General Input Unit Voltage Measurement

I have an Atco TM50 transformer connected to my GIU from a remote location on my property. The power source to the transformer is different to that into the house so I use this to let me know if the power is on (as it runs, amongst other things, my Biolytix (worm) waste system. Homegate shows me a status indicator letting me know if the group is active , just Green for Ok (ie: there is voltage), Red for problem (0 voltage - circuit/power/is off). All this works fine.

My query is that when I look at the level of the GA from a 'Show Current Level' in the Channel tab or the GA assigned to the channel, it reads as only 48 (19%) despite the fact that the voltage from the transformer is rated as 12v (and as measured from my multimeter is 14v) - this measurement is at the GIU so there is no loss over the cable.

I have the channel on the GIU set to input type 0-20v. Am I missing something obvious - I would have thought it should read 14/20 = 178 = 70%. Again no big deal as effectively I'm only interested in whether there is power or not but it would be nice to know why the reading doesn't seem correct.

 

Hi ABG, How long is the cable you're running? If it's longer than about 5 metres (for DC) or 10 metres (for AC, which is what your tranny is outputting), then you're going to experience voltage drop. I'm not sure what kind of load the GIU itself would put on the line, but it would pay to test your voltage at the GIU while the cable is connected.

If you can't figure it out, and this problem is still of concern to you, I'd recommend putting a 12v relay at the transformer end, and wiring the coil to the tranny, and the contacts of the relay to the cable to GIU. Then switch the GIU over to measuring resistance (ohms).

That way you dont have to worry as much about voltage drop, because there isn't really any voltage on the line.

 

How are you measuring the transformer output voltage. The GIU is not designed to measure AC, so applying AC to the input will result in strange readings. Any reading would be because of inadvertent rectification of the AC by the input protection diodes. It will also be dependent on which channel you are on as the time of measurement for each channel is locked to the mains frequency.

If you want to measure AC, then I suggest you half wave rectify, with a small filter cap and a light load (to discharge the cap when powered off). This should result in a peak reading (~1.4 x 12 for sine wave). The downlight transformer may not be outputing sine waves.

The GIU has about 200k input impedance so shouldn't load your network.

RDS

 

As NickD said it is and iron core transformer and I am measuring voltage at the GIU as 14 volts so there is no voltage drop. That's what's got me interested....(and I thought the atco TM50's were dc?)

 

Pretty much all downlight transformers output AC. Saves on the component count inside. And no, the Atco TM50 is no exception. It's nothing more than an iron-core transformer, and thats about it. 230v AC in > 12v(~) AC out.

 

Then as RDS said:

 

Doh. Ac reading 14.3, DC 0..04-05 (if accurate....)

As the current solution does provide a simple way to determine if power is present, is there any possibilty of damaging the GIU leaving the 12v AC transformer connected as is?

Otherwise....as I only want to know if there is power or not, ie: ON/OFF on the circuit I think the easiest option is the relay at the transformer end and measuring resistance.

 

One to add to the to-do list.

Many thanks to all for the excellent help,


Andrew

 

DC Measurement Fluctuates

Hey guys
Had a good read of this thread and I understand the AC issues and associated rectification stuff.
However, I tried using a DC plug pack (plugged into a pool solar controller until I get around to buying the new 4 port c-bus temp sensor) and I can stick a multimeter over the GIM terminals and get 12.4 volts consistently.
However the GIM reports anything from 3 to 34 volts. Do I need a load across it or what am I missing?
Thanks

 

Anyone?

 

Hi Msniper,

What are you trying to achieve plugging a power supply directly into a GIM input channel? Knowing the required application can help people posting reply to your question.

Usually you are measuring something that you want to act upon, like the change in resistance in a moisture sensor when it gets wet or the resistance change with temperature in a thermistor etc. Voltages are quite often measured for pseudo 'digital' event notification, like watching for an output being high (on) or low (off) to signal a certain event has occurred or is occurring.

Plug packs can be built in many ways, some have transformers but more modern low current / light weight ones use switch-mode power supplies. Many of these will shut down completely or can continually move around in output voltage with no load (or a minimal load like a high impedance input channel) as some have only half wave rectification, some full wave rectification and some are current controlled and regulated.

Other questions.
Is it attached to the A and the GIM 'channel' number, ie not 'B' on the GIM.
Is it an 'outdoor' unit? (as many of these have inbuilt circuit protection that almost shuts them off if they have no load or are shorted)?
Does the GIM have its 24VAC PSU in place?
How old is the Plugpack you are using (more than 2 years of hard use and the capacitors in the output stage could be shot making its output dirty)? Look at output on an oscilloscope. Clean up dirty output with an electolytic capacitor?
Is it definitely a DC PSU?
How long is the cable between and does it go near any 240Vac cables on it's cable route? Induced voltage?

Putting a load resistor of around 10K ohms (the lower the resistance in ohms, the more it will draw current and heat up) across the plugpack output will usually make most 'kick in' (ie pull enough current to stabilise / switch on), start heating the resistor/s and stabilise the output. Measure across the resistor with the GIM channel.

If you want to measure a larger voltage closer to the maximum of 20v on the GIM, you can put two resistors in series (a voltage divider circuit) and use the negative to the GIM negative and take the GIM positive input from the junction.

eg. With a DC plugpack of say 12v-24v

.......................................................................................................................................................
.......................................5K.............................................................................................................
plugpack +ve o__________/\/\/\____________o......................................................................................
...............................................................|......................................................................................
...............................................................|......................................................................................
...............................................................|......................................................................................
......................................5K......................|......................................................................................
plugpack -ve o___o_____/\/\/\______________o___________o GIM chan +ve (terminal A) ............................
..........................|...........................................................................................................................
..........................|............................................. (50% of Plugpack's Output Voltage).............................
..........................|..........................................................................................................................
..........................|..........................................................................................................................
..........................o___________________________________o GIM chan -ve (Terminal 1,2,3 or 4 - not B)......
.....................................................................................................................................................
.....................................................................................................................................................


A good place to start is putting a known voltage across the channels to start with to ensure the GIM is configured correctly. Put a (measured) 9v battery across the GIM channel. Are you getting a solid 9V reported from the GIM channel in the module in Toolkit?

What is the application that you specifically want monitor the plug pack voltage for?

Kind Regards,

Brad