30-07-2012, 08:08 AM
What's your mains supply Voltage in Geneva John - presumably between 220 - 240V? If that's so and you are keeping the set to run from 120V you'll have a drop-down transformer to run it from? From Robert's calculations, if you only need to drop 40V at 0.3A that's 12 Watts, so your two 270R 20 Watt resistors will be well within their 40 Watt capability. However, if you needed to drop another 120V to run off 240V mains, 120V+40V = 160V at 0.3A = 533R. Nearest preferred value is 560R, which - at 0.3A and assuming 240V mains - would drop 168V. 168V x 0.3A = 50.4 Watts.
If you wished to reduce the wattage by using two in parallel (as with the 270R resistors), you’d need one at 1.2k and one at 1K = 545.45R, which would drop 153.5V. The 1K resistor would pass slightly more current than the 1.2K one. (If you were to use metal-clad resistors, they only dissipate their rated wattage if attached to a heat sink).
All of the above is of course irrelevant if you plan to run the set on 120V!
I'd never heard of these sets until John mentioned his - fancy Robert having one as well! Presumably yours too will be designed for 120V mains Robert - what are your plans for dealing with that?
If you wished to reduce the wattage by using two in parallel (as with the 270R resistors), you’d need one at 1.2k and one at 1K = 545.45R, which would drop 153.5V. The 1K resistor would pass slightly more current than the 1.2K one. (If you were to use metal-clad resistors, they only dissipate their rated wattage if attached to a heat sink).
All of the above is of course irrelevant if you plan to run the set on 120V!
I'd never heard of these sets until John mentioned his - fancy Robert having one as well! Presumably yours too will be designed for 120V mains Robert - what are your plans for dealing with that?
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'
