Lessons learned building a 5VDC Power Supply for a 300B heater

[Michael Koster]

"The position of R10 is important - It gave a huge improvement by being on the -ve side of the supply (as opposed to an equivalent resistor between the caps on the +ve side or even one resistor of 1/2 value on each side) - A very usefull trick."

Interesting and clever

Doer R10 balance and cancel the residual ripple voltage across the sense resistor R7? It looks like they would cancel at the opamp input.

Michael

[sgerus]

The LT1085 has relatively low dropout voltage and even with the current setting resistor

OK, I see the LT1085CT drops about 1V at 1.2A

With 6.3vac we have 6.55vdc at the first capacitor.

It looks like that is enough, but I’m not sure how the 1.25 reference voltage factors in.

Does this mean we need a minimum of 5V+1.25V+1V=7.25Vdc at the first capacitor?

Also, what wattage resistor should I use for the 1R…. 4-5 Watts?

[kevinkr]

Originally posted by sgerus


OK, I see the LT1085CT drops about 1V at 1.2A

With 6.3vac we have 6.55vdc at the first capacitor.

It looks like that is enough, but I’m not sure how the 1.25 reference voltage factors in.

Does this mean we need a minimum of 5V+1.25V+1V=7.25Vdc at the first capacitor?

Also, what wattage resistor should I use for the 1R…. 4-5 Watts?

You'll need to add the drop out voltage and resistor voltage together, add a margin for improved regulator performance and for low line.. I recommend something like 8V or over.. The resistor should be a 2 - 3W type and will be dissipating 1.25W - something with good precision and low tempco is recommended. I typically use 1% MF or WW 2W or better.

[andyjevans]

I'm with Kevin here. I did try and get a DC supply off 6.3v but that's not quite enough. Close but very borderline.

I have the same setup as Kevin for my 10Ys (also 1.25A), for which I use a 12v transformer, Schottky bridge, 15,000uf cap and LM1084 as current source with 1 ohm resistor. I found that to get 7.5v I needed to go up to 12.5vDC going into the LM1084. Tested all this with bench supplies. That's a good 5v difference to get good stable regulation, which then stays stable up to 16vDC and beyond.

So I'm guessing you want around 10vDC into your LM1084 or LT1085 to get the regulation really stable. I don't see 6.3v as quite getting you there, even with a big transformer. And I do wholeheartedly agree with Kevin - use a transformer with double the current rating if you can. I once used a borderline one and it hummed like crazy.

So I'm using a 60VA 0-12 0-12 toroid for two 10Y, and when I do the next 300b I expect to use pretty much the same but 0-9 0-9. That way the transformer doesn't get hot!

Andy

[andyjevans]

I should add that for the 1 ohm (if you use a current source, which sounds better), a 2W resistor gets pretty damn hot! I've been using a 12w wirewound, but I would have thought 7W is ample. Nice to keep the whole setup fairly cool. Nevertheless, Kevin's longevity clearly means his setup is stable. I guess I like a nice margin.

Andy