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Old 12th December 2008, 03:24 AM   #31
lineup is offline lineup  Sweden
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If they are only rated 350 mWatt (Fairchild SOT-23)
then I would maybe run them at like 200 mW, max.

The Fairchild TO-92 are rated 625 mW, and 60% would give like 360 mWatt, max.
For 12 Volt across them, this would give an absolute max. for me, of 25-30 mA.

As you say Grey, there are several factors to decide how much current to run.
Maybe the most important is IDSS of your J310.

Here is the datasheet PDF for Fairchild J310 and J309.
http://www.fairchildsemi.com/ds/J3/J310.pdf
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File Type: png fairchild_j310.png (23.0 KB, 631 views)
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Old 12th December 2008, 04:02 AM   #32
lineup is offline lineup  Sweden
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Well, there is something wrong with that 625 mW rating for J310 in Farichild datasheet.
Onsemi & Philips have these figures: 350mW & 400mW.
So Grey is right, run them no harder than 150-200mW.

Max 12-16 mA with like 12 Volt across Drain-Source.
Gives:
12mA x 12V = 144mW
16mA x 12V = 192mW
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Old 12th December 2008, 04:04 AM   #33
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Interesting...the Siliconix (which are the parts I'm using) datasheet shows 350mW for the TO-92 and SOT-23; 500mW for the TO-52. I'm content with 250 on the TO-92, but that's as far as I care to push it.

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Old 12th December 2008, 04:06 AM   #34
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lineup,
I don't know that the Fairchild datasheet is wrong. It's not unusual for TO-92 devices to be rated as high as 625mW. Nonetheless, I'll stick with the Siliconix numbers, since they made the parts I'm using.

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Old 12th December 2008, 04:17 AM   #35
lineup is offline lineup  Sweden
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Well, 625 seems to be wrong
if we use the Fairchild thermal resistance value for junction-ambient (ja): 357 C/W
125 C / 357 = 0.350 !!!

So for a normal no heatsink mounting, the max is 350mW
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Old 12th December 2008, 04:58 AM   #36
lineup is offline lineup  Sweden
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See attachment.

If using the default 50 VDC supply and JFETs so are working at 12V drain-source,
then this simulation renders I should use >= 100 Ohm source resistors
for max 200 mWatt operation.

This is for the Philips IDSS= 50 mA spice models.

So, the advice to use 51.1 Ohm or more is correct. I guess for most J310.
Each one that will build SSTART may do his own measurments.
It is not very difficult math
--

Actually I could use 82 ohm resistor, because there is a voltage drop across this 82 Ohm resistor of like 1.5V.
So the JFET has 12-1.5= 10.5V drain-source.
18.4mA x 10.5V = 193.2 mWatt
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File Type: png j310_initial_spice-test.png (8.7 KB, 669 views)
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Old 14th April 2009, 09:41 AM   #37
Ipanema is offline Ipanema  Malaysia
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Hi Grey,

Is STAART strong enough to drive a 32ohms heaphone?

Thanks.
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Old 14th April 2009, 10:41 AM   #38
AndrewT is offline AndrewT  Scotland
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I wonder if the confusion with power dissipation numbers is that Fairchild have not specified Ta=25degC or Tc=25degC.

These two standards do give different dissipation.
We normally expect to see the 625mW with Ta=25degC, but I suspect that Fairchild have omitted to tell us that the 625mW applies when Tc=25degC.
The dissipation for Ta=25degC will be 350mW and then all the numbers quoted on the datasheet are correct.

If this is the case then Fairchild need to adopt the standard method for max dissipation, they must quote Ta or Tc as appropriate.
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Old 31st May 2009, 10:24 PM   #39
lineup is offline lineup  Sweden
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Quote:
Originally posted by Ipanema
Hi Grey,

Is STAART strong enough to drive a 32ohms heaphone?

Thanks.
Yes, and most probably even 16 Ohms.
32 Ohms or higher is an EASIER, Lighter load..

High quality Sennheiser are so easy to drive
because they are even almost 10 times higher impedance = 300 Ohms
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