audioguru2
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I think with a low output voltage at 5A then driver transistor Q2 will get too hot. I think the three 2N3055 transistors will also get too hot. Opamp U2 also might get too hot.
As to the 33v lm317, I wasn't clear. Let's look at the output. From ground, we need 1.35 volts at 5A across thecurrent sense resistor R7. We need 30v across the output. We need another .5v assuming 1.6A through each 2N3055, another .6v base to emitter in the 3055s and another .6 across the base to emitter of the driver Q2. That's a minimum of 33.05v at the U2 output. However, U2 can't swing to its upper limit, so we need another 1v at least, giving us 34.05v output for an lm 317, which needs 3v headroom, giving us 37.05 at any lm317 feeding U2. ... and those are all fairly conservative numbers.
I considered doing it - by using a smaller load sense resistor, I can get down to 1.0 to 1.25 v. By using the emitter resistors in the original supply (0.2R) I get 0.4v. I could use a low dropout regulator and large filter caps to get it to work.
I'm slowly heading to a question and comments in my next post.
I considered doing it - by using a smaller load sense resistor, I can get down to 1.0 to 1.25 v. By using the emitter resistors in the original supply (0.2R) I get 0.4v. I could use a low dropout regulator and large filter caps to get it to work.
I'm still in the middle of replying, but are you saying the other 5A designs I've seen won't work? I just finished testing at 1.3v 5A output across the load resistor, which equals 0v out at the output terminals. I ran for 5 minutes. It needs active cooling on the 3055 heat sink and a large heat sink on Q2, but it looks like it will work to me.
I'm slowly heading to a question and comments in my next post.
Ok, continuing on. Although I think U2 *could* be driven from a regulator, my current design doesnt do that. It uses a 44v OA, probably the TLE2141, although I haven't made a final selection.
The reasons are:
1) the voltage requirements
2) the null offset pins available.
3) the PSRR pwr supply rejection ratio is quite high (98 db IIRC), as recognized in the original design, so the fact that it's powered from the unregulated supply has little effect
4) one more thing as discussed below in my question, when I get to it
The comment about using the lm317 relates to U3 and U1, not U2. U3 only needs to get above the 11.2v voltage reference, and -1.3v to pull down D9 at the U2 input. So it can easily be powered by an lm317.
Further, there are several reasons why I want a regulated supply. I plan to eliminate U1, the voltage reference. And replace it with two tl431 adjustable voltage references. One will be the 11.2 reference relative to the top end of the current sensing resistor R7, at the negative supply output. The other will be 2.5v relative to ground.
The first of those references is used to provide the output voltage reference for U2. That reference voltage floats above the R7 current sensing resistor voltage. The second reference is used to provide the current limiting reference for U3. By using a current limiting reference that is fixed relative to ground potential, I solve the two problems discussed in my earlier post which are: 1) nonlinearity of the current limiting potentiometer and 2) the problem of the current limiting output voltage varying as the load current flowing through the current sensing resistor varies. That allows me to use the current meter to display the current limit setting. To do that, however, I need another op amp (U4). So, like the original design I end up using a 3 op amps, but two of them can be on a dual chip powered by the lm317 and operate at a much lower voltage than the 44 volt chip used for U2. The regulated output of the lm317 cooperates with the adjustable zener tl431s to give excellent voltage regulation. It also reduces the idling current through R7, so the voltage across R7 more accurately reflects the load current through the output terminals.
Edit: I should have said that the lm317 regulation was helpful in providing much more accurate tl431 voltage references, as compared to driving them from the unregulated supply. It wasn't all that critical to the dual op amps.
The reasons are:
1) the voltage requirements
2) the null offset pins available.
3) the PSRR pwr supply rejection ratio is quite high (98 db IIRC), as recognized in the original design, so the fact that it's powered from the unregulated supply has little effect
4) one more thing as discussed below in my question, when I get to it
The comment about using the lm317 relates to U3 and U1, not U2. U3 only needs to get above the 11.2v voltage reference, and -1.3v to pull down D9 at the U2 input. So it can easily be powered by an lm317.
