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Old 26th December 2017, 02:56 PM   #11
Marksd is offline Marksd  United States
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These are the schematics of what I am using foVoltage Regulator Circuits.pdfr experimentation.
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Old 26th December 2017, 03:08 PM   #12
Marksd is offline Marksd  United States
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Infinia, I get perfect regulation regardless of load at about 9.8 Vout with the TIP147 or TIP 42C. I notice the Vce drops to about 2.3 to 3 volts. I read this as moving slightly out of satusartion and maybe this could be causing Vout to droop with the TIP 147 and TIP42C. There is zero affect using the IRF 9540 P channel mosfet. I mentioned I am just a hobbyist. I read something and then experiment on my breadboard to see if I can make it work. I struggle with the the why it works and why it does not work question. I do not have a dual rail supply only a fixed voltage 15 volt 2.5amp supply.
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Old 26th December 2017, 04:45 PM   #13
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In the case of bipolar pass topology, note 741 error amp must rely on high state to drive the bipolar pass element, which looking at the data sheet Voh ~ 2 to 3V drop for a 10K/1mA load*. On top of that you must add both junction drops from the darlington to get the minimum headroom. This can be ~ 3.5V or more. Note the data sheet numbers are measured without fine grained linearity, E.g the error amp is resting at the stops. So call it ~4 volts of headroom. So the target regulator output voltage s/b less than or equal 11.0 Vdc Not counting any wiring drops or ripple, which can be significant when looking at millivolt changes at 1.3A. So youre reporting Vce ~ 2.3 -3 Volts which is much less than my rough estimate of 3.5-4.0 headroom, right? Consider to take more careful measurements. Are you sure there isn't AC ripple or wiring drops on your 15V source?
The MOSFET pass element works with less headroom because the 741 pulls the gate lower to drive the output harder. So 741 Voh/Vol is not at the end stops, it remains linear.

*worst case 741 load is Ib ~ Iout/Beta 1.3A /1000 or 1.3mA
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Last edited by infinia; 26th December 2017 at 05:07 PM.
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Old 26th December 2017, 04:58 PM   #14
infinia is offline infinia  United States
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Quote:
Originally Posted by Marksd View Post
Infinia, I get perfect regulation regardless of load at about 9.8 Vout with the TIP147 or TIP 42C. I notice the Vce drops to about 2.3 to 3 volts.
Vout+Vce = Vin
9.8+3.0 =12.8 V so your Vin is not 15V anymore.
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Last edited by infinia; 26th December 2017 at 05:08 PM.
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Old 26th December 2017, 05:24 PM   #15
Mooly is offline Mooly  United Kingdom
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Op amp Linear power supply
Try adding a zener (say 6.2v or thereabouts) in series with the base drive, and also add a B-E resistor of around 10k.

The PNP series pass transistor begins to turn on when the 741 output drops 600mv below the incoming supply... which is no good. The FET version overcomes that by reason of the FET having a high turn on voltage.

Also, the 3.9v zener is liable to fluctuation due to the incoming unregulated supply varying under load/ripple and should therefore be configured to be biased from the regulated output.
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Old 27th December 2017, 05:15 PM   #16
Marksd is offline Marksd  United States
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Thanks to all for the answers. I am off all week and will begin experimentation Friday. I am also experimenting with the TL431. These devices are very interesting also. For now one problem at a time. Thanks again and I will let everyone know how it turns out. Mark
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Old 27th December 2017, 05:19 PM   #17
Mooly is offline Mooly  United Kingdom
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Op amp Linear power supply
The TL431 is an excellent device if you need the level of performance it offers (stability and stability vs temperature). Lots of options and possibilities once you start looking.
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Old 29th December 2017, 09:18 PM   #18
Marksd is offline Marksd  United States
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Mooly, I tried the circuit you sent and it worked with one exception. I could not get the 3.9 volt zener to conduct/regulate from the regulated output so I connected it back to the V+ rail. Vout regulation is within .01 volt across the load conditions with a TIP 42c, and it did not matter if the circuit was cold or hot. Vout remains steady. I tried the TIP147 darlington and it could not manage the regulation across the load. Vout varied over 2 volts from the 50 ohm to 10 ohm load. I also changed out the 3.9 volt zener for a 4.7 Volt zener as I thought your circuit wanted me to do that. Here are more questions. What does the 6.2 volt zener do along with the 10K resistor? Why did the darlington not regulate as perfectly as the TIP 42C? Can this circuit work for NPN transistor use by moving the inverting and non inverting op amp connections and the V+ accordingly on the NPN?

Thanks again.
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Old 30th December 2017, 08:13 AM   #19
Mooly is offline Mooly  United Kingdom
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Op amp Linear power supply
Easy ones first...

I had not got a 3.9 volt model for the simulator so I used a 4.7 volt.

The 10k resistor guarantees the transistor base does not 'float' under any conditions. Its just good practice really. If the 6.2 volt zener was non conducting, and if the transistor was a high gain (or darlington) device then unexpected things might happen without the resistor.

The 6.2 volt zener is non critical, an LED or 8.2v or 2.7v zener should all work. All it does is force the opamp output to operate away from the positive rail.

For example, if the unregulated rail is 17 volts then the transistor will fully conduct when the base is pulled just 700mv or so below that value. That means the opamp output has to operate in the 17v to 16.3 volt region, something most opamps won't do. So we force it to work in the centre of its range by adding a 'volt drop' to the output. Now the opamp has to go below 6.2 +0.7 volts before the transistor conducts. So the opamp is happy and the opamp output is now at around 7 volts in normal operation.

The circuit should work feeding the zener from the output. Could the basic circuit be oscillating ? A scope check would show any problem there.

You could try adding a 10uF across the regulated output which would be good practice anyway. Also try a small cap (say 0.1uF) across the zener, again good practice.

If the circuit was oscillating then we cold look at a small cap across the feedback network.

Another odd problem that can occur in some circuits is 'failure to start'. If you had no output voltage when initially switched on then that could be a possibility... if you had voltage and it appeared just not to be regulating then oscillation is much more likely.

I would try the zener connected back to the output again and see if you can get it to work by adding the caps.

An NPN transistor can be used as long as there is enough 'headroom' or voltage differential between unregulated input and the regulated output voltage. 17 volts supply and 12 volts output leaves just 5 volts to play with. If you used a darlington NPN then you would need at least 13.4 or so drive voltage... and we are getting close to the supply. The PNP has no such restriction, we can pull the base as low as needed. PNP is the correct choice usually (for a positive regulator).
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Old 1st January 2018, 02:39 AM   #20
PRR is offline PRR  United States
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Quote:
Originally Posted by Mooly View Post
...I had not got a 3.9 volt model for the simulator so I used a 4.7 volt.....
You may already know this; or it may be difficult in your software....

Depending on your sim interface....

I use Edit, Model, Model text, and change the 4.7 to 13 or 3.9 as needed.

This will not be exact in all cases. The shape of the knee varies, especially from Zener to Avalanche zone. But to just set a voltage, or offset an opamp output, it works good enough for most cases.
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