#t41-ep Power rating for the 6 parallel 1Ohm sense resisters R300 in PA? #t41-ep


Wayde Nie
 

Hi Everyone,

The quest to source parts continues, and I don't see this specified in any of the provided BOM's or schematics, or list archive search :-)

For R300 in the PA, the 6 parallel x 1Ohm resistors used for current sense, can someone confirm power rating necessary (and probably less critically, the tolerance)? Since they are in series with the current pass transistor, I'm assuming they need to be bigger than "regular" 1/4W's?

Thanks!Wayde VA3NCA


Albert Peter
 

One or two watt units should work just fine.  Max dissipation at full load is less than 3W in the 6 resistors, so 6x 1W will be OK, especially at 50% duty cycle.

 

Al

 

From: Jack Purdum
Sent: Thursday, June 16, 2022 1:27 PM
To: Al Peter
Subject: Fw: [SoftwareControlledHamRadio] #t41-ep Power rating for the 6 parallel 1Ohm sense resisters R300 in PA?

 

??

 

----- Forwarded Message -----

From: Wayde Nie via groups.io <wayde@...>

To: "softwarecontrolledhamradio@groups.io" <softwarecontrolledhamradio@groups.io>

Sent: Thursday, June 16, 2022 at 12:26:43 PM EDT

Subject: [SoftwareControlledHamRadio] #t41-ep Power rating for the 6 parallel 1Ohm sense resisters R300 in PA?

 

Hi Everyone,

The quest to source parts continues, and I don't see this specified in any of the provided BOM's or schematics, or list archive search :-)

For R300 in the PA, the 6 parallel x 1Ohm resistors used for current sense, can someone confirm power rating necessary (and probably less critically, the tolerance)? Since they are in series with the current pass transistor, I'm assuming they need to be bigger than "regular" 1/4W's?

Thanks!Wayde VA3NCA

 


Wayde Nie
 

Thanks Al, appreciated!


Wayde Nie
 

A related question, for when you have a min :-)

R202 and R205 are 240Ohm resistors in the PA schematic. On the BOM they are shown as 1206 SMD, the gerbers have drill holes for them suggestion through hole. I see in figure 14.1 they appear as series pair of 120Ohm,  1W or 2W through hole resistors. Was this done for power dissipation? Should they be high(er) wattage through holes used here as in the photo?

Do  R106 and R107 have similar power requirements, or can they be SMD like in the BOM?

Thanks!W


Jerry Kornbau
 

Just a real quick calculation for R300 comes up to about 250 watts input to reach the .25 watt (actually .276 watts/resistor) resistor rating.

Jerry
aa4jk

On Thu, Jun 16, 2022 at 3:02 PM Wayde Nie via groups.io <wayde=va3n.ca@groups.io> wrote:
A related question, for when you have a min :-)

R202 and R205 are 240Ohm resistors in the PA schematic. On the BOM they are shown as 1206 SMD, the gerbers have drill holes for them suggestion through hole. I see in figure 14.1 they appear as series pair of 120Ohm,  1W or 2W through hole resistors. Was this done for power dissipation? Should they be high(er) wattage through holes used here as in the photo?

Do  R106 and R107 have similar power requirements, or can they be SMD like in the BOM?

Thanks!W


Wayde Nie
 

Hi Jerry,

Thanks for your help, I guess I should have probably made a second thread for my questions about R202 and R205 /(R106,R107) , instead of reusing the R300 thread, sorry for the confusion, but wondering is your answer about R202,R205 or is it about the 6 parallel resistors making up R300?

I'm trying to understand the numbers you provide (are they in relation to R300?) This project is going to be a stretch for my current abilities, but I'm stubborn enough to give it a try :-)

Wayde VA3NCA.


aa4jk@...
 

They were for R300. However my figuring went haywire, trying to do it in my head i did something wrong. The resistors should have a rating of at least 1/2 watt that would be the "get by" rating. If the PA is drawing 4 amps the power for each resistor would be 0.4 watts. I saw something about current draw being about 2.5 amps at 40 watts the dispassion at that setting would be 0.25 watts each. I think the best option is the 1/2 watt unit. Or I found a resistor from Digikey that is 0.2 ohms at 4 watts it however was pc board mount, no leads. I have not checked for a wire lead part as of now, maybe latter.

The R106,R107 and R202,R206 appear to be 1/4 watt 1206 resistors.

Jerry
aa4jk


aa4jk@...
 

For the resistors labeled R106 and R107 1206 style resistors can be used, for R202 and R206 use 1 watt resistors. Use carbon resistors, not metal film or wire wound.

Jerry
aa4jk


Wayde Nie
 

Thanks again!

Is the call for carbon resistors to minimize the inductance they may otherwise contribute? Is carbon composite the same as carbon film? (I'm seeing conflicting answers in my Google searches, and various sellers sometimes use one or the other in their descriptions...)

Wayde VA3NCA


Jerry Kornbau
 

Yes, that is the reason. Carbon film is made by depositing material on a substrate then setting the value by laser trimming. The cut is usually around the substrate which at higher frequencies increases the inductance. I have not worked with them on HF but above VHF they do have an impact.

Jerry
aa4jk

On Sat, Jun 18, 2022 at 7:05 AM Wayde Nie via groups.io <wayde=va3n.ca@groups.io> wrote:
Thanks again!

Is the call for carbon resistors to minimize the inductance they may otherwise contribute? Is carbon composite the same as carbon film? (I'm seeing conflicting answers in my Google searches, and various sellers sometimes use one or the other in their descriptions...)

Wayde VA3NCA