Further, there are several reasons why I want a regulated supply. I plan to eliminate U1, the voltage reference. And replace it with two tl431 adjustable voltage references. One will be the 11.2 reference relative to the top end of the current sensing resistor R7, at the negative supply output. The other will be 2.5v relative to ground.
The first of those references is used to provide the output voltage reference for U2. That reference voltage floats above the R7 current sensing resistor voltage. The second reference is used to provide the current limiting reference for U3. By using a current limiting reference that is fixed relative to ground potential, I solve the two problems discussed in my earlier post which are: 1) nonlinearity of the current limiting potentiometer and 2) the problem of the current limiting output voltage varying as the load current flowing through the current sensing resistor varies. That allows me to use the current meter to display the current limit setting. To do that, however, I need another op amp (U4). So, like the original design I end up using a 3 op amps, but two of them can be on a dual chip powered by the lm317 and operate at a much lower voltage than the 44 volt chip used for U2. The regulated output of the lm317 cooperates with the adjustable zener tl431s to give excellent voltage regulation. It also reduces the idling current through R7, so the voltage across R7 more accurately reflects the load current through the output terminals.
Edit: I should have said that the lm317 regulation was helpful in providing much more accurate tl431 voltage references, as compared to driving them from the unregulated supply. It wasn't all that critical to the dual op amps.
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Ok, here's my question. The D9 diode pulls down towards the -1.3v supply when current limiting is in effect. Depending on the op amp selected, the output of U3 can go as low as its negative supply (-1.3v) or 0.3 higher than that (-1.0v). The D9 drop is 0.7 volts max, so doesn't the input of U2 see a negative voltage under conditions of very low current flow through R7?
IIRC, the minimum idling current in ver7 was at least 12mA, producing insufficient voltage to ensure that the NI input of U2 remains positive. The specs of the two 44v opamps require it to be either no lower than the neg supply, or no lower than 0.3v below it.
Perhaps this is only a problem for my redesign, where I've reduced the idling current to below 2mA.
I mention it only because one of my simulations went into what looked like phase reversal. It went to max voltage and current under conditions where output current approached zero and R17 was small. I solved that by connecting U2 to the -1.3v supply. That made it almost impossible to use anything other than a 44v opamp for U2, and there was no real benefit to a lower voltage OA anyway.
Ok, that's it for now. I'm still finalizing the design, checking some things, like U2 power dissipation with almost 43 volts across it. I may need to adjust the drive circuitry. (IIRC, the TLE U2 needs 4.6mA, and Q2 needs another few mA to drive 100mA into the three 2N3055s with hfe at around 50-90. I don't recall the power dissipation rating of the TLE2141.
Just checked, and the plastic pkg TLE2141 has 84.6 °C/W so figuring 4.6mA plus 3mA drive I get 7.6mA x 43v= 326mW resulting in a 28°C rise above ambient temp for Tj. If I did that right (remember I'm a beginner) it looks OK to me.
IIRC, the minimum idling current in ver7 was at least 12mA, producing insufficient voltage to ensure that the NI input of U2 remains positive. The specs of the two 44v opamps require it to be either no lower than the neg supply, or no lower than 0.3v below it.
Perhaps this is only a problem for my redesign, where I've reduced the idling current to below 2mA.
I mention it only because one of my simulations went into what looked like phase reversal. It went to max voltage and current under conditions where output current approached zero and R17 was small. I solved that by connecting U2 to the -1.3v supply. That made it almost impossible to use anything other than a 44v opamp for U2, and there was no real benefit to a lower voltage OA anyway.
Ok, that's it for now. I'm still finalizing the design, checking some things, like U2 power dissipation with almost 43 volts across it. I may need to adjust the drive circuitry. (IIRC, the TLE U2 needs 4.6mA, and Q2 needs another few mA to drive 100mA into the three 2N3055s with hfe at around 50-90. I don't recall the power dissipation rating of the TLE2141.
