Dropper capacitor safety - UK Vintage Radio Repair and Restoration Discussion Forum

John G8MWF · 15th Jan 2018, 10:09 am

Hello,
I wonder if you could help me with a dropper capacitor issues again?

This question is more about safety than performance and I do not want to take any chances in getting it wrong.

The Bush DAC90A that I modified with a dropper capacitor is working fine.
The only trouble I have is that when it is unplugged there is still a charge in the capacitor that gives you a little jolt should you pick up the plug and touch the pins.
This is not very nice especially for a domestic user such as the wife!

A 680K 1W resistor? across the mains path would provide a discharge path when it is unplugged but I just wondered if there is a better way - possibly by putting a resistor directly across the capacitor itself?

Cheers

John

ms660 · 15th Jan 2018, 10:16 am

Across the capacitor.

Lawrence.

John G8MWF · 15th Jan 2018, 10:26 am

Thank you, and would 680K be a good value to use - I calculate it would draw 0.35mA, 85mW which shouldn't give performance issues?

boxdoctor · 15th Jan 2018, 10:57 am

I agree with MS660, but I would also suggest you use a 1 watt type, as there is quite a few volts across it, and some of the ¼ watt types seem physically rather small to have anything like mains voltages across them. Erring on the side of safety!
The increased current you point out is unlikely to be near the change in heater current caused by the normal variations in mains voltage in most areas, so I wouldn't fret about that. Tony

ms660 · 15th Jan 2018, 10:57 am

It all depends on how fast you want the capacitor to discharge, to discharge from a given voltage to approx 37% of that voltage then t = CR, in practical terms for full discharge it's 5t.

0.35mA wouldn't be an issue for a 100mA heater chain.

EDIT: Post crossed.

Lawrence.

John G8MWF · 15th Jan 2018, 11:11 am

So I make that around 1 second to 37% and 5 seconds to approx. zero.

There is still a very small chance of a "tickle" but a lot less risk than before.

A 470K/2W would give 0.5mA and a slightly smaller time - so I think I will give that a go.

BTW I am listening to Radio Caroline on medium wave while typing this and its a bit eerie hearing the ships bell sounding out again over a vintage radio!

Sideband · 15th Jan 2018, 10:50 pm

As an aside, the 'allowed' time for capacitor discharge is 2 seconds after the mains has been disconnected (this is according to EN60065 safety standard). The voltage across the capacitor after this time should be <35V.
Don't worry about this too much! I just wondered if someone might raise the question as to what the accepted discharge should be. The way we check this at work is by connecting a scope across the mains plug (100 megohm impedance), setting the timbase to give a 5 second trace and then disconnecting the mains at the peak voltage. The scope then shows a discharge curve against voltage and time.

I agree about the resistor size....use a 1 watt type. Strictly speaking it should have a 500V rating....remember on a scope, peak mains shows as 340V

John G8MWF · 15th Jan 2018, 11:05 pm

Well that is certainly a conclusive answer- thank you!

I will probably open up another can of worms here, the reason I want to use the capacitor dropper is that I have finally got hold of a Ivory Cased DAC90A that needs some TLC and I want to run it as cool as possible to reduce further cracking if that is possible.
At the moment there is only one crack in the RH top dial bezel, a chip to the speaker ring and damage to the internal dial glass pillar.

John G8MWF · 31st Jan 2018, 2:17 pm

Quote:

Originally Posted by Sideband

As an aside, the 'allowed' time for capacitor discharge is 2 seconds after the mains has been disconnected (this is according to EN60065 safety standard). The voltage across the capacitor after this time should be <35V.
Don't worry about this too much! I just wondered if someone might raise the question as to what the accepted discharge should be. The way we check this at work is by connecting a scope across the mains plug (100 megohm impedance), setting the timbase to give a 5 second trace and then disconnecting the mains at the peak voltage. The scope then shows a discharge curve against voltage and time.

