Safety Hazard with capacitive droppers. - UK Vintage Radio Repair and Restoration Discussion Forum

22nd Apr 2004, 7:51 pm

When fitting a capacitor in place of a dropping resistor on a mains radio, ensure that a discharge resistor is fitted across the capacitor.

Without a discharge path, a high voltage can remain across the pins of the mains plug if this is removed whilst the set is working, or if the plug is subsequently removed after switching off at a wall socket, as I often do.

The discharge resistor must be rated in voltage for full mains potential. If 1/4W components are used, this generally means fitting two in series.

Leon.

Paul Stenning · 26th Dec 2004, 11:14 pm

In fact the resistor should be rated for the peak mains voltage, which is 1.414 times the RMS. In the UK this is around 350V. Ideally use high voltage resistors, otherwise use two in series as suggested above. 220K or thereabouts should discharge the capacitor quickly while remaining cool.

G8UWM-MildMartin · 19th Dec 2005, 10:43 pm

The 4-uF LCR fluorescent lighting power factor correction capacitors I got from CPC have an internal resistor.Perfect!

zak · 21st Dec 2005, 12:03 pm

It is not good practice to use a capacitor as a dropper,especially true if there are harmonics and high RF on the AC supply,(like it is these days)the valves are completely unprotected from waveform and spikes.
I can not recall ever seeing this in any set I have worked on,and as far as I know no reputable manufacturer has ever tried this.
Its better to use a diode.

Ed_Dinning · 21st Dec 2005, 1:33 pm

The demise of valve sets came about just as suitably reliable capacitors became available at a low enough cost to make them economic, hence very few capacitive dropper sets.
Capacitice droppers are now appearing in some low cost, low power, supplies where isolation is achieved by other means.

Ed

flyingtech55 · 21st Dec 2005, 2:28 pm

Quote:

Originally Posted by zak

I can not recall ever seeing this in any set I have worked on,and as far as I know no reputable manufacturer has ever tried this.
Its better to use a diode.

A capacitive heater dropper was used in a Thorn transportable TV; I can't remember the chassis type now. I remember repairing them in the seventies. I never heard of the capacitor failing but they weren't very common where I worked.

Tim

Sideband · 21st Dec 2005, 3:01 pm

Quote:

Originally Posted by flyingtech55

A capacitive heater dropper was used in a Thorn transportable TV; but they weren't very common Tim

Yes I remember it well...but not the model number unfortunately.

I've also come across capacitive droppers in Central Heating controllers. There was one in the model I had which I discovered one day when the controller failed. The dropper was in series with the mains and fed a 12 volt zener which presumably stabilized the supply to the rest of the electronics. The capacitor was only small and was dry jointed. Once that was sorted, the controller carried on for many more years. So in UK at least, capacitive droppers are not unusual.

Rich

flyingtech55 · 21st Dec 2005, 4:07 pm

Hi Rich

Mike Phelan may well be able to help out with the Thorn chassis model. He probably wrote an article on it. I seem to remember that the capacitive dropper powered the heaters of the scan circuit valves. Then the HT current for the scan circuits passed through the signal valve heaters to light them. The sets came on in two stages; the raster appeared first then the sound and picture appeared some time later (about 90 seconds or so). Weird.

I too have seen that capactive dropper arrangement; I used to have a Pifco rerchargeable torch and the charger was built in. You pulled the end off the torch and it exposed two round pins which plugged into a shaver type socket to recharge the torch directly from the mains. However the torch was far too small to contain a mains transformer. I was young then and it took me ages to work out how the charger worked; of course a small capacitor was used as a wattless dropper.

I think regulations and the proliferation of inexpensive switch mode power supplies have killed off the use of capacitors for 'quick and dirty' power supplies.

