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Op amp Linear power supply
Op amp Linear power supply
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Old 18th December 2017, 11:32 PM   #1
Marksd is offline Marksd  United States
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Default Op amp Linear power supply

I have a question concerning the following. I am only a small time self taught hobbiest. I have a 741 op amp driving a TIP 102 NPN Darlington current pass transistor. I have two loads attached. One load draws .240 amps. The second load draws 1.150 amps. I can set the voltage output for the first load at 11.8 volts. When i switch the to the 1.150 amp load the voltage drops by almost .3 to .5 volts. My supply is 15 VDC. I use a 50 ohm resistor for the .240 Amp load and a 10 ohm resistor of for the 1.150 amp load. The transistors are heatsink mounted when testing. The heatsink temperature rises from about 23C to 77C with the higher amperage load. I also tried a PNP TIP 147 darlington and while it functions better suffers from the same larger than expected voltage drop changing from one load to the another. I have 2.4 to 3.2 volts on each darlington from collector to emitter so i think these are in saturation. I used the darligntons due to the op amp output current. The spec sheet says i only need about 300 microamps to get then current output. So, I tested a IRF9540 P channel mosfet with the same two loads and it holds within .04 volts regardless of load. Regulation from cold to hot is almost 99% accurate. Same temperature rise on the heatsink with the mosfet. I think I am operating the mosfet in saturation, but just barely.

Please help with any explanation as to why the darlington are losing, (not regulating) the voltage between current loads and any improvements I can make to allow the use of the darligntons. Regulation changes with an increase in temperature. How can I fix this too. I don't care whether I use NPN or PNP because they are inexpensive. Thanks.
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Old 18th December 2017, 11:45 PM   #2
rayma is offline rayma  United States
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Quote:
Originally Posted by Marksd View Post
why the darlington are losing, (not regulating) the voltage between current loads
Can you post a schematic?
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Old 21st December 2017, 03:45 AM   #3
dotneck335 is offline dotneck335  United States
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I still challenge anyone to beat this supply for $10!!
LM317 / LM337 +/-1.5V~37V Adjustable Dual Voltage Regulator Power Board | eBay
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Old 21st December 2017, 09:37 AM   #4
r_merola is offline r_merola  Brazil
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Quote:
Originally Posted by dotneck335 View Post
I still challenge anyone to beat this supply for $10!!
LM317 / LM337 +/-1.5V~37V Adjustable Dual Voltage Regulator Power Board | eBay
Do you have schematics of this power supply?

It is attractive for lower cost and good presentation, but what is the function of the DIP8 operational amplifier as it have 2 voltage adjusts for positive and negative rail voltage. (supposed some kind of noise filter)
One problem of this kind of symmetrical regulator is the lack of protection systems to turn off both regulators when one fails.
Regards
Ronaldo
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Old 21st December 2017, 10:34 AM   #5
infinia is offline infinia  United States
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Quote:
Originally Posted by Marksd View Post

Please help with any explanation as to why the darlington are losing, (not regulating) the voltage between current loads and any improvements I can make to allow the use of the darligntons. Regulation changes with an increase in temperature. How can I fix this too. I don't care whether I use NPN or PNP because they are inexpensive. Thanks.
Historically PNPs were double the cost at lesser specs than NPNs.

You are probably running out of "headroom" E.g. the voltage difference required between regulators pass input and the output and still remain in "regulation" under all operating conditions.( low line voltage, etc )
1) The regulators Vinput drops under load.
Consider the raw DC voltage will drop under load and it has AC ripple on it/. you can measure or estimate both. You must subtract from the average DC the lowest ripple valley. E.g. negative peak AC ripple component under full load is technically the correct Vin. Its normal to have 1 Vrms ripple at full load which is about 2.8 V peak-peak, so it's very possible for this to be worse if the bulk filter caps are wimpy.
2) The regulator needs the V error op-amp's output level and current to support the transistors Vbe's and any beta droop.
Look at the devices data sheets to calculate the level drop starting at the Op-amps Vout thru two Vbe's of the darlington. Next look at the op-amp current needed to support the full load. One trick is to tie the op-amps supply a few volts higher than the raw DC. The op-amps output current could still be too weak. But it's better to test it and see what headroom you have 1st. I'd suggest drop the output voltage by 2 or 3 volts and recheck.

