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Old 26th Jan 2013, 12:57 pm   #1
'LIVEWIRE?'
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Question 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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Old 26th Jan 2013, 1:30 pm   #2
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Default 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
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Old 26th Jan 2013, 3:26 pm   #3
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Default 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.
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Old 26th Jan 2013, 4:38 pm   #4
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Default Re: Use of Class X2 Capacitors as Droppers.

Quote:
Originally Posted by ukcol View Post
I have been advised by others on this forum that X2s are not the ideal component for this application
I wonder why?

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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Old 26th Jan 2013, 5:54 pm   #5
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Default 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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Old 26th Jan 2013, 6:19 pm   #6
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Default Re: Use of Class X2 Capacitors as Droppers.

Quote:
I've also wondered about putting a whole set "inside" a bridge rectifier
You may (will?) get the switching noise from the diodes all over the set and it will probably hum horribly when you move your hand or any other earthed/large object near it. This capacitivly couples the said noise to sensitive parts of the circuit.
 
Old 26th Jan 2013, 6:58 pm   #7
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Default Re: Use of Class X2 Capacitors as Droppers.

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Originally Posted by paulsherwin View Post
I also can't think of any reason not to use X2s in this application.
Neither can I! They are rated for continuous use and I'm using them very successfully in Pilot radio, replacing the original dropper. It's been running for about three years now. Motor run capacitors are also good to use as they are designed for continuous use. Motor start capacitors are not as they are designed for intermittent use.

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



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Old 26th Jan 2013, 7:59 pm   #8
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Default 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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Old 27th Jan 2013, 12:48 am   #9
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Default Re: Use of Class X2 Capacitors as Droppers.

Quote:
Originally Posted by 'LIVEWIRE?' View Post
I calculated the capacitor value, which actually came to 3.126uf, using the formulas on the 'Dropper Calculations' pages....
This is not a valid way of calculating for heaters + HT for the reason I gave in post 2. You may be able to get away with using 3 uF plus or minus X uF by tweeking the dropper capacitor value for the correct voltage across the heater chain and then checking that the HT voltage is somewhere near right.

Colin M
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Old 27th Jan 2013, 1:05 am   #10
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Default Re: Use of Class X2 Capacitors as Droppers.

Quote:
Originally Posted by ukcol View Post
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.

Colin M


I have just had a look back at where I got the information that justified the above quote.


This was it.

Quote:
Originally Posted by Studio263 View Post
In a word no, X2 capacitors are designed only for connection across the mains supply and have special characteristics to optimise them for that position.
But it was written in the context of replacing wax capacitors in TVs and not that of capacitive droppers.

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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Old 27th Jan 2013, 1:21 am   #11
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Default 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.
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Old 27th Jan 2013, 1:59 am   #12
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Default 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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Old 27th Jan 2013, 2:06 am   #13
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Default Re: Use of Class X2 Capacitors as Droppers.

Quote:
Originally Posted by ukcol View Post
This is not a valid way of calculating for heaters + HT for the reason I gave in post 2.
This is true. Paul Stenning's spreadsheet is designed to calculate capacitor values for heater chain droppers, not replacing curtain burner mains leads and the like. The current drawn by the radio will vary a lot as the valves warm up.

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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Old 27th Jan 2013, 2:12 am   #14
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Default Re: Use of Class X2 Capacitors as Droppers.

Quote:
Originally Posted by Herald1360 View Post
Sounds like that could call for a sacrificial X-cap in the traditional across the mains position ahead of the dropper.
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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Old 27th Jan 2013, 6:06 pm   #15
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Default 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.
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Old 27th Jan 2013, 7:46 pm   #16
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Default 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.
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Old 27th Jan 2013, 8:23 pm   #17
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Default 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
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Old 27th Jan 2013, 9:04 pm   #18
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Default 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.
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Old 27th Jan 2013, 9:24 pm   #19
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Default Re: Use of Class X2 Capacitors as Droppers.

Quote:
Originally Posted by Leon Crampin
A poor man's true RMS meter:
Excellent idea!
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Old 27th Jan 2013, 9:51 pm   #20
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Default Re: Use of Class X2 Capacitors as Droppers.

Search for "Bunsen grease-spot photometer"!
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