Transorbs

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Re: Transorbs

Postby n4lta » Tue Sep 28, 2010 8:00 pm

Steve,

I could use ten transorbs also. Just let me know how much the Paypal will be. They are hard to find.

Email is pbunn@matrixei.com

Pat
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Re: Transorbs

Postby kf1z » Wed Sep 29, 2010 1:50 pm

K9ACT wrote:Upon further investigation, it seems that I may be missing the forest for the trees.

Ponder this photo...Image

When I first noticed this pattern I assumed that it was just an imprint from the pattern of the silpad.

Under the microscope it is clearly reformed aluminum and not just rubber. These things got so hot, the aluminum melted; so it obviously is not a transient problem but a more basic heat sink problem. Notice how the pattern does not appear where the screw applies direct pressure but away from the screw.

These two would indicate that the one will less of the pattern was screwed down more tightly than the other. This sort of presents a quandry as screwing down too tight can cock the device and produce the same problem.

BTW, these are the last two devices to fail.

Any thoughts?

js



That plating on the back of the fets is a very soft alloy... maybe containing berrilium, or bismuth ( bismuth makes more sense)

It is made to conform to the mating surface, for thermal conductivity.

It is very soft, and melts at a VERY low temperature... (lower than 60/40 solder)

Tighten down your fets !
:D
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Re: Transorbs

Postby K9ACT » Wed Sep 29, 2010 2:32 pm

kf1z wrote:

That plating on the back of the fets is a very soft alloy... maybe containing berrilium, or bismuth ( bismuth makes more sense)

It is made to conform to the mating surface, for thermal conductivity.

It is very soft, and melts at a VERY low temperature... (lower than 60/40 solder)

Tighten down your fets !
:D


Awesome! As the kids would say.

It scratches easily with a piece of aluminum fence wire that will not leave any mark on a piece of aluminum.

I am not about to say for the 10th time or so that I fixed it but I have logged about 5 hours of air time with no problems since torquing down the screws.

Thanks for solving the mystery.

js
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Re: Transorbs

Postby K9ACT » Tue Oct 05, 2010 12:06 am

It has been exactly one week since I torqued down the FET mounting screws and I have been running the transmitter every day at the full, 200W power level with modulation always 150% or more.

I have had no problems whatever since.

The only change I have made was to replace the 120V light bulb bleeder/load with a 220V. I was getting nervous about lamp failure in the standby mode at higher voltages. The calculated MTBF for this one at 120V is about 4000 years.

Of course, being a conservative pessimist I bought two of them to make sure I have a spare when it fails.

Thanks for all the help and ideas.

js
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Re: Transorbs

Postby Gilly » Tue Oct 05, 2010 4:30 am

Hi Jack

Good to hear about the TX now being reliable....

Just wondering what the turns ratio on the transformer is now...
Going from 100Watts to 200Watts is it something like 4:6 ? ?

Also did you have to change the value of the capacitors across the Drains for max efficiency.....

On my class D cannot get 90% + ( given up now ) ....

What is your efficiency of your TX.... do the FETs gets hot at all ?


Wayne
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Re: Transorbs

Postby K9ACT » Tue Oct 05, 2010 8:17 am

Gilly wrote:Hi Jack


Just wondering what the turns ratio on the transformer is now...
Going from 100Watts to 200Watts is it something like 4:6 ? ?


I believe it is exactly as W1VD specifies, i.e. 4t ct : 5t out. I had to fiddle with the ratio on the 100W but this worked as described.

>Also did you have to change the value of the capacitors across the Drains for max efficiency....

I put them in thinking they would solve the blown fet problem. Didn't know it had anything to do with efficiency.
I don't think I had them in the 100W. I now have two 1000pF in series for about 500 across the drains. I have no idea what is supposed to be there.

>On my class D cannot get 90% + ( given up now ) ....

I have no problem but 95% is hard to believe so there must be a measurement problem.

>What is your efficiency of your TX....

30V @ 7A = 200W

I trust the DVM and the Bird and calibrated the Ameter on the bench so don't know what to think.

> do the FETs gets hot at all ?

Not at all but that's what I thought when they were blowing up.

I scanned them regularly with an IR thermo with a peak hold and never see anything above 110F and they are only warm to the touch. The heat sink is about the same temp after long tests also.

Jack
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Re: Transorbs

Postby kf1z » Tue Oct 05, 2010 9:38 am

I would not use a DVM to try to measure the modulator voltage (input to rf deck), while at full power carrier. (that's when to measure.)

Do you have a good analog meter to try?
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Re: Transorbs

Postby K9ACT » Tue Oct 05, 2010 10:14 am

kf1z wrote:I would not use a DVM to try to measure the modulator voltage (input to rf deck), while at full power carrier. (that's when to measure.)

Do you have a good analog meter to try?


Sure but are you concerned about the meter or the accuracy of the reading?

js
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Re: Transorbs

Postby kf1z » Tue Oct 05, 2010 10:24 am

Most DVMs (not all... maybe really expensive ones are exempt..) have a hard time with RF present....

Maybe the bypass caps at the input inductor aren't quite bypassing all the rf to ground.....

