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Is 30mV still acceptable as the lowest voltage for a linear power supply?
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duak:
Samuel, you have designed and built a very good power supply with some interesting specifications.  You have used a single supply DAC and a single supply precision autozeroing opamp and expect it to give repeatable results.  I know that your supply is not two quadrant and cannot accept current from the load.  What I propose is a simple experiment to see what happens when pulses are injected into the supply at low voltage settings.  I expect that it will be fine.  However, it may act strangely.

The LM358 circuit I mentioned above is a friend's that misbehaved in an unexpected way when he connected to a purchased PWM servo.  The conducted EMI from the servo was enough to cause a problem.  He is using it now so I can't try different opamps in it.   I wouldn't be surprised if a rail to rail opamp shows anomalous operation with its inputs and output at 0 V and some high frequency noise injected into its various ports.

Another little story: in the 80's I developed a PWM servo using MOSFETs for the switching devices.  I used LEM Hall effect current sensors to measure the motor currents instead of series sense resistors.  The LEM sensors were specified to better than 1% accuracy and stability.  On the bench with a DC or AC test current they were perfect.  In the servo, they had an unpredictable error caused by the high frequency noise on the load current.  Some sensors were better than others.  I found that if I added a 1n0 capacitor inside the LEM sensor, I could eliminate the problem.  LEM analyzed the problem and produced a shielded version that also worked.  Later on, I found that LEM replaced the LM301 opamp with an OP37 opamp in an unshielded case and it worked fine.  I will also say that some 7912 negative three terminal regulators used in the servo would have unstable output voltages when the PWM was on.  The regulators were from one manufacturer but we didn't have a long enough product run to nail it down - they were just replaced.  That part worked fine everywhere else; some just had a problem in a high EMI environment.

Cheers,
bloguetronica:

--- Quote from: duak on April 12, 2019, 04:49:33 am ---Samuel, you have designed and built a very good power supply with some interesting specifications.  You have used a single supply DAC and a single supply precision autozeroing opamp and expect it to give repeatable results.  I know that your supply is not two quadrant and cannot accept current from the load.  What I propose is a simple experiment to see what happens when pulses are injected into the supply at low voltage settings.  I expect that it will be fine.  However, it may act strangely.

The LM358 circuit I mentioned above is a friend's that misbehaved in an unexpected way when he connected to a purchased PWM servo.  The conducted EMI from the servo was enough to cause a problem.  He is using it now so I can't try different opamps in it.   I wouldn't be surprised if a rail to rail opamp shows anomalous operation with its inputs and output at 0 V and some high frequency noise injected into its various ports.

Another little story: in the 80's I developed a PWM servo using MOSFETs for the switching devices.  I used LEM Hall effect current sensors to measure the motor currents instead of series sense resistors.  The LEM sensors were specified to better than 1% accuracy and stability.  On the bench with a DC or AC test current they were perfect.  In the servo, they had an unpredictable error caused by the high frequency noise on the load current.  Some sensors were better than others.  I found that if I added a 1n0 capacitor inside the LEM sensor, I could eliminate the problem.  LEM analyzed the problem and produced a shielded version that also worked.  Later on, I found that LEM replaced the LM301 opamp with an OP37 opamp in an unshielded case and it worked fine.  I will also say that some 7912 negative three terminal regulators used in the servo would have unstable output voltages when the PWM was on.  The regulators were from one manufacturer but we didn't have a long enough product run to nail it down - they were just replaced.  That part worked fine everywhere else; some just had a problem in a high EMI environment.

Cheers,

--- End quote ---
Hi duak,

I'm guessing that this power supply might be unstable when dealing with the residual currents that are found in many SMPS. I've had a weird experience with a power supply what used a similar topology as this one, and had some weird fluctuations when it was connected to a constant current load that was being supplied by a SMPS. See the last post of this thread:
https://www.eevblog.com/forum/projects/usb-controlled-precision-power-supply-(or-voltage-reference)/msg2007782/#msg2007782

However, I think this is an issue with the opamp that was used there. This supply doesn't show that susceptibility.

On another note, what voltages do you have in mind for the test (output and square wave voltages)? I don't wish to fry the signal generator, BTW.

Kind regards, Samuel Lourenço
David Hess:

--- Quote from: duak on April 12, 2019, 04:49:33 am ---I will also say that some 7912 negative three terminal regulators used in the servo would have unstable output voltages when the PWM was on.  The regulators were from one manufacturer but we didn't have a long enough product run to nail it down - they were just replaced.  That part worked fine everywhere else; some just had a problem in a high EMI environment.
--- End quote ---

Negative regulators built on the common NPN process have a common emitter output so they have special stability requirements which are reflected in the datasheets always showing an output capacitor with a minimum ESR.  They have more design, process, and application variations in performance.
duak:
Samuel, to inject a signal into the power supply output, I would use a 10n capacitor in series with a 47R resistor from a square wave generator set to 1 kHz 1 Vp-p.  The impedance of the generator and the RC network will limit current and prevent any DC current from entering the output the generator.  The power supply output capacitance will attenuate the signal so you will have to observe the output with a 'scope to see the actual signal and if it triggers something unusual.  I recomend 100 mVp-p as a target measured on the power supply because that was the voltage that I observed on the LM358 circuit.

Regarding the other power supply, I don't understand the test conditions and what is connected together and how.

Cheers,
bloguetronica:
Hi duak,

Setting the supply to 500mV, would be good enough for the test? Anyway, what it is to be expected? I'll test this as soon as I can, and post the results.

Meanwhile, here are some more photos of the tests done so far. Voltage was set to 1, 2, 4, 8 and 16V. Precise as usual. I'm surprised!

Kind regards, Samuel Lourenço
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