DC voltage sensor

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Hello, I am currently working on an audio amplifier that makes use of +/- 40 VDC.
This project encorporates a digital control panel, where I would like the system to be able to alert the user when one of the supply rails is at an unusual voltage.

I guess wha Im looking for is a device or circuit that can monitor each 40vdc rail and alert the user if, most importantly, the rail drops to zero and if possible, also alert the user if it is say, 30v or less or 50v or more, although this is less important.

I realise that a comparator may be the answer, but is this voltage too high for it? How could I provde a voltage reference for thr comparator?

I also considered a heavy duty resistor and optocoupler to detrtmine when the power rail fails, but this seems 'unprofessional' or at best inefficent.

Surely there is a better way.
I apprciete any and all feedback

-Michael
 
Yes, use a comparator, run from a lower voltage using the a regulator such as LM317 and LM337 or a zener diode and monitor the supply voltages using potential dividers.

It's possible to power a single LM339 using a +/-15V supply which will enable you to use two comparators for monitoring each rail.
 
hi Mike,
I would have thought the listener would know almost at once if one of the power rails went to zero.

Why do think its necessary to monitor these rails.?
 
If am to use two resistors to divide the voltage, they would have to be very beefy resistors, no? The power supply is +/- 40vdc @ 5A. If I had to use resistors, I may as well just use a reistor to lght the led of an optocoupler and do that, or am I missing something?
 
Hi Eric, the system is often left on most of the day, and people are coming and going. I am not sure if it is healthy for a power amp to be missing its a power rail for considerable time. Is this true, or is is this unneccesary?
 
mike11298 said:
Hi Eric, the system is often left on most of the day, and people are coming and going. I am not sure if it is healthy for a power amp to be missing its a power rail for considerable time. Is this true, or is is this unneccesary?
Click to expand...

hi,
If the equipment is left unattended for long periods, you need an auto shut down system if a power rail is lost.

As you may already know audio equipment can fail in a number of catastrophic ways other than a rail failure.

I would use say 30v zeners as droppers to a resistor, detect the volts across the resistor.
This would give you the 'low' as well as the zero detect.
 
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No, if you use high enough values, the current can be under 1mA so standard 250mW resistors can be used.

The LEDs can be powered using the regulator which probably won't even need a heatsink.
 
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mike11298 said:
Hi Eric, the system is often left on most of the day, and people are coming and going. I am not sure if it is healthy for a power amp to be missing its a power rail for considerable time. Is this true, or is is this unneccesary?
Click to expand...

It's not needed - what you DO need is speaker protection, which is usually built-in most amps. This checks for a standing DC voltage on the speaker outputs and disconnects them (using a relay) is any is found.

Check out **broken link removed** for one of many examples on the net.
 
Ok thanks- now Ive got some good starting points.
Since my amp utilises LM4780s, I dont think they have any speaker protection inbuilt, so Ill definately check that out.

Ill also gather up a few ideas and test a few to see which ones I like - although the relay looks good, the digial control panel can shut the system down as soon as a signal is applied to one of its pins- hence the detection circuit.

So a 30v zener doesnt conduct until there is 30v present, when this occurs (indcates a 'good' rail) the outpu of the zener is connected to high-value resistor revider which then operates a optocoupler. Is this the simplest but effective way of going about it?
 
You're overcomplicating things.

You could just put the zener in series with an LED, which will only light when the supply voltage exceeds its breakdown voltage, plus the forward voltage of the LED which is normally 1.8V for a red LED.

You could also use a potential divider and an LED but it won't be as accurate.
 
Ok, Hero, but I guess I would need a beefy resistor to drop the 40v down to 1.8, right?

Please forgive what may seem like repeated questions, but I am not at all familiar with zeners or discreete parts, really.

Regards,
Michael
 
mike11298 said:
Ok, Hero, but I guess I would need a beefy resistor to drop the 40v down to 1.8, right?

Please forgive what may seem like repeated questions, but I am not at all familiar with zeners or discreete parts, really.

Regards,
Michael
Click to expand...

hi,
hero's suggestion of a zener/diode would self destruct if the supplies line were slightly over +/-40V.

Also it would only give an indication, no shut down action of the audio system would happen.
 
Ok, thanks Eric, since the supplies are cpable of going up to about 45v wihout damage, I wold need a more robust solution. I assume the comparator would achieve this.

As previously stated, the digital control panel can shut the system down as soon as a signal or lack thereof occurs (theres a 'sense' pin for both active high and low). So this isnot a problem. Is it possible to obtain a 40v zener that "doesnt" self destruct a 45v?
 
The zener diode will do half of that.

Just connect a 680R resistor in series with the LED and a 30V zener.
 

hi Mike,
Do you have details of the control panel, ie: signal levels that the panel expects as high/low or missing signal.
If the audio system is worth saving/protecting then I would have thought a more reliable solution to detecting the +/-V loss would be justified.
 
Eric- Just a logic level signal will control the system. So if I can use a resistor or whatever to light an LED, that LED would be part of an optocoupler, which would be used to send a logic signal to the microcontroller.

Hero - Sorry, the zener diode will do half of what? A 30V zener would do it - because the system is still normal at +/- 30V also. So all I can see at the moment is:

Power rail with 30V zener connected with the other end of the zener to a 680R resistor, then an LED (in fact an optocoupler, in order to provide a logic level signal)

But will this zener diode be damaged at a particular voltage? what voltage?

Regards,
Michael
 

Morning Mike,
Would you confirm at what 'low rail voltage' you want the change in logic level to occur.?
 
Anything less than about 30V is regarded as low. Anything higher than about 45V is regarded as too high. 40V is the nominal voltage.
 
mike11298 said:
Anything less than about 30V is regarded as low. Anything higher than about 45V is regarded as too high. 40V is the nominal voltage.
Click to expand...

hi,
Look at this image, its for the +40Vrail, the -40Vrail would similar.

I have used two 15V zeners for the 30V.

 
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