Light bulb(s) as dropper resistor - UK Vintage Radio Repair and Restoration Discussion Forum

daviddeakin · 24th Apr 2007, 4:59 pm

I recently picked up a DAC 90A and thought about this dropping problem.
I did some experiments and it seems an ordinary 35W mains bulb, wired in series with the heater chain, will drop the voltage by just the right amount. (Two 15W bulbs in parallel will also do). I was thinking a pigmy bulb would fit under the glass dial quite neatly, and do two jobs at once.

Paul Stenning · 24th Apr 2007, 6:53 pm

I repaired a radio several years ago which had the heater chain in two halves, each fed with a separate bulb, 25W each I think. I can't remember what set it was now, though.

Nickthedentist · 24th Apr 2007, 7:04 pm

Quote:

Originally Posted by daviddeakin

I was thinking a pigmy bulb would fit under the glass dial quite neatly, and do two jobs at once.

Nice idea! But I wonder whether the heat there would be detrimental to the dial glass and/or cabinet

Nick.

igranic · 24th Apr 2007, 7:58 pm

Probably not a good idea imho. I suggest you measure the cold resistance of a tungsten lamp, and compare it with the calculated hot resistance. The initial current surge at switch-on is sure to shorten the life of those expensive or hard-to-get valves. On the other hand, dropper resistors, being made of constantan, nichrome, or similar, have a fairly constant resistance regardless of temperature.

paulsherwin · 24th Apr 2007, 8:24 pm

I agree, there's too much variability using a bulb, and you finish up dumping just as much heat inside the cabinet. It's better to fit a diode or capacitor dropper if you're making a major modification like this.

Paul

daviddeakin · 25th Apr 2007, 1:20 pm

But a tungsten bulb reacts MUCH faster than valve heaters, so will absorb the inrush quite effectively I think.

Plus you don't produce nearly as much heat because most of it is given off as light- that was really the whole point.

John M0GLN · 25th Apr 2007, 4:48 pm

If a tungsten filament bulb did work, and I've no idea whether it would or not, would it look ok in a small table lamp wired in series with the wireless, or would it be too dim?

John

ppppenguin · 25th Apr 2007, 5:08 pm

Quote:

Originally Posted by daviddeakin

Plus you don't produce nearly as much heat because most of it is given off as light....

About 95% as heat, 5% as light. Not much better than a resistor.

G8DLG · 25th Apr 2007, 6:49 pm

Quote:

Originally Posted by daviddeakin

But a tungsten bulb reacts MUCH faster than valve heaters, so will absorb the inrush quite effectively I think.

Plus you don't produce nearly as much heat because most of it is given off as light- that was really the whole point.

That is true,the inrush is short lived,but a dropper is best.
A tungsten lamp produces only 12-20% light,so not much saving on heat,if you can make use of the light,all well and good.A lamp resistance varies with temperature,so if the current increases the resistance of the filament also increases,so the increase in voltage does not produce the same increase in heater current.A Baretter on the other hand will completely stabilise heater chain current from wide mains voltage changes.
Dont be tempted to use a capacitor as a dropper,harmonics on the line will blow your heaters,also high surges will shorten valve life.

igranic · 25th Apr 2007, 6:50 pm

David

I am not going to do the sums and draw the current vs. time graphs, but tungsten lamps do not react quickly enough, which is why they almost always 'blow' at switch on - the initial inrush current causes the filament to rupture before it has reached working temperature (and resistance).

Ask yourself why, in all the years that they were striving to reduce the cost of their AC/DC sets whilst maintaining reliability, did radio designers and manufacturers not opt for a cheap pygmy lamp instead of the more expensive mains dropper with relatively constant resistance?

Also, consider why, as well as the dropper resistors, some set designs include negative temperature coefficient thermistors in the heater chain?

Ed_Dinning · 25th Apr 2007, 8:14 pm

Hi David, next to the resistive dropper a capacitor is the most practical bet, very many sets have been converted this way and function well. It is, however important to only use "X" class capacitors or motor strt types for this function. Unlike being used on DC, there is a much reduced inrush on AC due to the more limited rate of rise of the waveform. There should only be troubles with harmonics if you are in a mains area when these are particularly bad. This will give rise to a higher current than calculated but will not CAUSE the problem. I often "pad" the current value with about a 5W resistor, this further reduces surges.

