Do DC input PLC's work with AC?

86turbodsl · Nov 22, 2008

I've got an automation direct/koyo plc that I am working on that I accidentally applied 24V ac to and the inputs seemed to work ok. Is this normal? Am I going to damage anything by doing this? The AC signal is what I have as an input, and I've been trying to rectify it to input to the PLC.

OZEE · Nov 22, 2008

I wouldn't expect it to work for very long.

BryanG · Nov 22, 2008

OK two problems. 24V a.c. isn't actually 24V, it is 24V RMS (route mean square, try a Google) which means that at its peak the input will rise to about 34V, is the PLC input rated for that voltage. Second, the input will actually see 50/60 on's and off's each second. As long as you set the filtering on the input to ignore that cycling then you would be OK, otherwise whatever code you have being operated by the input will be operated 50/60 times per second. Rectifying will fix problem 2 but will still leave you with problem 1.

So the answer to your question is you may be OK but if it was me I would just add a AC relay and let that switch into the PLC.

Bryan

86turbodsl · Nov 22, 2008

So what you're saying is the circuitry is incorrect for the input?

OZEE · Nov 22, 2008

yes, that's what we're saying.

86turbodsl · Nov 22, 2008

There's too many inputs to add AC relays to each one. The application is as a HVAC controller. There are 9 thermostats that have relays in them, they send the 24AC voltage back to the controller. These thermostats will work on DC. The thermostats are supplied voltage from the PLC. I put a diode inline with the voltage for the t-stats and got half wave DC out of the t-stat. The plc didn't like that. It turned inputs on and off depending on what the voltage was when it read the input. So at this point, it looks like my choices (without a lot of reconfiguring) are AC input or swap a full wave bridge into the t-stat feed line. The plc is rated for 6-240V DC input. I'm good there.

jtn · Nov 22, 2008

Whoever inherits this after you would thank you greatly for getting the correct input card for the voltage instead of adding a lot of extras into the circuit.

If the T-stats have dry contacts in them, why not send 24VDC to them so that you get 24VDC back? A cheap power supply is all you would need to add.

86turbodsl · Nov 22, 2008

Good idea. Know where I can find a cheap 24V supply? Don't have a good idea where to get one. Transformers are easy to get. 24VDC supplies, not so much...

bernie_carlton · Nov 22, 2008

By 'easy to get' do you mean they aren't generally available anywhere or that they cost more than a 24VAC transformer? Since you are already working with AutomationDirect equipment why not give them a try. They sell power supplies including 24VDC models.

By the way, which model of AD PLC do you have. Many have a current limited 24DVC supply which produces more than the system itself needs and has terminals for that voltage. If power just inputs that may be enough.

keithkyll · Nov 22, 2008

24AC won't hurt the inputs. They're Optoisolators. Put a 1N4004 (1N4005, 1N4007) in series with the signal. Cathode (band) towards PLC. Add 4.7 Uf, 50 Volt cap across input. Use Tantalum caps.
Someone draw this for me.

If you're really handy with electronics you could do this inside the module. Lift one leg of the 2.4K (or is it 2.2K?) resistor and put diode in series. Clearly mark module 'Modified'.

Doing this inside the Thermostat is clearly the best choice.

86turbodsl · Nov 22, 2008

It's a DL06 DR. I already tried adding a diode. The plc didn't like the half-wave back from the t-stat. I saw that the PLC has a 24VDC output. I don't know if thats enough since I don't have a good idea of the current draw on the t-stats. I've got 3 modules in the PLC, so I need to do a power budget and see what's left over.

It's good to know I won't kill it with AC input.

keithkyll · Nov 22, 2008

A diode by itself isn't enough. You need the cap too. The PLC supply might not be enough to drive the stats.
Edit: The stats should only be 20mA or so. I assume you will still be using the 24AC to drive the furnace/Compressor contactors. I think the PLC 24DC IS 500mA.

86turbodsl · Nov 22, 2008

I put a 4.7 uf cap across the diode and got no improvement. Ok, maybe slight. The voltage on my VOM was reading about 7V. I tried moving the tstats to the plc 24V supply and it works perfect. I still haven't found the power budget info in the manual yet. I won't leave it like that long unless I find info that it's got some headroom. The tstats run for a couple of years on two AAA batteries, so they can't be pulling much.

Edit: Yes, the 24VAC transformers will be running the zone valves and furnace contacts through the PLC relays. That's another question, do I need anything to prolong contact life on the relay outputs if it's running AC? My understanding is there isn't a big voltage surge on AC as compared to DC. My relays won't be moving that much. typical would be switch on for 15 minutes and off for 15 minutes. Repeat during heating season.

rpoet · Nov 22, 2008

I have done exactly what you are trying to do with an AD PLC. It's a really easy solution. Place a 2.7k 1/4w resistor in series with each input - no caps needed. That will limit current to a safe value, since the peak voltage of a 24vac RMS signal is around 34v. That takes care of the hardware. In DirectSoft, use the "Off-Delay" I-box to filter the inputs, since they are actually turning on and off 120 times a second.

--X0-----------------------------OFF-DELAY IBOX, K30---C0 output

--CO-----------------------------Your program here.

Use the C-bit output of each Off-Delay timer to control your program. Note - each input will need its own timer. Works like a charm, and very simple. Just make sure to document this well in DirectSoft with comments.

Hope this helps.

rpoet

86turbodsl · Nov 22, 2008

Ok, measured the current on the tstats. All 9 stats measured at 125mA. The D0-06DR has a 200mA budget. I'm in great shape.
My total power budget on the 5V side is 1300mA and I can take 2000mA.

Now the only question left is do I need debounce/MOV's on the output side. Output is 24VAC transformers running 24VAC motorized zone valves and contactors through the relays on the DR.

Do DC input PLC's work with AC?

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