Help with monitoring voltage

[truekaos]

Ok, I have checked the posts and can't seem to find a quick and dirty solution.
I am trying to monitor several 28 volt lines while operating under heat and vibration. the circuit doesnt have to whithstand the vibration and temp but I would like to build an independant voltage monitor for each line. I would like to use a LED to indicate the voltage dropping below 28v. I have tried a LM 317 but the leakage keeps the LED on below 28 volts. I was looking into LM 393 but I don't think this will work for me. If I go with a 24 volt regulator will it shut off under 24 volts? Any help would be appreciated.

Thanks
C

 

[kchriste]

A LM339, combined with a single voltage reference, will let you monitor 4 lines. Using the LM393 will let you monitor 2 lines.
You should also think about how much hysteresis you want. ie: If the LED comes on when the voltage drops below 28v, then it should turn off at a higher voltage, say 29V to prevent flicker at the trigger point.

 

[ericgibbs]

Ok, I have checked the posts and can't seem to find a quick and dirty solution.
I am trying to monitor several 28 volt lines while operating under heat and vibration. the circuit doesnt have to whithstand the vibration and temp but I would like to build an independant voltage monitor for each line. I would like to use a LED to indicate the voltage dropping below 28v. I have tried a LM 317 but the leakage keeps the LED on below 28 volts. I was looking into LM 393 but I don't think this will work for me. If I go with a 24 volt regulator will it shut off under 24 volts? Any help would be appreciated.

Thanks
C

hi,
You say a QDS solution.
How close to under 28V is required and will the 28V rise above 28V.?

 

[truekaos]

hi,
You say a QDS solution.
How close to under 28V is required and will the 28V rise above 28V.?

Hello, so mabye not so QD, but I was going crazy trying to watch 3 multi meters at a time until I realised that I was no longer able to distinguish numbers. I think I was just short of drooling with a detached retina or something.

Thanks for the reply. I am trying to quickly evaluate voltage over 26 volts if below 26 then I know I have a problem. the maximum voltage rise will be to 30 volts.

Cheers
Cory

 

[truekaos]

I looked into the LM 339 but it confused me about how it referenced the voltage, I mostly work with high voltage AC so the low voltage DC stuff is somewhat greek to me. I am now researching hysteresis.

Thanks for the suggestion, I am learning to buy components in bulk and for the most part the worst that can happen is some smoke and sometimes minor component explosions.

Cheers
Cory

 

[Boncuk]

Hi Cory,

using a TLC274 you can monitor four voltages.

The trip points for the LED are: ≤26.5V = ON, ≥27.88V = OFF.

You might adjust for your requirements using RV1.

Boncuk

 

[kchriste]

Good example Boncuk, but why the complex divider made up of RV1, RV2, R3, R4, & R5, when you can do it with just 2-3 parts instead of 5?

 

[truekaos]

Hello Boncuk,

Thanks for the reply. I have yet to try any TI chips and this looks fairly new guy tolerant. I like your moto however proper planning saves me from running to the electronics store every 5 minutes!

Cheers
Cory

 

[Boncuk]

Good example Boncuk, but why the complex divider made up of RV1, RV2, R3, R4, & R5, when you can do it with just 2-3 parts instead of 5?

Good question.

You may have noticed the blue frame around RV2 and R5. This is the simulated 28V (to 30V) source which is not part of the circuit.

 

[Boncuk]

Hello Boncuk,

Thanks for the reply. I have yet to try any TI chips and this looks fairly new guy tolerant. I like your moto however proper planning saves me from running to the electronics store every 5 minutes!

Cheers
Cory

I wish my wife could plan better for her shopping tours. Would certainly save two tank fills per week.

 

[kchriste]

Good question.
You may have noticed the blue frame around RV2 and R5. This is the simulated 28V (to 30V) source which is not part of the circuit.

Doh!
(filler to extend message to at least 10 characters.)

 

[MRubenzahl]

Hi, someone from Maxim Tech Support spotted your question and gave me the following reply.

You can consider the MAX16043 (which operates with a maximum of 28V supply) which can monitor up to 4 inputs. It has 4 open drain outputs which you can use to drive LEDs to indicate a particular input is below threshold. See the QuickView data sheet for this part: **broken link removed**.

Moe Rubenzahl, Maxim

 

[truekaos]

Hello,

Thanks for the info. I am getting some grey hair over this project. I am having a problem as I have no reference voltage to operate a comparitor with. I had tried a few different circuits but it seems my load (the LED) doesn't draw enough current to make the circuit not flaky.

 

[kchriste]

You could use a simple voltage regulator such as a 7805/08/12 etc as a reference if you don't need uber accuracy.
The Maxim chip would work too.