Author Topic: Mystery Voltage Monitor Board  (Read 288 times)

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Offline danlb_2000

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Mystery Voltage Monitor Board
« on: June 26, 2021, 12:28:43 am »
I found a bunch of these boards in my surplus bin, no idea where I got them but based on the date codes they date back to the late 70's. I finally decided to play with them a bit so I traced out the circuit and did some experiments. They have 4 LED's, +V and GND inputs but no other inputs. I hooked it up to a variable power supply and found that LED's 8 and 20 come on around 6V, at 9V LED 8 goes out and 20 stays on, at 12V all the LED's go out, and at around 13V, LED 14 comes on. The part that perplexes me is, what condition would cause LED 28 to come on? The element of the comparator it's tied to has it's minus input tied to ground, so based on my understanding of comparators, there should be no way for that element to turn on. Anyone have and ideas on this?

« Last Edit: June 26, 2021, 12:32:53 am by danlb_2000 »
 

Offline Renate

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Re: Mystery Voltage Monitor Board
« Reply #1 on: June 26, 2021, 12:24:16 pm »
On the schematic, if you move the 2nd from the top op amp to the bottom and put R23, R21, R18, R17 in a vertical line it will be easier on our brains.
LEDs 8 & 20 should come on at different voltages, but it's a small difference.
Maybe you have the top opamp circuit wrong? Maybe it's a blinker. Are you sure the - input doesn't go to the same bias as the other ones?
 

Offline danlb_2000

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Re: Mystery Voltage Monitor Board
« Reply #2 on: June 26, 2021, 04:01:17 pm »
Good suggestion on the schematics, I have made that change and attached the new version, it does make it clearer. I double checked and Pin 6 definitly goes to ground. In my experimentation I have also not found any scenario that causes LED 28 to come on.
 

Offline Renate

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Re: Mystery Voltage Monitor Board
« Reply #3 on: June 26, 2021, 04:50:57 pm »
Do you have the wiring for pin 8 & 9 backwards?
You say that that LED only goes on with higher voltage.

What is this +5? Is there a regulator not shown?

Is C11 electrolytic in the microfards or tiny?
 

Offline danlb_2000

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Re: Mystery Voltage Monitor Board
« Reply #4 on: June 26, 2021, 06:59:37 pm »
I double checked 8 & 9 and they are correct. +5 was a mistake, there is a single voltage input so that should have been labeled +V like everything else. C11 is a 10uf, 35 V electrolytic.
 

Offline Renate

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Re: Mystery Voltage Monitor Board
« Reply #5 on: June 26, 2021, 08:37:58 pm »
Oh, so I was confused which was the reference.
D27 is a Zener for the reference.
Run the voltage up and see what it peaks at.

D28 will only go on if there is lots of spikes on the power.
I don't know why they are doing that.

D14 is on for voltage OK.
D8 is for low voltage.
D20 is for really low voltage.

If you can't get D8 & D20 to behave somewhat independently try playing with the pot.
It may be that the voltages they are trying to sense is lower than the Zener voltage.
 


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