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Author Topic: Eliminating a heatsink or heat  (Read 3464 times)

Offline magnetman12003

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Eliminating a heatsink or heat
« on: June 20, 2016, 04:24:54 AM »
Below is a video of a project I made.  It works as described but I need to eliminate the heat sink and transistor heat up problem.

https://www.youtube.com/watch?v=4UmVLfFNx7U

Is there a very simple way of using a single transistor (and or) a small circuit that can replace the transistor and large resistance?  The power lost through heat loss is terrible.  Surely with the thousand of transistors on the market there must be what's needed to do this job. 

Offline derricka

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Re: Eliminating a heatsink or heat
« Reply #1 on: June 20, 2016, 10:07:56 AM »
Switching from a bipolar transistor to a power MOSFET, should keep your drive circuit much cooler. Something like a IRFZ30 comes to mind, but there are others that will work just as well, or possibly better.  To keep any switching transistor from getting hot, it's important to drive it into saturation. Hard on, hard off, nothing in between.  This mean driving the base current (or MOSFET gate voltage) to near the maximum the device will allow, or none at all. I usually Google the part number followed by "datasheet" to obtain the device specifications. As a rule of thumb, I usually drive at 80% of maximum (as a safety margin), because exceeding maximum, even a little bit, risks permanently destroying the device.  If your signal source doesn't provide enough current (bipolar) or voltage (MOSFET) to saturate the transistor, use an OP amp to boost the drive signal.  As the maximum gate voltage for an IRFZ30 is listed at 20 volts, you will want to drive it with a 12 to 16 volt signal.

Offline magnetman12003

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Re: Eliminating a heatsink or heat
« Reply #2 on: June 21, 2016, 12:33:56 AM »
Switching from a bipolar transistor to a power MOSFET, should keep your drive circuit much cooler. Something like a IRFZ30 comes to mind, but there are others that will work just as well, or possibly better.  To keep any switching transistor from getting hot, it's important to drive it into saturation. Hard on, hard off, nothing in between.  This mean driving the base current (or MOSFET gate voltage) to near the maximum the device will allow, or none at all. I usually Google the part number followed by "datasheet" to obtain the device specifications. As a rule of thumb, I usually drive at 80% of maximum (as a safety margin), because exceeding maximum, even a little bit, risks permanently destroying the device.  If your signal source doesn't provide enough current (bipolar) or voltage (MOSFET) to saturate the transistor, use an OP amp to boost the drive signal.  As the maximum gate voltage for an IRFZ30 is listed at 20 volts, you will want to drive it with a 12 to 16 volt signal.

I understand that the gate acts like the base on a bi polar transistor. How does the drain and source correlate to the emitter and collector on a bi polar transistor?   Plan to use the IRF30Z

Offline TinselKoala

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Re: Eliminating a heatsink or heat
« Reply #3 on: June 21, 2016, 06:43:44 AM »
Are you sure about that part number? I can't find a data sheet for "IRF30Z". If you have one please give a link or attach it.

Your JT wireless transmitter/transformer is nice and interesting. I guess you realize that your spiral coil is essentially a "Tesla Bifilar" with a center-tap. Although it could be better if you stripped the red and black wires out of the outer insulation and carefully wound them in spiral fashion without overlap or twisting.

To answer your question about mosfets: Comparing an N-channel mosfet and a NPN bipolar junction transistor, the Gate of the mosfet is analogous to the Base of the transistor, the Drain corresponds to the Collector and the Source is like the Emitter.

However.... it is important to understand the functional differences between a BJT and a mosfet. The BJT is a _current_ controlled device, and the mosfet is a _voltage_ controlled device. That is, the BJT works by having a small current flowing through the Base-Emitter junction, which then allows a larger current to flow from Collector to Emitter. The mosfet does not have any real current flowing between Gate and Source in that sense. Instead, the Gate acts like a capacitor that must be charged to turn the mosfet "on" and discharged to turn the mosfet "off". The Drain-Source channel is opened and closed depending on the charge (voltage) that is put on the Gate. In most JT circuits you can't just replace the BJT with a Mosfet and expect it to work the same, because of this difference. I haven't tried it with your configuration using the spiral "TBF" coil, so I'm interested to see what you can come up with when you try it.


Offline gyulasun

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Re: Eliminating a heatsink or heat
« Reply #4 on: June 21, 2016, 05:39:32 PM »
The part number is IRFZ30  (magnetman made a typo, in earlier post of derricka IRFZ30 was correctly mentioned)

http://media.digikey.com/pdf/Data%20Sheets/International%20Rectifier%20PDFs/IRFZ30,%20IRFZ32.pdf

Gyula