AC or DC

[tkb]

I have some CO detectors to install in a parking garage.
They can run on 24VAC or 24VDC.
I need to connect the contacts to an interposing relay for fan control.
The run will be about 300' to the last detector in #16 multiconductor cable connecting multiple detectors..
One power supply at the motor starters.

I want to make sure that the relay will pull in and the sensor has enough power given the length.

Would you use AC or DC for the power supply?

 

[templdl]

I can't see any difference in voltage drop between AC and DC but using an AC power supply involves only a basic transformer which is easy to replay where DC requires a power supply to change AC to DC which may be more prown to failure and not as commonly available when attempting to find a replacement.
Regarding the interposing relaying , whether it us a n AC or DC coil there would be no advantage in one or the other.
As such, I would lean toward AC control.
You do have to consider the length of run and the potential voltage drop to assure that you can pull in that interposing relay.

 

[broadgage]

I would go for DC.
Although ohms law applies to both AC and DC, in practice the use of DC may give a significantly larger margin for voltage drop, for the following reason.

An AC supply is normally from a simple stepdown transformer that gives no voltage regulation.
If the utility voltage is 5% low, and about 5% is lost in the premises wiring, the 120 volts AC nominal will be only about 108 volts at the transformer terminals. This will reduce the output voltage of the transformer by about 10% from 24 volts to 21.6 volts.
That could be problematic and require large and costly cables to maintain the required voltage at the load.
Use of a slightly higher voltage, such as 25 volts helps to an extent, but there are limits to this, or the voltage at the 24 volt load may become too high if the utility supply is at 105%, and the drop in the premises wiring reduced at times of low load.

A DC power supply will almost certainly be switched mode with a regulated output.
It will provide a constand 24 volts at any reasonable input voltage.
Voltage drop calculations may therefore be based on a constant 24 volt supply with DC rather than a rather variable voltage with AC.

 

[Besoeker]

I would go for DC.
Although ohms law applies to both AC and DC, in practice the use of DC may give a significantly larger margin for voltage drop, for the following reason.

An AC supply is normally from a simple stepdown transformer that gives no voltage regulation.
If the utility voltage is 5% low, and about 5% is lost in the premises wiring, the 120 volts AC nominal will be only about 108 volts at the transformer terminals. This will reduce the output voltage of the transformer by about 10% from 24 volts to 21.6 volts.
That could be problematic and require large and costly cables to maintain the required voltage at the load.
Use of a slightly higher voltage, such as 25 volts helps to an extent, but there are limits to this, or the voltage at the 24 volt load may become too high if the utility supply is at 105%, and the drop in the premises wiring reduced at times of low load.

A DC power supply will almost certainly be switched mode with a regulated output.
It will provide a constand 24 volts at any reasonable input voltage.
Voltage drop calculations may therefore be based on a constant 24 volt supply with DC rather than a rather variable voltage with AC.

Points noted, Gar.
And a couple of thoughts.
All things being equal, the voltage drop with DC would be lower than AC - there's no reactive component to consider. Whether this is significant in this particular case, I don't know.
But, a variation on your suggestion. What about sending the 120Vac to the garage and installing the SMPS at that end?
Possibly smaller conductors for the 120Vac and then no concerns about voltage drop on the 24Vdc.

 

[T.M.Haja Sahib]

Greater is the complexity in a system, lesser is its reliability. As templdl rightly pointed out, AC system here has fewer number of components and so more reliable. As for the voltage drop concerns, it may be stated that the holding coil in the relay is usually rated for voltage drop up to 15% and its contacts drop out only when the voltage decreases beyond 15%.

 

[broadgage]

A regulated switched mode power supply is potentialy a weak point compared to a simple transformer, though they are much more reliable than was the case a few years ago.
I would purchase a good qaulity one, with a long warenty, and not fully load it.

A relay should indeed tolerate about 15 % voltage drop, but in the example I gave, the voltage could already be 10% low due to low line voltage and voltage drop in the premises wireing.
That would only give 5% allowance for perhaps long runs of 24 volt cable.

It would be unwise to count on the equipment tolerating 15% voltage drop, unless this is confirmed by the supplier.

 

[MichaelGP3]

Do these CO detectors have their own backup battery?

 

[templdl]

A regulated switched mode power supply is potentialy a weak point compared to a simple transformer, though they are much more reliable than was the case a few years ago.
I would purchase a good qaulity one, with a long warenty, and not fully load it.

