I added a section to the instructions for the mod this morning that echo Jack's
suggestion to check the dissipation of Q10. Because of the great variability of
specs with the J310, it is a good idea to check it.
Jack's example is not representative of any actual measurements, but rather
just an example of how to calculate the dissipation. In my case, I found a
calculated dissipation of about 125mW... a quite safe value. My original SPICE
simulation predicted 180mW. The device is rated at 350mW, and my goal was to
keep dissipation to half that value.
Larry N8LP
Message: 24
Date: Fri, 22 Aug 2008 09:13:40 -0700 (PDT)
From: Bill W4ZV <[EMAIL PROTECTED]>
Subject: Re: [Elecraft] K3 Buffer Mod
To: elecraft@mailman.qth.net
Message-ID: <[EMAIL PROTECTED]>
Content-Type: text/plain; charset=us-ascii
Lyle Johnson wrote:
>
>> Elecraft is well aware of N8LP's mod, Wayne saw no problems with it at
>> first
>> glance and is now considering whether to make it a production mod. Both
>> N8LP and K8ZOA seem to feel it is not "damaging the radio, etc".
>
> Remember to make the measurements Jack points out on his web page if you
> do the mod. This is to be sure the buffer transistor does not exceed
> its thermal ratings.
>
Yes, here are the details:
http://cliftonlaboratories.com/elecraft_k3_and_panadapters.htm#Alternative_to_increasing_K3_IF_sample_port_output
**********************************************************
"If you make this modification, I suggest measuring the voltage across the
new R9. Q10's drain current is set by a combination of the voltage drop
across R9 and the individual characteristics of the J310 part in your K3.
FETs have a notoriously wide part-to-part spread in parameters and it's
possible that the J310 device in your K3 has parameters sufficiently far
from the mean that Q10's power dissipation limits will be reached or
exceeded. The surface mount J310 has a maximum power dissipation rating of
350 mw, and for reliability a safe operating value is 200 mw or so. Q10's
current can be easily determined by measuring the voltage drop across R9. If
changed to 49.9 ohms, Q10's drain current Id is 1000*Vs/50 (in milliamperes)
where Vs is the voltage measured from ground to Q10's source pin. To
calculate the power dissipated in Q10, measure its drain voltage. The power
is then (Vd-Vs)*Id in milliwatts, where Id is in milliamperes.
An example. After replacing R9 with a 49.9 ohm resistor, the following
voltage readings are found: Vs = 2 volts, Vd = 12 volts. The idle current
through Q10 is thus 2 / 49.9 = 40 mA. The power dissipated in Q10 is (12-2)
* 40 = 400 milliwatts. This exceeds the J310's maximum permissible power
dissipation and would not be a good design practice."
**********************************************************
In my unit the voltage across R9 was 460 mV, which resulted in 0.46/51 = 9
mA and power of 90 mW, well below the maximum rating of 350 mW and below
Jack's recommendation of 200 mW.
73, Bill
_______________________________________________
Elecraft mailing list
Post to: Elecraft@mailman.qth.net
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com
