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Old 18th Nov 2019, 6:20 pm   #1
Viscount
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Default Capacitive dropper.

hi there all ,ive just got hold of a marconi t11da ,and it has got a cap dropper which has been fitted by the previous owner,ive not dealt with one of these before,it seems to be well constructed,just wondered how these units work ,i havent operated this radio yet as im waiting for a replacement valve to arrive
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Old 18th Nov 2019, 7:00 pm   #2
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Default Re: cap dropper

It really is a treat the way they work.
I tried diodes, plus resistors, high wattage resistors etc.
AFAIC, it is the only way to perform the task. The capacitor-reactance is even a surge limiter to protect the valve heaters.
No one uses DC mains anymore, anyway.
Dave, USradcoll1, been playing with this rubbish way too long.
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Old 18th Nov 2019, 9:44 pm   #3
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Default Re: Capacitive dropper.

If the attachment works, here is a spreadsheet to work them out. Also a .doc explaining all.

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File Type: doc Dropper Calculations.doc (105.5 KB, 167 views)
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Old 18th Nov 2019, 9:46 pm   #4
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Default Re: Capacitive dropper.

use an x rated capacitor!!!!

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Old 19th Nov 2019, 7:31 am   #5
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Default Re: Capacitive dropper.

No, do not use an X-rated capacitor! There is no need for the X-rating when a series resistor is used, and the self-healing property is detrimental in this application.
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Old 19th Nov 2019, 8:51 am   #6
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Default Re: Capacitive dropper.

So, all the caps are X rated, is this not good then?
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Old 19th Nov 2019, 9:30 am   #7
David G4EBT
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Default Re: Capacitive dropper.

X rated capacitors are ordinarily meant to go across the mains input and Y rated ones go both from line to ground and neutral to ground, as depicted below.

In capacitive droppers, the capacitor is in series. That said, the excellent Word document on how to calculate the value of the capacitor does mention X Class. Whether or not that's because unpolarised mains rated X Class caps are available up to 2uF, or whether they're desirable in this application, because they're designed for continuous use at mains Voltage, I've no idea.

Here's the relevant clip of what the notes say, and it does make reference twice to X Class capacitors:

Quote:

8-<

The capacitor value will probably be somewhere in the range of 1uF to 2uF. Motor run capacitors are available in this range and have adequate voltage ratings. Alternatively, Class X suppression capacitors are available up to 1uF and possibly higher. Neither of these types are available in a vast range of values (for example motor run capacitors are available in 1uF, 1.5uF and 2uF). The tolerance is 10% or 20% so do not expect great precision.

Do NOT use electrolytic capacitors! The capacitor must be non-polarised and must be rated for continuous mains voltage operation. You will be unlikely to get a capacitor of exactly the correct value. You will probably need to use two in parallel.

For capacitors in parallel:

For example, if you required 1.6uF you could use a 1.5uF motor run capacitor with a 0.1uF Class X suppresser capacitor in parallel.

8-<

End quote.

Incidentally, 'Motor Run' capacitors are meant to run continuously - 'Motor Start', only for a brief time when the motor is started.
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Old 19th Nov 2019, 11:01 am   #8
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Default Re: Capacitive dropper.

Thanks Dave for that diagram. That information about X and Y rated capacitors. I have been thinking about building a mains filter in a suitable plug top as an experiment to reduce mains noise.
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Old 19th Nov 2019, 10:03 pm   #9
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Default Re: Capacitive dropper.

My Pilot Model 10 radio has been running on a capacitive dropper for around 10 years. I used a 2uF motor run capacitor with a 0.47uF X class in parallel to bring the value up to 2.47uF...the actual value I wanted was 2.5uF which was close enough.

I have a Pye television and I've used a capacitive dropper to replace the failed heater section of the dropper. Again no problems using a couple of X class caps (don't remember the value now but I think it was around 3uF).

No problem with either of these and of course they both run much cooler.
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Old 20th Nov 2019, 11:22 am   #10
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Default Re: Capacitive dropper.

I seem to remember Thorn used a Series capacitor in the Heater line of one TV in the late 60's.. was a really strange value though.
At tech we had to calculate the value of a series heater chain.
Maths is and was not one of my strengths..
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Old 20th Nov 2019, 6:07 pm   #11
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Default Re: Capacitive dropper.

ok so the new valve arrived today(KT71 ),and all appears to work fine ,so the dropper seems to work ok,although volume is low and dial lamp is a little dim ,need to do more checks ,all in all good though
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Old 20th Nov 2019, 6:15 pm   #12
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Default Re: Capacitive dropper.

I've never used a capacitive-dropper for series heaters - I'm happier with the approach used in some 1970s colour-TVs which was to use a series-diode in the heater chain so the valves got half-wave-rectified AC, along with a Brimistor or similar to tame the cold-heaters inrush current.

One place I *do* use capacitive droppers is when I need a relatively-low-voltage bias supply [100V or so] in a valve amplifier: a capacitor from one side of the HT secondary to a suitably-wired diode and capacitor and a Zener can happily provide 10mA or so of -ve bias which will always be there when there's HT available.

[In the same sphere I've also used a capacitive *multiplier* powered from the same 25.2V AC supply that powers the valve-heaters [I was using 4X150D], so there's bias whenever the heaters are energised]
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Old 21st Nov 2019, 3:23 am   #13
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Default Re: Capacitive dropper.

