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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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18th Nov 2019, 6:20 pm | #1 |
Heptode
Join Date: Jan 2018
Location: Folkestone, Kent, UK.
Posts: 709
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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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18th Nov 2019, 7:00 pm | #2 |
Heptode
Join Date: Mar 2017
Location: Cedar Grove, Wisconsin, USA.
Posts: 823
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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. |
18th Nov 2019, 9:44 pm | #3 |
Pentode
Join Date: May 2006
Location: Poole, Dorset, UK.
Posts: 129
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Re: Capacitive dropper.
If the attachment works, here is a spreadsheet to work them out. Also a .doc explaining all.
JohnB G8JMB |
18th Nov 2019, 9:46 pm | #4 |
Pentode
Join Date: May 2006
Location: Poole, Dorset, UK.
Posts: 129
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Re: Capacitive dropper.
use an x rated capacitor!!!!
johnb |
19th Nov 2019, 7:31 am | #5 |
Dekatron
Join Date: May 2008
Location: Derby, UK.
Posts: 7,735
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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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19th Nov 2019, 8:51 am | #6 |
Heptode
Join Date: Jan 2018
Location: Folkestone, Kent, UK.
Posts: 709
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Re: Capacitive dropper.
So, all the caps are X rated, is this not good then?
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19th Nov 2019, 9:30 am | #7 |
Dekatron
Join Date: Feb 2003
Location: Cottingham, East Yorkshire, UK.
Posts: 5,737
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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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19th Nov 2019, 11:01 am | #8 |
Nonode
Join Date: Jan 2004
Location: 1966-1976 Coverack in Cornwall and Helston Cornwall. 1976-present Bristol/Bath area.
Posts: 2,965
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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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19th Nov 2019, 10:03 pm | #9 |
Dekatron
Join Date: Oct 2003
Location: Croydon, Surrey, UK.
Posts: 7,549
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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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20th Nov 2019, 11:22 am | #10 |
Octode
Join Date: Jun 2013
Location: Huddersfield, West Yorkshire, UK.
Posts: 1,782
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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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20th Nov 2019, 6:07 pm | #11 |
Heptode
Join Date: Jan 2018
Location: Folkestone, Kent, UK.
Posts: 709
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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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20th Nov 2019, 6:15 pm | #12 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 13,953
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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] |
21st Nov 2019, 3:23 am | #13 | |
No Longer a Member
Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 2,679
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Re: Capacitive dropper.
Quote:
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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21st Nov 2019, 3:48 am | #14 |
Dekatron
Join Date: Dec 2007
Location: Haarlem, Netherlands
Posts: 4,185
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Re: Capacitive dropper.
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. |
21st Nov 2019, 8:54 am | #15 |
Dekatron
Join Date: Nov 2010
Location: Oxford, UK.
Posts: 4,941
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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. Craig |
21st Nov 2019, 9:08 am | #16 |
Dekatron
Join Date: Nov 2010
Location: Oxford, UK.
Posts: 4,941
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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. Craig |
21st Nov 2019, 12:35 pm | #17 |
No Longer a Member
Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 2,679
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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. |
21st Nov 2019, 5:16 pm | #18 |
Heptode
Join Date: Jan 2018
Location: Folkestone, Kent, UK.
Posts: 709
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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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21st Nov 2019, 10:20 pm | #19 |
No Longer a Member
Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 2,679
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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. |
21st Nov 2019, 10:50 pm | #20 |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
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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. |