltage drop across the supply resistor, the current needs to go either through the load or the shunt. I suspect the 5 mA I tested with is a bit high, and designing for 11.2 V may be overdoing it a bit, so we can probably increase the resistor value (which would reduce the consumption).
Another approach would be to replace the 100 ohm supply resistor with a small NPN transistor and a 1k resistor, turning the circuit into a precision series regulator:
I tested this with a few loads from from 5 to 50 mA, and the total consumption is now close to the actual current through the load; the shunt gets just over 1 mA via the 1k resistor, to make sure it regulates, but the rest goes to the load.
I used a BC337 in my tests, but pretty much any NPN should work (BC547 etc, 2N3904, …). The trimmer setting is the same as before.
If you reduce the 100ohm resistor you can handle larger loads with a lower input voltage.
You basically want that resistor to be just under (Vin_min - Vout)/(I_load_max+1mA)
(The 1mA is for the TL431)
That way a minimum of current is wasted through the shunt but if Vin increases or the load decreases, the the TL431 shunts additional current to increase the drop through the resistor and maintain the desired output voltage. (This explanation is for any onlooker who would not already know, i.e. not for Fredrik.)
That’s why I asked about the maximum and minimum currents above if the load current doesn’t vary much, it’s all good, but if it varies significantly, then it might be worth implementing a series regulator as you describe.
Well, actually … while cogitating about this circuit, I made the unfortunate mistake of consulting the data sheet for the LM7812 which is commonly used in Kosmo and Eurorack power supplies, including in the Frequency Central power supply, and I found out that the minimum voltage that it might put out is 11.5V so in theory we would have to design to that constraint.
But with a small enough series resistor I believe the TL431 based circuit can handle a dropout of less than a volt.
