I'm thinking about making a lil board for my "doomsday" bomb-proof linear tracking regulator as it's often handy to have four rails with two that track halfway below the higher ones:
Feed it with +50/-50 from a two-output flyback and make two the resistors a dual isolated digipot to offer some adjustment.
This is a single with rail-rail output and rated for 36 volt supply and
+125 degree C temperature range:
cheaper than a CA3140. it would be OK if the amp wasn't low-power as I learned the hard way that unless there's sufficient loading on the rails to pull the regulators into regulation (or one of the DIP-style amps isn't properly seated in its socket) the op-amp get whapped with the full "high voltage", bye-bye CA3140.
The old LM358 seems a lot more tolerant of over-voltage oopsies than the CA3140 but it can't get within hardly 2 volts of its own positive supply even with a light load at 30 volts fugedaboutit.
Aren't the low voltage rails going to be a bit higher than half the higher voltage rails. You are supplying U4 adj pin with half the "30V" output, so the -15V output will be half the -30V output with the 1.25V reference adde d (or subtracted, depending on how you look at it). The resistors for sett ing U3 output voltage puts it at the -15V output plus 14 volts.
Is that intentional? The whole thing seems a bit cyclical. Why not have o ne regulator set for a fixed voltage independent of the others, then the re st can leverage off that. Putting so many regulators in series like this w ith two in a loop seems like a bad idea.
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Rick C.
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Loaded voltages in the sim with the LT1014 that is RRIO:
+30: 29.44
+15: 14.68
-15: -14.61
-30: -28.02
The regulators are within the negative feedback loop of the op amps.
The rails track each other and come up and go down in unison check out the wave forms.
If you remove all the nanofarard-ish bypass caps it will oscillate or at least looks like it's trying to in the sim there's a bunch of ringing on startup.
It still seems to work OK with an amp like the LM338 without a RR-output stage but the output ends a couple volts short of where it should be. And if I'm not wrong the PSRR of amps without RR-output stages gets degraded when their output stages have to operate up against their Vcc-whatever limits.
her voltage rails. You are supplying U4 adj pin with half the "30V" output , so the -15V output will be half the -30V output with the 1.25V reference added (or subtracted, depending on how you look at it). The resistors for setting U3 output voltage puts it at the -15V output plus 14 volts.
Why are the voltages off? They may not be exactly 15 and 30, but shouldn't the opposite sign supplies track? Can you explain these voltages? In the sim it doesn't impose tolerance variations, so all the voltages should be pretty close, no?
I'm curious. Could you measure the voltages on the four ADJ pins? Maybe I 'm completely missing how this circuit works.
I don't follow that. U5 is just a voltage follower with nothing in the neg ative feedback loop but wire. U6 is an inverting voltage follower. There' s a cap in the feedback loop acting as a low pass. How are any of the regu lators in the inverting feedback loop?
The op amps do remove the small offset from the ADJ pin current on the low voltage regulators versus using just the resistor divider. With the added
0.25 volts for the ADJ pin current the two high regulators should output 14 volts higher than the low side regulators. The feedback point for the low neg reg is half the high neg output which is
Vh- = Vl- + 14
The feedback voltage then is
Vf- = (Vl- + 14)/2
so the final low output should be 1.25 volts higher or...
Vl- = 1.25V + (Vl- + 14)/2
Vl- = 8.25V + Vl-/2
Vl- = 16.5V
The high output should be 30.5V, which is close to the 30 volts intended.
ve one regulator set for a fixed voltage independent of the others, then th e rest can leverage off that. Putting so many regulators in series like th is with two in a loop seems like a bad idea.
Not saying they don't. I'm saying that to get the initial voltage on the n egative side, the master if you will, I'd have to write and solve a couple of simultaneous equations, one for the low rail dependent on the high rail and one for the high rail dependent on the low rail.
Once you get those voltages you can then find the low positive voltage and then the high positive voltage. That high positive voltage depends on the operation of all three of the other circuits.
The op amp shouldn't need RR outputs once it is operating, but U6 will be r unning the inputs at ground. Why not power it from +/- voltage?
Why not have the low negative voltage regulator regulate its own voltage ra ther than sensing the high voltage? Everything else in the circuit will be the same but it takes the neg high voltage regulator out of the regulation path for the two positive regulators. R1 = 1.1k and R2 = 15k will get you to within 1%.
Am I completely misunderstanding this design? I just can't see a good reas on for putting two regulators in a loop regulating each other.
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Rick C.
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You're making it too complicated I think you don't need to solve no equations. U1 and U3 can just be replaced by +/-15 volt voltage sources. at DC U4 and U5 function as a unit and acts like one big honkin' power-op amp with two equivalent inputs, one at the output of U4 and at U5's non-inverting input. There is an input to a three-terminal regulator feedback loop there see e.g.:
Take an ideal op amp and connect its inverting input to its output, then take the output and connect a 15 volt voltage-source in series with it. Take a 0.5 resistor divider lower end to ground and mid-point back to the op amp non-inverting input. What's the output of the op-amp gonna be. What else can it be.
