audioguru

The 10A darlington will have a voltage drop of about 2V at 3A which might be OK.

I have used very high power audio amplifiers and they do not limit the inrush current to the main filter capacitor. But they used torroid transformers that might not saturate like an ordinary power transformer. When the power switch is turned on then maybe the inrush current almost saturates the core of the transformer then after a short time the main filter capacitor is charged and the transformer current becomes much less which causes it to develop a flyback voltage that we are measuring. A power resistor will reduce the inrush current and a relay can short it for normal operation.

The voltage drop of 180 ohms at 3A is 540V! If you use 1.8 ohms then its voltage drop at 3A is 5.4V which might prevent the output from reaching 30V. With 1.8 ohms then the size of the zener diode must be huge.

I guess you know the 180 ohm resistor reduces the maximum output current to almost zero for an output voltage of 30V. The maximum output current with an output of 5V is only about 150mA.

For anyone who has the same problem that I discovered and fixed. I designed this project almost 10 years ago. This problem occurred on two projects that were 8 and 9.5 years old. The first LED remained lit when it should be turned off and each blinked LED had a light trail behind it. The cause was one LED leaks current when it has a reverse bias voltage. I didn't want to replace the leaking LED because the replacement LED will probably look different. The output of the 74HC4017 driver IC goes high to turn on an LED when the transistor that drives the cathode low blinks it for a moment. Then during the pause in the chasing when all the LEDs should be turned off the transistor is turned off. During the pause the IC driver is reset which causes its first output to go high. The first LED should not light because the transistor driving its cathode low is turned off. BUT all the other IC outputs are low during the pause then the leaking LED conducts which lights the first LED. I fixed it by adding a 1.8k to 10k resistor parallel with the first LED and adding a 10k resistor from the collector of the transistor to the positive supply.

With a 30V power transformer that is 31V when the project has no load then the peak voltage is 43.8V and the bridge rectifier drops it to about 42.6V which is the positive supply for U2. Then what voltage will you use for a spike cutting zener diode? A 28V transformer will be 29V when the project has no load has a peak of 41V and the bridge rectifier drops it to about 39.8v. Zener voltage?

Please provide dynamic details. The internal resistance rise of the battery as it runs down? The voltage drop at the output of the battery as it runs down? The temperature rise of the discharging battery? What else?

The circuit at diyfan.blogspot has some of the problems of the original circuit here plus one: 1) The new problem is that the datasheet for the TL081 shows that its negative supply should be a minimum of 4V so that its inputs work properly all the way down to ground. Then when you add the +33V supply the resulting 37V is higher than its maximum allowed supply of 36V. The TLE2141 has a max supply of 44V and does not need a negative supply. 2) When you calculate the minimum beta for the driver and output transistors then the maximum current required from the TL081 is 1.6mA and its maximum voltage drop is about 2V. The total maximum voltage drop of the Vbe of the driver and output transistors is 2.1V. The maximum voltage drop of R14 is 1.6V. Then the total voltage drops are 2V + 2.1V + 1.6V= 5.7V so the maximum output at 3A is only 33V - 5.7V= 27.3V if the main filter capacitor has no ripple (but it does have some ripple). 3) If the output has a low voltage or is shorted then at 3A the output transistor heats with 40.4V minus 1V for R7 = 39.4V x 3A= 118W. It will melt.

The output of the project should go down to exactly 0V when it has 2.7K for a load and C7 is a film cap. Then the output of U2 will be about +1V (it can go down to at most 100mV).

The maximum input offset voltage of a TLE2141C is only 1.4mV so the input offset adjustment should change the output plus and minus 1.4mV or more. The minimum output voltage of the TLE2141C (when the output is saturated as low as it can go) is about +100mV. U2 has no negative feedback when the output of this project has no load. A small leakage current in the driver or output transistors will cause the output voltage to rise then the negative feedback will force the output of U2 to go as low as it can go and maybe oscillate up to 1V and back down over and over. We should add a low current load from the output of the project to ground. Maybe 2.7k at 1W.

Why does Texas Instruments show the RV1 circuit we are using for nulling the input offset voltage of their TLE2141 opamp then? Please double-check your pcb that pin 4 of U2 and R10 both connect together and to the 0V of the main filter capacitor. If C7 is an electrolytic then its dielectric absorption will produce a positive output of the project when there is no load. Since U2 no longer has a negative supply then it cannot cause an output negative offset voltage so adjusting the output offset should be zero to positive only. I think the output of U2 should be monitored when RV1 is adjusted because it should never go below about +1V (two low current base-emitter voltage drops) when the voltage pot is zero. With too much negative input offset adjustment on U2 then the output of the project will be 0V (it can't go negative) but the output of U2 will saturate at close to 0V instead of being linear at about +1V. Also check that adjusting RV1 slightly increases or slightly decreases an output voltage.

Certainly, But we are not using the low max supply voltage TL081 opamp that has the inversion problem. The opposite. An inductor does not producer an inrush current. Charging the main filter capacitors quickly produces the inrush. The inductance of a transformer slows the rise in current for each half cycle. I don't know how many half cycles it takes to fully charge the huge main filter capacitor. I do not know what produces a 36V spike or high frequency oscillation at power off. Maybe you will kill the spike or oscillation with Q1 without knowing why it happened.

Doesn't the linear regulator eliminate the voltage spikes caused by U1? Redwire said it did. Does the linear regulator produce less noise than the zener diode/U1 circuit? When the mains power is turned off then the 47uF filter capacitor C3 for the negative supply takes time to discharge. The speed of any little NPN transistor is much faster. Then the slow discharge time of C3 delays activation of Q1. Then the voltage spikes are not completely squashed. When a powered inductor is suddenly unpowered then it produces a voltage spike. Then does a powered transformer that is suddenly unpowered when its mains current is at a peak also produce a voltage spike?

