Use of Class X2 Capacitors as Droppers.

[julie_m]

The method of matching the intensity of light bulb filaments fed from DC and AC was used in "The Boy Electrician" to calibrate the home-made (200 ohms per volt! But still better than no meter at all) test meter on AC.

[AC/HL]

It was also used in some Strowger era exchange test equipment and is surprisingly sensitive.

[Sideband]

Originally Posted by ukcol

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

Ahh that makes sense! I couldn't see any plausible reason why they could not be used! The loss of capacitance (should it actually occur when they are used as a dropper) will, as stated by Paul, cause a reduction in heater current so they are still 'fail safe' components in this position.


Rich

[Studio263]

I wouldn't do it.

The "self healing" process that X2 capacitors employ involves a burst of current which burns away the problem area. This is done so that the capacitor doesn't form a resistive path which would result in it getting hot whilst not drawing enough current to activate any other protective devices (fuses etc).

During the self healing process the capacitor is effectively short circuit so you can expect damage to your valve heaters if it is being used as a dropper in an AC/DC receiver. The resistance of the heaters may be sufficient to stop the process being completed (remember that X2 is designed to go between L and N, where the source impedence is close to zero), so you can expect the short to be a prolonged one.

Of course for most of the time it doesn't matter because self healing events are fairly rare. However, if I were choosing a capacitor to use as a dropper then one that is designed to fail "short" would not come high on my list!

[kalee20]

True - but the energy needed for self-healing is small (else the thing would get hot each time it happened). So the time of being short-circuit would be very brief.

You are right that adding heater resistance may limit the current so that the self-healing doesn't happen properly - it would be prudent to investigate further.

I would suggest the best way to evaluate the capacitor is to connect in series with perhaps 100Ω and then put the combination across the mains, with either a data logger or some latching circuit to detect spikes. And then leave running for several months. Access to some means of superimposing spikes and surges would be useful.

A metal-oxide varistor across the valve heater chain could protect the valves in case the capacitor fails short-circuit - the fuse would then get blown.

[dinkydi]

You need to be careful using X2 capacitors because some types (eg Rifa, Wima) may have paper dielectrics. As noted elsewhere on this forum, these types are hygroscopic (absorb moisture), are unreliable and fail prematurely (independent of use) with a solid short circuit and in an uncontrolled (potentially explosive) manner.

Before using any X2 capacitor, first check that it is not a paper type. Beware of old stock because while the X2 capacitors of some manufacturers no longer have paper dielectrics, the same capacitor models previously did.

Peter

[GMB]

I think the above comments on film capacitors could be misleading.

Many capacitors will fail "short", the idea that film capacitors are "designed" to do this is misleading in my view. They are designed to recover from that situation, in which other types would maintain the short and thus result in a bang.

The healing recovery time is very short so I doubt that under normal situations anything would notice.

The only concern may be that given that they are better at recovering from failure then I expect they are built with less margin.
The physically bigger a capacitor is the better it is likely to be, for the same rating and era.

[turretslug]

Sorry to play devil's advocate on what I actually find to be an appealingly elegant and efficient solution but an aged, reduced value dropper cap restricting heater string current might be deleterious to rectifier and output cathodes, in particular. Maybe a periodic "MOT"check on this area, I don't suppose any more than once every couple of years. It might simply show that it wasn't an issue, after all.

[TrevorG3VLF]

I am intrigued by the idea of using a capacitive dropper to provide both heater current and HT. What program would be able simulate this and provide measurement of power dissipation in a transient analysis?

I have a copy of a DOS program (ECA) which will calculate power dissipation with AC but it is a pig to use and I have forgotten how and I don't know whether the computer could still get steam up. Instantaneous power is not wanted.

[paulsherwin]

I can't see how this could work using a valve rectifier. The cap value would need to be calculated for the heater current and HT current combined, which would be difficult enough in itself, but there would be no HT current before the rectifier warmed up so the heaters would be overrun for a not insignificant period.

The only way I can think of for this to work would be to use separate capacitive droppers for the heaters and HT, but it's hairy stuff. A simple autotransformer will save all this grief.

[Glowing Bits!]

I've just read the OP's scenario, with the set being from Canada/USA, the safest way to power it would be through an auto-transformer, if it was a British AC/DC radio only, the HT will end up on 240V with a capacitor for the heaters.
Owing to the fact the set in question runs on 120v, it would be quick and safe to run it through a transformer.