I agree about the resistor size....use a 1 watt type. Strictly speaking it should have a 500V rating....remember on a scope, peak mains shows as 340V

I finally had a quick chance to fit the cap and resistor this morning although I had to fit a 1M ohm resistor as I couldn't find the pack of 680K.

The results are excellent and without any tweaking to the 1.5uF cap value I am getting 96V ac across the heater chain which works very well and I may just leave it, possibly giving the heaters a easier time, that way - unless it is doing some harm?

PsychMan · 1st Feb 2018, 9:00 am

Might it be wise to fit say a 200ma fuse in series with the dropper? Otherwise if the capacitor failed shorted your valves might be in trouble

John G8MWF · 1st Feb 2018, 9:58 am

Sounds like a good idea to me - although I was under the impression that X caps failed to the O/C state when they fail.

PsychMan · 1st Feb 2018, 10:55 am

They should, but I tend to be a bit on the paranoid side when it comes to these things! A pico fuse would probably be very simple to install, though of course more of a pain if it blows. You'd probably need to take some measurements and experiment a little to size it for inrush current. Though perhaps someone more knowledgeable will chime in and say its a waste of time

, in which case, ignore me!

John G8MWF · 1st Feb 2018, 2:17 pm

It is probably one of those topics where you cannot win whichever decision you make!
Saying that I have just put the cream DAC90A back in its case and it is just running slightly warm so that may save the case from cracking further down the line.
The other thought I had was if Modulation Hum is reduced by using a dropper cap? I wouldn't have thought that it would as it is 1.5uF in series with the mains and it wouldn't have much effect on RF entering.

Meanwhile I am going to cook the newly restored DAC90A which is now looking and working very well. Pictures enclosed of original condition - new ones to follow when it is cooked.

julie_m · 1st Feb 2018, 3:26 pm

X-rated capacitors should not be used as mains droppers, because the self-healing property is undesirable in this application. But you should use a fusible resistor in series with the capacitor, lest the heater in the most expensive of your valves take on this role! (X-rated caps can fail short-circuit; they just must not set fire to anything else. It's Y-rated capacitors, which might be connected to user-accessible metalwork, that only ever fail open-circuit.)

John G8MWF · 1st Feb 2018, 3:52 pm

So how is the value of the fusible cap calculated and would it blow before the heaters?
And would there be a problem with the start up surge current accidentally blowing them?

PsychMan · 1st Feb 2018, 5:10 pm

Quote:

Originally Posted by julie_m

X-rated capacitors should not be used as mains droppers, because the self-healing property is undesirable in this application. But you should use a fusible resistor in series with the capacitor, lest the heater in the most expensive of your valves take on this role! (X-rated caps can fail short-circuit; they just must not set fire to anything else. It's Y-rated capacitors, which might be connected to user-accessible metalwork, that only ever fail open-circuit.)

I thought the OP had said Y rated, not sure why as I see they said X rated...I cant tell from the picture if its X or Y

PsychMan · 1st Feb 2018, 5:16 pm

Looking up that capacitor it looks like a motor run capacitor, I cant see any X or Y rated info in the specs. One spec states self healing, so maybe it is an X then. I thought if X or Y it would say on the body

John G8MWF · 1st Feb 2018, 5:42 pm

You are quite right it is a CBB61 motor run capacitor that I assume(d) is a X type cap.

usradcoll1 · 1st Feb 2018, 5:52 pm

Quote:

Originally Posted by John G8MWF

So how is the value of the fusible cap calculated and would it blow before the heaters?
And would there be a problem with the start up surge current accidentally blowing them?

I find by using a cap dropper, it minimizes the inrush current to the valve heaters.
It seems that all the British electronic devices used double pole mains switches, so there wouldn't be any potential across the plug prongs.
I can see the bleeder resister across the cap dropper being a good practice.

John G8MWF · 1st Feb 2018, 6:03 pm

The double pole switch works well if you use it!
I got a tickle after unplugging it and then picking up the plug a short while later.