Quote:
Originally Posted by julie_m View Post
No, do not use an X-rated capacitor! There is no need for the X-rating when a series resistor is used, and the self-healing property is detrimental in this application.
Agree, do not use an X rated capacitor for any dropper application like this. They lose value over time, you can expect by 1 to 3 years a drop to 1/2 the original uF value. (Been there done this many times)

Use a 1200v or 1500V (Better) rated Polypropylene or Polyester cap and you won't likely have any trouble with it. Don't go below a 1000V DC rating. Also, add a fuse.

A motor run cap is also a good option.
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Old 21st Nov 2019, 3:48 am   #14
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Default Re: Capacitive dropper.

Quote:
Originally Posted by JohnBG8JMB View Post
use an x rated capacitor!!!!
johnb
Unless you want the equipment to suffer from the Senseo Syndrome *), don't. At least not without thinking it through and looking up the specs of the capacitor to be used.

X-rated capacitor's aren't usually designed to carry current or to stay within tolerance over the long run.

If you must use one for liability reasons, look up the datasheets and spend the extra few cents on one that is explicitly rated for mains series operation by the manufacturer. They will usually last.

Otherwise, a good quality (preferrably impulse rated) polypropylene foil capacitor would be a good fit. As a rule of thumb, use a 1000V rated one or when the voltage across the load is relatively high and no significant inrush current is expected: a DC rating of at least 3 times the calculated AC voltage across it (which is NOT equal to mains voltage minus the voltage across the load!) This means, if you have a 127V heater chain, you can just barely get away using a 630V capacitor (see example calculation in the word document).


*) The Philips Senseo is a coffee maker which uses a capacitive dropper for the controls. They used to go low capacitance so regularly (especially when left connected to the mains) and then turn up at 'repair cafes' that the term Senseo Syndrome was re-coined to mean 'X2 mains dropper going o/c'.

Last edited by Maarten; 21st Nov 2019 at 4:12 am.
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Old 21st Nov 2019, 8:54 am   #15
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Default Re: Capacitive dropper.

X2 avaliable to 4.7uF

http://www.farnell.com/datasheets/102502.pdf

But whether you use an X2 or something else low loss like a motor run, put 1M (or 3 resistors in series to handle the voltage reliably) in parallel with the cap to discharge it. If not, if you unplug the unit at the peak of the mains cycle and then a finger is poked in the mains socket at the back you will get a belt.

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Old 21st Nov 2019, 9:08 am   #16
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Default Re: Capacitive dropper.

Also note the very comprehensive applications note by Panasonic relating to their X2 capacitors.

http://industrial.panasonic.com/cdbs...BD0000C249.pdf

There is a section to do with inrush current (for example when the heaters are cold), and the very last sentence to do with using them as a capacitive dropper.

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Old 21st Nov 2019, 12:35 pm   #17
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Default Re: Capacitive dropper.

Many years ago I experimented with a series capacitor as a dropper to make a "wattless" method for dropping the voltage to a series heater chain in a valve TV.

The heat dissipation from the series resistor seemed awfully unfavorable and the capacitor a better option.

And it works well (with a suitable capacitor not an X2 type if you want it to last).

However the capacitor does not obviate the requirements for initial current limiting for the current peaks that occur when the heaters are cold and their resistance is very low at initial turn on.

It still requires the addition of a power NTC resistor (Brimistor). The common one used for this application was the CZ1, but there are many other variants. If you don't use an NTC, by the time the resistor has a large enough Ohmic value to prevent a turn on current surge, when in use , with warm heaters, the resistor's heat dissipation will be higher than you would like.

So with this scenario take the lead from people who have fallen into the traps of things like the X2 capacitor degradation over time, the turn on current surge in a heater string, which turns the heaters in small valves in the RF amplifier into flash bulbs, while the heaters in the power Pentodes in the audio and line output stages and audio output stages had barely warmed up.

Or you can find out for yourself, at the expense of a few valves and learn the lesson that way, either way you will learn it, because if you don't, as the Buddhists say, the lesson will return to you over and over again, until you do.
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Old 21st Nov 2019, 5:16 pm   #18
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Default Re: Capacitive dropper.

with regards to comment about getting a poke when switched off and unplugged,yes ive just discovered that one ,
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Old 21st Nov 2019, 10:20 pm   #19
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Default Re: Capacitive dropper.

Oh, I forgot to mention, there is another way to get the heat dissipation in a "dropper" down in a series heater chain, in a nearly wattless manner, especially running off 240V, is to half wave rectify the line power with a silicon power rectifier, a 6A10 diode works fine and the losses are low. (if there is not already one in the circuit) I have also used this method (also works well for a fixed amount of lamp dimming to) in the application the diode barely gets warm because of its low firward voltage drop when conducting.

This eliminates every second half cycle of the mains power waveform.

It is easy to calculate the effective or new equivalent rms voltage of this waveform, you can use calculus, but its even easier (a quick trick) to think that since over the time of one full cycle the voltage is missing for half that time, then the power delivery is halved for this reason and since the power is proportional to the square of the voltage then V^2 has been halved, so root((V^2)/2) is the new effective voltage or simply 1(root 2) = 0.7071.

So "half waving" the mains, from the perspective of heat in a resistive load, like a heater chain, creates a new rms voltage source of 240v x 0.7071 = 170V.

Mostly you will find it calculated a as the peak or max voltage, Vm / 2. see attached, but since the original rms voltage is 0.7071 Vm, you can also see that Vm x 0.7071 x 0.7071 = Vm/2. So both methods agree.
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Old 21st Nov 2019, 10:50 pm   #20
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Default Re: Capacitive dropper.

You can fit a crowbar diode for diode dropper, just in case. Mains...fuse...dropper diode and crowbar diode...dropper resistor....heaters.

Lawrence.
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