Tim

Biggles · 21st Dec 2005, 4:14 pm

Capacitor droppers seem fairly common in domestic units and I have seen them in PIR sensors and the like. It sounds like the capacitor may let spikes through because I have repaired PIR outdoor lighting sensors which commonly have the zener regulator diode s/c. This suggests voltage spike damage, or maybe just under rated diodes. The capacitors are usually low value to pass small currents to low demand units, and obviously can't provide isolation from the mains.

Biggles.

Paul Stenning · 21st Dec 2005, 6:58 pm

In modern electronic equipment with capacitive droppers, there should be a small resistor in series as a surge limiter. This should also absorb the spikes and other rubbish that would otherwise be coupled straight through the capacitor into the zener diode or whatever.

The capacitive dropper arrangements discussed on my website includes a 100-ohm wirewound surge limiting resistor in series with the capacitor.

Mike Phelan · 22nd Dec 2005, 9:36 am

Quote:

Originally Posted by flyingtech55

A capacitive heater dropper was used in a Thorn transportable TV; I can't remember the chassis type now. I remember repairing them in the seventies. I never heard of the capacitor failing but they weren't very common where I worked.

Tim

I have a vague idea that was the 980 chassis? I do remember them, and did not think we had any failures.

dazzlevision · 23rd Dec 2005, 9:08 am

Hello,

The Thorn 980 VHF/405 only all-valve 12" portable chassis used resistive heater droppers and a series silicon rectifier. The Thorn 960 405/625 16" series chassis (first production version), used a 4.3uF series capacitive dropper, with some of the heaters in the -ve HT line! This gave rise to a two stage warm up, where the raster would appear first and then the sound and vision signals! The later version used more conventional arrangements. The 960 was a variant of the 950 chassis, with a cut down PCB at the bottom of the chassis and the power supply, frame and sound output stages on a wired chassis bit at the top. It also used a small Mullard/Philips VHF turret tuner in lieu of the usual standard Thorn type.

The 4.3uF cap was also used in the Thorn 158 12" UHF/625 hybrid mono portable chassis, but in a straightforward series heater chain. Thorn did put a resistor across the 4.3uF cap - I think it was 47kOhm, 0.5Watt.

Regards,

Dazzlevision

22nd Apr 2004, 7:51 pm	#1
Leon_Crampin Guest Posts: n/a	Safety Hazard with capacitive droppers. When fitting a capacitor in place of a dropping resistor on a mains radio, ensure that a discharge resistor is fitted across the capacitor. Without a discharge path, a high voltage can remain across the pins of the mains plug if this is removed whilst the set is working, or if the plug is subsequently removed after switching off at a wall socket, as I often do. The discharge resistor must be rated in voltage for full mains potential. If 1/4W components are used, this generally means fitting two in series. Leon.

26th Dec 2004, 11:14 pm	#2
Paul Stenning Administrator Join Date: Dec 2002 Location: Cardiff Posts: 9,071	Re: Safety Hazard with capacitive droppers. In fact the resistor should be rated for the peak mains voltage, which is 1.414 times the RMS. In the UK this is around 350V. Ideally use high voltage resistors, otherwise use two in series as suggested above. 220K or thereabouts should discharge the capacitor quickly while remaining cool. __________________ Paul Stenning Forum Admin/Owner and BVWS Webmaster

19th Dec 2005, 10:43 pm	#3
G8UWM-MildMartin Heptode Join Date: Jan 2004 Location: Stockport, Greater Manchester, UK. Posts: 827	Re: Safety Hazard with capacitive droppers. The 4-uF LCR fluorescent lighting power factor correction capacitors I got from CPC have an internal resistor.Perfect! Last edited by G8UWM-MildMartin; 19th Dec 2005 at 10:45 pm. Reason: missing word

21st Dec 2005, 12:03 pm	#4
zak Retired Dormant Member Join Date: Feb 2005 Location: Monmouthshire,on South Wales border Posts: 85	Re: Safety Hazard with capacitive droppers. It is not good practice to use a capacitor as a dropper,especially true if there are harmonics and high RF on the AC supply,(like it is these days)the valves are completely unprotected from waveform and spikes. I can not recall ever seeing this in any set I have worked on,and as far as I know no reputable manufacturer has ever tried this. Its better to use a diode.