That is what gave me the reason to fit the resistor just in case some innocent person did the same - I don't think that it would have given vintage radios a good reputation if it had shocked Joe public.

15th Jan 2018, 10:09 am	#1
John G8MWF Hexode Join Date: Aug 2014 Location: Dartford, Kent, UK. Posts: 435	Dropper capacitor safety Hello, I wonder if you could help me with a dropper capacitor issues again? This question is more about safety than performance and I do not want to take any chances in getting it wrong. The Bush DAC90A that I modified with a dropper capacitor is working fine. The only trouble I have is that when it is unplugged there is still a charge in the capacitor that gives you a little jolt should you pick up the plug and touch the pins. This is not very nice especially for a domestic user such as the wife! A 680K 1W resistor? across the mains path would provide a discharge path when it is unplugged but I just wondered if there is a better way - possibly by putting a resistor directly across the capacitor itself? Cheers John

15th Jan 2018, 10:16 am	#2
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Dropper capacitor safety Across the capacitor. Lawrence.

15th Jan 2018, 10:26 am	#3
John G8MWF Hexode Join Date: Aug 2014 Location: Dartford, Kent, UK. Posts: 435	Re: Dropper capacitor safety Thank you, and would 680K be a good value to use - I calculate it would draw 0.35mA, 85mW which shouldn't give performance issues?

15th Jan 2018, 10:57 am	#4
boxdoctor Rest in Peace Join Date: Sep 2010 Location: Ripley, Derbyshire, UK. Posts: 785	Re: Dropper capacitor safety I agree with MS660, but I would also suggest you use a 1 watt type, as there is quite a few volts across it, and some of the ¼ watt types seem physically rather small to have anything like mains voltages across them. Erring on the side of safety! The increased current you point out is unlikely to be near the change in heater current caused by the normal variations in mains voltage in most areas, so I wouldn't fret about that. Tony

15th Jan 2018, 10:57 am	#5
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Dropper capacitor safety It all depends on how fast you want the capacitor to discharge, to discharge from a given voltage to approx 37% of that voltage then t = CR, in practical terms for full discharge it's 5t. 0.35mA wouldn't be an issue for a 100mA heater chain. EDIT: Post crossed. Lawrence.

15th Jan 2018, 11:11 am	#6
John G8MWF Hexode Join Date: Aug 2014 Location: Dartford, Kent, UK. Posts: 435	Re: Dropper capacitor safety So I make that around 1 second to 37% and 5 seconds to approx. zero. There is still a very small chance of a "tickle" but a lot less risk than before. A 470K/2W would give 0.5mA and a slightly smaller time - so I think I will give that a go. BTW I am listening to Radio Caroline on medium wave while typing this and its a bit eerie hearing the ships bell sounding out again over a vintage radio!

15th Jan 2018, 10:50 pm	#7
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 7,580	Re: Dropper capacitor safety As an aside, the 'allowed' time for capacitor discharge is 2 seconds after the mains has been disconnected (this is according to EN60065 safety standard). The voltage across the capacitor after this time should be <35V. Don't worry about this too much! I just wondered if someone might raise the question as to what the accepted discharge should be. The way we check this at work is by connecting a scope across the mains plug (100 megohm impedance), setting the timbase to give a 5 second trace and then disconnecting the mains at the peak voltage. The scope then shows a discharge curve against voltage and time. I agree about the resistor size....use a 1 watt type. Strictly speaking it should have a 500V rating....remember on a scope, peak mains shows as 340V __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman.....

15th Jan 2018, 11:05 pm	#8
John G8MWF Hexode Join Date: Aug 2014 Location: Dartford, Kent, UK. Posts: 435	Re: Dropper capacitor safety Well that is certainly a conclusive answer- thank you! I will probably open up another can of worms here, the reason I want to use the capacitor dropper is that I have finally got hold of a Ivory Cased DAC90A that needs some TLC and I want to run it as cool as possible to reduce further cracking if that is possible. At the moment there is only one crack in the RH top dial bezel, a chip to the speaker ring and damage to the internal dial glass pillar.

