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redwire

0-30V Stabilized Power Supply

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Why do you use 30V instead of the collector voltage of approx 40V (with a 28V transformer) when computing the power?

Did I calculate it wrong?
28VAC has a peak voltage of 39.6V and the rectifier bridge drops 2V to 37.6V and the capacitor ripple drops it to maybe 35.6V. if the current sensing resistor is 0.47 ohms then it drops 1.4V to 34.2V. Then the 0.33 ohm emitter resistor drop 0.5V to 33.7V so the output transistors have a total dissipation of 101.1W. I DID calculate it wrong. Sorry. 

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I have two questions:
1) can the circuitry associated with RV1 be omitted?  What would be the consequence?
2) when mounting in a metal box with the chassis earthed to mains earth (green/yellow), should the -ve on the electronics also be connected to mains earth?

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I have two questions:
1) can the circuitry associated with RV1 be omitted?  What would be the consequence?

RV1 nulls the input offset voltage of the voltage setting opamp U2 so that the output is exactly 0V when the voltage pot is set to zero. Without RV1 then the input offset voltage of the opamp (if there is any) will cause a small positive or negative voltage output instead of 0V.
But some people say RV1 does not do anything.

2) when mounting in a metal box with the chassis earthed to mains earth (green/yellow), should the -ve on the electronics also be connected to mains earth?

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Thank you for the helpful response.
I am not sure what you mean by 'electrical outlets that were wired backwards'.  In Australia mains outlets have three wires - live, neutral and ground/earth (for earth leakage protection).  Would you recommend that I simply do not connect the earth to the chassis.  The danger is that if there is a fault (fingers in the wrong place!)  on the mains side of the transformer then the full 240V could be fatal.

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Guest liquibyte

Ground the chassis and the transformer frames to mains earth but leave the circuit ground isolated and floating.  I have built mine this way and would be the way I'd recommend doing it.  What audioguru is talking about is sometimes outlets in houses can have neutral and hot reversed which can lead to odd issues with certain equipment.  I live in a house that seems to have been worked on by prior homeowners so I actually checked my outlets for correctness before I built this to make sure that the fuses would be correct in the circuit.  So, make sure that your hot and neutral are correct so that you fuse on the hot side and make sure you don't connect the chassis or transformer frames to neutral inadvertently.

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I am not sure what you mean by 'electrical outlets that were wired backwards'.  In Australia mains outlets have three wires - live, neutral and ground/earth (for earth leakage protection).  Would you recommend that I simply do not connect the earth to the chassis.  The danger is that if there is a fault (fingers in the wrong place!)  on the mains side of the transformer then the full 240V could be fatal.

In Canada we also have three wires, live, neutral and earth ground. I have seen electrical outlets wired wrong by drunk electricians or by homeowners who do not know how to do it correctly so the "earth" terminal is actually "live". Then if a person touched the metal chassis and an earth ground, the person would be electrocuted if the outlet was wired wrong and if the chassis was connected to the earth wire on the plug. If the chassis is not connected to the earth wire then there will not be a problem unless the transformer live wire or live wire in the cord shorted to the chassis.

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Earthing matter sorted thanks.
Next question:  our DIYFAN option shows a 24V Zener and resistor across input +ve and -ve to provide a stable 24V supply voltage to pin 7 of U1.  Remove the connection of pin 7 to +ve. Since the +ve input voltage can drop at high loads, what do you think of this idea?

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Next question:  our DIYFAN option shows a 24V Zener and resistor across input +ve and -ve to provide a stable 24V supply voltage to pin 7 of U1.  Remove the connection of pin 7 to +ve. Since the +ve input voltage can drop at high loads, what do you think of this idea?

Then you must replace the 10V zener diode with a resistor.
I do not think U1 needs its power supply voltage regulated because a TLE2141 opamp has a minimum Supply-Voltage Rejection Ratio of better than 90dB so 2V of ripple causes an output of less than 62.5uV (typically less than 2uV) which is very close to zero.

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Guest liquibyte

A member (homebrew) over at EEVBlog put together an LTSpice simulation of this circuit.  I've been trying to do this off and on for a little while but was never able to get it to run right.  I'm going to put this up here as well in the hopes that someone might find it useful, I know I will.  The screenshots are what was posted over there and the zip file contains the .asc for the spice simulation.


