Electronics > Beginners
Bogus parts from Reputable Dealer?
Jwillis:
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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nigelwright7557:
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.
Jwillis:
--- Quote from: nigelwright7557 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.
--- End quote ---
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 .
exe:
--- Quote from: Cerebus on January 20, 2020, 09:05:00 pm ---For measuring offset the best bet is a circuit like this:
--- End quote ---
Does it require matched resistors?
Cerebus:
--- Quote from: exe on February 11, 2020, 09:57:57 am ---
--- Quote from: Cerebus on January 20, 2020, 09:05:00 pm ---For measuring offset the best bet is a circuit like this:
--- End quote ---
Does it require matched resistors?
--- End quote ---
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).
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