I think with ideal components you can redraw this circuit with two voltage sources, two resistors and two unity-gain buffers, one inverting and one non-inverting. that's all it is! (ideally)
There's something weird with the LM models connected this way perhaps I've built the physical circuit on a breadboard seems to work fine but I'll have to measure the voltages again, the "high voltage" rails are closer than that IRL, IIRC
igher voltage rails. You are supplying U4 adj pin with half the "30V" outp ut, so the -15V output will be half the -30V output with the 1.25V referenc e added (or subtracted, depending on how you look at it). The resistors fo r setting U3 output voltage puts it at the -15V output plus 14 volts.
dn't the opposite sign supplies track? Can you explain these voltages? In the sim it doesn't impose tolerance variations, so all the voltages should be pretty close, no?
be I'm completely missing how this circuit works.
negative feedback loop but wire. U6 is an inverting voltage follower. Th ere's a cap in the feedback loop acting as a low pass. How are any of the regulators in the inverting feedback loop?
low voltage regulators versus using just the resistor divider. With the ad ded 0.25 volts for the ADJ pin current the two high regulators should outpu t 14 volts higher than the low side regulators. The feedback point for the low neg reg is half the high neg output which is
d.
have one regulator set for a fixed voltage independent of the others, then the rest can leverage off that. Putting so many regulators in series like this with two in a loop seems like a bad idea.
he negative side, the master if you will, I'd have to write and solve a cou ple of simultaneous equations, one for the low rail dependent on the high r ail and one for the high rail dependent on the low rail.
and then the high positive voltage. That high positive voltage depends on the operation of all three of the other circuits.
at
on
ut
be running the inputs at ground. Why not power it from +/- voltage?
e rather than sensing the high voltage? Everything else in the circuit wil l be the same but it takes the neg high voltage regulator out of the regula tion path for the two positive regulators. R1 = 1.1k and R2 = 15k will get you to within 1%.
reason for putting two regulators in a loop regulating each other.
Yes, but they are designed for 14 volt outputs, not 15. However the voltag e at the feedback point between R1 and R2 is a function of both regulators.
The ADJ pin bias is ~50 uA so the offset voltage is only 0.021 volts which can be ignored.
So U1 and U3 are 13.75 volt sources not 15 volts. This is straight from th e data sheet and your resistor values.
U5 is just a voltage follower. 1 volt in is 1 volt out, so it is actually transparent in this design other than sinking the ADJ pin current. The reg ulator is the same regulator it is without the op amp except the ADJ pin bi as current is now absorbed by the op amp without affecting the voltage at t he ADJ pin. Don't know why you need to try to change your thinking of the regulator to an op amp.
If the feedback were based on the output of U4 it would be completely obvio us how it is working. By using the output of U3 as the feedback you now ha ve to factor in the 13.75 volts along with the 1.25 volts of the internal r eference and the voltage divider.
You don't mention the 1.25 volt internal reference that must be added to th e ADJ pin voltage. That's how the regulator operates which is not the same as an op amp... unless you treat it as 1.25 volts of input offset maybe.
Sure, but you have to do it correctly factoring in the reference voltage in ternal to the regulators. That's how you get 14 volts out of U1 and U3. T ake away that reference voltage and they will output zero volts.
"Real Life" has many variation from tolerances. The simulations should not .
I don't see where you addressed my calculations. I'm pretty sure they are correct. It's just a consideration of how each part works without trying t o apply undue simplifications which may or may not be valid. Why don't you try writing the equations yourself and see what you get?
I'm not trying to be a PITA. I just think this circuit isn't doing what yo u think it should. I also think making the feedback loop for U4 include th e feedback loop for U3 is looking for trouble with stability. And I can't see any advantage to doing it this way.
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Rick C.
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It is good to investigate it. Hey at least it's on-topic!
But I don't understand where your calculations come from. Two of the regulators are configured as voltage sources; if the op-amp inside them were ideal and they drew no input current then they would be essentially ideal.
If I make R9 and R8 non-symmetric and change R8 to 5.2k to try to get the voltage-source regulators more symmetric in isolation because the LM317 and '337 aren't identical mirror-images of each other. Then I get:
+30: 30.62
+15: 15.86
-15: -15.79
-30: -30.54
The bottom -15 regulator has its reference inside a the feedback loop I don't think it matters what it is exactly so long as it's approximately similar between the '317 and '337. The op amp U5 tries to make the voltage at its non-inverting terminal and the voltage between the -30 and -15 rails equal. Set up like this the voltage between those two rails is -14.78. The voltage at the non-inverting input off the dividers is -14.58. The voltage of the negative voltage-source regulator in isolation with R8 tied to ground is -14.76.
If I then multiply the voltage at U5 non-inverting terminal by -1 and add 1.25 to it then the +15 rail must be approximately equal to the -15. if the two internal references are approximately equal. Then if the upper voltage-source regulator is approximately the same as the bottom one then its output must be approximately the same, too.
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