More than a year ago, after having decided to construct two more remote switches, I quickly made a list of the parts required and shot off half a dozen orders to Ebay. One thing I could not source on Ebay was the needed mains transformers, which was the reason that, despite proclamations in earlier posts, I ended up going back to Aliexpress, tail firmly between legs. Although Ebay would be nothing without Asian Sellers, many more of these Sellers only seem to sell their wares through Aliexpress. Anyway, transformers were ordered and promptly delivered.
Only, I had ordered the wrong ones! At the time, I took a quick look inside my one remote switch, had seen 6v written on the transformer, so ordered two of the 220v @ 6v variety. Only many months on, when I had finally received the PCB's from JLCPCB, and had started building, did I realise that what I needed was 12v transformers, not 6v, to power the 12v relays, and that the transformer in my working unit was in fact supplying 12v - 3-wire 6v-0v-6v. Duh! Unperturbed, I went a-searching on Aliexpress, found the correct transformers, or more accurately, the same spec 220v @ 6v-0v-6v transformers used in my working unit, but at less than half the price of what I had paid first time round, so shot off another order. These also arrived quickly and seemed fine apart from the fact that there was adhesive all over the wires, probably that used to secure the iron core laminations, plus said leads were a little shorter than I would have liked. But hey, I'd gotten them for half the price, so no biggie.
On connecting one up to my build under construction, what immediately became apparent was that, although they 'worked', you could hear that the 12v relay was being switched 'hard-on'. Measuring the voltage across it while energised, I was shocked to see that it was over 18v DC. De-energised, that rose to almost 20v. The voltage across the 6v - 6v wires of the secondary was 16.4v AC, not the 12v AC that I was expecting. Note that this is on a 240v mains supply, nominally around 238v, though the extra volts don't significantly change things.
I wasn't happy with this in the least, even after I realised that the relay would happily trigger when I used the 6v - 0v wires, therefore working from just 8.6v DC, I concluded that the 'cheapness' of the transformers was the culprit. Particularly after checking the first 220v @ 6v transformers purchased and finding that they provided a reasonable 6.5v AC on their secondaries. So, back to Aliexpress I go and order another 2 'expensive' transformers from the original seller, this time, 220v @12v, 2-wire, in the fervent hope that these would provide the expected voltage.
They didn't, and I'm pissed!!!
Below are a series of pics taken of measurements from the various transformers in play. To be as unbiased as possible, I am powering them from a variac, supplying 220v AC.
Below are the voltage measurements taken from the working unit, but with the transformer in-circuit. The first shows it being supplied from the variac @ 220v AC. The circuit is being powered (@ 15mA's), though the relay is not energised. The secondary of the transformer sits at 11.47v AC.
Here I adjust the variac to provide 240v to the circuit. The voltage increases to 12.27v AC, which had me wondering if these particular transformers, bought from Maplin (RIP) almost 30 years ago, were specifically manufactured in/for the UK. Regardless, these are the voltages I expect to see from a 12v transformer, not some 3+ volts over the stated RMS value.
Finally, Also shown below are measurements across the transformer's secondary with the relay powered on & off, to show loading effects. The maximum current drawn from the secondary by the PCB is 75mA AC.
Edit1:
Been playing with my scope's Save/Recall function, and aided by Linux's ever-versatile Gimp, grabbed waveforms & measurements from the 3 offending transformers, on the mains supply, and shown below.
Given the lottery that seems to be involved when buying transformers from China, I instead decided to try dismantling one of the 220v @ 12v 2-wire ones, to see how difficult it would be to adjust the turns-ratio of its secondary. I had low hopes going into this - I figured there was a 70% chance of screwing it up entirely - instead, it proved to be surprisingly easy!
This was the first transformer I'd taken apart in this manner. The wire to be removed from the secondary I calculated as follows;
19 Ohms (actual secondary resistance) - 12v (desired volts) / 14.98v (actual volts) * 19 Ohms = 3.76 Ohms.
This is the resistance of the wire segment that needed to be removed. I then used this Round Wire Resistance Calculator site to determine the length of (0.17mm) wire that would have a resistance of 3.76 Ohms - turned out to be a little over 5 meters. After removing this amount and reassembling, I was pleased to find that its voltage after rectification, with the relay de-energised, had dropped to a much 'safer' level of 15.9v, one that wouldn't pop my 16v 220uF reservoir capacitor over time - the voltage had been over 19v in the 'relay-off' state.
However, the values I had used in the above calculations had been with a 220v supply, not the 240v I will actually be using. So, working on the second 220v @ 12v transformer, and re-doing the above calculation with 15.8v as the 'actual volts', gave me a 'wire length to be removed' of 6.175 meters. After doing so, i was presented with an RMS value of just over 13v AC on the secondary, which, in-circuit, rises to no more than 15.1v DC. With the relay engaged, it actually drops to around 11.5v DC. So, yay, I'm happy with this result.
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