Hi Liquibyte, On your measurements it looks like Q1 turns on a little too late to completely squash the voltage spike but it reduces it a little. Are we barking up the wrong tree?? (I haven't said that for about 50 years) Without Q1 the 'scope photo from Redwire shows 24MHz ringing when the power is turned on and when it is turned off. But in post #? (this site does not have post numbers) he shows no ringing when he replaced U1 with a voltage regulator. I don't think he uses Q1.

I found Redwire's 'scope photos of the 24MHz output ringing at turnon and shutdown on page 141 on May29/2014. The output goes -30V then +30V back and forth many times. I cannot read the settings on his 'scope so I don't know how much delay occurs from when shutdown occurs to when the ringing begins. What causes the ringing? The 2N3055 output transistors are slow with an fT (no gain) at about 3MHz but the BD139 driver is fast with an fT at 190MHz. The TLE2141 opamp has an fT at 6MHz but its output cannot slew faster than 700kHz.

I do not know how a meter can show a spike. It can show a good long pulse but not a short duration spike. Didn't Redwire discover and measure the spike with his 'scope? Where is the post?

Hi Liquibyte, Your Rev7 looks good. You have R13 as 10k but I calculated with 12k but it doesn't make much difference. Leave it as 10k. When Q1 shorts the output of U2 and the base of Q2 to ground then R15 and D10 will prevent the charge on C7 (and on an external load capacitor) from causing excessive reverse voltage on the base-emitter junction of Q2. But Q1 causes a nightmare: 1) Q1 shorts the output of opamp U2 to ground. 2) When the output of U2 is externally forced to ground then its feedback forces its output to go high with all the current the opamp can provide. A nice fight. R15 reduces the current in the fight to a safe amount but causes a voltage drop so the output of the project might not go as high as 30V when it is loaded with 3A. It might be better if Q1 shorted the input pin 3 of U2 to ground then there will be no fight and R15 is not needed. Test it with Q1 shorting the input of U2 to ground to see if there is still no spikes.

Hi Liquibyte, I am glad it is fixed. I didn't build this project so please can you show a 'scope photo of the output noise from U1 and a comparison to the output noise of a voltage regulator? I think the old zener diode is a noise source. Can you also show a 'scope photo of a comparision in time of when the power is turned off and when the collector of Q1 drags down the output voltage? I think there will be a delay while C3 discharges. Please update the schematic to Rev.7 showing the new voltage regulator as the reference voltage. My copy of Redwire's schematic is 2008 with at least one error so I do not know what he has done lately. Please post his updated schematic too.

Sorry, I made a mistake converting the original schematic to the fixed and improved one. You do not want the negative supply pin 4 of U2 to be negative -1.3V because then U2 will have its total supply voltage very close to its maximum rating of 44V (+42.6V plus -1.3V= 43.9V). U2 does not need a negative supply so connect its pin 4 and R10 to the 0V from the unregulated power source. Most little NPN silicon transistors have avalanche breakdown (like a zener diode) of their base-emitter junction when it is reverse biased more than about 6V so maybe you have the emitter and collector pins of Q1 reversed? Here is my corrected schematic:

To add Q1 back in I calculated the voltage divider values like this:

The original circuit used R13 and R14 to keep Q1 turned off when the positive unregulated supply was about +28V and the negative supply was -5.6V. When the mains power was turned off then the negative supply disappeared first allowing R13 to turn on Q1. To add Q1 to the latest circuit then the value of R14 must be decreased a lot so that the base voltage of Q1 is at a negative voltage when the circuit is working. The 0V (ground) of both circuits is the output 0V, except the adjustable reference voltage for U3 from R17. The original circuit had -5.6V for the same negative supply for U2 and U3 (R10 was connected wrongly to 0V instead of to the negative supply). The latest circuit uses an opamp for U2 that works perfectly without a negative supply so it does not have a supply voltage more than its maximum rating of 44V. U3 needs to have its output go negative one diode voltage drop so that D9 can pull down the drive voltage during current regulation. Then the negative supply for U3 was made -1.3V. R15 was removed because it didn't do anything useful and it wasted valuable output drive voltage. D10 also never did anything and still doesn't do anything.

On the Tektronics circuit it looks like transistor Q15 shorts the output of the voltage error amplifier opamp U45 to ground when the main power is turned off. I did not see their discussion of it. I do not know which opamp they use since they do not use standard part numbers. The original circuit of this 0V-30V power supply here used the TL081 opamp that has the problem called "Opamp Phase Inversion" which causes its output to suddenly go high when an input voltage goes outside its allowed negative input common mode voltage range. The allowed negative input voltage range for the TL081 is a maximum of 4V more positive than its negative power supply voltage which was -5.6V. Then when the main power is turned off the negative -5.6V supply disappears quickly and causes the input of the voltage error amplifier opamp U2 to have a voltage outside its allowed input common mode voltage range. Then it would cause the output voltage of the project to go as positive as it can while the huge main positive filter capacitor was discharging. They added Q1 to short the output of U2 to ground when the main power was turned off. The new MC33071 and TLE2141 opamps do not have this problem so transistor Q1 was removed. Adding Q1 back might not fix the existing voltage spike because it might not activate quickly enough. Liquibyte, can you try it please?

The dual rows of pins need a pcb designed for it. Maybe you can make a little pcb for the IC pins then use wires to connect to the pcb you purchased.

Not enough details. Is the IC too big or is it too small to fit on the pcb? What is the IC? Is it an audio amplifier with two rows of terminals?

Thanks, Liquibyte. I hope you or Peter K will select and add one of the excellent pcb designs and place it in the sticky.