12VDC machine

[Mark York]

We used a T100MD1616 with an MX-RTC on our last machine and it worked perfectly. We are building the same machine but the system voltage is 12VDC (last time it was 24VDC) and we can't get the PLC to function properly. Everything is the same except all inputs and outputs are 12VDC. When we attempt to run the machine the outputs blink every 5 seconds. The 3 LEDs on the board (PLC err, RTC, etc) also blink.
What are we doing wrong? Do the outputs require 24 volts to function correctly? This is the same program, the same input devices (all dry contact inputs) and the same outputs (except all coils are 12VDC). We use interposing relays on the outputs so the plc sees only the relay coils as loads.
Any thoughts?
Thank you
Mark

[Roger Shaffer]

Did you set the jumper near the power connector?  If you are not using 24 volts that needs to be addressed.  Second, did you place a diode across the coils?

[Mark York]

Roger
Yes I put the jumper in the position for 12VDC as shown in the literature. Upon further testing we have found that one of the loads (a 12VDC motor) is drawing a huge amount of current. I suspect that the current draw is dropping the voltage down to a level that is causing the PLC to reset. Power to the machine is supplied by a deep cycle marine battery. We took a second battery and connected it as the power supply to the PLC only and the machine worked as we expected it to (perfectly). The literature says that the PLC will reset itself if the voltage falls below 9 volts which I assume is happening. I can't find any PLC specs regarding low voltage "ride-through" time, are we talking miliseconds for the voltage drop? The literature also shows a capacitor connected across the incoming PLC supply terminals, do you think that would help the PLC "ride-through" the dip in voltage (what size capacitor would be recomended)? I could also add a regulated wide range DC-DC converter with input voltage ranges of 5-15 volts and a regulated output of 12 volts, but if a simple capacitor would work it would be an easy fix.
Your thoughts?
Regards
Mark

[support]

From what you described, the PLC definitely has been reset by voltage dip to its power supply. Try a 470uF 50V E-cap and the 1N4007 diode combo as shown on the installation guide, which should eliminate the self-reset caused by voltage dip on the power supply, unless the voltage dip is much longer than the capacitor can hold the voltage above 9V (such as engine starter application). If this does not resolve the problem then a separate power pack or DC-DC converter will be good solution.

[evanh]

A little note on reliability:  An average LM78xx regulator can not handle large spikes in either direction.  It is strongly advised you add some real protection between the battery and the PLC.  And you might have to be careful with ground bounce too.

At the minimum, this would be something like a 33 volt zenor or  low voltage schotty diode forward biased in series with an inductor or resistor supplying a large storage cap and another 33 volt zenor clamping the cap.

Maybe these components could be in future PLC board designs.  The existing capacitor could be user replacable for a larger one that will hold up for longer brown-outs.


Evan

[evanh]

Err, the existing design doesn't appear to have a main capacitor but instead seems to have many large caps for various supply rails which I feel is a little duplicative.  But then I don't actually have the schematics so I might be spouting garbage.


Evan

[cdenk]

I have been working on a UPS that goes the other direction. 120VDC (International Power IHC12-3.4) normal power, with a 12 VDC marine battery as backup)( See my post ot this forum some months ago for more details). To do the switching I use a "ice cube" relay (Square D 8501KP12P14V20) DPDT 120VAC coil). NC contacts to battery, NO contacts to 120VAC supply.  The coil is series with 12VDC coil DPDT DIP relay (Clare LX200000) the NC contacts. One of the PLC's outputs controls the DIP relay. When the 120VAC drops, the big relay coil power drops, and the PLC power is switched to the battery. The PLC is monitoring the 120VAC frequency and voltage already since this is a standby generator controller, and recognizing the 120VAC is out, will power the DIP coil, keeping the PLC power to the battery until the 120VAC power is good for a period of time say 15 minutes, to prevent chatter and the fluctuations that might occur as the power company is working on the lines. At this time I have bench tested the setup, and the 120VAC power supply bridges the gap, and even 12 VDC the PLC does not reset.  Hopefully in about a week I should have the installation complete and be able to verify the design, will keep posted.