24VDC vs. 24VAC


Thread Starter

Mike Hubbell


Can anyone break down all the advantages and disadvantages of using 24VAC vs. 24VDC for industrial control circuits? I can see some advantages on both sides but 24VDC seems to be more common. Why?


Mike Hubbell
[email protected]

Vitor Finkel

In a nutshell, Mike.

In Industrial control circuits you usually want to have a reliable means to guarantee you will not loose control power, whatever happens to
your electrical supply.

All the AC UPS systems I've seen so far have week spots, common failure points, that sooner or later will lead you to loose that control power.
DC circuits can be arranged in such ways, to be quite less expensive and much more simple and reliable than anything I so far have ever seen done with AC.

Need further clarification ? Just ask for it.

Vitor Finkel
[email protected]
P.O. Box 16061 tel (+55) 21 285-5641
22222.970 Rio de Janeiro Brazil fax (+55) 21 205-3339

Curt Wuollet

Hi Mike.

FET's are cheaper and more reliable than triacs and DC can be faster since one half cycle of AC @ 60Hz is comparable to a scan. Typically AC inputs are DC inputs with a rectifier and fiilter If you do a lot of solonoids and relays, they may be a little cheaper for AC as an AC coil does not rely on wire resistance for current limiting. It is also less erosive on mechanical contacts and tends to extinguish arcing better by nature.
DC is simpler for many reasons and that's why it has become prevalant. AC is often used for cost reasons as a transformer is cheaper than a DC
supply. It has few other advantages and as devices contain more and more electronics which want DC, it will fade a little more except for special applications where it's attributes matter.


Hello Mike,
Our company designs automated machinery for
plastic printing. Our designs are usually
24VDC. Our control outputs are almost always
transistor outputs. The reason is simple:
Long life and high speed sensing and switching.
24VAC can be used but is much less common
because sensors and outputs would be sending a pulse train to whatever they were wired to, due
to the constant zero-voltage crossover.
The only advantage I can think of for AC might
be contact life on mechanical relays.

Charlie Griswold

A lot of small devices are Direct Current. One reason is safety. Switches, lights, small devices....these often come into direct contact with personal. Alternating Current makes you grab and hold on, Direct Current throws you off.
Oh no let's not go there again with the "Alternating Current makes you grab and hold on" story. We been there - done that. One important reason not mentioned so far for 24VDC is where simple circuits reach into the thousand feet or more to a contact and back (for a pull-cord switch for instance) the capacitive coupling of the field wiring would keep a small control relay energized regardless of the state of the distant contacts. So it can be a safety issue.


Nucciarone, Joe

Using 24vdc to activate relays and solenoids can and will cause noise on your signal lines if not properly snubbed. Back emf induced when a dc voltage is removed from an inductor will cause havoc on your signal lines. If you cannot use diodes or some other voltage clipping device directly on the solenoid then go with 24vac.
> Alternating Current makes you grab and hold on, >Direct Current throws you off.

First, what happens when you touch an energized surface depend on what muscles are effected by the current. For example, if they are extensor
muscles, the victim will be thrown away from the energized surface rather than if they were contracting muscles which would clamp to the

Secondly, I know of no reason why DC would "throw you off". DC current values for the "Let-Go-Current" are typically about 4 times higher than
the 60 Hz values but DC current can still cause you to be unable to release a grip on a energized surface.

If you have some technical references to support this statement, I would be interested in hearing about them.

If anyone is interested in some basic technical references regarding effects of AC and DC currents to the human body, I would suggest:

"Lawrence Livermore Laboratories ES&H Manual, Chapter 23",

Grounding and Bonding, Michel Mardiguian, Interference Control
Technologies,Inc. Gainsville, VA
ISBN 0-944916-02-3

Electrical Instruments in Hazardous Locations, 4th ed., Ernest C.
Magison, ISA, ISBN 1-55617-638-4

Bill Mostia
William(Bill) L. Mostia, Jr. PE
Principal Engineer
WLM Engineering Co
Independent I&E Consultant
P.O. Box 1129
Kemah, TX 77565 USA
E-Mail: [email protected]
These opinions are my own and are offered on the basis of Caveat Emptor.