Just checked, and the plastic pkg TLE2141 has 84.6 °C/W so figuring 4.6mA plus 3mA drive I get 7.6mA x 43v= 326mW resulting in a 28°C rise above ambient temp for Tj. If I did that right (remember I'm a beginner) it looks OK to me.
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audioguru2
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The output current is not shutoff when the current exceeds the current setting. Instead the current is regulated to drop to and stay at the setting:
1) Voltage is set to 20.0V and current is set to 1.0A. No load. The output of U3 goes as high as it can, D9 is reverse-biased and does nothing.
2) Voltage is set to 20.0V and current is set to 1.0A. A load of 10 ohms is applied. The output voltage must drop to 10 ohms x 1A= 10V and the output of U3 drops to about +3.13V. The input of U2 is about +3.73V.
3) Voltage is set to 20.0V and current is set to 1.0A. The output is shorted. The output voltage must be 0V and the current must be 1A. The output of U3 drops to about -0.04V and the input of U2 is about +0.56V. The output of U3 will never go below -0.04V which does not harm it since its negative supply is -1.3V.
The gain of the voltage amplifier is 30V/11.2V= 2.68 and the output of U2 never goes below about +1.2V due to the driver and output transistors Vbe voltage drops. The non-inverting input of U2 is always at least +1.2V/2.68= +0.45V when the voltage and current settings are zero and there is no load. Many ordinary opamps have inputs that do not work when they are a few volts from the negative supply pin, but the inputs of the TLE2141 work all the way down to the negative supply pin voltage.
Isn't the idling current simply the 3.4mA to 4.4mA current of U2? You are correct, U2 gets hot but not too hot. The driver and output transistors need huge non-enclosed heatsinks or a fan.
1) Voltage is set to 20.0V and current is set to 1.0A. No load. The output of U3 goes as high as it can, D9 is reverse-biased and does nothing.
2) Voltage is set to 20.0V and current is set to 1.0A. A load of 10 ohms is applied. The output voltage must drop to 10 ohms x 1A= 10V and the output of U3 drops to about +3.13V. The input of U2 is about +3.73V.
3) Voltage is set to 20.0V and current is set to 1.0A. The output is shorted. The output voltage must be 0V and the current must be 1A. The output of U3 drops to about -0.04V and the input of U2 is about +0.56V. The output of U3 will never go below -0.04V which does not harm it since its negative supply is -1.3V.
The gain of the voltage amplifier is 30V/11.2V= 2.68 and the output of U2 never goes below about +1.2V due to the driver and output transistors Vbe voltage drops. The non-inverting input of U2 is always at least +1.2V/2.68= +0.45V when the voltage and current settings are zero and there is no load. Many ordinary opamps have inputs that do not work when they are a few volts from the negative supply pin, but the inputs of the TLE2141 work all the way down to the negative supply pin voltage.
Isn't the idling current simply the 3.4mA to 4.4mA current of U2? You are correct, U2 gets hot but not too hot. The driver and output transistors need huge non-enclosed heatsinks or a fan.
Yes, I understand this.
Responding to your numbered points:
1) Agreed.
2) "Voltage is set to 20.0V and current is set to 1.0A. A load of 10 ohms is applied. The output voltage must drop to 10 ohms x 1A= 10V and the output of U3 drops to about +3.13V. The input of U2 is about +3.73V."
I think we agree here, too. The input of U2 has to be just low enough that the positive output is 10v, measured relative to the negative output terminal. Measured relative to ground, we add the 1A times the resistance of the R7 current sense resistor, or 10.47 volts. To get nominal 10v output, the input of U2 is down by the gain of the U2 amp stage set by the feedback resistors R12, R11, RV2. It should be around 3+ volts and the output of U3, setting the CL will be 0.6 lower, so those numbers look right.
3)" Voltage is set to 20.0V and current is set to 1.0A. The output is shorted. The output voltage must be 0V and the current must be 1A. The output of U3 drops to about -0.04V and the input of U2 is about +0.56V. The output of U3 will never go below -0.04V which does not harm it since its negative supply is -1.3V."
This looks right, too.
