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Author Topic: push/pull amp using IRF511  (Read 39309 times)
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WU2D
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CW is just a narrower version of AM


« Reply #25 on: October 19, 2011, 07:39:57 PM »

Frank what should I do. DC flows though my caps  Shocked


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WA1GFZ
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« Reply #26 on: October 19, 2011, 08:03:16 PM »

Attach each cap to a line cord and plug it in.
This will release excessive smoke built up in the package
Once excessive smoke is released no current of any type will flow.
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ssbothwell KJ6RSG
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« Reply #27 on: October 20, 2011, 08:41:25 PM »

WA1GFZ, can i accelerate that process using a blowtorch to gently singe the capacitors?  Grin  Grin  Grin

but seriously. i wasnt thinking when i added those caps but with 10k resistors gate->ground the output waveform looks funky: http://i.imgur.com/QYiyE.png
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WA1GFZ
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« Reply #28 on: October 20, 2011, 08:47:58 PM »

Looks like crossover distortion. one fet turns off before the next one turns on.
I suspect you need to increase the bias voltage a bit. What is the no signal resting current?
You can add some RF feedback by putting a resistor in series with a cap say 100 ohms and .01 uf.
Connect between gate and drain of each FET. This will reduce the gain a bit
1 ohm source resistors will also require slightly higher bias
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ssbothwell KJ6RSG
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« Reply #29 on: October 20, 2011, 09:15:30 PM »

with no signal there is 80pA at the gates and 10pA at the source.

ahh i see. correcting the crossover distortion is a matter of getting the bias pots (R3 and R4) set to just the right level.

also those gate shunt resistors really drop my total power output. i go from ~22W to ~14W. how do i get that 50W you mentioned earlier?
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WA1GFZ
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« Reply #30 on: October 21, 2011, 08:30:45 AM »

Yup you need more bias voltage. Yes the 10K resistors will pull the bias down a tad. They still need to be in the circuit. I'm thinking your drain current should be around 100 ma.
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ssbothwell KJ6RSG
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« Reply #31 on: October 21, 2011, 03:50:37 PM »

so i went back through the thread and tried to add all the modifications you mentioned. here is the circuit: http://i.imgur.com/yiI47.png

 i had originally put 1ohm series resistors on the drain rather then the source (i mixed up p and n channel). i switched that resistor to source and added gate-drain feedback as you mentioned. 

the good news: i get the same gain regardless of the input signal's frequency. before i was getting more gain with lower frequencies.

the bad news: i am only getting ~1.5W output now. Sad
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WA1GFZ
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« Reply #32 on: October 21, 2011, 07:56:54 PM »

I think the feedback resistors  are too low reducing the stage gain. Try changing the 100 ohm resistors to 470 ohms and see what happens. Gate Z is about 100 ohms and feedback 100 ohms  so that is heavy feedback.
Waveform looks good.
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ssbothwell KJ6RSG
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« Reply #33 on: October 21, 2011, 09:26:16 PM »

cool! that seemed to do the trick. i think i am about ready to assemble a prototype. how big of heatsinks do you think i am going to need?

i have some of these guys. will they do the trick: http://www.allelectronics.com/make-a-store/item/HS-6030/COPPER-HEATSINK-FOR-TO-220/1.html

until i can get a larger supply, i am going to test the circuit on a 12V 3A regulated supply.
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WA1GFZ
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« Reply #34 on: October 22, 2011, 09:54:56 AM »

No you will need a lot bigger heat sink. 2 to 3 square inches of surface area for every watt of dissipation.
Enclosed my 300W 3 stage amp (modified MRI amp)
The heat sink has 3 inch fins to dissipate up to 400 W PEP.
This is overkill so I can run class A at 100 watts
The heat sink has 750 square inches of surface area.
I don't think i will need a fan.


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WU2D
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CW is just a narrower version of AM


« Reply #35 on: October 22, 2011, 11:03:06 AM »

You can do a lot with something like this for what you are talking about. These things are all over surplus on the net and at hamfesters. Black anodized aluminum is good. Drill and tap for the TO-220's and use a good insulator and shoulder washer and chickeSH%^.

http://www.ebay.com/itm/Black-Heatsink-6x-1-3-4-x-3-5-/120364128285?pt=LH_DefaultDomain_0&hash=item1c0642d81d#ht_500wt_922
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ssbothwell KJ6RSG
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« Reply #36 on: October 22, 2011, 01:36:41 PM »

oh wow that is a huge heatsink. considering i am only planning on 50W from two IRF510s, would two of these heatsinks be okay: http://www.allelectronics.com/make-a-store/item/HS-167/T0-220-HEATSINK-W/MOUNTING-LEGS/1.html

