30-07-2012, 10:13 AM
(This post was last modified: 30-07-2012, 10:15 AM by Robert Darwent.)
Hi David,
From a quick internet search, I believe Switzerland has a 220V, 50Hz mains supply.
So in John's case, the calculations to drop 220 -80 = 140V at 0.3A come out as;
Resistive Dropper: 467 ohms at 42W
Capactive Dropper: 4.76 uF with a 33 ohm, 3W surge limiter (10V drop)
As you say David for a 240V supply, the calculations for a 240 - 80 = 160V drop at 0.3A come out as;
Resistive Dropper: 533 ohms at 48W
Capacitive Dropper: 4.22 uF without a surge limiter, or 4.25 uF with a 17 ohm, 1.5W limiter (5V drop)
Both of those values could be easily made up from a 4 uF motor run capacitor and either a 0.22 uF or 0.25 uF X2-type in parallel.
Of course all of the above just refers to the LT supply. It still leaves the problem of dropping the HT supply to 120V as well. According to the service sheets, the total consumption of the set is 0.43A. So assuming the HT current is in the region of 0.43 - 0.3 = 0.13A, the calculations for a 240 - 120 = 120V drop are;
Resistive Dropper: 923 ohms at 15.6W
Capacitive Dropper: 1.99 uF without a surge limiter, or 2 uF exactly with a 15 ohm, 0.3W limiter (2V drop)
Obviously, 2 uF motor run capacitors are available but the exact HT current of the set will have to be measured and the values 'tweaked' accordingly.
However, I read somewhere that you cannot use a capacitive dropper to drive inductive loads, such as the rectifier anodes directly, without having an additional resistive component. I have little previous experience with capacitive droppers so I am not sure why that is so, but if correct looks like it could rule out using the capacitor dropper method unless the surge limiter itself could serve as the "resistive component"?
Taking everything it account, it may prove easier to fit a small 240V to 120V transformer for the HT and run a 40V dropper off of that for the LT. But that all depends whether a small enough transformer can be obtained to fit in the confined chassis space available.
Also, in view of the small dimensions of this set, I would prefer to fit two seperate capacitive droppers and keep the heat produced to a minimum.
Regards
(30-07-2012, 08:08 AM)Yorkie Wrote: What's your mains supply Voltage in Geneva John - presumably between 220 - 240V?
From a quick internet search, I believe Switzerland has a 220V, 50Hz mains supply.
So in John's case, the calculations to drop 220 -80 = 140V at 0.3A come out as;
Resistive Dropper: 467 ohms at 42W
Capactive Dropper: 4.76 uF with a 33 ohm, 3W surge limiter (10V drop)
(30-07-2012, 08:08 AM)Yorkie Wrote: 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?
As you say David for a 240V supply, the calculations for a 240 - 80 = 160V drop at 0.3A come out as;
Resistive Dropper: 533 ohms at 48W
Capacitive Dropper: 4.22 uF without a surge limiter, or 4.25 uF with a 17 ohm, 1.5W limiter (5V drop)
Both of those values could be easily made up from a 4 uF motor run capacitor and either a 0.22 uF or 0.25 uF X2-type in parallel.
Of course all of the above just refers to the LT supply. It still leaves the problem of dropping the HT supply to 120V as well. According to the service sheets, the total consumption of the set is 0.43A. So assuming the HT current is in the region of 0.43 - 0.3 = 0.13A, the calculations for a 240 - 120 = 120V drop are;
Resistive Dropper: 923 ohms at 15.6W
Capacitive Dropper: 1.99 uF without a surge limiter, or 2 uF exactly with a 15 ohm, 0.3W limiter (2V drop)
Obviously, 2 uF motor run capacitors are available but the exact HT current of the set will have to be measured and the values 'tweaked' accordingly.
However, I read somewhere that you cannot use a capacitive dropper to drive inductive loads, such as the rectifier anodes directly, without having an additional resistive component. I have little previous experience with capacitive droppers so I am not sure why that is so, but if correct looks like it could rule out using the capacitor dropper method unless the surge limiter itself could serve as the "resistive component"?
Taking everything it account, it may prove easier to fit a small 240V to 120V transformer for the HT and run a 40V dropper off of that for the LT. But that all depends whether a small enough transformer can be obtained to fit in the confined chassis space available.
Also, in view of the small dimensions of this set, I would prefer to fit two seperate capacitive droppers and keep the heat produced to a minimum.
Regards
