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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 4th Feb 2006, 12:39 pm   #1
Radio_Dave
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Default Valve heater dropper calculations

Hi,

How do I work out the size of a heater dropper if the chain is using different currents?

Thanks
David


See also this related thread.
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Old 4th Feb 2006, 12:51 pm   #2
Paul Stenning
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Default Re: Valve heater dropper calculations

The whole point of a series heater chain is that the individual heaters have the same current rating, as the current goes through them each in turn.

If you need to have a series heater chain with valves having different heater currents, you need to find the highest current one then put resistors in parallel with the other heaters to make them up to same current as the highest current one. Then you can work it out based on the highest current.

You may be able to come up with some series/parallel combinations, but it still gets complicated.

If you have a set with a mix of heater currents in a series arrangement, it is likely that someone has put sosme wrong valves in it at some time.
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Old 4th Feb 2006, 1:00 pm   #3
paulsherwin
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Default Re: Valve heater dropper calculations

All Paul says is true, but if you just want help in calculating dropper values you should read his excellent page on the subject (there's a downloadable spreadsheet at the bottom): http://www.vintage-radio.com/repair-...per-calcs.html

Maybe he was too modest to refer to it himself

Best regards, Paul
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Old 4th Feb 2006, 1:01 pm   #4
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Default Re: Valve heater dropper calculations

Oh er, it's not that complicated . The valves are 0.1mA but the dial lamps are 0.15mA.


Thanks
David
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Old 4th Feb 2006, 1:13 pm   #5
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Default Re: Valve heater dropper calculations

It's normal practice to underrun dial lamps, especially in a series heater string. If this isn't done they'll have a short life because of the switch on surge. You can just treat them as 100mA components.

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Old 4th Feb 2006, 1:29 pm   #6
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Default Re: Valve heater dropper calculations

Thanks guys, I knew it would be easy

David
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Old 4th Feb 2006, 2:52 pm   #7
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Default Re: Valve heater dropper calculations

There may be a thermistor in parallel with the lamps. If so you can ignore it in the calculations.

In fact you can assume that the lamps ate a bit lower voltage than their rating (as they are under-run), but that's only a couple of volts wich is irrelevant when you are thinking of 230V or so across the chain.
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Old 4th Feb 2006, 2:59 pm   #8
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Default Re: Valve heater dropper calculations

Quote:
Originally Posted by paulsherwin
Maybe he was too modest to refer to it himself
It didn't seem applicable to the question, which was about the mix of currents. Fortunately it was much simpler once we had all the information!
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