Nick Haddock

Surely at 24 volt there is no question of A.C. grabbing hold of a person. One reason for having 24 volt supplies is that at such a low voltage there is no risk of electric shock be it A.C. or D.C.
Regards Nick Haddock

I respectfully disagree. The combination of voltage, current, frequency, and body resistance must be considered before assessing danger in electrical systems.

A former acquaintance of mine violated our employer's electrical safety standards while working on a 24 volt AC system with a 2 amp fuse. We found him, quite dead, some hours later. It seems the 60 Hz did something to his heart. I do not know if the low voltage ac "grabbed" him or not. It killed him just the same.

Until that time, I did not think that 24 volt ac or dc could be dangerous.
Conditions for potentially lethal shock across a critical path (heart)
1. 42.4 VAC peak to peak, or 60 Vdc at a total impedance of less than 5000 ohms.
2. 10-75 ma
The worst possible frequency for humans is 60 HZ At 60 Hz humans are six times as sensitive to alternating current at 5000HZ, above 100kHZ things
start to go the other way again.

Anthony Kerstens

Well, I'll be blunt...
What deserves respect is electricity.

Not just body resistance counts, but also the path
between then entry and grounding points.
I won't go into gory details because they make me nauseous just thinking about it, but electricity will take the path of least resistance which can be through bone, muscle, nerves, blood, and other fluids. Whether it grabs hold of you or not is irrelevant and ridiculous.

Another thing to think about with dry winter air
coming: generate some static electricity and
discharge yourself to a metal object. If you get a good enough static shock, you might feel queasy to your stomach. Take the knowledge of the feelings in the end of your finger and your
stomach, and imagine that sensation and the
damage caused wherever electricity may flow
through your body, amplified.

Remember that damage can't be repaired.

I know as I've had small shock (not static
electric) and survived. Days after that I still
felt queasy and tingly, and quite lucky.

Anthony Kerstens P.Eng.
It actually only takes 10's of microamperes to fibrillate a heart. Depending on the circumstances, this can happen with pretty low source voltages.
Well, not much risk of death anyway. You can get a shock from 24 vdc or 24vac under the right circumstances. You are very unlikely to get the 10 ma. of current that would cause death as your body resistance is fairly high even with perfect contact. By the way, a hot sweaty hand gives nearly as good a contact as can be had. Since 50-60 volts is commonly quoted as the threshold I would guess min. body resistance as approx. 5 or 6 kOhms. High voltages can cause carbonization, etc. that will lower this somewhat.


Curt Wuollet

Anthony Kerstens

I'd also like to point out the instant lethal
shock to the heart is not the only way to die.

Infections of entry/exit wounds (exit being point
of ground contact) and damage to other organs can
be quite lethal, if not immediate.

Anthony Kerstens P.Eng.
One of the links mentioned the '1/10/100' rule I had once been told, but long forgot. "1mA can be felt, 10mA contracts muscles, 100mA can stop the heart."

I would presume that regardless of the type of low voltage, once your body conducts sufficient current it could be fatal.


Paul handy guy

I am considering powering my sprinkler valve with 24VAC vs 24VDC. My question is can this be done? I normally just hook up all the lines to the sprinkler control unit which sends 24VDC the sprinkler valves and don't really know if I should keep it at 24VDC or 24VAC. What is your advice?
Hi Paul,

First, you should refer to your sprinkler valve data sheet or user manual, some solenoid valve only accept DC voltage. some other only work with AC voltage and also there is solenoid that will be work with either AC or DC voltage, as an example of solenoid datasheet please see below link from Tyco :


it shows that there are several cat no of solenoid from their product list and each cat no can only be working with certain of voltage type and rating