Capacitor dropper .

[vinrads]

What's your thoughts on using a capacitor from an old fluorescent fitting as a dropper, they seem very light weight but does work ,the value is 5.5 mf /250v ac I can just about squeeze it in on this set A french one 120volt Mick.

[G8HQP Dave]

I assume you mean 5.5uF? What dropper value have you calculated for your radio?

[TrevorG3VLF]

This is quite complicated. Capacitor droppers are used to replace resistors in some sets with series heaters. Running the whole set through a capacitor means that there is cross coupling between the heaters and high tension.

An accurate value to get RMS heater voltages within 5% may be difficult.

If it is a set with a transformer, then drive it with an external transformer. How about a circuit diagram?

[vinrads]

Hi thanks for the replies . The value of the capacitor is not in question , I came to the value by using my variac I have loads of this type of capacitor salvaged from scrap lights . I just wondered is it because they are mostly plastic construction they seem so light, I suppose they are doing less work , than being across the mains . Mick.

[Nuvistor]

Lots of information on capacitor droppers, scroll down the page.
http://www.vintage-radio.com/repair-...per-calcs.html

Frank

[vinrads]

Thank's Frank so it looks like the one in question which is rated at 250 v ac will do , the jury is out weather to fit it in the set or leave it as a 120 volt radio . Mick.

[Herald1360]

One way round the problem of the dc component in the mains loading of an ac/dc set would be to use the capacitor in series with the ac input to a bridge rectifier and feed the whole set with the resultant full wave rectified dc. Remember to connect -ve to the chassis!


This does result in the chassis being at an indeterminate voltage to ground, but in its original format it could have been live anyway.

[PJL]

Leave it as 120V and get yourself an isolated 240-120V transformer. Many of the US sets were a bit lapse on isolating the operator from the chassis!

[McMurdo]

there's no problem if the capacitor feels light. And they're quite tough, as they're either across the mains or in series with the light depending on the size of light. Usually metallised polypropylene, so ideal as a dropper.

[vinrads]

Thanks for all the info chaps , Mick.

[Ed_Dinning]

Hi Mick, "typical " value for a 5 valve set is normally about 3uF, so you may want to put 2 of you 5.5uF types in series.

Ed

[Graham G3ZVT]

11 posts in the thread and no one has mentioned line cord!

(I'll get me coat)

Graham.
G3ZVT

[AC/HL]

Or "curtain burners" as they were nicknamed. They were popular on the wartime American imports, but you'd be lucky to find NOS. You might want to restore an example, but far too wasteful these days compared with the alternatives.

[Oldcodger]

There was an article in a very old edition of ( from memory ETI circuits no 1) which discussed using a capacitor dropper in LED circuits ( along with a series diode). All that was needed was to know the voltage drop across the capacitor and the required current. The capacitor supplied the needed impedance calculated from V /Z , WHERE Z = IMPEDANCE =1/(2*pi*f*c). For your 5.5 uF Cap, I'd calculate that you'd need a current of approx 200mA to drop 120v. But ,would you need a capacitor that could handle that current ?
Vinrads- another suggestion- starting at low voltage, with a meter across the set input, and another meter in series with the set, turn the variac up until you get 120 v AC across the set and note the current.
You now know those parameters , all you have to do,using OHMS law is to calculate the required impedance ,and hence the capacitance .
Might be a good starting point, as often theory and practice don't like each other .

[ms660]

?

Lawrence.

[bluepilot]

Originally Posted by Oldcodger

OHMS law

Ohm's law ?

[Herald1360]

Not exactly of course, since the capacitor's reactance is at 90 deg to the resistive-ish load of the radio. In practical terms this means that the capacitive reactance has to be about 1.75 * the load resistance to get half volts on the resistance.

[boxdoctor]

I would guess that to obtain a true reading of the output on the load side of the capacitor, a true RMS type of meter will be required, as the "draw" of the load is unbalanced on each half cycle due to the H.T. load being applied on one half of the mains cycle only (via the rectifier).
I would expect this to seriously affect any theoretical calculation, as one is not working with a symetrical, purely resistive, load. Tony.

[Herald1360]

T'other problem of a series capacitor on a mixed dc/ac load like a 120v ac/dc set is that even if you get the rms heater chain voltage correct the HT probably won't be the same as it would have been with a low impedance 120v ac supply.

Time for a play with a simulator, I think. Easier than doing the maths......

[TrevorG3VLF]

There was another thread "Use of class X2 capacitors as droppers" Started by livewire 26 Jan 2013. This went rambling on for quite a while. I put some analysis in using 5spice. I have not used it since so would need a lot of re-education.

I also did some analysis using LTsice in another thread where I was helped by Merlinmaxwell. LT spice will calculate dissipation with funny waveforms. I had trouble getting the colour graphs to show clearly. I do remember what I had for breakfast this morning but that is because it is always the same.