Electronics > Beginners
Bogus parts from Reputable Dealer?
Jwillis:
I don't want to mention the North American dealer just yet out of respect until I figure what might have gone wrong.
I bought 10 pieces of TLV2371IP as suggested for a parallel mosfet linear electronic load. My intention was to parallel 2 mosets with 2 TLV2371IP to control.I was simply wanted to test the circuit design and expand to 4 mosfets when things go correctly with 2 . Seems simple enough right?
Now on the first try 1 of the op amps worked correctly, but I couldn't get the second one to work . It seems around 250mV is leaking back from pin 3 and the out puts is the same as the supply 9V on the one that wasn't working right .No voltage is applied to the number 3 pin . I changed 6 of them with similar results with slightly different voltage leakage into pin 3 . The one that worked continues to work. So figuring it may be a bread board issue. I re orientated the bad circuit but got the same results .So I changed out the bad one for another one . Same results .6 out of 10 got changed . Same results. So I switched the two op amps in the two circuits .Well the good op amp works as it should in the other circuit now and bad op amp doesn't work where the good one used to be. :wtf: So its not the circuit . So I changed the "bad " again (# 7) and low and behold it worked . So I pulled it out to test the last one left out of the 10 and same results as before it won't work. 8 out of the 10 are uncontrollable .Ok so I have 2 out of 10 that work. Now I admit I'm not an expert on op amps but I was being really careful about static and making sure everything was hooked up right before power up. Both circuits are exactly the same Resistors are all 1% . Capacitors are new and measure fairly close. Checked the mosfets and both are functioning .Both circuits receive a parallel voltage at pin 3 . I tried to get the ones that don't work by adding an offset thinking that may be the problem but the offset with a 200k trim pot don't seem to do anything.i tried a voltage divider at pin 3 but that does'nt make any difference either. The voltage is at pin 3 is coming from the chip.
Now before I go and make a complete fool out of myself to the dealer I would like to test each failed op amp for leakage and functionality.I don't have really spectacular gear. So what would be the simplest way of doing that so I have evidence to show to the dealer in case they decide to get nasty about it. :box:
Now, from I understand when I supply the op amp with 9V there should be no voltage at the inverting and non inverting pins and no voltage at the out put relative to ground correct ? When used in common mode I should only get a voltage at pin 6 when a voltage is applied to pin 3 ,correct?
I haven't had this sort of thing happen before with other op amps I've used and I'm out of ideas.
nigelwright7557:
I would have thought a reputable dealer wouldn't buy rubbish.
I have bought IC's off ebay in the past and had bad experiences like you in that some worked and others didnt.
On a previous project I bought in some op amp's for RS (reputable dealer.)
The circuit behaved very strangely with impossible voltages on op amp pins.
Turned out my fast op amp was also bi polar and had quite a bit of input current which messed up my potential divider voltage.
Another problem I found with op amps is the bandwidth, which in reality is only small signal bandwidth.
If you have a large signal going in the bandwidth is much less.
E-Design:
not at all likely "bogus" parts. Post your circuit.
ebastler:
--- Quote from: E-Design on January 19, 2020, 02:09:23 pm ---not at all likely "bogus" parts. Post your circuit.
--- End quote ---
Agree, and could you post a photo of the breadboard setup too please?
Something oscillating maybe?
ArthurDent:
If you did get bogus parts from a major supply channel, this isn’t new. Twenty plus years ago I worked for an international company and we made some electronic test equipment for utilities and we had open parts orders from major parts suppliers. All of a sudden almost all of this equipment we made was failing test and it was found that one shipment of a particular fairly expensive IC that we had been ordering for years from the same supplier was the problem. After an investigation it was found that all those ICs in this one shipment were relabeled crap. We never found out how this happened but the supplier certainly wouldn’t have done it knowingly and they made good on the repairs we had to make.
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