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Part Number: XTR106
The recommended transistor packages are all too big/too tall for my device. Is there a substitute in surface mount package I can use? The smaller the better as I have space constrain. Please also educate me what specifications of external transistor I need to watch out for and why.
In reply to Katlynne Jones:
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In reply to Jason Chang4:
the junction to ambient thermal resistance of your TO-92 transistor is 156K / W, under optimum conditions (see the datasheet). So at short circuit condition the transistor will heat up 156K / W x 0.48W = 75K over the ambient temperatur. With other words, if your ambient temperatur is 40°C, then the transistor temperature will be 115°C. This is too much for reliable operation.
In reply to kai klaas69:
My operation would falls in 24Vdc excitaiton, 4-20mA output, -40°C to 105°C. What recommendation would you give that uses TO-92? Can I use ZTX450? It has Vceo=60Vdc, hfe=300, Pd=1W, Ic=1A. Using your calculation, 105°C+75°C=180°C which is under Operation temperature Tj=200°C
Kai is correct here, although I believe the value is 156°C/W. Remember, the point of the external transistor is to reduce on-chip thermal-induced errors. Allowing the transistor to heat up this much would add error to the XTR106, even if it is under Tj= 200°C max of the transistor. Do you plan to operate the system up to 105°C? Increasing the ambient temperature usually decreases the power rating of the transistor that is listed in the datasheet. As you can see in the previous transistor you proposed, the KSC1008CYTA, the power dissipation value needs to be derated 6.4mW/°C, which would put you at only 288mW.
I don't believe any transistor in a TO-92 package would be viable for your application. They are going to get too hot, and will not support the power dissipation requirements needed for the temperature range you require.
If you take a look at the SOT-89 package, the transistor will heat up much less, especially with the proper heat sink.
I understand now. Can I use the recommended 2N4922, but not assemble it all the way to the board, leaving some space in the Z direction to have some components under the casing. Here is an illustration of what I have in mind.
Your picture did not come through. Can you post again using the Insert Media button.
I was able to find this application note from the manufacturer. I think the information on pages 16 and 17 is relevant to you. It looks like their power semiconductors are ok with no heatsinking if the average power dissipation is around a watt.
This leads me to think it's ok if the part is not soldered flat to the board. However I'm not sure they would recommend bending the part at an angle as you propose (I still haven't seen your image but I believe I understand what you mean). I cannot comment on the mechanics of the package itself, but it looks like it would function electrically without a heatsink. I'm going to point you to the article where they talk about support of the package as well as bending the leads, and you can decide if you would like to mount the part in the way you describe. AN1040-D.PDF
The TO-225 package is only 3mm thick, and the pads of the transistors will take up some of that space. I'm not sure if there is room to slide parts under the transistor package like your drawing depicts.
If you want a confirmation on this set up you will have to contact the manufacturer. I don't have any more details, other than the application note I found, that would allow me to confirm if this set up is ok.
Please let us know if you have any other questions about the XTR106.
I have an additional question on zero offset. I have a closed bridge sensor and the offset is 1%, but I would like to adjust the offset to be .5%. How do I do that specifically? I looked at bridge balance, but not quite sure how I can accompliash that. Also, the unit is already temperature compensated; will this zero adjustment affact my temperature compensation accuracy?
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