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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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26th Jan 2013, 12:57 pm | #1 |
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Use of Class X2 Capacitors as Droppers.
I am in the process of converting a Canadian built Mitchell 1260 (117vac)Table lamp radio to work from UK 230/240v Mains supply. For space reasons I cannot use a Motor Run Capacitor as a dropper, since it needs to fit inside the radio. Instead, I am planning to use 3 x 1uf 275vac Class X2 capacitors in parallel, with a parallel 'discharge' resistor of maybe 470k, and possibly a surge limiting resistor in series. Since these capacitors will only have to pass ca. 125vac, they should be O.K. IMHO, but are there any possible consequences I've overlooked?
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26th Jan 2013, 1:30 pm | #2 |
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Re: Use of Class X2 Capacitors as Droppers.
I have been advised by others on this forum that X2s are not the ideal component for this application although I run my workshop radio (Bush DAC90A) with one.
How did you calculate the capacitor value you will need? Normally a capacitive dropper is used in the heater chain only but if you are using one to drop the mains for the whole set you will run into difficulty calculating the capacitor value. This is because of the non-linear nature of the current drawn from the mains by the HT circuit. Colin M |
26th Jan 2013, 3:26 pm | #3 |
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Re: Use of Class X2 Capacitors as Droppers.
Yes, a capacitor can't pass DC so the HT current for the set has to be subtracted from a (modified, lopsided) heater current. Not only is it difficult to calculate, it will also be difficult to measure unless you have a True RMS AC meter.
Your best option may be to construct a Spice model of the heater chain and PSU stages, with the rest of the set as a resistor to draw some DC current. Adjust the capacitor value until the RMS heater current is 150mA (or whatever it is supposed to be). You may find that the HT voltage is off a bit, but that should not matter too much. |
26th Jan 2013, 4:38 pm | #4 | |
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Re: Use of Class X2 Capacitors as Droppers.
Quote:
They'll be passing less current than they would otherwise do connected straight across the supply and they're designed to fail open circuit rather than short. The Spice Model idea is good for an hour or two of amusement, playing around with values, especially if you want to keep the HT the same as original. There are some useful free simulation packages about- either as limited functionality demos or pretty much full featured ones with a bias towards a particular manufacturer like TI's TINA. I've also wondered about putting a whole set "inside" a bridge rectifier fed via the dropping capacitor- with the covers in place it's no more dangerous than the original setup.......
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26th Jan 2013, 5:54 pm | #5 |
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Re: Use of Class X2 Capacitors as Droppers.
I also can't think of any reason not to use X2s in this application.
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26th Jan 2013, 6:19 pm | #6 | |
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Re: Use of Class X2 Capacitors as Droppers.
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26th Jan 2013, 6:58 pm | #7 | |
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Re: Use of Class X2 Capacitors as Droppers.
Quote:
Very often, X capacitors can be used to 'trim' the value of a motor-run capacitor (by connecting them in parallel with it) when a precise value can't be obtained Rich
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26th Jan 2013, 7:59 pm | #8 |
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Re: Use of Class X2 Capacitors as Droppers.
Thanks for the replies, folks. Most helpful as ever. I calculated the capacitor value, which actually came to 3.126uf, using the formulas on the 'Dropper Calculations' pages, and, have established that, with a 3uf Motor run capacitor, the HT voltage as measured with an analogue multimeter on it's 250vac range(9k/volt) a reading of 120v is obtained. The circuit of the Mitchell Radio is essentially an 'All-American Five', using octal valves. Heater current is 150mA, and HT I about 50mA. As noted in my post #1, I cannot fit a motor run capacitor inside the radio due to lack of space, so will have to use 3 x 1uf Class X2s. All I can do is fit them, replace the wax paper caps, and test run it out of it's cabinet to establish, hopefully that all is well, and the AC supply voltage remains sensibly constant using this method.
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27th Jan 2013, 12:48 am | #9 | |
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Re: Use of Class X2 Capacitors as Droppers.
Quote:
Colin M |
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27th Jan 2013, 1:05 am | #10 | ||
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Re: Use of Class X2 Capacitors as Droppers.
