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In Reply to: 6c33c heater with 12vdc posted by se6c33c on February 20, 2007 at 00:43:18:
Hi ,
You would need to ensure that the SMPS could take the cold resistance of a 6C33 heater first .cheers
Follow Ups:
A first approximation is that cold resistance will be around 1/7th of hot resistance for an oxide coated filament, so in theory the inrush surge is about 7 times the rated current.The current limit on the supply can be much less than this depending on the foldback characteristics of the supply. In practice I have found a 50% margin to be sufficient for smaller V/Ts on current limited supplies, I haven't tried anything as large as the 6s33s.
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it will only increase as it warms up, yes?
Hi.So what is the next step?
SMPS that are internally current limited are not uncommon at all. I'd rather pick one that was limited at something over the maximum hot load and take advantage of what would be a slow start up.
Hi.That's what I would do.
(1) measure the DCR of the tube heater, OUT of its socket.
(2) calculate the theoretical inrush current of the heater.
Since I do not have a 6C33C-B with me right here, so I just measure
EL-34 (6.3V 1.5A) which I have a few sleeping in my parts bins, which is 0.7R. So the theoretical cold start heater current inrush factor will be 6.3V/1.5A/0.7R=6 ie. 6 times the rated heater current.So taking this aproximation guideline factor, the theoretical inrush
current of a cold start 6C33C-B heater (12.6V 3.3A) would take 12.6V/3.3AX6=22.9A !!!!(3) get the right SMPS to handle such huge inrush.
It is hard to find a convention analogue PS to deliver such cold start inrush 6 times of rated current. In fact I blew a few conventional anaologue PS before I switched to SMPS now, to charge up my SLA cells. I learnt it the hard hard way.
But surprisingly a quality designed SMPS can handle very tough inrush load punishment.
Let me take a look at the specs of a 210W (70KHz with 72% efficiency)
external switching adaptor (rated to IEC801-4 transients specs).It is rated 5A 115V I/P norminal max load & O/P voltage 30V 7A peak. It is also rated 50A max cold start I/P current at 115V, which will give cold start I/P power surge of 5,750W!
To test how 'robust' a SMPS can be, I tried with my 9.86V2.7A SMPS (scraped from a used camercorder battery charger), it delivered up to 4.5A on initial charging my 6V5AH WIHTOUT adding any current limiter !!!! It got very hot but worked fine.
So get the right SMPS to deliver 3.3A which is a piece of cake, & get a limiter resistor of the right value, to prevent damaging the PS on cold star. For seried 6C33C heaters, I would recommend adding an inrush current limiter, e.g. Ametherm MS355R025 rated max SSI 25A, R(hot) 0.03R upstream of the heaters.
c-J
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