From: [email protected] [mailto:[email protected]] On Behalf Of Jerry Gaffke via Groups.Io
Sent: Friday, March 23, 2018 2:22 PM
To: [email protected]
Subject: Re: [BITX20] Common heatsink for IRD510's #ubitx

?

Not a problem.
Impedance from drain to ground is quite low, my guess is the push-pull output transformer gives it 12.5 ohms
because the center of the primary is effectively at ground.? (Some push pull amps power the finals through
a center-tap which is at AC ground, instead of using the extra chokes at L8, L9).

How much not a problem?
Check this out:??

Capacitances around the IRF510 gate do mean it requires more drive at 30mhz than
some of the other FET's.? But I've never heard of the drain-to-ground capacitances being
a problem.? Allison recommends extremely short source leads to a good ground plane,
and keeping the gate far far from the drain to avoid oscillations.?
She clips off the drain lead and solders any drain connections directly to the tab.

Jerry, KE7ER


On Fri, Mar 23, 2018 at 11:43 am, Al Duncan VE3RRD wrote:

This may have been answered already. The mounting tab on the IRF510 is internally connected to the DRAIN which is the output of the transistor (connected to the output transformer). Therefore would it not be hot both with 12V and also with RF; and if mounted on a common heatsink, wouldn't they be capacitively coupled together through the insulating material? Would this be a problem on the higher bands, like 10m?
I guess the way to check would be to measure power out on 10m with individual heatsinks, and then with a common heatsink. The dielectric properties of different insulators (mica, silicone, ceramic etc) could also affect things.
73? AL