Quote:
I have just had a look back at where I got the information that justified the above quote. This was it. Quote:
So no one had suggested that class X capacitors were not suitable for droppers. I'm sorry if anyone was misled by that. Colin M |
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27th Jan 2013, 1:21 am | #11 |
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Re: Use of Class X2 Capacitors as Droppers.
One possible practical objection I've encountered with X2 capacitors used as droppers (in things like central heating timers and plug in digital timers) is that they seem to be so good at self-healing without obvious manifestation under many moons of mains splats that their value slowly decreases over time- eventually, control relays no longer pull in and the device is passed to me with "can you just have a look at this...?". I've also noticed that X2 types in their "normal" placement in long-serving kit can end up with very low values.
One to be wary of. |
27th Jan 2013, 1:59 am | #12 |
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Re: Use of Class X2 Capacitors as Droppers.
Sounds like that could call for a sacrificial X-cap in the traditional across the mains position ahead of the dropper. Maybe feed the dropper via a small choke as well to make sure the splats stay on the mains side.
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27th Jan 2013, 2:06 am | #13 | |
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Re: Use of Class X2 Capacitors as Droppers.
Quote:
Self healing caps do gradually reduce in value in applications where they are constantly exposed to mains voltages, but used as a heater dropper they will only be connected to the mains for relatively short periods, and even if the capacitance goes down this will just result in lower voltages so not cause catastrophic damage. |
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27th Jan 2013, 2:12 am | #14 |
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Re: Use of Class X2 Capacitors as Droppers.
And a VDR for good measure. I sprinkle them around all my kit, but they need feeding through a closely rated fuse for safety- in my professional capacity, I've encountered temporary three-phase installlations where neutral has been lost (accidentally or maliciously). On the leg that drifts high, VDR's in equipment can disintegrate in a molten shower if fusing is lax.
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27th Jan 2013, 6:06 pm | #15 |
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Re: Use of Class X2 Capacitors as Droppers.
Don't adjust the cap for correct HT voltage. Adjust the cap for correct heater current, which requires a True RMS meter. You can adjust the balance between heater and HT by adding a wirewound resistor on the side which needs dropping a bit more/doesn't need boosting a bit more.
The reason why it needs a True RMS meter is that the heater current won't be a sine wave, but some combination of AC, DC and charging pulses. This is so the total current drawn from the supply has no DC component. A normal DMM usually measures the average of rectified AC but is then scaled into RMS assuming the waveform is pure sine, so no good here. |
27th Jan 2013, 7:46 pm | #16 |
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Re: Use of Class X2 Capacitors as Droppers.
A poor man's true RMS meter:
Obtain two pilot lamps from the same source, rated to light below full brightness at the required RMS heater current. For a set with 100mA heaters, I would use a 150mA lamp. Place one in the set's heater chain and connect the other to a DC power supply and supply it with the required current - 100mA in this case. Either subjectively or (better) use a film camera exposure meter to judge when the two lamps are at equal brightness, adjusting the dropper capacitance to achieve this. The heater RMS current in the set then has the required value. It's a remarkably effective method. You can interchange the lamps and take an average value to mitigate against the effects of mismatch, but within the allowable tolerance of heater current, it's good enough. Leon. |
27th Jan 2013, 8:23 pm | #17 |
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Re: Use of Class X2 Capacitors as Droppers.
Hi,
I don’t know if this will help but I got a unit through LVD (BS EN 60065) approvals with an X2 used as dropper with no problems. Terry |
27th Jan 2013, 9:04 pm | #18 |
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Re: Use of Class X2 Capacitors as Droppers.
Very clever, Leon, thanks for that.
I remember Goodmans used 2 LES lamps as a tuning meter in their Module 80 receiver (and possibly other units too). Best tuning was achieved when both lamps were of equal brilliance. It was indeed remarkable how precise this method proved to be. N. |
27th Jan 2013, 9:24 pm | #19 | |
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Re: Use of Class X2 Capacitors as Droppers.
Quote:
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27th Jan 2013, 9:51 pm | #20 |
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Re: Use of Class X2 Capacitors as Droppers.
Search for "Bunsen grease-spot photometer"!
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