2v DC signal to operate an alarm

Thread Starter

ross.williams

Joined May 19, 2021
8
Hi everyone, I am stuck on a project in work. I need an alarm to sound when our DC voltage drops below 2V. Its operating voltage is around 2.1V and we need an alarm to sound if it drops below 2VDC. I have been looking at relays however I can't seem to find anything that switches at 2VDC. Any and all suggestions welcome. Thanks, Ross.
 

Yaakov

Joined Jan 27, 2019
3,155
Welcome to AAC

.1V is a very tight threshold, how accurate does it have to be?

You probably just need a comparator and a voltage reference.
 

MisterBill2

Joined Jan 23, 2018
8,761
Y is right, certainly you need both a comparator and a stable reference voltage, and a stable operating voltage for the comparator, as well. One alternative would be to have a stable DC amplifier with a fairly high gain and a very stable DC offset, and use that to drive a relay.
What sort of application is it that uses2.1 volts but can not get by with 2.0 volts?
If you can use an LM339 quad comparator you could even arrange to get a warning when the voltge dropped just a small amount.
There are a number of threads showing comparator circuits and they should be quite useful.
 

Thread Starter

ross.williams

Joined May 19, 2021
8
Welcome to AAC

.1V is a very tight threshold, how accurate does it have to be?

You probably just need a comparator and a voltage reference.
It would need to sound at 2V, or as close to 2V as possible. I have more of an electrical background rather than electronics, so some of this stuff is a bit confusing
 

Thread Starter

ross.williams

Joined May 19, 2021
8
Y is right, certainly you need both a comparator and a stable reference voltage, and a stable operating voltage for the comparator, as well. One alternative would be to have a stable DC amplifier with a fairly high gain and a very stable DC offset, and use that to drive a relay.
What sort of application is it that uses2.1 volts but can not get by with 2.0 volts?
If you can use an LM339 quad comparator you could even arrange to get a warning when the voltge dropped just a small amount.
There are a number of threads showing comparator circuits and they should be quite useful.
Basically they are using a modified 0-10V DC analogue meter. Someone has stuck amps stickers on there e.g. 2VDC is now 54A. I have only recently started so I have not had time to go through everything (no drawings at all). When the voltage drops bellow 2V i.e. 54A our extrusion screw stops allowing material down it. Once the amps/volts drop below 2V/54A the operator needs to turn off a motor to allow volts/amps to build back up, however it isn't very practical for the operator to be watching the meter all day, therefore the need for the alarm. Also if you could link any of these threads that would be a great help. Thanks for your reply.
 

BobTPH

Joined Jun 5, 2013
3,315
That is what is affectionately known as a Kludge.

God knows what is connected to that meter and whether it is dangerous or not.

I think you need more info before doing anything with this.

Bob
 

MisterBill2

Joined Jan 23, 2018
8,761
It is clearly a bit more complex, if 2 volts indicates 54 amps, then it may be reading the current through some heaters, or else the current of the extruder screw drive motor. It may also be that the meter is connected in place of a temperature display. It might be that the meter is reading pressure.
So we do not even know if the voltage is AC or DC, nor if it is the voltage from a current transformer, an ammeter shunt, a temperature monitor, or from a pressure gauge.
We can presume that it does need to be an isolated monitor arrangement and that it does need to provide an isolated relay contact output, because the meter is watching some process variable..

So now I suggest a more expensive approach, a digital meter relay with a settable switch point. I have used the ones from Red Lion company with great success in the past. One caution iis that various other companies privately-brand the Red Lion products and sell them for a much higher price. Red LIon will sell directly so there is no reason to pay that 40% extra for exactly the same thing. So you need to know if the 2 volt signal is AC or DC, and then you can order the meter/controller and the system can be automated. The standard unit resolution will be 2 millivolts, which should be adequate.
 

drc_567

Joined Dec 29, 2008
1,110
...Here is an alternative approach, using the Arduino microcontroller:
There is a command in the Arduino programming language, and a voltage divider modification such that it is possible to utilize a reference voltage of your choosing. So setup a 2.5 voltage reference voltage range, then read the 2.0 volt sensor output value on the internal 1024 count analog to digital converter, and determine the critical ADC count number that indicates the low ampere condition ( ... approximately 820 counts). Then set up a programming loop to test the ADC count number. If the sensor ADC number is below the critical number, (2.0 volts) then turn on an output pin that can sound an audible alarm.
... There are always a few details to consider, so you will have to try this scheme out and see if it actually works. The advantage is that it should provide the resolution that you require. If there turns out to be excessive false triggering of the alarm, try reading the sensor count using a longer delay period ... maybe 60 seconds between readings, or require two or three consistent low sensor counts to switch on the alarm.
 
Last edited:

BobTPH

Joined Jun 5, 2013
3,315
I don’t think we can assume that the meter actually has 2V across it when it has been relabeled as 54A.

It is probably actually a voltmeter with the series resistor removed and a shunt
wired across it.

Bob
 
The LT6700 series comparitor https://www.analog.com/en/products/lt6700.html#product-overview will work with that low of a voltage.

You basically use a high impeadance voltage divider to bring the 2.1 V to 400 mV and use the fixed reference of the chip. You have to pick the varient that works for you. It has an open drain output, then there is the question what to do with it and what constitutes an alarm. You would likely need another power source for the alarm. The output (open drain) could pull down a higher voltage than the chip's supply.

The cool part, is the chip is protected against input voltages greater than the supply voltage of the comparitor.

Now, the question is: What is an alarm?
Separate power supply?
 