Maybe a better simulation model might help to debug the circuit. I have attached you a full simulation of your supply in LTSpice. Feel free to mess around :-)

It has quite good regulation performance in voltage mode! However, in current mode it starts to oscillate ...


I'm leaving this simulation up but you'll want to see this post for the latest simulations.

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post-107142-14279144819087_thumb.png

post-107142-14279144819202_thumb.png

post-107142-1427914481932_thumb.png

SupplyEEVBlog.zip

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The very old LT1001A opamp is very slow like a lousy old 741 opamp. The TLE2141 is MUCH faster.
You have C6 as only 33pF but it should be 330pF.
Why does the circuit have C11? It is causing the current spike and maybe is causing the oscillation.

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Guest liquibyte

Changed C6 to 330p and re-uploaded.  I didn't do this simulation, I'm just putting it here for folks to play around with.  I asked about why some of the stuff, specifically the mosfet, was on the output but haven't heard back yet.  Like I said, I tried to simulate this but couldn't get it to work.  As far as the TLE2141 goes, if someone can get it out of UltraLibrarian from TI's website, I'll gladly add that in too.  I can't get UltraLibrarian to run on my system because it won't work under WINE (it is a VB6 app, ancient and crappy).  I've emailed them about this issue and was basically told to stuff it.  I think that it really shouldn't matter for the most part because the op amps are theoretical parts in spice and what we're looking for in this is a way to tweak the model in ways we like before trying it out in the real world.  In other words, we aren't going to be blowing these up.  If you can get the model out of its "packaging" from TI, I'll certainly use it instead.

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About a month ago I asked if anyone could explain why my 30V 5A (with 3 x TIP3055 output transistors) suddenly failed at 45V and no current.  Both Audioguru and Redwire (I think) suggested a short in one of the 3055s.  Finally I have isolated the fault to a 'popped' 3055.
At the time that the PS failed, I was trying to charge a 'golf cart' battery which was nearly fully charged so no significant current.  I set the volts on 14.1V and current limit at 1A.  When touching the PS leads to the battery there were a few sparks (possibly back to front polarity), and maybe not, but PS popped the 3055.
Has anyone had experience with the PS and 'active' loads like batteries, as opposed to passive loads like resistors?

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About a month ago I asked if anyone could explain why my 30V 5A (with 3 x TIP3055 output transistors) suddenly failed at 45V and no current.  Both Audioguru and Redwire (I think) suggested a short in one of the 3055s.  Finally I have isolated the fault to a 'popped' 3055.
At the time that the PS failed, I was trying to charge a 'golf cart' battery which was nearly fully charged so no significant current.  I set the volts on 14.1V and current limit at 1A.  When touching the PS leads to the battery there were a few sparks (possibly back to front polarity), and maybe not, but PS popped the 3055.
Has anyone had experience with the PS and 'active' loads like batteries, as opposed to passive loads like resistors?

I've never tried anything like that but I'd think that this particular power supply wouldn't want to be used like that.  If I were to try and charge a battery, I'd research a specific charging circuit or at least protection circuitry to add to the output of a power supply to allow this.

On an unrelated note, I've been playing with the simulation and found that if I change the value of C6 in the sim schematic (C4 in the original schematic) to a larger value I can get the output to slowly ramp up.  I tried introducing the spike like I had it in the mosfet circuit but it doesn't really affect the output so it's as if the circuit doesn't suffer from the problem.  But, if we reduce the time the output can rise with a larger value here wouldn't the issue be solved for the most part?

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I've been playing with the simulation and found that if I change the value of C6 in the sim schematic (C4 in the original schematic) to a larger value I can get the output to slowly ramp up.  I tried introducing the spike like I had it in the mosfet circuit but it doesn't really affect the output so it's as if the circuit doesn't suffer from the problem.  But, if we reduce the time the output can rise with a larger value here wouldn't the issue be solved for the most part?

C6 in the latest SIM circuit prevents the current regulator from quickly cutting back or quickly allowing more current. Then if the output is suddenly shorted the current in the circuit will skyrocket until the slowly ramping opamp can catch up which is very bad.
Maybe the current sensing resistor should be non-inductive (not wire-wound).