A relay should indeed tolerate about 15 % voltage drop, but in the example I gave, the voltage could already be 10% low due to low line voltage and voltage drop in the premises wireing.
That would only give 5% allowance for perhaps long runs of 24 volt cable.

It would be unwise to count on the equipment tolerating 15% voltage drop, unless this is confirmed by the supplier.

A good warranty doesn't contribute anything to the party other than covering the expense of a replacement component after a failure which most likely doesn't cover the cost of labor.
A failure is a failure regardless of warranty. What you are looking for is reliablitiy, reducing the chance of a failure in the first place. You target is that you want the thing to work. And consider if you are splitting hairs when comparing AC to DC weighing the trade offs between the two the ones that are realistic and the ones that are not.

 

[Besoeker]

A good warranty doesn't contribute anything to the party other than covering the expense of a replacement component after a failure which most likely doesn't cover the cost of labor.
A failure is a failure regardless of warranty. What you are looking for is reliablitiy, reducing the chance of a failure in the first place. You target is that you want the thing to work. And consider if you are splitting hairs when comparing AC to DC weighing the trade offs between the two the ones that are realistic and the ones that are not.

I'm all for simplest is best but....
Pretty much everything that has electronics in it these days has switch mode power supplies. Computer, printer, television, phone charger, variable frequency drives....
The SMPS units we've used have much more built in self protection than a simple transformer rectifier.

 

[templdl]

I'm all for simplest is best but....
Pretty much everything that has electronics in it these days has switch mode power supplies. Computer, printer, television, phone charger, variable frequency drives....
The SMPS units we've used have much more built in self protection than a simple transformer rectifier.

Yes SMPS' certainly have become more reliable. When compared to a simple transformer to provide AC power what would be the advantage of the SMPS power supply?

 

[Besoeker]

Yes SMPS' certainly have become more reliable. When compared to a simple transformer to provide AC power what would be the advantage of the SMPS power supply?

Voltage stability.

 

[iwire]

Voltage stability.

Is that an issue for the OP to be worried about? My experience with devices like CO detectors is that they typically list a voltage range.

If it was my job I would go with an AC transformer, less up front cost, more reliable than even the highly reliable DC supply and if it does fail easier to get a replacement for.

Now if voltage stability was important, or I had the need to provide battery back up DC would be my choice.

 

[Besoeker]

Is that an issue for the OP to be worried about? My experience with devices like CO detectors is that they typically list a voltage range.

If it was my job I would go with an AC transformer, less up front cost, more reliable than even the highly reliable DC supply and if it does fail easier to get a replacement for.

Now if voltage stability was important, or I had the need to provide battery back up DC would be my choice.

It's a fairly long run (600 feet there and back) and I was thinking about voltage drop which is why I made the suggestion of fitting the SMPS at the garage end.
Then you have the interposing relay if it's AC the inrush current will be several times the holding current.
I don't know what current will be taken at 24V but assume it is say, 5A including the relay inrush.
For that current rating we'd use 1.5mm² cable. This has a voltage drop of 29mV/A/m. In round figires the 600 feet is 200 metres. So 29V drop. A bit of a problem if you wanted to send 24Vac. Clearly, you'd have to put in bigger cable and that would have a bearing on the economics.

 

[don_resqcapt19]

Do the issues of long AC control circuits as discussed here come into play for this application?

I guess I should have re-read the paper before posting. It looks like the shunt capacitance issue does not apply for a 24 volt circuit. In some cases the shunt capacitance of an AC control circuit will keep the coil pulled in even when the control contact is open.

 

[T.M.Haja Sahib]

No need to be obsessed with SMPS, since a voltage regulator, simpler one, can serve the purpose here.

 

[Besoeker]

Do the issues of long AC control circuits as discussed here come into play for this application?

I suppose it does. From table 15 onwards, maximum control distance in feet due to series impedance gets quite short for 24V.

 

[Besoeker]

No need to be obsessed with SMPS, since a voltage regulator, simpler one, can serve the purpose here.

What sort did you have in mind?

 

[T.M.Haja Sahib]

What sort did you have in mind?

Servo.

 

[Besoeker]

Servo.

Using what technology?
Perhaps that's the question I should have asked.

 

[don_resqcapt19]

I suppose it does. From table 15 onwards, maximum control distance in feet due to series impedance gets quite short for 24V.

Series impedance is just the votlage drop issue, and I was thinking more of the shunt capacitance issue that can make a open contact act like a closed one and that does not seem to be an issue at 24 volts.