AC control circuits

I've just had a hell of a job trying to fault find on an AC control circuit centre tapped 12v either side of 0V for the 24v coils. Why the hell do
designers use AC control circuits? Is it sadism or do they simply want to save the cost of a power supply?
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Could be it was just more readily available for that applications time & place.
there are still some items & methods from simpler times cheaply available all around the world., the ones that have not been banned are tempting and more likely to employed.
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See my reply to TimPerry. Ohms law is a fundamental law which predicts the current flow at a certain voltage through a resistive circuit It doesn't have very much to do with trouble shooting a machine that will not start generally speaking. Testing line with respect to zero volts is easy with dc wher e you have +24v and 0v. 24vac line to 0vac neutral is easy too. On the other hand 12vac line to 12vac neutral to give 24vac pd in a centre tapped supply is a nightmare. You can for example have a contactor with it's line voltage of 12v present but it will not energise your neutral will also have 12v present even if it is open circuit as there is no volt drop across the coil in an open circuit. So for all intents and purposes it appears to be a faulty contactor when in fact it may not be.

true but only if you don't factor in the cost of lengthier downtime if the machine fails to work.
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the
dc
the
have
a
a
it is only lengthier if the tech/worker is not familiar with the controls. I have worked on and designed controls for 30 years. I have used AC and DC. It all depended on what is available and who is going to work on it. Some older SES's have upwards of 300 V DC control systems. Work around with them some time and see what happens.
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Ok, familiarity helps but why do you suppose that 24vdc is by far the most commonly used control voltage?
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Maybe in industry, but in power generation and distribution, 110Vdc is by far the commonest voltage (with a small amount of 24, 48 and 220Vdc in some plants)
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In almost 20 years as a controls engineer, I've never seen a 24vac system with a grounded center tap. As for 24vdc, I can think of three reasons:
1) It's pretty much the standard for powering 4-20mA loop-powered analog instrument loops. 2) It simplifies battery backup systems. Where I used to work, instead of a 120VAC UPS, we powered remote telemetry units using an alarm system power supply/battery charger with a separate transformer and homemade battery shelf. Now I see that Phoenix Contact has come out with DIN rail models. 3) Some electricians will work with 24v live, but never with 120v. This is handy when you have to power the entire control panel to put part of a plant on-line before all of the field wiring is terminated.
Mike
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On Sun, 16 Jan 2005 03:02:23 GMT, "Mike Lamond"

I can say the same, in 30 years in the machine tool biz I have never ran across that, no matter where the machine tool was made (country)
Regards

Daveb
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What is the SES you're referring to? 10 pages of Google search results didn't turn up anything that made sense to me.
Mike
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the controls.
Of course...naturally. But these days who is familar with all the stuff on the market, much of it crap...and the best of it changing every 6 months... at 4 years old its obsolete... worth maybe 500 dollars new for the component...now a new one costs 200 dollars... not too many people are going to want to mess learning all the different and ever changing crap.
In my primary business, even the major controls companies, honeywell and Johnson have 100 different versions each...and you can spend 6 months of your life with just one version...so you will not become a pro on the other 99 versions...and all of their glitches, bugs, fiasco's, and sub assembly issues.
its not going to happen.
a quality company does not design things that you need a PhD and a 2" thick manual, and two months to study it to troubleshoot. Its a waste.
Controls need to be as simple, obvious, intuitive and direct as possible imho.
Phil Scott

used AC and DC.

work on it. Some

around with them

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message

in the

which predicts the

circuit It doesn't

will not start

is easy with dc

easy too. On the

centre tapped

with it's line

will also have

drop across the

appears to be a

all else being said, it is a screwy configuration they stuck you with there...very unusual and one would wonder why they split it that way....Ive investigated such things a lot...its usual just idiots in the background.

transformer than a

downtime if the

Correct... seldom considered, even on jobs where down time can cost them a million dollars a day... I always ask that question. and design accordingly.. not too many people do though. so we get a lot of junk built.
Phil Scott

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Dunno but they went to the trouble of installing a 24vdc power supply for the temperature controller. Why not use 24vdc for the control circuit too?
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applications time &

cheaply
been banned are

power supply for

control circuit too?
The VDC temp controller is a more expensive device needed for some critical applcations...the rest of the job may not have needed such precision...and suffered from the variable resistance issues in DC systems....the units being interfaced with could also have been ac...virtually all air conditioning systems for instance.
Many hvac mfgrs use both vdc and ac in thier controls.. dc for sensing, ac for controlling contactors and dampers etc.
Phil Scott

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often industrial relay control circuits are called "control ladders" due to the appearance of the schematic diagram. it looks like a ladder with the "rails" drawn vertically and the "rungs" are the coil loads which are interrupted by switches, contacts, and whatever odds and ends of devices the particular system calls for.
troubleshooting consists of placing a meter (or even a light bulb or buzzer) across various points to determine the state (on or off ... picked or dropped ... open or closed).
as to the fuse example... and open fuse for DC or AC yields the same results. open fuse = voltage on both sides (providing a load exists and the circuit is energized) good fuse = zero volts across the fuse.
you simply need to adjust your thinking at this point and stop making measurements with one lead attached to the chassis ground. hook your reference lead to a "rail" and life will be better :)
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Yeah join the "real freaking world". Stop living in yesterday. Stop crying! Try working on a piece of crap made in a foreign country. Then cry to a newsgroup for help because tech support does not have English translators.

often industrial relay control circuits are called "control ladders" due to the appearance of the schematic diagram. it looks like a ladder with the "rails" drawn vertically and the "rungs" are the coil loads which are interrupted by switches, contacts, and whatever odds and ends of devices the particular system calls for.
troubleshooting consists of placing a meter (or even a light bulb or buzzer) across various points to determine the state (on or off ... picked or dropped ... open or closed).
as to the fuse example... and open fuse for DC or AC yields the same results. open fuse = voltage on both sides (providing a load exists and the circuit is energized) good fuse = zero volts across the fuse.
you simply need to adjust your thinking at this point and stop making measurements with one lead attached to the chassis ground. hook your reference lead to a "rail" and life will be better :)
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Actually I wasn't crying at all Brian. I was simply engaging in conversation with these people with regard to the merits or otherwise of AC control. My apologies if your clearly lacking social skills have failed to allow you to grasp this.
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