Author Topic: Bogus parts from Reputable Dealer?  (Read 7454 times)

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Offline wraper

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Re: Bogus parts from Reputable Dealer?
« Reply #50 on: January 21, 2020, 12:27:32 am »
... and BTW, this particular circuit with meter in series with input is not suitable for CMOS op amps like TLV2371 due to very high input impedance. You are basically connecting voltmeter in series with tiny capacitor.

Erm, that's a voltage source symbol, symbolising the offset voltage. Why would you even believe that was a meter? There would be no point of adding gain to then just go and read the unamplified signal at the input.
OK, I had a brainfuck.
« Last Edit: January 21, 2020, 12:33:42 am by wraper »
 

Offline wraper

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Re: Bogus parts from Reputable Dealer?
« Reply #51 on: January 21, 2020, 12:30:46 am »
Please measure input offset voltage properly. First of all, it's as ebaster said. Secondly, "good opamps" most likely simply have input offset voltage of opposite polarity.

Before I get started on this experiment . Do I need a split supply , single supply or no supply .? I'm guessing VDD must be 9V  but I want to get this right . I want to be sure as to probe orientation . The  voltage at pin 3 must be exactly 4.5V ? I'll try to find exact or as close as I can for the resistors.
I would like to do this right so I give you the right measurements.
With circuit I posted you don't need split power. 2 resistor divider create a reference point at half of the power source voltage, which you can also consider as virtual ground. With split power, you don't need those resistors and can simply tie non-inverting input to ground.
 
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Offline rstofer

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Re: Bogus parts from Reputable Dealer?
« Reply #52 on: January 21, 2020, 12:56:38 am »
Before I get started on this experiment . Do I need a split supply , single supply or no supply .? I'm guessing VDD must be 9V  but I want to get this right . I want to be sure as to probe orientation . The  voltage at pin 3 must be exactly 4.5V ? I'll try to find exact or as close as I can for the resistors.
I would like to do this right so I give you the right measurements.

As Wraper's circuit is drawn, you need +9V at the Vdd pin of the op amp and 0V (the other side of the power supply) at Vee but we will call that "Ground".

Then you need to get a voltage APPROXIMATELY equal to Vdd / 2 (ie 4.5V) with 2 resistors.  Close is good enough.  Remember, I am advocating for a potentiometer with end resistors.  How well matched will that be when you twiddle the knob from end to end?

We're trying to get Vos but, really, we're looking for op amp functionality.  With the follower circuit, the output of the op amp should match the non-inverting input, more or less, due to offset voltage.  Again, close is good enough.  We know that Vos should be < 4.5 mV when the non-inverting input is almost exactly Vdd/2 but it shouldn't be wildly different anywhere else.

 
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Offline JwillisTopic starter

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Re: Bogus parts from Reputable Dealer?
« Reply #53 on: January 21, 2020, 02:26:35 am »
I noticed that you made a mark from pin 3 to pin 6 so I took 2 measurements  with opposite polarities.
Vdd 9.06V    Resistors are 4.707K and 4.706K  Voltage at divider 9.06 V  4.536V Pin #3
Chip # 1 and # 2 are the ones that work in the circuit
Measurements in Volts

   Chip#         Output to grnd           Negative of probe at  pin#6  across to pin # 3          Negative of probe at Pin#3  across to pin # 6

     1                   4.535                                           0.033                                                                          -0.046
   
     2                   4.535                                           0.033                                                                          -0.046

     3                   4.535                                           0.041                                                                          -0.049

     4                   4.535                                           0.039                                                                          -0.048

     5                   4.535                                           0.040                                                                          -0.047

     6                   4.536                                           0.038                                                                          -0.048

     7                   4.535                                           0.036                                                                          -0.049

     8                   4.535                                           0.040                                                                          -0.046

     9                   4.536                                           0.038                                                                          -0.049

    10                  4.535                                           0.036                                                                          -0.048

Not sure what to make of this but chip 1 and 2 have identical measurements .
 
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Offline wraper

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Re: Bogus parts from Reputable Dealer?
« Reply #54 on: January 21, 2020, 02:58:45 am »
I noticed that you made a mark from pin 3 to pin 6 so I took 2 measurements  with opposite polarities.
What are you using for measurements? It's really strange that you are getting different reading by just switching the leads. Not to say that simply measuring output voltage shows only 1mV variation but measuring between non-inverting input and output 10+mV, or did you miss one zero? Also why it's in a volts range. Doesn't your meter have millivolt range?
« Last Edit: January 21, 2020, 03:02:03 am by wraper »
 

Offline rstofer

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Re: Bogus parts from Reputable Dealer?
« Reply #55 on: January 21, 2020, 03:26:49 am »
As pointed out, all measurements between pin 2 and pin 6 seem to be off by an order of magnitude.  If so, all of the offset voltages are in spec.  The simple output voltage test leads me to believe all of the op amps are operating correctly.