The maths are not too difficult, but you will need an understanding of vectors if you want to fully appreciate what is happening.

Best regards, Ed

24th Apr 2007, 4:59 pm	#1
daviddeakin Hexode Join Date: Apr 2007 Location: W Yorks, UK. Posts: 407	Light bulb(s) as dropper resistor I recently picked up a DAC 90A and thought about this dropping problem. I did some experiments and it seems an ordinary 35W mains bulb, wired in series with the heater chain, will drop the voltage by just the right amount. (Two 15W bulbs in parallel will also do). I was thinking a pigmy bulb would fit under the glass dial quite neatly, and do two jobs at once.

24th Apr 2007, 6:53 pm	#2
Paul Stenning Administrator Join Date: Dec 2002 Location: Cardiff Posts: 9,073	Re: DAC90A Light bulb(s) as dropper resistor. I repaired a radio several years ago which had the heater chain in two halves, each fed with a separate bulb, 25W each I think. I can't remember what set it was now, though. __________________ Paul Stenning Forum Admin/Owner and BVWS Webmaster

24th Apr 2007, 7:58 pm	#4
igranic Rest in Peace Join Date: Jan 2005 Location: North Bedfordshire, UK. Posts: 395	Re: DAC90A Light bulb(s) as dropper resistor. Probably not a good idea imho. I suggest you measure the cold resistance of a tungsten lamp, and compare it with the calculated hot resistance. The initial current surge at switch-on is sure to shorten the life of those expensive or hard-to-get valves. On the other hand, dropper resistors, being made of constantan, nichrome, or similar, have a fairly constant resistance regardless of temperature.

24th Apr 2007, 8:24 pm	#5
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,970	Re: DAC90A Light bulb(s) as dropper resistor. I agree, there's too much variability using a bulb, and you finish up dumping just as much heat inside the cabinet. It's better to fit a diode or capacitor dropper if you're making a major modification like this. Paul

25th Apr 2007, 1:20 pm	#6
daviddeakin Hexode Join Date: Apr 2007 Location: W Yorks, UK. Posts: 407	Re: DAC90A Light bulb(s) as dropper resistor. But a tungsten bulb reacts MUCH faster than valve heaters, so will absorb the inrush quite effectively I think. Plus you don't produce nearly as much heat because most of it is given off as light- that was really the whole point.

25th Apr 2007, 4:48 pm	#7
John M0GLN Octode Join Date: Jan 2004 Location: Southport, Merseyside, UK. Posts: 1,156	Re: DAC90A Light bulb(s) as dropper resistor. If a tungsten filament bulb did work, and I've no idea whether it would or not, would it look ok in a small table lamp wired in series with the wireless, or would it be too dim? John

25th Apr 2007, 6:50 pm	#10
igranic Rest in Peace Join Date: Jan 2005 Location: North Bedfordshire, UK. Posts: 395	Re: DAC90A Light bulb(s) as dropper resistor. David I am not going to do the sums and draw the current vs. time graphs, but tungsten lamps do not react quickly enough, which is why they almost always 'blow' at switch on - the initial inrush current causes the filament to rupture before it has reached working temperature (and resistance). Ask yourself why, in all the years that they were striving to reduce the cost of their AC/DC sets whilst maintaining reliability, did radio designers and manufacturers not opt for a cheap pygmy lamp instead of the more expensive mains dropper with relatively constant resistance? Also, consider why, as well as the dropper resistors, some set designs include negative temperature coefficient thermistors in the heater chain?

25th Apr 2007, 8:14 pm	#11
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,195	Re: DAC90A Light bulb(s) as dropper resistor. Hi David, next to the resistive dropper a capacitor is the most practical bet, very many sets have been converted this way and function well. It is, however important to only use "X" class capacitors or motor strt types for this function. Unlike being used on DC, there is a much reduced inrush on AC due to the more limited rate of rise of the waveform. There should only be troubles with harmonics if you are in a mains area when these are particularly bad. This will give rise to a higher current than calculated but will not CAUSE the problem. I often "pad" the current value with about a 5W resistor, this further reduces surges. The maths are not too difficult, but you will need an understanding of vectors if you want to fully appreciate what is happening. Best regards, Ed