1st Feb 2018, 9:00 am	#10
PsychMan Octode Join Date: Jun 2013 Location: Fleet, Hampshire, UK Posts: 1,765	Re: Dropper capacitor safety Might it be wise to fit say a 200ma fuse in series with the dropper? Otherwise if the capacitor failed shorted your valves might be in trouble

1st Feb 2018, 9:58 am	#11
John G8MWF Hexode Join Date: Aug 2014 Location: Dartford, Kent, UK. Posts: 435	Re: Dropper capacitor safety Sounds like a good idea to me - although I was under the impression that X caps failed to the O/C state when they fail.

1st Feb 2018, 10:55 am	#12
PsychMan Octode Join Date: Jun 2013 Location: Fleet, Hampshire, UK Posts: 1,765	Re: Dropper capacitor safety They should, but I tend to be a bit on the paranoid side when it comes to these things! A pico fuse would probably be very simple to install, though of course more of a pain if it blows. You'd probably need to take some measurements and experiment a little to size it for inrush current. Though perhaps someone more knowledgeable will chime in and say its a waste of time , in which case, ignore me!

1st Feb 2018, 2:17 pm	#13
John G8MWF Hexode Join Date: Aug 2014 Location: Dartford, Kent, UK. Posts: 435	Re: Dropper capacitor safety It is probably one of those topics where you cannot win whichever decision you make! Saying that I have just put the cream DAC90A back in its case and it is just running slightly warm so that may save the case from cracking further down the line. The other thought I had was if Modulation Hum is reduced by using a dropper cap? I wouldn't have thought that it would as it is 1.5uF in series with the mains and it wouldn't have much effect on RF entering. Meanwhile I am going to cook the newly restored DAC90A which is now looking and working very well. Pictures enclosed of original condition - new ones to follow when it is cooked. Attached Thumbnails

1st Feb 2018, 3:26 pm	#14
julie_m Dekatron Join Date: May 2008 Location: Derby, UK. Posts: 7,735	Re: Dropper capacitor safety X-rated capacitors should not be used as mains droppers, because the self-healing property is undesirable in this application. But you should use a fusible resistor in series with the capacitor, lest the heater in the most expensive of your valves take on this role! (X-rated caps can fail short-circuit; they just must not set fire to anything else. It's Y-rated capacitors, which might be connected to user-accessible metalwork, that only ever fail open-circuit.) __________________ If I have seen further than others, it is because I was standing on a pile of failed experiments.

1st Feb 2018, 3:52 pm	#15
John G8MWF Hexode Join Date: Aug 2014 Location: Dartford, Kent, UK. Posts: 435	Re: Dropper capacitor safety So how is the value of the fusible cap calculated and would it blow before the heaters? And would there be a problem with the start up surge current accidentally blowing them?

1st Feb 2018, 5:16 pm	#17
PsychMan Octode Join Date: Jun 2013 Location: Fleet, Hampshire, UK Posts: 1,765	Re: Dropper capacitor safety Looking up that capacitor it looks like a motor run capacitor, I cant see any X or Y rated info in the specs. One spec states self healing, so maybe it is an X then. I thought if X or Y it would say on the body

1st Feb 2018, 5:42 pm	#18
John G8MWF Hexode Join Date: Aug 2014 Location: Dartford, Kent, UK. Posts: 435	Re: Dropper capacitor safety You are quite right it is a CBB61 motor run capacitor that I assume(d) is a X type cap.

1st Feb 2018, 6:03 pm	#20
John G8MWF Hexode Join Date: Aug 2014 Location: Dartford, Kent, UK. Posts: 435	Re: Dropper capacitor safety The double pole switch works well if you use it! I got a tickle after unplugging it and then picking up the plug a short while later. That is what gave me the reason to fit the resistor just in case some innocent person did the same - I don't think that it would have given vintage radios a good reputation if it had shocked Joe public.