01-09-2015, 09:27 AM
This has turned out to be quite an interesting and instructive thread - well to me, anyway!
Almost everything I build in terms of electronic projects rather than radio restoration are simple projects (small things amuse small minds!), which aren't safety critical and use low voltages. However, occasionally I have built projects which use high voltages and in an earlier post I was trying to say (not very well perhaps), that just because a resistor is connected to a high voltage point - say 230V - doesn't mean that the PD across it will be 230V or anything like that, but as Terry points out in his table, it isn't simply a case of 'oh well, these are rated at 250V so they'll be fine in all circumstances'.
Just to give a little example, some years ago I built a couple of nixie clocks from a design on the excellent 'Mike's Electric Stuff' website. The design goes back to 1999, and was initially for use in America, with a lower mains voltage, but the circuit and PCB allows for use on UK mains. Nick is familiar with the design, and expressed some disquiet at it being run straight from the mains with no isolating transformer, and Mike does show two back-to-back transformers to achieve that objective. From a safety perspective, the two clocks I made (one for me, one for my son) are in non-metallic enclosures have no external metalwork and the time is set externally using a small button magnet to operate two reed relays internally
Setting those considerations aside, and concentrating on the voltage rating of the 47k resistors used to drop the voltage for each cathode on each tube, they have to cope not with 230V, but with the voltage dropped across them and the current drawn. (Obviously). The strike voltage of the cathodes is 170V, so the voltage dropped across each resistor is 230 - 170 = 60V, so the current drawn is 60/47K = 0.00128A. Hence, 60V x 0.00128A = 0.077W. I appreciate of course that there will be lots of situations where a higher voltage drop will be called for, at a higher current rating, but in the case I've mentioned, even 1/8 Watt resistors would cope.
Not to take the thread even further off topic, (not that it matters here in happy valley!), here's the nixie clock design:
http://www.electricstuff.co.uk/nixclock.html
Thanks for the erudite input to the thread, and the table from Terry and the Vishay link by Lawrence.
Almost everything I build in terms of electronic projects rather than radio restoration are simple projects (small things amuse small minds!), which aren't safety critical and use low voltages. However, occasionally I have built projects which use high voltages and in an earlier post I was trying to say (not very well perhaps), that just because a resistor is connected to a high voltage point - say 230V - doesn't mean that the PD across it will be 230V or anything like that, but as Terry points out in his table, it isn't simply a case of 'oh well, these are rated at 250V so they'll be fine in all circumstances'.
Just to give a little example, some years ago I built a couple of nixie clocks from a design on the excellent 'Mike's Electric Stuff' website. The design goes back to 1999, and was initially for use in America, with a lower mains voltage, but the circuit and PCB allows for use on UK mains. Nick is familiar with the design, and expressed some disquiet at it being run straight from the mains with no isolating transformer, and Mike does show two back-to-back transformers to achieve that objective. From a safety perspective, the two clocks I made (one for me, one for my son) are in non-metallic enclosures have no external metalwork and the time is set externally using a small button magnet to operate two reed relays internally
Setting those considerations aside, and concentrating on the voltage rating of the 47k resistors used to drop the voltage for each cathode on each tube, they have to cope not with 230V, but with the voltage dropped across them and the current drawn. (Obviously). The strike voltage of the cathodes is 170V, so the voltage dropped across each resistor is 230 - 170 = 60V, so the current drawn is 60/47K = 0.00128A. Hence, 60V x 0.00128A = 0.077W. I appreciate of course that there will be lots of situations where a higher voltage drop will be called for, at a higher current rating, but in the case I've mentioned, even 1/8 Watt resistors would cope.
Not to take the thread even further off topic, (not that it matters here in happy valley!), here's the nixie clock design:
http://www.electricstuff.co.uk/nixclock.html
Thanks for the erudite input to the thread, and the table from Terry and the Vishay link by Lawrence.
Regards, David.
BVWS Member.
G-QRP Club Member 1339.
'I'm in my own little world, but I'm happy, and they know me here'
BVWS Member.
G-QRP Club Member 1339.
'I'm in my own little world, but I'm happy, and they know me here'