I know my DVM (not expensive OR really good :) ) does NOT read the voltage correctly on the mmodulator output,
Even though at pure DC , like a battery or good bench supply, the analog meter and DVM agree on voltage...
But on the modulator output, the DVM gets flaky.

This may or may not be your case.... but I would certainly try it...

95% is almost too good to believe... though it's not a problem by any means..
The rig is working well, there isn't excess heat.. so all is well ! :D

Just thought I'd throw that in because you wondered IF there was a metering snafu.


I myself will trust the analog meter, and oscilloscope over my dvm in the presence of RF.

Bruce
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Re: Transorbs

Postby K9ACT » Wed Oct 06, 2010 12:32 am

K9ACT wrote:
Gilly wrote:Hi Jack


Just wondering what the turns ratio on the transformer is now...
Going from 100Watts to 200Watts is it something like 4:6 ? ?


I think I gave you some bum info here. As I now recall, I removed one turn from each side of CT on the primary which means it is only two turns center tapped not 4 as specified.

Today's project was to make the 160 output filter and see how it works there.

So far, no joy.

First of all, I thought it was not working because the Fet drivers only draw about 400 ma compared to the 1500 ma of the 75 meter version. This seems to have nothing to do with anything and wonder why this would be.

The problem is that I can not get a clean sine wave output. I modeled the filter on Spice and it seems to be OK; flat to about 2.5 MHZ then drops off rapidly. Diddling around with various caps, I came up with something that looks fairly clean but it draws 10A at 20 V for well under 100W.

I am thinking that the transformer might have to be changed also to work on 160. Guess I need to ask Jay about this.

js
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Re: Transorbs

Postby kf1z » Wed Oct 06, 2010 12:04 pm

The impedance is about 4.28 ( 30 volts @ 7 amps )

So the impedance ratio of your output transformer will need to be around 11 or 12 to 1

If your primary is 2 turns center tapped... the secondary would need to be around... 3 1/2 turns

So try 3 turn secondary or 4 .



The transformer should not have to change between 75 and 160 meters.

Only the output filter and the shunt cap.
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Re: Transorbs

Postby K9ACT » Thu Oct 07, 2010 9:22 am

kf1z wrote:So try 3 turn secondary or 4 .

The transformer should not have to change between 75 and 160 meters.

Only the output filter and the shunt cap.


I can't seem to find a happy compromise.

I rewound the primary with the original 4 turns and 5 t secondary. I get a nice clean output but not enough current to get 200W at 30v. But even if I find the right number of secondary turns, I know it won't work on 80 unless I remove two turns from the primary based on what worked before.

A real problem experimenting with this is the very limited amount of #14 wire that can be gotten through the cores. As it is, I am using 18 magnet wire on the secondary.

BTW, I checked the carrier voltage with a needle meter and it agrees with the DVM.

js
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Re: Transorbs

Postby kf1z » Thu Oct 07, 2010 10:17 am

Very odd Jack...

The number of turns... or the turns ratio is based on your impedance...
Jay was very likely running more voltage than you... I guess around 45 volts at carrier.
Which makes his impedance higher than yours, so you need a different ratio.

If he was running 45 volts @ 5 amps.. that's 225 watts input to the rf deck
45 / 5 = 9 ohms

50 / 9 = 5.56 ( so to get from 9 ohm imp to 50 ohms... we need a step up of 5.56 x )

Then 4 turns CT and 5 turn secondary
Is really 2 turns to 5
5 / 2 = 2.5 turns ratio
2.5 x 2.5 = 6.25 impedance ratio

so 6.25 is pretty close to 5.56 close enough......



This is what is not stated in his schematic or written docs...

This should not be frequency dependent... to a point.


So for your 30 v @ 7 amps

30 / 7 = 4.28 ohms

50 / 4.28 = 11.68 x step up in impedance needed

so that requires a turns ratio of 3.42 to 1 ( sq rt of 11.68)

the a 2 turn ct primary would mean a 3.4 turn secondary ..... (thats why I said try 3 or 4 turn secondary.)

If you now have 4 turns ct that would call for a 7 turn secondary !


Raising your voltage, and in turn your impedance is about the best way to make the output transformer a little more manageable.
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Re: Transorbs

Postby K9ACT » Thu Oct 07, 2010 11:50 am

When you say, "raise the voltage", I assume you mean the voltage at which the carrier will produce 200W.

Using Steve's transformer and my supply, I can get about 120VDC under load.

My understanding is that to get 200% modulation, (ignoring efficiency) my carrier can not exceed 30V. That and 7A gives me 200W and 5 ohms. I don't really need 200% but 150 and some heard room looks about the same.

Am I missing something here?

js
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Re: Transorbs

Postby kf1z » Thu Oct 07, 2010 4:57 pm

Yes, that's what I mean...
raise your carrier voltage... drop the current.


At 30 volts carrier, 60 volts total supply voltage will produce 100% modulation

if your supply can handle 120 volts under full load...
Then you can run 40 volt carrier and still get 150% modulation ..... just.

150% is plenty.... :D

30 volts at 7 amps is 4.25 ohms 210 watts input

35 volts at 7 amps would be 5 ohms 245 watts input

then for your transformer.........
at 5 ohms, that would mean a turns ratio of 2 turns CT primary, to 3 secondary
Or 4 turns ct, to 6 turn secondary.
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