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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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Old 19th Dec 2015, 10:00 pm   #1
Stylo N M
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Default Capacitive droppers how do they work?

Hi,

i have never had anything to do with this type of circuit, so how do they work and what would a typical circuit look like?

Paul.
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Old 19th Dec 2015, 10:05 pm   #2
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Default Re: Capacitive droppers how do they work?

This may help:-

http://www.vintage-radio.com/repair-...per-calcs.html
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Old 19th Dec 2015, 10:06 pm   #3
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Default Re: Capacitive droppers how do they work?

Here... http://www.vintage-radio.com/repair-...per-calcs.html is a good start.
 
Old 19th Dec 2015, 10:07 pm   #4
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Default Re: Capacitive droppers how do they work?

Great minds and the like!
 
Old 19th Dec 2015, 10:34 pm   #5
Stylo N M
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Default Re: Capacitive droppers how do they work?

Thanks graham, thank you merlinmaxwell, yes that is helpful, it seems the capacitor values could be quite critical for the end result

paul.
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Old 19th Dec 2015, 11:04 pm   #6
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Default Re: Capacitive droppers how do they work?

They are indeed fairly critical, but you can measure the voltage directly with a simple AC voltmeter (unlike with a diode dropper). This means you can choose a main cap which gives a slightly lower voltage than you want, then experiment by adding small capacitors to get the correct voltage. X2 caps recovered from scrap equipment can be used for this.
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Old 19th Dec 2015, 11:17 pm   #7
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Default Re: Capacitive droppers how do they work?

To put things in perspective, bear in mind that the original dropper resistor would itself have been a bespoke item, produced especially to the manufacturer's specification with precisely calculated values for the bulk resistance and resistance between voltage taps to ensure the valves were operating with optimum heater voltage for effective operation and reasonable working life. If they could have got away with off-the-shelf E6 power wirewounds, say, they surely would have done.
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Old 19th Dec 2015, 11:43 pm   #8
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Default Re: Capacitive droppers how do they work?

The value of the dropper capacitor is no more critical than the value of a dropper resistor would have been for the same job.

A resistor exhibits resistance, a property of resisting current being passed through it by demanding an appropriate voltage to push that current through it.

Inductors and capacitors exhibit reactance when driven with AC currents. Reactance sort of works like resistance, in demanding voltage to push current, but the current is out of step with the driving voltage. This sounds odd until you think a bit further about it.

If I apply AC current to a capacitor, the voltage on the capacitor rises as it is charged. It keeps on rising as long as the current is going in the forwards direction. We reach the peak voltage when the charge in the capacitor is at its maximum, when the forwards current period has finished and all the charging there is ever going to be has finished. What happens next is the negative going current part of the waveform, sucking charge out of the capacitor.

Do it with a sine wave current and the capacitor voltage is also a sine, but delayed 90 degrees compared to the driving current.

With inductors, it works the other way. Apply voltage and the current builds up. With a sine wave voltage drive, the current waveform is delayed 90 degrees compared to the voltage waveform. This is the opposite shift to what capacitors do (As a rule of thumb, inductors always seem to do everything in the opposite way to capacitors)

So the reactance of an inductor is opposite to the reactance of a capacitor. They can cancel, and depending on the circuit may at one spot frequency create either infinite impedance (parallel circuit) or zero impedance (series circuit) This behaviour is the fundamental mechanism of a tuned circuit.... used in every radio to select and filter signals.

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Old 21st Dec 2015, 4:19 pm   #9
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Default Re: Capacitive droppers how do they work?

Quote:
Originally Posted by Radio Wrangler
The value of the dropper capacitor is no more critical than the value of a dropper resistor would have been for the same job.
This might seem like nitpicking, but the quadrature voltage means that a dropper capacitor value is more critical. Not too much to worry about, except that caps are generally available in fewer values per range so you might have to choose a bigger cap than you wish and add a small dropper resistor too.

Just to give an example, assume we need to drop half the voltage of a 240V supply at 100mA current. Our load is equivalent to a 1.2k resistor, so we need another 1.2k resistor dropper. Alternatively, we can use a capacitor with an impedance of 2078 ohms. Both arrangements give a total impedance magnitude of 2.4k so we get our wanted 100mA.

Now suppose that the values are 10% out. The resistor dropper is 1080, so we get total impedance of 2280 and a current of 105.3mA - a 5.3% error. The capacitor would have impedance 1870 so the total impedance magnitude is 2222 and the current is 108.0mA - 8% error.

As I say, not too serious but as this website is supposed to be building up a body of knowledge I felt it ought to be corrected.
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Old 21st Dec 2015, 5:53 pm   #10
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Default Re: Capacitive droppers how do they work?

Yes, there is a difference, but considered in terms of 10% tolerance on power resistors, the unstated tolerance on valve heater hot resistance and the tolerance of the mains voltage it is sufficiently small that capacitive droppers don't need any more care than do resistive droppers. I suspect that the 50% voltage drop case is also the worst case and that the difference diminishes as the heater chain voltage goes above or below the 50% mark.

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Old 21st Dec 2015, 6:07 pm   #11
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Default Re: Capacitive droppers how do they work?

My two pennorth- capacitive droppers are easy to design for resistive loads like heaters, but if a half wave rectified HT supply gets in on the act (as it would if trying to replace a line cord on a 120V American set) it all gets rather interesting. If HT load is significantly less than heater load, it's not too bad, given that there's reasonable leeway on exact HT voltage, but if not it can be useful to resort to a simulation program for some interesting tinkering without any leakage of magic smoke.
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Old 21st Dec 2015, 9:57 pm   #12
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Default Re: Capacitive droppers how do they work?

I've lashed up a CCTV and power it from an oldish 12 Volt Netgear wall wart and I'm wondering if it's a cap dropper? It's too small to be a linear and it's stone cold when running and makes no hash on AM other than a feint ticking ?
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Old 21st Dec 2015, 10:02 pm   #13
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Default Re: Capacitive droppers how do they work?

The other thing to watch with capacitive droppers is the peak current and voltage at switch on; this varies with where on the mains cycle the contact is made. If the contacts first make when the voltage as it positive or negative peak the capacitor instantaneous voltage is 0 (assuming any charge from previous operation has leaked away) therefore the peak voltage applied to the load could be 340V. Though probably not for very long, it could cause damage

Ian
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Old 22nd Dec 2015, 12:24 am   #14
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Default Re: Capacitive droppers how do they work?

You should always use a fusible resistor in series with a capacitive voltage dropper. This will also act as a surge limiter if contact is made at one or other extreme of the mains cycle.

Also, despite the presence of a dielectric layer, a capacitive dropper provides no electrical isolation from the mains. But in some applications -- e.g. mains appliance control modules, and lighting effects controllers in the days before LEDs -- this is not a consideration.
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Old 22nd Dec 2015, 11:24 am   #15
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Default Re: Capacitive droppers how do they work?

Being a novice regarding valve radio, I ask, just how critical is the heater voltage 6.3 or 12.6 volts percentage wise. Ted
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Old 22nd Dec 2015, 1:26 pm   #16
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Default Re: Capacitive droppers how do they work?

About +/- 10% should be OK for domestic kit not run 24/7.
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Old 4th Jan 2016, 9:30 pm   #17
Stylo N M
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Default Re: Capacitive droppers how do they work?

Hi,

Thank you all for the responses all very helpful, as ever a treasure trove of information.

Paul.
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