"The gain of the voltage amplifier is 30V/11.2V= 2.68 and the output of U2 never goes below about +1.2V due to the driver and output transistors Vbe voltage drops. The non-inverting input of U2 is always at least +1.2V/2.68= +0.45V when the voltage and current settings are zero and there is no load. Many ordinary opamps have inputs that do not work when they are a few volts from the negative supply pin, but the inputs of the TLE2141 work all the way down to the negative supply pin voltage."
OK, put that way, assuming that both transistors are always forward biased base to emitter, I understand. But is that assumption warranted? Are there no conditions under which the input of U2 might be low enough to drive the output low enough to cut off the transistors? I can see that assuming that they aren't in cutoff requires an input that is safe, but I don't understand where that assumption comes from (not disagreeing, just haven't worked through it yet, and it was the only way to explain my simulation problem I could come up with.)
"Isn't the idling current simply the 3.4mA to 4.4mA current of U2? You are correct, U2 gets hot but not too hot."
The rev 7 design has the idling current of U2 going direct to ground, so the current limiting circuit doesnt see it. It's the idling current of U1 that goes through R7, so there is 3.4 to 4.4 mA for the TLE 2141. Also, there is another 5.6mA idling through zener D8 and R4. The rest comes from gain resistors on both U1 and U2 and pot P1, but its minor. Most of that is eliminated by removing U1.
"The driver and output transistors need huge non-enclosed heatsinks or a fan."
Yes. I even considered adding another 2n3055. (4 instead of 3). The present heatsink is roughly 7"x8".
Also, my transformer has two additional 10 vac rms windings, each rated for 5A, I intend to connect them in series and provide a 20v second supply in the same housing that I will use for lower voltage high current conditions. I might even use a relay with the windings to autoswitch so that they are in series only when needed and at the lowest voltages, high current conditions, only a single 10vac rms winding is in use. That way the main supply will seldom run at low voltage high current, except when accidentally shorted
Thanks for taking the time to reply. I really appreciate it.
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I thought it might be interesting to look at the input voltages of U2 under various conditions in my current design (iteration 18) in Multisim. I could get the NI input of U2 as low as -1.1v by setting current and voltage pots to 0.0 and reducing R17 to zero. In my current design, R17 goes to ground as the voltage ref for U3 is relative to ground (to solve the nonlinear problem and allow me to display the CL setting on the current meter).
That means my sim doesn't directly relate to the rev 7 design. Plus, R17 won't be zero. Nonetheless, I see no real harm in powering U2 from the -1.3v supply instead of ground, so I think I'm going to do that. It increases the supply voltage by 1.3v closer to the 44v limit, so I may drop the upper supply to U2 by a bit with two diodes or a small zener.
That means my sim doesn't directly relate to the rev 7 design. Plus, R17 won't be zero. Nonetheless, I see no real harm in powering U2 from the -1.3v supply instead of ground, so I think I'm going to do that. It increases the supply voltage by 1.3v closer to the 44v limit, so I may drop the upper supply to U2 by a bit with two diodes or a small zener.
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Hello friends!
I'm Brazilian, I'm sorry if I made a mistake in translation.
I have a source that has a problem. It turns out that when I put a 12V voltage in a lamp 12V / 1W. The voltage drops to 10V and no use trying to make up for POT.VOLTAGE. She does not rise! Already I changed the 2N3055, D9, amplifiers and can not find what is causing it.
The same happens with other loads and stresses that apply.
Any suggestions of what might be causing this problem?
I'm Brazilian, I'm sorry if I made a mistake in translation.
I have a source that has a problem. It turns out that when I put a 12V voltage in a lamp 12V / 1W. The voltage drops to 10V and no use trying to make up for POT.VOLTAGE. She does not rise! Already I changed the 2N3055, D9, amplifiers and can not find what is causing it.
The same happens with other loads and stresses that apply.
Any suggestions of what might be causing this problem?
audioguru2
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12V at 1W is a current of only 1W/12V= 83mA. Is the current pot set higher than 83mA? When the voltage drops does the current regulation warning LED light up?