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Guest liquibyte


C6 in the latest SIM circuit prevents the current regulator from quickly cutting back or quickly allowing more current. Then if the output is suddenly shorted the current in the circuit will skyrocket until the slowly ramping opamp can catch up which is very bad.
Maybe the current sensing resistor should be non-inductive (not wire-wound).

What about the sims C9 (orig C6)?  I can get the same results there as well.

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What about the sims C9 (orig C6)?  I can get the same results there as well.

It slows down changes in the voltage regulation. Add a load then the voltage suddenly drops and when this capacitor charges then the voltage slowly comes up to normal. Disconnect a load and the voltage suddenly increases and when this capacitor charges then the voltage slowly comes down to normal.
It prevents high frequency oscillation, maybe because the output transistors are very slow.

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Guest liquibyte

Well, up until I can get my hands on a couple of 0.47R non inductive resistors to test I'll keep playing with the simulation and trying to find a way around this.  When you get down to it, non-inductive resistors at this resistance and power rating end up costing half as much each as the 10-turn pots I used.  Is there no way to minimize or mitigate this with a wirewound resistor?

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Guest Ronaldinie

Hello guys. Since i'm new to electronics, may I know what are the function of the op-amps in this circuit?

Thank you.

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Hello guys. Since i'm new to electronics, may I know what are the function of the op-amps in this circuit?

We cannot teach the basics of opamps in one post but here goes:
1) Everyone who modifies this circuit uses a different parts numbering system so I will describe the three opamps as the voltage reference circuit, the voltage amplifier and the current regulator.

1) The voltage reference opamp provides a constant current to the 5.6V reference zener diode and has a gain of 2 times so the reference voltage is 11.2V.

2) The voltage amplifier opamp drives the BD139 driver transistor which drives the two 2N3055 output transistors. Two resistors in the amplifier allow the amplifier to have a gain of 2.68 times so that the 11.2V reference is amplified up to 30.0V at a high current.

3) The current regulator opamp compares the voltage produced by the load current in the 0.47 ohms current sensing resistor with the voiltage of the current setting potentiometer. If the sensed voltage is too high then this opamp reduces the voltage from the voltage setting potentiometer through a diode until the load current is the same as is set. If the output is shorted then the current regulator opamp causes the output voltage setting to drop to almost zero so the current is not higher than the setting of the current setting pot. 

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Guest liquibyte

Maybe the current sensing resistor should be non-inductive (not wire-wound).

On some advice, I shorted across the resistor and I'm still getting the same results so I don't think that's what is causing this.

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Guest liquibyte

Here's a corrected and slightly modified version of the simulation.  There were a few errors in the original and I've rearranged things to more closely resemble the original schematic.  I didn't renumber the parts because it's a pain to do.  It's easy to renumber them top to bottom, left to right which is why my Eagle schematics are renumbered the way they are but to do this based off of the original would take quite awhile and if you add or remove something it just screws up the flow.  I've been trying to get a decent spike simulated but it's not going very well so far.  I have noticed that the output of the current control op amp oscillates where the other two don't.  Anyone else have any experience with LTSpice IV?

See this post for the latest simulations.

post-107142-14279144819865_thumb.png

post-107142-14279144820491_thumb.png

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The current regulator opamp is not oscillating. Its output is saturated as high as it can go which is about +28V because it is not regulating the current so of course it shows the 120Hz ripple from the unregulated +39V, which is reduced to +29V by the 10V zener diode.
The moment the current regulator opamp begins to work then its output will have no ripple.

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The current regulator opamp is not oscillating. Its output is saturated as high as it can go which is about +28V because it is not regulating the current so of course it shows the 120Hz ripple from the unregulated +39V, which is reduced to +29V by the 10V zener diode.
The moment the current regulator opamp begins to work then its output will have no ripple.

Yep, you're right, as I lower the current pot to about .48 it starts to kick in and smooth out the output.  It's neat to be able to simulate this in a somewhat accurate way.  I've got the trimmer on the voltage set so it outputs right at 30V when the voltage pot is set at 1 (.7025) but I haven't messed with the current side of things yet.  I'm still trying to figure out how to emulate a voltage spike on the input without drawing the time frame out with the soft start mosfet circuit.  For some reason when I change the time to ramp the voltage up, the spike stretches out along with it instead of firing and just shutting off.  It's kind of frustrating really.

Edit: Here's a nice plot showing that if anyone's interested.

post-107142-14279144820689_thumb.png

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