Minimum spacings between buses

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Minimum spacings between buses

Postby W1AC » Tue May 06, 2014 10:05 am

I'm building a Class E rig, and thinking about mechanical arrangements for the FQA11N90 FET's.

The Fairchild data sheet (which I assume covers all TO3PN cases) warns about bending the leads within 3mm of the case, so I'm wondering what the minimum spacings are for the drain and gate buses, assuming a 400 watt RF deck on 75 meters.

Come to think of it, has anyone used the case heat sink as a connection point?

Bill, W1AC
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Re: Minimum spacings between buses

Postby kf1z » Tue May 06, 2014 11:00 am

I usually bend the drain leads almost back over the device....
"Good" layout? probably not for the sake of easy replacement.
But I have almost never had to replace FETs

the Source buss can be screwed right to the heatsink, AFTER soldering the source leads.

Then the gate buss hangs nicely in between....
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Re: Minimum spacings between buses

Postby W1AC » Tue May 06, 2014 5:16 pm

kf1z wrote:I usually bend the drain leads almost back over the device....
"Good" layout? probably not for the sake of easy replacement.
But I have almost never had to replace FETs

the Source buss can be screwed right to the heatsink, AFTER soldering the source leads.

Then the gate buss hangs nicely in between....


OK, potential red-faced newb question, but I'm still on my honeymoon here, so ...

... I thought the heat sink mounting pad was connected to the drain on the 11N90. Not so?

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Re: Minimum spacings between buses

Postby n1ps » Tue May 06, 2014 6:33 pm

Yes correct with a silpad between with compound. Some guys also use kapton. As Bruce mentioned, solder the fet source lead to the source bus first. I like to set the source bus in place but don't tighten the screws yet. Then solder the source leads and follow by tightening the screws. Use a screw about every inch or so and use stainless. Make sure the copper strip is clean. Oh and use copper, not brass for the source bus.

Good luck with the project. Steve's designs are great. This is a good venue to get help.
Peter
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Re: Minimum spacings between buses

Postby Gilly » Tue May 06, 2014 6:39 pm

Hi Bill

The only devices I find that you have to be careful with are the driver Chips usually the IXDD414....

If the pins are bend at the base of the IC they can break off or weaken anyway.....

What I do with these is make a 30 degree bend mid way along and another appox 1mm from there on until you get the angle required....

This works you me anyway :D


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Re: Minimum spacings between buses

Postby kf1z » Tue May 06, 2014 8:01 pm

W1AC wrote:
OK, potential red-faced newb question, but I'm still on my honeymoon here, so ...

... I thought the heat sink mounting pad was connected to the drain on the 11N90. Not so?

Bill, W1AC



Hmm
I might have confused you....

What I mean is.... the fet has to be insulated from the heatsink ( ground) because the case of the fet is internally connected to the Drain.
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Re: Minimum spacings between buses

Postby kf1z » Tue May 06, 2014 8:04 pm

n1ps wrote: Oh and use copper, not brass for the source bus.

Peter



Why Pete?

I use brass for everything and AVOID using copper. ( unless I am using a sheet of PC board for everything....)

Copper corrodes pretty quick in contact with Aluminum.
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Re: Minimum spacings between buses

Postby n1ps » Tue May 06, 2014 8:21 pm

Why Pete?

I use brass for everything and AVOID using copper. ( unless I am using a sheet of PC board for everything....)

Copper corrodes pretty quick in contact with Aluminum.[/quote]

Hiya Bruce. Are you still in central VT these days?

Brass actually is a lousy conductor, much worse than aluminium. So unless you are using it near salt water I don't see an issue. I think brass for the other bus strips is fine. I use it a lot too, but not for a ground/source bus.

My .02
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Re: Minimum spacings between buses

Postby kf1z » Tue May 06, 2014 9:48 pm

Point taken Pete.
I had trouble finding copper strips ( I didn't want to cut strips myself)
And remembered that the NEC calls for anytime you put Aluminum wire into a Copper lug on a power panel, that you put
"No-OX" or some similar anti oxidation paste to prevent corrosion
( this is for the service entrance part)

So, I thought, "Gee, I wonder how long it'll be before a copper strip looses contact with a heatsink...." :-)

It all works for us, or I should say me... because no one rig stays built for long! :-)


No.... not in VT now...

I'm out in the high desert of Ely, Nevada for the past year!
( WAY out in nowhere.... heck, it's a 3 1/2 hour, one way, ride to the nearest Walmart! )

Where the Scorpions are out already... but yet right now... it's snowing .... yup, pretty strange weather here.
( Elevation above sea level is 6,244 feet...... my driveway is 44 feet lower than the summit of Mt Washington.)
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Re: Minimum spacings between buses

Postby n1ps » Wed May 07, 2014 7:06 pm

Holy sh!t Batman :shock: How did you end up there? Is there any activity on 75 around there? Looks like a nice area....I like the Ghost Train. I have been wondering why I have not heard you on the air. Thought maybe you pulled a Dirk and escaped up to 40 :roll:

Yeah copper is a PITA to get. I am running low and have to go back to the roofer where I got the last batch. He gave me lots of scraps back when I built the E rigs. BUT! The price of copper has risen just a smidgen since then :D :D :D so we'll see.

