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Old 31st Jan 2013, 7:29 pm   #41
G6Tanuki
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Default Re: Use of Class X2 Capacitors as Droppers.

The problem of X2-capacitors being used as droppers [and failing after a while] is well-known in the area of certain central-heating control-valves.

http://www.seered.co.uk/sunvic_detailed_failure.htm

As the capacitors take repeated line-spikes and subsequently self-heal the remaining capacitance reduces - and eventually the series-impedance rises to a point where insufficient current flows through the capacitive-dropper for the subsequent circuits to operate correctly.

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Old 31st Jan 2013, 8:12 pm   #42
G8HQP Dave
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Default Re: Use of Class X2 Capacitors as Droppers.

If you use an extra diode to deal with the 'passing DC through a cap' problem then you have made a voltage doubler, so there is a risk of DC overvoltage under low load. Better to use the heaters for this purpose.
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Old 31st Jan 2013, 8:55 pm   #43
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Default Re: Use of Class X2 Capacitors as Droppers.

The part about the CH valves is interesting.
23 years ago, I worked for 12 months for a ceramic kiln manufacturing company. For the previous six decades, gas and oil control valves had been operated by "Satchwell Valves". It could be any actual "valve", but the operation was via a 180 degree rotation of a geared down electric motor which was linked to the valve to move it from open to closed. A new kiln was built requiring five such valve arrangements, but somebody decided to use the NEW air conditioning valve actuators they had read about. Not sure the make, not sunvic, but flat blue units. One by one they failed in the first month, so they were to be scrapped and Satchwells bought. I was curious to look inside, but was told "No" as they were all to be returned to the manufacturer. However, I did check around with a testmeter, and found the mains input connections were "open circuit". However, that was the same with the good spare, so I concluded that they must have a capacitor dropper inside. The units did not weigh enough for a transformer, so I guess they were an equivalent design to the Sunvik units in the cited link.
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Old 1st Feb 2013, 12:15 am   #44
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Default Re: Use of Class X2 Capacitors as Droppers.

I have had a play with 5spice with nominal values for heater and a resistor for HT. I used a transient analysis and was unable to output dissipation with the free version I have.

I have also looked at the voltage doubler for HT with a capacitive divider on the input, this does not seem to be worth the trouble.

The heater voltage did not seem to vary much with HT current. At switch on, the heater resistance will be low and with the restriction that the capacitor gives, there will be little if any overvoltage.

The resistive mains lead apparently works to control heater and HT so a capacitor may work also.

I may have a go at showing a graph if anyone is interested.
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Old 1st Feb 2013, 11:36 am   #45
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Default Re: Use of Class X2 Capacitors as Droppers.

Quote:
Originally Posted by G8UWM-MildMartin View Post
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...
I have twice tried to visit the site in your link only to have AVG Secure Search block access to it, stating that a threat caused by 'Exploit Blackhole Exploit kit(type2364)' had been removed successfully.
Because of this I am not able to read the article in question. Frustrating!
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Old 1st Feb 2013, 1:16 pm   #46
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Default Re: Use of Class X2 Capacitors as Droppers.

Quote:
Originally Posted by TrevorG3VLF
The resistive mains lead apparently works to control heater and HT so a capacitor may work also.
A resistor can pass DC, so the complications of a capacitor dropper do not arise.

The WA2ISE site seems to have been 'got at'. Let's hope the site owner soon gets it cleaned up and back in use.
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Old 2nd Feb 2013, 12:48 am   #47
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Default Re: Use of Class X2 Capacitors as Droppers.

As we on this thread are in danger of reinventing the wheel, here is a clean copy of Robert Casey's article, as referred to by MildMartin. As I previously posted, Robert gives some better configurations for the design of droppers.

Peter
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File Type: pdf AA5 AM tube radios on 240V AC.pdf (750.7 KB, 787 views)
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Old 2nd Feb 2013, 3:02 am   #48
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Default Re: Use of Class X2 Capacitors as Droppers.

Quote:
Originally Posted by merlinmaxwell View Post
Quote:
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?
Yes, it would, but I think (I'm not an electronic engineer) that the effect would be worse with a capacitor. As other have suggested though, it may not be significant.
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Old 2nd Feb 2013, 11:25 am   #49
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Default Re: Use of Class X2 Capacitors as Droppers.

Peter, thanks for the 'clean' copy of Robert Casey's article. It makes interesting reading, but I don't think I'll be going down the route of replacing the existing valves with types having lower heater current. Use of relatively high wattage dropper resistors is not really an option since space inside the 'rocket shaped' case of the Mitchell 1260 is limited. The chassis mounts vertically in the cabinet, which has an E.S. Lamp-holder at the top, fitted with a 40 or 60 watt bulb so that the product is an AM radio and Table lamp combined.
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Old 2nd Feb 2013, 11:55 am   #50
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Default Re: Use of Class X2 Capacitors as Droppers.

Ahhhhh!

Then why not poke around for a suitable 120V light bulb and wire it in series with the set? Presumably not being able to use lamp and radio independently won't be much of an issue.
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Old 2nd Feb 2013, 12:37 pm   #51
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Default Re: Use of Class X2 Capacitors as Droppers.