Things to consider , either all or in combination of.
a) Use a beefier' Xfmr and filter capacitor.
b) Use a better op-amp and /or run it from a higher voltage.
c) Check the pass and driver transistors betas. Change the whole topology for "low drop out" performance. one thing you said a MOSFET pass element works with your op-amp and reference, right?
d) Reduce your specs E.g Output current and voltage to account for poor headroom. (this is the easiest solution)
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Last edited by infinia; 21st December 2017 at 11:00 AM.
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Old 21st December 2017, 05:00 PM   #6
Jenyok is offline Jenyok  Russian Federation
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LM317 load capacitance
.
See those circuits. They specially disigned for low noise application. MIC preamp, RIAA, playback amp, DAC converters.
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Old 21st December 2017, 08:17 PM   #7
MorbidFractal is offline MorbidFractal
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You make a reference to the spec sheet for the Darlingtons. Did you consider the spec sheet for the operational amplifier?

http://www.ti.com/lit/ds/symlink/lm741.pdf

Check the output voltage specifications. Page 5) 6.5] Minimum output swing is +/-12V.

As infinia mentions your NPN will drop an extra 1.2V so it will likely be slightly rubbish. Your PNP may be better because you gain an extra 1.2V, pull down to turn it on. The P-Channel mosfet will be happiest of the three because the gate will need to be about 3V/5V below source for it to start conducting. It will also need less drive.

You also need to check Fb/Ft for your chosen transistors.

Also the 741 only has a Gain Bandwidth Product of 1MHz

The following is an 'idealised' LTSpice model which may give you some ideas about how to do it. Also includes a 'concept' of foldback current limiting. It also lets you check the Loop Gain. Plot V(A)/V(B).

Loop stability is important.

Code:
Version 4
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TEXT -536 792 Left 2 !.LIB OPAMP.SUB
TEXT -536 816 Left 2 !.ac dec 50 100 1E6
TEXT -536 768 Left 2 !.MODEL ZID D(Ron=10m Roff=1E9 Vrev=5V)
Attached Images
File Type: png Screenshot at 2017-12-21 21:16:59.png (42.7 KB, 916 views)
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Old 23rd December 2017, 12:43 PM   #8
dotneck335 is offline dotneck335  United States
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Quote:
Originally Posted by r_merola View Post
Do you have schematics of this power supply?
....... but what is the function of the DIP8 operational amplifier as it have 2 voltage adjusts for positive and negative rail voltage. (supposed some kind of noise filter)
LF353 provides feedback to lower noise.
Attached Files
File Type: doc LowVoltageRegulatorWithOpampFeedback.doc (87.0 KB, 87 views)
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Old 23rd December 2017, 02:04 PM   #9
Mooly is offline Mooly  United Kingdom
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Op amp Linear power supply
Quote:
Originally Posted by rayma View Post
Can you post a schematic?
This is the most important point. We need to see the circuit to advise.

The most important points are that the output voltage itself is monitored and included within the feedback loop to the opamp. Secondly, if using an NPN pass transistor then you must ensure that there is sufficient headroom available for the opamp which simply means 'is the supply voltage high enough'.

If those points are covered then the circuit should show no appreciable change in output voltage with varying load.

Here are the two scenarios. See how the opamp output voltage is quite high in the second example as it looks to drive the darlington pair.
Attached Images
File Type: jpg R1.JPG (160.3 KB, 855 views)
File Type: jpg R2.JPG (161.7 KB, 841 views)
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Old 23rd December 2017, 05:20 PM   #10
infinia is offline infinia  United States
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The most important thing is the measured voltages DC + AC under load.
The schematic alone wont tell us were the problem is. From the OP symptoms points to what the fundamental problem is, loses regulation under load. The best advise was to lower the output voltage until it regains regulation, If it doesn't happen then the circuit is broken. Infact he could use a potentiometer to determine the required headroom for any reasonable Vreg schematic. I agree schematics are helpful but necessarily mandatory given the OP's level and experience.
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