Later you can try a 1k load to Vdd and then again to Vee and do the simple voltage measurement again.  This will show whether the output stage can actually drive a load.

All of the devices look good to me - assuming there is a decimal point error in the offset voltages.
 

Offline JwillisTopic starter

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Re: Bogus parts from Reputable Dealer?
« Reply #56 on: January 21, 2020, 04:04:47 am »
I don't know whats going on . The meter is auto ranging . I only have meters that go down that far .
« Last Edit: January 21, 2020, 04:10:50 am by Jwillis »
 

Offline JwillisTopic starter

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Re: Bogus parts from Reputable Dealer?
« Reply #57 on: January 21, 2020, 04:36:54 am »
So if can I  compensate for the offset  externally with a single supply or do I still need a negative rail to adjust the offset ?
 

Offline ebastler

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Re: Bogus parts from Reputable Dealer?
« Reply #58 on: January 21, 2020, 05:46:35 am »
I don't know whats going on . The meter is auto ranging . I only have meters that go down that far .

Your offset readings (measurements between pins 3 and 6) look implausible, especially in view of the very precise and reproducible output voltage (pin 6 to GND). As you have separately measured pin 3 and pin 6 to be at 4.535 .. 4.536 V, stable to within a mV, why would you see more than a mV of difference between them?

Please have another look at your multimeter(s). Most auto-ranging meters have a manual range selection pushbutton which you can use to select the most sensitive voltage range, and avoid the potentially confusing auto-ranging.

Also, could you connect a scope in parallel to your voltmeter when you take those readings between pins 3 and 6? Just to check that no AC noise gets introduced during this measurement. You are connecting some nice long "antennas" there, with the multimeter's probe leads. [Edit: And no oscillations due to an unwanted positive feedback loop, coupled via the probe leads.]
« Last Edit: January 21, 2020, 05:58:13 am by ebastler »
 

Offline wraper

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Re: Bogus parts from Reputable Dealer?
« Reply #59 on: January 21, 2020, 09:08:47 am »
In addition to said above, try attaching some 0.1-1uF capacitor in parallel to one of 4.7k resistors (does not matter which) in voltage divider and measure offset again.
 
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Offline JwillisTopic starter

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Re: Bogus parts from Reputable Dealer?
« Reply #60 on: January 22, 2020, 04:21:56 am »
Good news. I concluded that the op amps are fine. No need to freak out to Digikey. Why 2 would work in the old schematic and the others didn't I want to find out and will continue to to learn why.
Any way what I did  do was make a couple changes to the circuit that work with all the op amps now.  First of was to fix the open loop. so I added a 10K from the inverting input to the output. But like before that didn't  make a difference until I lowered the voltage to the non inverting input. So I added an adjustable voltage divider  to the signal wire that goes to my circuit to lower that voltage to prevent  saturation .Now I have to figure out how to balance the op amps  so every mosfet is taking the same load.

I don't want to go through the agony of reverse engineering the control board right now . But when the load dial is turned the voltage to the gate immediately goes to 7.3 Volts without load . With load a can't determine what the voltage would be  unless I put the  mosfet back into the board . Then as the dial is turned that voltage rises by a few millivots which opens up the mosfet more allowing more  current to be sunk to ground. Why the control board is configured this way I don't know. I should have observed what the control board was doing a  lot better.

This video help me understand what was happening
.

 
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Offline nigelwright7557

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Re: Bogus parts from Reputable Dealer?
« Reply #61 on: February 10, 2020, 01:20:34 am »
I use RS components and Farnell.
I know with them an IR part will be from IR and not an unknown/unreliable source.

The only component I have had a slight problem with was Chinese valves from RS.
Some of them were microphonic but still worked OK.

 

Offline JwillisTopic starter

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Re: Bogus parts from Reputable Dealer?
« Reply #62 on: February 11, 2020, 09:22:00 am »
I use RS components and Farnell.
I know with them an IR part will be from IR and not an unknown/unreliable source.

The only component I have had a slight problem with was Chinese valves from RS.
Some of them were microphonic but still worked OK.

I was still having problems with these units so I took them to a communications tech and he tested them for me. 4 function correctly are with in tolerance,   2 exploded with 12V supply ,5 leak voltage to the output. 1 leaked voltage back to the non inverting side. They are indeed faulty . I contacted digikey and they are sending replacements .   
 

