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woodchips · 24th Jan 2023, 2:17 pm

Doesn't seem to have been mentioned but don't leave the secondary of a mains isolating transformer floating. Why? if you earth one side then you can use an RCD before you start holding the wires, get the considerable protection these afford if you do dob your finger where it shouldn't.

On a similar point about capacitive coupling I have recently had an interesting example of this. My last readout I designed 20+ years ago was battery powered using a 1/2AA lithium cell. The readout, being used on machine tools and similar is usually in a fairly dark environment, to the display used were LED, I finf LCD to be rubbish. Whilst you can wave a DVM around to get the reflection to work, not quite so easy with 3 tonnes of drilling machine. The readout had power saving to blank the display but many customers found that irritating so I offered a mains version. This used a 9V wall wart with regulator to drop it to 3.6V.

No problems for years, then customer said the readout was jumping positions, not really much use. In the end I discovered that it was the capacitive coupling through the PSU that was upsetting the readout.

The opto sensor uses power saving, turning the LEDs off, also the display is software scanned so there is no digits displayed if a low reading. This meant that the readout current consumption was zero for a considerable time as the readout was used. This allowed the capacitive coupling to blatt the ADC input and give the false reading.

Testing produced an about 80V pk-pk 50Hz sinewave on the power connector, plenty to overload the ADC input. This was at minimal current, testing suggested 10uA or less. Further testing showed that a 1k resistor across the 9V, so about 1mA, was enough to completely stop the 50Hz noise voltage and solved the jumping problem.

As an aside, when I started selling these I supplied a 9V 300mA PSU, but transformer based, these never had the capacitive problem, so no unhappy customers. Then transformers were out of fashion and switched mode PSUs were in, so it took many years to use up my stock of PSUs.

Something to be aware of? It was the time based change in current drawn, and being zero for blanked digits that caused the head scratching, you see a digit and automatically assume they are on 100% of the time, nope!

24th Jan 2023, 2:17 pm	#24
woodchips Octode Join Date: May 2010 Location: Grantham, Lincolnshire, UK. Posts: 1,177	Re: Isolator Transformer Query Doesn't seem to have been mentioned but don't leave the secondary of a mains isolating transformer floating. Why? if you earth one side then you can use an RCD before you start holding the wires, get the considerable protection these afford if you do dob your finger where it shouldn't. On a similar point about capacitive coupling I have recently had an interesting example of this. My last readout I designed 20+ years ago was battery powered using a 1/2AA lithium cell. The readout, being used on machine tools and similar is usually in a fairly dark environment, to the display used were LED, I finf LCD to be rubbish. Whilst you can wave a DVM around to get the reflection to work, not quite so easy with 3 tonnes of drilling machine. The readout had power saving to blank the display but many customers found that irritating so I offered a mains version. This used a 9V wall wart with regulator to drop it to 3.6V. No problems for years, then customer said the readout was jumping positions, not really much use. In the end I discovered that it was the capacitive coupling through the PSU that was upsetting the readout. The opto sensor uses power saving, turning the LEDs off, also the display is software scanned so there is no digits displayed if a low reading. This meant that the readout current consumption was zero for a considerable time as the readout was used. This allowed the capacitive coupling to blatt the ADC input and give the false reading. Testing produced an about 80V pk-pk 50Hz sinewave on the power connector, plenty to overload the ADC input. This was at minimal current, testing suggested 10uA or less. Further testing showed that a 1k resistor across the 9V, so about 1mA, was enough to completely stop the 50Hz noise voltage and solved the jumping problem. As an aside, when I started selling these I supplied a 9V 300mA PSU, but transformer based, these never had the capacitive problem, so no unhappy customers. Then transformers were out of fashion and switched mode PSUs were in, so it took many years to use up my stock of PSUs. Something to be aware of? It was the time based change in current drawn, and being zero for blanked digits that caused the head scratching, you see a digit and automatically assume they are on 100% of the time, nope!