[Radio Wrangler]

On the assumption that things like mains switches etc in any North American (meaning the continent) radio will only be rated for 120v and that a capacitive dropper would expose them to the full 240v when switched off, whichever order things were connected in, I would want a proper transformer to keep the voltage controlled if it went in my bedroom.

Besides, most X2 capacitors don't do a very good job of even being, um, X2 capacitors.

David

[G8UWM-MildMartin]

There's an interesting and detailed article about modifying this type of set for 240V at:
http://pw2.netcom.com/~wa2ise/radios/aa5240v.html

I agree about an auto-transformer (like the Maplin 100-VA one) being the simplest option, cheap and "plug & play."

A suitable isolating transformer is generally better in that it avoids the hazard in US/Canadian sets of the chassis typically being connected to the input via a 0.2-uF capacitor in parallel with 220K and having exposed screws, but I think the Mitchell doesn't have this problem.

The US switches had to cope with 110V D.C., so are not quite as much a problem at 240V A.C. as it might appear, though still a questionable practice...

[dinkydi]

MildMartin gives an interesting link to Robert Casey's article on this subject. Robert gives several alternative configurations which may be preferable to the simple "capacitive dropper" considered above.

The load placed by a radio on the mains supply has two separate parts: first, the power supply (inc. ht load), and second, the valve heater string. In the capacitive-dropper approach the two load parts, the supply and the heaters, are connected in parallel.

In a better configuration the load parts are connected in series. Normally the load parts require equal voltage (115V, for example), and to get this the load parts need to be adjusted with a suitably chosen parallel impedance: a resistor which dissipates power (but less power than in a resistive-dropper configuration), or alternatively, a capacitor which avoids power dissipation. (The capacitor is smaller than required by the capacitive-dropper configuration.) In addition, heater "overrun" is avoided.

I recommend Robert's article for full details.

Peter

[G8HQP Dave]

Originally Posted by TrevorG3VLF

I am intrigued by the idea of using a capacitive dropper to provide both heater current and HT. What program would be able simulate this and provide measurement of power dissipation in a transient analysis?

As I said, Spice.

Originally Posted by paulsherwin

The only way I can think of for this to work would be to use separate capacitive droppers for the heaters and HT, but it's hairy stuff.

Exactly how much DC current will the HT dropping capacitor pass?

['LIVEWIRE?']

A quick reply to the 'why not use a transformer?' question is that it's not my radio, and the owner would like it to be run via capacitive dropper(s). I had actually operated it via such a transformer prior to starting work on the modification. In answer to G8HQP Dave, the HT current is nominally 50mA(Heater I is 150mA) so maybe a 1uf dropper for HT and a 2uf one for the heaters. I would still have to use X2s as they're the only non-electrlytics small enough to fit in the available space!

[julie_m]

Note that if you are using a capacitive dropper to supply the HT, you will need a 1N4007 diode wired with its anode to chassis and its cathode to the anode of the rectifier valve, to provide a discharge path for the capacitor.

It's good practice to add a fusible resistor (220R will be fine) in series with the capacitive dropper to limit the power-up surge current when the capacitor is discharged, and a resistor in parallel to discharge it when not powered (470K is OK).

[Herald1360]

Having poked about with Spice a bit, I reckon that the original idea of 3uF feeding the whole set with a flywheel diode from the rectifier anode to HT- as suggested will be pretty close to requirements. The capacitance may need a tweak to get exactly 150mA rms through the heater chain but when that's right, the HT should be near enough.

To be honest, even without the flywheel, it won't be far out and a tweak for correct heater rms will do. The heaters will be slightly overrun until the set warms up, but I doubt that will cause any real issues. The currents in various bits of the circuit will have odd waveforms and the HT may be a bit higher or lower than standard but so what?

but there would be no HT current before the rectifier warmed up so the heaters would be overrun for a not insignificant period.

Wouldn't the same happen with a resistive dropper?

Back to reality, valves do cope well with short term overloads, a capacitive dropper either calculated, simulated (LTspice) or suck and see (empiricaly) would be fine.

[kalee20]

Originally Posted by ajs_derby

Note that if you are using a capacitive dropper to supply the HT, you will need a 1N4007 diode wired with its anode to chassis and its cathode to the anode of the rectifier valve, to provide a discharge path for the capacitor.

Well that may work, but it also gives the configuration for a voltage doubler! So if the load on the HT drops massively for any reason, the voltage can increase vastly beyond what it can reach now.