21st Dec 2005, 1:33 pm	#5
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,194	Re: Safety Hazard with capacitive droppers. The demise of valve sets came about just as suitably reliable capacitors became available at a low enough cost to make them economic, hence very few capacitive dropper sets. Capacitice droppers are now appearing in some low cost, low power, supplies where isolation is achieved by other means. Ed

21st Dec 2005, 4:07 pm	#8
flyingtech55 Octode Join Date: Nov 2005 Location: Ayrshire, UK. Posts: 1,096	Re: Safety Hazard with capacitive droppers. Hi Rich Mike Phelan may well be able to help out with the Thorn chassis model. He probably wrote an article on it. I seem to remember that the capacitive dropper powered the heaters of the scan circuit valves. Then the HT current for the scan circuits passed through the signal valve heaters to light them. The sets came on in two stages; the raster appeared first then the sound and picture appeared some time later (about 90 seconds or so). Weird. I too have seen that capactive dropper arrangement; I used to have a Pifco rerchargeable torch and the charger was built in. You pulled the end off the torch and it exposed two round pins which plugged into a shaver type socket to recharge the torch directly from the mains. However the torch was far too small to contain a mains transformer. I was young then and it took me ages to work out how the charger worked; of course a small capacitor was used as a wattless dropper. I think regulations and the proliferation of inexpensive switch mode power supplies have killed off the use of capacitors for 'quick and dirty' power supplies. Tim

21st Dec 2005, 4:14 pm	#9
Biggles Rest in Peace Join Date: Feb 2004 Location: Hexham, Northumberland, UK. Posts: 2,234	Re: Safety Hazard with capacitive droppers. Capacitor droppers seem fairly common in domestic units and I have seen them in PIR sensors and the like. It sounds like the capacitor may let spikes through because I have repaired PIR outdoor lighting sensors which commonly have the zener regulator diode s/c. This suggests voltage spike damage, or maybe just under rated diodes. The capacitors are usually low value to pass small currents to low demand units, and obviously can't provide isolation from the mains. Biggles.

21st Dec 2005, 6:58 pm	#10
Paul Stenning Administrator Join Date: Dec 2002 Location: Cardiff Posts: 9,071	Re: Safety Hazard with capacitive droppers. In modern electronic equipment with capacitive droppers, there should be a small resistor in series as a surge limiter. This should also absorb the spikes and other rubbish that would otherwise be coupled straight through the capacitor into the zener diode or whatever. The capacitive dropper arrangements discussed on my website includes a 100-ohm wirewound surge limiting resistor in series with the capacitor. __________________ Paul Stenning Forum Admin/Owner and BVWS Webmaster

23rd Dec 2005, 9:08 am	#12
dazzlevision Dekatron Join Date: Sep 2003 Location: Near Swindon, North Wiltshire, UK. Posts: 3,612	Re: Safety Hazard with capacitive droppers. Hello, The Thorn 980 VHF/405 only all-valve 12" portable chassis used resistive heater droppers and a series silicon rectifier. The Thorn 960 405/625 16" series chassis (first production version), used a 4.3uF series capacitive dropper, with some of the heaters in the -ve HT line! This gave rise to a two stage warm up, where the raster would appear first and then the sound and vision signals! The later version used more conventional arrangements. The 960 was a variant of the 950 chassis, with a cut down PCB at the bottom of the chassis and the power supply, frame and sound output stages on a wired chassis bit at the top. It also used a small Mullard/Philips VHF turret tuner in lieu of the usual standard Thorn type. The 4.3uF cap was also used in the Thorn 158 12" UHF/625 hybrid mono portable chassis, but in a straightforward series heater chain. Thorn did put a resistor across the 4.3uF cap - I think it was 47kOhm, 0.5Watt. Regards, Dazzlevision