Don't change parts without measuring what is wrong. The 2N3055 is simply an emitter-follower that has its base voltage about 0.62V higher than its emitter voltage at this low current. The driver transistor (I do not know which one you used) is also an emitter follower with its base voltage also about 0.62V higher than its emitter. Then for 12V output the output of opamp U2 should be about 12V + 0.62V + 0.62V= 13.24V. What is the base voltage of the driver transistor and the output voltage of U2 when the voltage drops? Does the unregulated supply at C1 measure 32V or more when the output voltage drops?
Don't change parts without measuring what is wrong. The 2N3055 is simply an emitter-follower that has its base voltage about 0.62V higher than its emitter voltage at this low current. The driver transistor (I do not know which one you used) is also an emitter follower with its base voltage also about 0.62V higher than its emitter. Then for 12V output the output of opamp U2 should be about 12V + 0.62V + 0.62V= 13.24V. What is the base voltage of the driver transistor and the output voltage of U2 when the voltage drops? Does the unregulated supply at C1 measure 32V or more when the output voltage drops?
audioguru2
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I revised the circuit, not the pcb. Others have done an Eagle pcb.
I agree that there should be a place for storing the revised project.
I agree that there should be a place for storing the revised project.
Dissertation Help
- Sep 20, 2016
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Hello
I am new here
Regards
Dissertation Help
I am new here
Regards
Dissertation Help
audioguru2
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A dissertation is an essay. Why do you want an essay? Don't you understand what the parts in the circuit do? Do you know how transistors and opamps work?
audioguru2
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Hi Army Vet.
Did you build the original circuit or the modified circuit? Which power transformer and opamps did you use? The dim power LED indicates that the transformer voltage is too low maybe because it is overloaded with something shorted or because the rectifier diodes are connected wrong. Maybe the main filter capacitor is connected backwards?
When the current regulator light is turned on what is the load, the voltage setting and current setting?
When the output voltage is 1V what is the load, the voltage setting and current setting?
Did you build the original circuit or the modified circuit? Which power transformer and opamps did you use? The dim power LED indicates that the transformer voltage is too low maybe because it is overloaded with something shorted or because the rectifier diodes are connected wrong. Maybe the main filter capacitor is connected backwards?
When the current regulator light is turned on what is the load, the voltage setting and current setting?
When the output voltage is 1V what is the load, the voltage setting and current setting?
Janisstals
- Nov 18, 2016
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Hello. This is my first post here. So HI!
im building this power supply, but I have ordered already premade board from eBay(the red ones). I have soldered and it's all working correctly (sort of). But I'm having trouble with current limiting. When the pot is turned all the way up the current can spike to 10-15 amps depending on the load and then fry the power transistor. I doo have a transformer that can supply more than 3a but that shouldn't work like that. So does anybody have an idea how to fix it?
Sorry if it has already been dicused in this forum, but google didn't provide answers and 75 pages is just too much.
im building this power supply, but I have ordered already premade board from eBay(the red ones). I have soldered and it's all working correctly (sort of). But I'm having trouble with current limiting. When the pot is turned all the way up the current can spike to 10-15 amps depending on the load and then fry the power transistor. I doo have a transformer that can supply more than 3a but that shouldn't work like that. So does anybody have an idea how to fix it?
Sorry if it has already been dicused in this forum, but google didn't provide answers and 75 pages is just too much.
audioguru2
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I do not know which circuit the ebay pcb uses, maybe it is wrong. The original circuit and the ones modified on this website use a 0.47 ohm current sensing resistor R7. Then when the current setting pot is turned to maximum the current is limited to about 3.0A. The Chinese modification for the Banggood kit might do it differently.
Janisstals
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@audioguru would you be so kind to point me to the latest revision schematic? I will attempt to transcribe it to KiCad and create a repository on GitHub so that people will be able to easily access the latest version. Later perhaps we'll even find people to perform the pcb routing in KiCad.
Thanks!
Thanks!
audioguru2
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February 23 above on this page has the latest schematic of the revised 3A lab power supply.
