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lm331 output not stable when connected to instrumentation amplifier

Rufaida

New Member
Output of LM331 not stable. The frequency keeps on changing for 1 minute approx. until it reaches a stable value. The input of LM331 is taken from AD620 (instrumentation amplifier)
 

audioguru

Well-Known Member
Most Helpful Member
Maybe your LM331 and frequency-setting parts are fake or defective from ebay?
Its spec's show that its frequency is very stable.
 

gophert

Well-Known Member
Most Helpful Member
Is that your photo? The Signetics NE556 looks like it has a 1979 date code.
 

ronsimpson

Well-Known Member
Most Helpful Member
Is that your photo?
No just a stock photo. Kind of a joke. But the picture sums up what I want to say.
I see wireless breadboards too much. Long wires. No bypass caps. Loose connections. Questionable grounds. Extra pin to pin capacitance.
When a part is unstable I first check the PCB layout. Check the grounds. Look at the grounding on the timing capacitors. Long leads, long traces.
 

gophert

Well-Known Member
Most Helpful Member
When a part is unstable I first check the PCB layout. Check the grounds. Look at the grounding on the timing capacitors. Long leads, long traces.
That checklist might be more effective than my method. I usually grab a handful of hair with each hand and scream, "nooooo" and then drink until a possible solution comes to mind.
 

dknguyen

Well-Known Member
Most Helpful Member
That checklist might be more effective than my method. I usually grab a handful of hair with each hand and scream, "nooooo" and then drink until a possible solution comes to mind.
First thing I do is check the power, then open check connections. Then I scream nooooo and go to sleep.
 

atferrari

Well-Known Member
First thing I do is check the power, then open check connections. Then I scream nooooo and go to sleep.
Saving the drinking to celebrate?
 

MichaelaJoy

Active Member
I miss those days...Beer and pizza while working. :)

These days, I have to settle for coffee and the occasional doughnut.

In general, the novice doesn't understand that the key to a functional design is a well built prototype.
Every new engineer should spend as much time as possible honing their prototyping skills.

ronsimpson: I have a bunch of those proto-boards which I hardly use, except for the most simplistic of things.
 

audioguru

Well-Known Member
Most Helpful Member
Most of my prototypes got sold as the finished product. Only one or two were ordered as custom circuits so I made them neatly on stripboard (Veroboard).
When tens of thousands of copies of my design were ordered then I took my stripboard circuit to a local pcb/assembly place who made their own pcb prototype for me to test, then made the thousands where each one was thoroughly tested with a tester I also built on stripboard. All passed except two that had assembly errors and of the thousands that were sold, none were returned. Money! The profit was dripping out of my mouth, eyes and ears.
 

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