Quite true on the al-cu concerns. I have been keeping an eye on the buses with no issues yet. Whereas current is not really flowing from one to the other I think all is OK so far.

p
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Re: Minimum spacings between buses

Postby kf1z » Thu May 08, 2014 12:02 am

n1ps wrote:Holy sh!t Batman :shock: How did you end up there? Is there any activity on 75 around there? Looks like a nice area....I like the Ghost Train. I have been wondering why I have not heard you on the air. Thought maybe you pulled a Dirk and escaped up to 40 :roll:

Yeah copper is a PITA to get. I am running low and have to go back to the roofer where I got the last batch. He gave me lots of scraps back when I built the E rigs. BUT! The price of copper has risen just a smidgen since then :D :D :D so we'll see.

Quite true on the al-cu concerns. I have been keeping an eye on the buses with no issues yet. Whereas current is not really flowing from one to the other I think all is OK so far.

p


My Wife took an USFS Engine Captain job here.... not enough Wildfires in Vermont.
The power company here is real slack, too many arcing poles. A little AM on 75 in the morning, but hard to hear through the noise.
Nothing like back east when you can chat on 75 all day long though.
The "usual" amount at night, but I don't get on at night still.

The area is "ok"... we only had maybe 40" of snow all year... shoveled the steps 3 times, never touched the driveway ( always windy...)
In the Summer it gets plenty hot, but usually a 40-45 degree difference between day and night temps. And of course, it's DRYYYYY


Sorry Bill.... really didn't mean to highjack the thread. ! :P
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Re: Minimum spacings between buses

Postby W1AC » Thu May 08, 2014 8:36 am

kf1z wrote:
n1ps wrote:Holy sh!t Batman :shock: How did you end up there?


My Wife took an USFS Engine Captain job here.... not enough Wildfires in Vermont.

The area is "ok"... we only had maybe 40" of snow all year... shoveled the steps 3 times, never touched the driveway ( always windy...)

In the Summer it gets plenty hot, but usually a 40-45 degree difference between day and night temps. And of course, it's DRYYYYY

Sorry Bill.... really didn't mean to highjack the thread.


It's OK: I'm looking for a place to retire myself, and I'm tired of shoveling "partly cloudy" off my steps, and I need a location with zero pollen, so it's all good. 8)

Bill, W1AC
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Re: Minimum spacings between buses

Postby n1ps » Thu May 08, 2014 7:05 pm

OK Bruce....go beat on some poles with a sledge.... :D :D :D

And Bill....enjoy your build. Its fun making these transmitters. I assume you are making a VFO too as you are using digital drive. Make sure you use a solid case for the VFO, something like cast aluminium. But Bruce is the guy to really listen too (with a keyboard ear). Bypass all connections in and out of the chassis (plural).

This really is the venue to ask questions. The site has been dormant.

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Re: Minimum spacings between buses

Postby W1AC » Tue May 13, 2014 5:37 pm

n1ps wrote:Bill....enjoy your build. Its fun making these transmitters. I assume you are making a VFO too as you are using digital drive. Make sure you use a solid case for the VFO, something like cast aluminium. But Bruce is the guy to really listen too (with a keyboard ear). Bypass all connections in and out of the chassis (plural).


I think it'll be more fun to have made this transmitter, but time will tell! :?

I hadn't planned on using a VFO just yet. I have an FT-890 transceiver that will serve as an exciter, although I'll need to get a pad to drop the power down to a manageable level. The old RF deck was being fed directly from an exciter, so the toroids are set up for split-phase drive, but I don't want to risk breaking any of the driver chips.

Speaking of bypassing: I have an idea for a way to reduce the chance of ground loops. If I mount the heat sink on insulators, and use it as an "isolated" ground connection, that will "float" the RF FETs above the chassis, and since the input and output are both transformer coupled, that seems a good way to keep the power feed clean.

As always, opinions welcome.

73,

Bill, W1AC
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Re: Minimum spacings between buses

Postby Gilly » Wed May 14, 2014 7:05 am

Hi Bill

If it was my TX would probably rebuild it a bit to make it look neater ... thats just me :D

Not sure what the minimum power the FT890 puts out but 5 watts would be a safe bet for a start......8 11N90 with a gate capacitance of appox 3900pF each is a large total capacitance to drive even at 3885Khz...
Don't apply any Drain voltage just drive to the gates and adjust for 24 volts peak to peak or so......checking all FETs gates etc:
Give all the input wiring a bit of a giggle to make sure the drive levels are consistant on all the FETs too.......

The picture here is of my 40M Sinewave drive TX....a little baby compared to yours but has been a very reliable Transmitter to this day really....

Image

A little different to yours but thought it would give you some ideas.......

You probably know all this anyway.....

Apply a bit a drive and see how you go.....good luck...


Wayne
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