Last edited:

MisterBill2

Joined Jan 23, 2018
8,761
What should not at all be assumed is that the meter circuit common mode voltage can be ignored. It may be reading the output of a current transformer, or possibly an ammeter shunt in series with a multi-horsepower DC motor connected to a VS drive. Any serious voltage would fry the toy arduino board, which is not suited for anything like a production environment, which this setup apparently is part of. That is why I suggested the Red Lion controller/meter.
But now we need the TS to explain a lot more about the actual application. It is always interesting to get a serious issue dumped in one's lap on the first day of a new job.
 

click_here

Joined Sep 22, 2020
190
It sounds like you are using a voltage drop across a resistor to measure current - Nothing more complicated than that.

A good solution to that would be to use a differential amp and pass the output from that straight into a comparator.
 

MisterBill2

Joined Jan 23, 2018
8,761
It sounds like you are using a voltage drop across a resistor to measure current - Nothing more complicated than that.

A good solution to that would be to use a differential amp and pass the output from that straight into a comparator.
I doubt that it is a resistor drop. Consider: 2 volts x 54 amps is 108 watts, which is quite a bit of power in a resistor. And consider, in post #5 an extrusion screw is mentioned , and "when it drops below 54 amps" which tells me that the current is related to some process variable. So really it would be handy to understand a bit more.
And still, for the money, a digital process meter with a variable setpoint relay could provide the required switching function with no design cost, and no calibration requirement, and it would not be a fragile assembly in a hobbyist package. The added advantage would be that the input would be isolated.
So perhaps the TS can provide a bit more information including a bit about how much they could aford to pay for a long lasting fix.
 

click_here

Joined Sep 22, 2020
190
The op said that they were measuring DC voltage using an analogue voltage meter and wants an alarm when the voltage gets down to 2.0VDC from 2.1VDC.

The digital process meter is a good idea, but because it's a one off utility device, I'd just quickly knock up something on a protoboard for cheap
 

Thread Starter

ross.williams

Joined May 19, 2021
8
That is what is affectionately known as a Kludge.

God knows what is connected to that meter and whether it is dangerous or not.

I think you need more info before doing anything with this.

Bob
After doing some digging yesterday the 0-10V DC voltmeter is connected to the Inverter drive for the motor, it has a +10V reference and -10V reference terminals which i believe this DC Voltmeter is connected too.
 

Thread Starter

ross.williams

Joined May 19, 2021
8
It is clearly a bit more complex, if 2 volts indicates 54 amps, then it may be reading the current through some heaters, or else the current of the extruder screw drive motor. It may also be that the meter is connected in place of a temperature display. It might be that the meter is reading pressure.
So we do not even know if the voltage is AC or DC, nor if it is the voltage from a current transformer, an ammeter shunt, a temperature monitor, or from a pressure gauge.
We can presume that it does need to be an isolated monitor arrangement and that it does need to provide an isolated relay contact output, because the meter is watching some process variable..

So now I suggest a more expensive approach, a digital meter relay with a settable switch point. I have used the ones from Red Lion company with great success in the past. One caution iis that various other companies privately-brand the Red Lion products and sell them for a much higher price. Red LIon will sell directly so there is no reason to pay that 40% extra for exactly the same thing. So you need to know if the 2 volt signal is AC or DC, and then you can order the meter/controller and the system can be automated. The standard unit resolution will be 2 millivolts, which should be adequate.
After doing some digging yesterday the 0-10V DC voltmeter is connected to the Inverter drive for the motor, it has a +10V reference and -10V reference terminals which i believe this DC Voltmeter is connected too.
 

Thread Starter

ross.williams

Joined May 19, 2021
8
I don’t think we can assume that the meter actually has 2V across it when it has been relabeled as 54A.

It is probably actually a voltmeter with the series resistor removed and a shunt
wired across it.

Bob
After doing some digging yesterday the 0-10V DC voltmeter is connected to the Inverter drive for the motor, it has a +10V reference and -10V reference terminals which i believe this DC Voltmeter is connected too.
 

Thread Starter

ross.williams

Joined May 19, 2021
8
What should not at all be assumed is that the meter circuit common mode voltage can be ignored. It may be reading the output of a current transformer, or possibly an ammeter shunt in series with a multi-horsepower DC motor connected to a VS drive. Any serious voltage would fry the toy arduino board, which is not suited for anything like a production environment, which this setup apparently is part of. That is why I suggested the Red Lion controller/meter.
But now we need the TS to explain a lot more about the actual application. It is always interesting to get a serious issue dumped in one's lap on the first day of a new job.
After doing some digging yesterday the 0-10V DC voltmeter is connected to the Inverter drive for the motor, it has a +10V reference and -10V reference terminals which i believe this DC Voltmeter is connected too.
 

Thread Starter

ross.williams

Joined May 19, 2021
8
I doubt that it is a resistor drop. Consider: 2 volts x 54 amps is 108 watts, which is quite a bit of power in a resistor. And consider, in post #5 an extrusion screw is mentioned , and "when it drops below 54 amps" which tells me that the current is related to some process variable. So really it would be handy to understand a bit more.
And still, for the money, a digital process meter with a variable setpoint relay could provide the required switching function with no design cost, and no calibration requirement, and it would not be a fragile assembly in a hobbyist package. The added advantage would be that the input would be isolated.
So perhaps the TS can provide a bit more information including a bit about how much they could aford to pay for a long lasting fix.
After doing some digging yesterday the 0-10V DC voltmeter is connected to the Inverter drive for the motor, it has a +10V reference and -10V reference terminals which i believe this DC Voltmeter is connected too. Could you possibly point me in the right direction for a digital process meter that would suit my application, many thanks.
 
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