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Is this a really bad design?

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Diver300

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I recently bought one of these:-https://library.e.abb.com/public/9e.../Royce Thompson Astro Nova City datasheet.pdf
and I found that it was quite easy to get it to crash, with the display locked up, and no function at all.

I looked at it a bit more today. It's got a transformerless power supply, with an electrolytic capacitor running at around 40 V There is a 3.3 V supply to the electronics that is kept going with a rechargeable battery. It seems to crash when the mains power is removed, and the electrolytic capacitor drops below the voltage. Once it has crashed, the electronics needs resetting by waiting for the battery to run down, or by opening up the timer and temporarily disconnecting the electronics from the rechargeable battery.

It looks like the 3.3 V supply dips and upsets the microcontroller, and it seems to be worse when the backup battery isn't well charged. Perhaps with a fully charged battery, the microcontroller would keep running correctly when the mains is removed.

There doesn't seem to be a low voltage reset, or a watchdog. Disconnecting the power and waiting for the battery to run down seems to be the only option, and with a 4 year battery life, that could be tedious.

Is the circuit as poor as I think it is?
 

Diver300

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On that timer, the video at
says to plug in for a minimum time, which must be to charge the battery. Also it shows having a reset button, which could be how that one gets over the problem of the software locking up if the battery is partially charged.

I was looking for a sunset/sunrise timer like this
but I wanted a wired one, not plug in, and of course I need it to be 230 V 50 Hz
 

Tony Stewart

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I think if the installation setup is followed , with min. charge time, there should be no issues.
You can shop for wired digital clock relays , but plug-in units will be cheaper and thus more common.
 

Diver300

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I left the unit to charge for a few days, and it was still the same.

When I looked more carefully, I found that it wasn't a rechargeable battery. It's a CR2477, 3 V 1000 mAh, that had was at or near end of life. There is no way of isolating the battery, so the time on the shelf will take time from the reserve time. The description in the data sheet of the battery being lithium-ion is wrong or at best misleading.

I had bought the timer from ebay. http://www.ebay.com/itm/122054775276 It is about 4 years old, and doesn't seem to have been used, but the box it was in was in a terrible state. The timer has two circuit boards, a main board and a display board, and where the power from the battery enters the display board, a track had corroded. I suspect that it got damp in storage, and there was condensation on the display board, causing the corrosion.

With the track fixed, and a temporary battery fitted, the display part of the timer works without the mains being connected.

I'm going to order a new CR2477 and I'll also disable one of the output relays, and reduce the capacitor in the transformerless power supply. I only ever want one relay, and the timer runs quite warm, so a lower value capacitor will reduce the heating in the zener regulator.
 
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