Offline exe

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Re: Bogus parts from Reputable Dealer?
« Reply #63 on: February 11, 2020, 09:57:57 am »
For measuring offset the best bet is a circuit like this:

Does it require matched resistors?
 

Offline Cerebus

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Re: Bogus parts from Reputable Dealer?
« Reply #64 on: February 11, 2020, 01:42:10 pm »
For measuring offset the best bet is a circuit like this:

Does it require matched resistors?

Yup, ideally.

General rule for all differential configurations of op amps is that resistors that share the same rĂ´le want to be matched. So in this case you want the 10R pair and the 10k pairs matched.  If they're not matched it lowers the common mode rejection ratio. If you've got an theoretically perfect op amp with infinite common mode rejection ratio a mismatch of 0.1% will reduce the CMRR to 60dB, a 1% mismatch to 40dB.

What matters most here is the ratio of input to feedback resistor. If you use 1% resistors you'll get a 1% uncertainty in gain, so a 1% basic uncertainty in your measurement. For this particular application the contribution from common mode errors is less important than the contribution from gain errors - you know that you have no common mode signal on the input (it's grounded), so any common mode errors will be from bias current effects on the input resistors which will be negligible in the face of the likely range of input offset voltages. For a [huge] 10uA bias current you'll have a common mode input voltage of 100 uV reduced to 1 uV by the 40dB CMMR.

If you're dealing with ultra-precision op amps with single digit microvolt offset voltages then of course you're going to need to move to much better resistor matching but if you're paying for those kind of op amps you can afford to pay Vishay for some precision matched resistor dividers.

I'd say if you're dealing with >100uV input offset voltages and are happy with results in the region of 1% accuracy then 1% resistors will do you fine. If you're after 0.1% accuracy or dealing in smaller offset voltages then you probably want to go down the route of hand matching some sets of resistors with a good meter (Or you can pay Vishay or someone similar lots of money for top notch matched resistors with low tempcos).
Anybody got a syringe I can use to squeeze the magic smoke back into this?
 
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Offline nigelwright7557

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Re: Bogus parts from Reputable Dealer?
« Reply #65 on: February 19, 2020, 12:24:34 am »
10K is a high value for gate resistor.
I tend to use 390R.
 
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Offline JwillisTopic starter

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Re: Bogus parts from Reputable Dealer?
« Reply #66 on: February 19, 2020, 12:55:22 am »
10K is a high value for gate resistor.
I tend to use 390R.

The current draw at the gate of a mosfet is negligible . In the nanoamps. The Voltage drop across 10K resistor would have little effect to the voltage at the gate.
 

Offline rstofer

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Re: Bogus parts from Reputable Dealer?
« Reply #67 on: February 19, 2020, 01:54:55 am »
But it could have an effect on rise/fall times and, thus, heating.  If it matters...
For power applications like H-bridges, we want to drive Amps into the gate to overcome the capacitance.
 

Offline JwillisTopic starter

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Re: Bogus parts from Reputable Dealer?
« Reply #68 on: February 19, 2020, 06:36:57 am »
But it could have an effect on rise/fall times and, thus, heating.  If it matters...
For power applications like H-bridges, we want to drive Amps into the gate to overcome the capacitance.

Yes if your running a pulse.But not when running in linear. The way its used here in in linear mode.
 

Offline exe

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Re: Bogus parts from Reputable Dealer?
« Reply #69 on: February 19, 2020, 09:32:23 am »
In a linear application the gate resistor becomes a part of feedback loop and need to be chosen wisely, imho. Although, I can't say if too big value can be a problem or not. Probably not as it seems to be pushing "first pole" to lower frequencies (as together with gate capacitance it makes a low-pass filter), making the whole loop more stable, but I advice use a simulation to confirm this.
 

Offline JwillisTopic starter

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Re: Bogus parts from Reputable Dealer?
« Reply #70 on: February 19, 2020, 10:21:09 am »
In a linear application the gate resistor becomes a part of feedback loop and need to be chosen wisely, imho. Although, I can't say if too big value can be a problem or not. Probably not as it seems to be pushing "first pole" to lower frequencies (as together with gate capacitance it makes a low-pass filter), making the whole loop more stable, but I advice use a simulation to confirm this.

The whole circuit has been changed from the original design anyway.https://www.eevblog.com/forum/beginners/protection-for-electronic-load-circuit/. The 10K was replaced with 10 Ohm. But it had no effect on the performance of the circuit . The whole problem turned out  to be faulty op amps.
Since replacing the components the circuit has been functioning properly at 500 Watts for extended periods of time so far.
 
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