Hi Livewire,

Robert gives several configurations. The one which may be best for you requires NO dropper resistor, NO changing of valves, and needs only a small capacitive dropper (eg 1.5uF). You do have to swap a few wires around but this is minor. Of course I know little about your particular radio model or any possible peculiarities, but Robert's configurations are quite general.

Peter
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Old 2nd Feb 2013, 1:10 pm   #52
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Default Re: Use of Class X2 Capacitors as Droppers.

Paul, I'd expect the resistor and the capacitor versions to be equally bad before the heaters get warm. The only difference would be phase shift, the RMS and peak volts would come out the same for a sine-wave mains.

But you've made me think of one difference. Capacitive droppers do less dropping for higher frequency components. So if the mains waveform is a bit nasty, the harmonic components will come through with a lot less attenuation than a resistive dropper would give. So a radio with a capacitive dropper would be much more open to mains harmonics affecting heater voltage as well as sundry clicks pops and bangs getting in.

For a 110v set on 240v I wouldn't think of anything but a properly rated transformer. Imagine that the worst happened, that someone got a bad shock, of a fire started,even if the dropper had nothing to do with it, and the authorities got involved through the 999 call.
How would anyone defend themselves from the questions that would be inevitable? Hop in a time machine, go back and do the job in a defensible manner? No it's too late. The only time to do the job in a way you can feel comfortable is right now. The problem is nothing's gone wrong, so there isn't the compelling need to do the job properly. But then, safety precautions are unnecessary 99. (lots of 9s percent) of the time. The problem lies in the lack of time machines.

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Last edited by Radio Wrangler; 2nd Feb 2013 at 1:11 pm. Reason: My fingers can't spell 'Paul', sorry
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Old 2nd Feb 2013, 2:52 pm   #53
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Default Re: Use of Class X2 Capacitors as Droppers.

Other advantages of Robert Casey's preferred capacitive-dropper configuration are that the heaters never overrun - the initial heater voltage is LESS than the final (steady-state) voltage after warm-up, and the mains-harmonic feedthrough problem described by David is reduced due to the lower capacitance required.

However, all dropper configurations share a lack of electrical isolation and the associated possible safety hazards, as David again describes.

Peter
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Old 3rd Feb 2013, 4:54 am   #54
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Default Re: Use of Class X2 Capacitors as Droppers.

Quote:
The WA2ISE site seems to have been 'got at'. Let's hope the site owner soon gets it cleaned up and back in use.
Agreed.
I've had it bookmarked for many years.
It "threw a wobbly" when I looked at it at work on Friday, but had seemed OK at home just before posting, so assumed it was just the corporate censorship kicking in...
Looks like I'd better run a virus scan!
Very sorry for posting a dodgy link & thanks to dinkydi for the clean version.
Martin.
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Old 3rd Feb 2013, 3:50 pm   #55
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Default Re: Use of Class X2 Capacitors as Droppers.

Quote:
Ahhhhh!

Then why not poke around for a suitable 120V light bulb
This very thought occured to me, two votes for it then.
 
Old 3rd Feb 2013, 3:51 pm   #56
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Default Re: Use of Class X2 Capacitors as Droppers.

A second thought, you can get (in the USA) twin fillament bulbs for dim and bright, so the light switch (if fitted) will do something.
 
Old 3rd Feb 2013, 5:01 pm   #57
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Default Re: Use of Class X2 Capacitors as Droppers.

I haven't asked the radio's owner, but I assume he'll want to be able to listen to the radio without the lamp on (or vice-versa) so using a 120vac lamp as a dropper isn't feasible
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Old 3rd Feb 2013, 6:10 pm   #58
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Default Re: Use of Class X2 Capacitors as Droppers.

Changing tack..... the Maplin auto transformer has been mentioned elsewhere- it comes with an American socket as its outlet.

http://www.maplin.co.uk/100w-uk-to-u...onvertor-31899

Why not simply fit the appropriate plug to the set, a 40W 120V lightbulb, and just plug it all in? Apart from anything else, the set itself is then as the maker intended which is always helpful if any problems arise.......
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Old 3rd Feb 2013, 8:12 pm   #59
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Default Re: Use of Class X2 Capacitors as Droppers.

Chris, as I mentioned in an earlier post(#36)the owner has said that he wants his radio to be run from 240vac via a capacitive dropper. If it were mine I would use a 240/120v transformer. In fact I test ran it from a Sunpower MW2P045 Transformer(o/p 120vac @ 45W) before starting to modify it. The Maplin one would no doubt be needed when the bulb was fitted. (incidentally your link doesn't work-when I click on it all I get is a blank page headed 'Bad request')
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Old 4th Feb 2013, 12:18 am   #60
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Default Re: Use of Class X2 Capacitors as Droppers.

Oh well. Sounds like someone with a bit of an idee fixe, but the customer's always right.....

Dunno about the link, I thought I just copied it from the page:

http://www.maplin.co.uk/100w-uk-to-u...onvertor-31899
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