VCCS filament supply for 813

[Fuling]

Hi

Found this cct and got curious if/how it can be modified to deliver 10V 5A for 813 filaments.

[Tekko]

Build it and mod it till you get the volts and amps u need.

[Fuling]

I guess that´s the only way to find out for sure!

A few thoughts: During warmup the FET will need to dissipate 70-100W for a few seconds so using two or more high power devices in parallel might be clever.

I´m a bit troubled by the lack of a negative voltage rail for the operational amps, would anything be improved by adding one?

[Zen Mod]

count on 6V across CCS for proper operation

[Fuling]

Zen Mod:

Thanks, I suspected that the CCS takes quitea few volts to operate as intended. I have a 2x15V 300VA transformer and some caps and chokes that should provide about the right voltage.

[adalin]

Is there any advantage in this circut vs a regular lm350 with a 2n3055

Smaler heatsink?

That would be a great advantage indeed

[Fuling]

Power loss (heatsink size) should be equal but this circuit feeds the filament with constant current rather than constant voltage which is beneficial according to several sources.

[Michael Koster]

I'm building a pair of these for 300Bs, and also looking at building
a version for Eimac 4-65A and 3C24s, admittedly smaller than what
you need, but still beyond the capability of this circuit as is.

The first thing that occur to me is that with 10A current everything
will need beefing up. The rectifiers will dissipate 4W and will need
heatsinking.

The MOSFET needs to be something like this that will operate with
approximately 0.5 volts drain to source at 10A current (5W diss.)

http://www.irf.com/product-info/data...rfp3206pbf.pdf


The opamp is speced for rail to rail inputs and output swing from
0V to 1.5 V below V+, plenty to drive the MOSFET into conduction.

Circuit traces and connections also need beefing up for 10A current.
I built a single-sided PCB for a power supply once that had diodes
soldered in through the holes. After 24 hours of use, the heat from
the diodes had melted the solder and charred the board. The
customer noticed the front panel lights flickering and a funny smell...

Cheers,

Michael

Oh and of course R4 should be 0.01 ohms 5 watts

[Fuling]

Thanks for your reply!
Regarding the fets I have a few high power, high current types (IRFP260, 75645P and others). As i mentioned earlier I´m planning to use two or more Fets per channel to spread the heat and increase their chances to survive the warmup procedure.

The PCB layout is of course useless in this application so my plan is to hardwire the whole thing on a pair of huge heatsinks using a tiny piece of veroboard for the IC. I have some 35A bridge rectifiers, a 2x15V 300VA toroidal transformer and some big caps and chokes to provide a fairly clean 16-18VDC for the active circuitry to work with.

I have some experience of building class A transistor amps so cooling mosfets and building high current power supplies is not new to me. Most of my concerns were regarding the single rail operation of the IC.

[Michael Koster]

Sorry, I'm sure you know what you're doing.

I guess my point, besides the opamp thing, was that this VCCS does
not need much voltage to work with. It should be possible to do what
you need with 7-8 volts and low DCR components. You can avoid
dissipating another 100W in the filament supply itself.

Cheers,

Michael