(i live near allelectronics so its really easy for me to pickup stuff from their storefront.)
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WA1GFZ
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« Reply #37 on: October 22, 2011, 05:20:39 PM »

That still looks small you want about 80 square inches of surface area
the formula you want to use is 130 degrees C / Watt / Square inch

So say you have 80 square inches and the amp will be about 50% efficient so 40 watts dissipation That will be 65 degrees C temperature rise making the heat sink 100 degrees C at 100% duty cycle. SSB is around 30% AM linear would be higher.
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W4AMV
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« Reply #38 on: October 22, 2011, 05:53:21 PM »

Here is one example, 4x5 x2 inch sink and sits atop a FAN. 70W out on 20 M and key down forever. This is a bit extreme but replacing the devices dead bug style is a chore. 


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ssbothwell KJ6RSG
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« Reply #39 on: October 24, 2011, 07:38:28 PM »

this is the biggest heatsink i was able to find: http://i.imgur.com/BYcSC.jpg

its about 3"x2.5"x1" plus the fan. will it dissipate enough heat?
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WA1GFZ
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« Reply #40 on: October 24, 2011, 07:44:04 PM »

that might work with a fan
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ssbothwell KJ6RSG
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« Reply #41 on: October 24, 2011, 08:24:27 PM »

hmm. well in my initial experiments i will be running it off a 12V 3A power supply which will limit total power output to a level that this heatsink can handle. i saw a 28V 15A power supply at the store today that i will eventually be using but i didnt have the cash to buy it today.
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ssbothwell KJ6RSG
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« Reply #42 on: October 26, 2011, 03:14:14 AM »

i started putting the circuit together today. i am taking my time with the assembly to make sure i dont burn anything out when i turn it on. ugly style circuit construction is fast but it sucks when you have to replace a part. Sad

here is a photo of the amp in its current state: http://i.imgur.com/MH3gx.jpg

tomorrow i might have time to finish it.
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ssbothwell KJ6RSG
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« Reply #43 on: October 26, 2011, 09:04:11 PM »

i found another decent sized heatsink today.

http://i.imgur.com/x20e9.jpg
this one is 4.5"x5" with 10 5"x1" fins. i guess that makes the total surface area 72.5"

the smaller one i posted earlier is 2.5"x3" with 26 2.5"x1" fins. oddly enough it also happens to have 72.5" of surface area. however the fins are much more closely spaced.

which one do you guys think i should use?
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WA1GFZ
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« Reply #44 on: October 27, 2011, 11:42:34 AM »

A 5 X 1 inch fin would be 10 square inches since it has two sides.
The larger deck gives you more room to play if you decide to go to 4 FETs
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ssbothwell KJ6RSG
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« Reply #45 on: October 27, 2011, 01:13:13 PM »

oh cool. in that case would these heatsinks work without fans at 50W? i dont understand the formula given earlier for heat dissipation.
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WA1GFZ
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« Reply #46 on: October 27, 2011, 05:50:55 PM »

Basic formula for temperature rise VS area VS power dissipation
Say your area is 1 square inch and dissipation is 1 watt, ther temperature rise in C is 130 degrees C
increase area to 10 square inches and the temperature rise drops to 13 degrees
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ssbothwell KJ6RSG
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« Reply #47 on: October 27, 2011, 07:00:05 PM »

so to restate what you said: 1 watt dissipated into 1 square inch of surface area will raise the temperature 130 degrees celsius? 

is the watts dissipated the difference between the power into the amplifier and the power out?

would a 50% efficient amplifier with 100watts input need a heatsink that can dissipate 50watts?
would such an amplifier would require 100 square inches of heatsink to reduce the temperature down to 65 degrees celsius?

do i understand all of that correctly?

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WA1GFZ
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« Reply #48 on: October 28, 2011, 04:57:25 PM »

Yes.  130 * (Watts / Square Inches)
Fins vertical is the most efficient heat radiator
Fins up second best
Fins down useless without a fan.
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ssbothwell KJ6RSG
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« Reply #49 on: October 28, 2011, 06:06:24 PM »

i've got everything but the bias system setup. should the bias be: "78L05->5kPot->Diode->fet Gate" or should i have an rf choke in there somewhere?

oh and, can i run both bias systems off of one 78L05 or should they have separate regulators?
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