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TC-08 Detecting a mains On

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TC-08 Detecting a mains On

Postby pnb » Fri Mar 30, 2018 1:09 pm

This maybe a cheeky posting, but anyway!
I have a TC-08 and am using it for a range of temperature measurements. The temperatures changes are the result of a heat source being switched on and vary in their reaction delay times. I would like to know when the Mains event occurs and the varies delays.
I thought one way to do this (without buying another Aq' unit) would be to switch a relay with the Mains, the relay contacts switch a 1.5 V cell and a resistor potential divider drops the ~1.5 V down to say ~6 mV which is fed into the TC-08.
Is there any reason why this would not do what I want?

Thanks
PS 2205, TC-08, Win10 64bit
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Re: TC-08 Detecting a mains On

Postby Glovisol » Mon Apr 02, 2018 6:01 pm

Your scheme will work fine, save for the cell finite lifetime.

Perhaps it could be simpler to connect a 50 KOhm resistor (drawing 4 mA approx.) across the 220V mains and sense the resultant temperature change with another thermocouple tightly coupled to the resistor. You could then use the alarm on this channel to detect when the mains are on or off. This scheme has some time lag.

Another alternative, just use a small AC transformer with a 220 V primary and a low voltage (e.g. 6V) secondary. Rectify/filter secondary voltage with a diode/capacitor and bring voltage down to the few mV needed for the TC-08 input. This option is fast and has no moving parts.

Cheers,

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Re: TC-08 Detecting a mains On

Postby sbabic44 » Tue Apr 03, 2018 7:16 am

You can use one of the Photovoltaic optoisolators..
On primary side you connect it as you would an ordinary optocoupler. On isolated side, instead of phototransistor, it has photovoltaic cell that produces voltage... It can be used to directly drive mosfet gates..
You can attenuate that to needed voltage levels and you're good to go..

Regards

Sinisa
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Re: TC-08 Detecting a mains On

Postby pnb » Tue Apr 03, 2018 8:59 am

Thanks for the replies.
I considered a low voltage power supply but thought the battery solution simpler, even though the voltage would be changing. Hadn't thought of the opto solution though, I will investigate this.

Thanks again
PS 2205, TC-08, Win10 64bit
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Re: TC-08 Detecting a mains On

Postby Glovisol » Wed Apr 04, 2018 12:58 pm

Hi pnb,

I do not wish to impose my ideas, but only to better express myself in case I had not been clear before.The value of an add-on mains presence detection system should be in the maximum practicality coupled with the least cost, ease of implementation and maximum reliability, as well as safety for circuitry/DA equipment/personnel.

1. Your proposed solution requires an AC relay, a battery holder, a cell, two resistors and the time to build the unit. It is prone to falsing when the battery is exhausted and the lower the battery current the higher the impedance and susceptibility to noise; the higher the battery current, the lower the battery lifetime. Furthermore the lower the current, the higher the danger of relay contact oxidation which makes the solution very unreliable in the long term.

2. The optocoupler solution uses a very small dual in line package which requires a PCB or a breadboard. Small dimensions between input (220V mains) and output (a few mV) do not help safety of anything following, DA included. Opto output is also AC, unless you rectify before or after the opto. It can be done, but for one unit it is in practice quite a job.

3. The transformer solution is the simplest, safest, most reliable and practically requires no work. You can get an AC miniature transformer in the size of less than a cubic inch for less than £ 10, solder one diode, one capacitor and two resistors across the pins and you are done. No work and no fuss and extremely high reliability & safety.

4. But your own idea of a small PSU is also very good. You can get a small switch mode AC/DC power supply, a little bigger than a postage stamp for £ 5 approx. and you just have to solder two resistors across the output and you are done. I would just add a Varistor and a fuse in the output just to make sure that no harm happens to the DA in case of a catastrophic failure of the PSU.

Kind regards,

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Re: TC-08 Detecting a mains On

Postby sbabic44 » Wed Apr 04, 2018 7:29 pm

Dear glovisol,

I would just like to comment on your post.

1. Of course photocoupler circuitry would need some kind of PCB. As would circuitry on a secondary of transformer. Unless you just want to simply hang stuff in the air, flopping in the wind...
2. Photocouplers with 5000V RMS isolation go for 2-3 €, in DIP case.. That includes PCB distances. To be sure, it is easy to drill/saw a slot between the isolated sides.. It is very standard industry practice... On the other hand, most of the small transformers won't be more than 4KV/60sec specified... So no, I wouldn't say it would be much different.
3. Opto wouldn't be AC.. It can't be.. It would be DC pulses.. And if you would rectify on primary (diode bridge), a simple capacitor (primary or secondary, you pick) would be enough.
4. It all can be done in less than a cubic inch and 10€.
5. A transformer and/or circuit for transformer crammed into cubic inch would have much smaller breakdown voltage than 5kV for an opto.

That being said, if you have a good transformer already and don't need it so small, put it in a nice plastic box, and do what you said, and you're good to go.. Nothing wrong with that. It's just far from being hands down superior solution the way you make it look...

Best regards,

Sinisa
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Re: TC-08 Detecting a mains On

Postby pnb » Thu Apr 05, 2018 8:23 am

Thanks for all the details.
This is for a one off project so a short life solution would be ok.
I happen to have a psu left over from an audio project which would need a few mods to make it suitable, so that was on my list of solutions, but then I thought battery, which I think is still my favoured one!

Is there any possibility that Pico would ever incorporate a 'Trigger' input of this type in to a futureTC-08?
PS 2205, TC-08, Win10 64bit
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Re: TC-08 Detecting a mains On

Postby Martyn » Thu Apr 05, 2018 9:02 am

Using the Single-channel TC-08 terminal board gives a few options for connecting other devices to the TC08. It would be unlikely that we will design anything into the unit, but an alternative board may be an option in the future, although not in the short to medium term.
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Re: TC-08 Detecting a mains On

Postby Glovisol » Thu Apr 05, 2018 9:41 am

With regards to the observations of sbabic 44, my final comments are as follows.

Let us not carry this too far...One thing is to assemble one diode and one capacitor to the terminals of a small AC transformer, another to design the artwork for a PCB and obtain a one off PCB specimen. Of course a box would be needed, as for most of the other solutions.

I totally agree about the DC pulses (in fact they should be truncated sine waves) and we should appreciate the fact that, as soon as the LED of the photocoupler is fed with 220 VAC, the PIV of the LED must be taken into consideration, so that some kind of voltage reduction must be provided and a single resistor is not enough, not mentioning protection from high voltage transients.

I never stated that the entire AC detector would or should fit into a cubic inch, in fact miniaturisation is not an issue here, but less than one cubic inch AC transformers fit for this job do exist.

Finally I observe that small transformers with screw terminals do exist, which solution would do away with the box as well as unnecessary PCb's, without danger of components flopping in the wind.....

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Re: TC-08 Detecting a mains On

Postby sbabic44 » Thu Apr 05, 2018 3:34 pm

Glovisol,

thank you for your clarification.
I already agreed trafo route is a good one.
Just wanted to clarify that opto is not that much more complicated or less robust like it looked from your post..


Tanti saluti

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