Schematic and parts list for DC heater supply?

[Structo]

So, here's what I've found...

I strung three 1N4007 diodes together in series after the DC cap(s), and with the tubes in and warmed up, I'm getting about 8.5vDC before the diodes (this being down from 9.56vDC...thus indicating the load of the tubes), and about 6.07vDC on the tube filament pins. SO, this is great! That's just where I want to be.

Maybe I'm missing something here but shouldn't you have the diodes before the filtering caps?

[paulster]

These are post-rectifier diodes used as a voltage dropping string because the unloaded DC voltage was too high.

[dehughes]

You'll need to drop about 2.2 volts (8.5-6.3).

At .9 A draw, that's 1.98 ohms. 2 ohms would be plenty close, as would 2.2.

Power dissipated would be 1.62 watts, so a 5 watter would be a good safe bet.

W

Tsk, tsk, ampgaragers. Either no one caught my mistake, or you're all just being polite! (if the latter is the case, I thank you)

The correct resistor is 2.4 ohms (meaning that 2.2 would still be close enough).

When I was younger I had trouble obeying some laws - "You can't drive that fast, son" and "Hey - don't smoke that - it's illegal" come to mind. I never had trouble until now adhering to Ohm's law!

W

You're welcome.

[dehughes]

Great news.

As regards the regulator, what you need to look at is the dropout voltage.

You've got 8.5Vdc available and you want to get 6.3V, so you've currently got 2.2V headroom. You want to allow for some line voltage fluctuation so if you allow for -10% then your 8.5Vdc would drop to 7.65V and your headroom would be reduced to 1.35V.

You'll also still have some ripple in that 8.5V, although you've got it down to a very low level by using 30,000uF of filtering (coincidentally the same as I use on my regulator boards). If we say there's 0.15V ripple (which it won't have, but it's good for the calculations) then you're left with 1.2V headroom.

So long as you have a voltage regulator which has less than a 1.2V dropout voltage at 1A drawn then it will give you a fully regulated 6.3Vdc down as low as 108Vac mains.

Get a 3A rated regulator, and use the chassis wall as a heatsink ideally. Make sure you read the datasheet to see whether the tab of the package is connected electrically to the ground pin or to any of the other pins (sometimes it's +V), in which case you might need an insulating washer.

Then go and enjoy that sucker!

Cool. I'm ordering some of each of these:

http://www.mouser.com/Search/ProductDet ... M%237806FA

and

http://www.mouser.com/Search/ProductDet ... -LD1085V50

I figure one of those should work...hopefully. I'll let you all know how it goes.

[Structo]

These are post-rectifier diodes used as a voltage dropping string because the unloaded DC voltage was too high.

Yes but wouldn't it make for better rectification with all of them before the caps?

Might need less filtering?

For instance, my D'Lite has three diodes on a side before the caps.

Not questioning the way he did it, just trying to understand why he did it that way.

[dehughes]

Those diodes were just a temporary solution to find out how much voltage I'd need to drop once the tubes were in the circuit. I'll replace them with a resistor, or not (if I use a regulator).

[paulster]

These are post-rectifier diodes used as a voltage dropping string because the unloaded DC voltage was too high.

Yes but wouldn't it make for better rectification with all of them before the caps?

Might need less filtering?

For instance, my D'Lite has three diodes on a side before the caps.

No, rectification is rectification. The D'Lite will have three diodes in series to keep each of the diodes well within its voltage rating, not to get a higher quality rectification or anything like that. That's the only reason you see more than one in series in a power supply. It also gives some added protection in that if one diode fails short then there's at least another to pick up the load before the electrolytic filter caps get unhappy at having AC across them.

You'll notice that on the Trainwreck Express there are 3 1N4007 diodes in series on the plate connections to ground. This is to get the correct voltage again such that the diodes will conduct if their combined reverse voltage is exceeded and limit flyback voltages. The three are needed or the usual plate voltages would cause them to start conducting with normal music signal.

[paulster]

Cool. I'm ordering some of each of these:

http://www.mouser.com/Search/ProductDet ... M%237806FA

and

http://www.mouser.com/Search/ProductDet ... -LD1085V50

You do know that the first one is 6.0V/1.5A and the second is 5.0V/3A, don't you? 6.0V would be okay for voltage but you might find some issues with it at startup when there'll be a massive inrush current until the heaters are up to temperature.

The UCC283-ADJ ones I used to use were 3A rated and would start to get upset at about 2A worth of heaters on power-up, whereas the Sipex ones I changed to have been happy with all sorts of abuse but they're a 5-wire one so are only practical for use on PCBs.

[Wayne]

Structo - More diodes in each leg isn't for better rectification - that is done when the diodes being used can't handle enough reverse voltage. If, for instance, 3 1000V diodes are used on each leg, the string of 3 as a whole could withstand 3KV - in theory, at least. I would imagine that in cases where the diodes being used are already rated for enough reverse voltage, it is done to build in some safety factor so that the voltage each diode sees is well within, as opposed to barely within, it's rating.

dehughes - If you're happy with the voltage obtained by using those diodes to drop the excess, technically you could quit now. If it were me, I would swap them out for 3A diodes, but 3A diodes are cheap enough.

And yes, I did double check this post

W

[Structo]

Uhggg, yeah, a minute after I typed that it kind of dawned on me what was going on with them in series.
Getting old and not remembering things sucks.....

[Wayne]

Uhggg, yeah, a minute after I typed that it kind of dawned on me what was going on with them in series.
Getting old and not remembering things sucks.....

^^^

+ about a zillion or so!

[dehughes]

Cool. Yeah, the regulators were mostly for tinkering with for curiosity's sake. I think it's quiet enough even with 20000uf, so what I'll do is just build a C-R-C setup with maybe a 680uf filter last, before the tubes. I think that'll do just fine...

I'll post back next week after the parts come in...

Thanks!