Week #14: (12/4-8) Triacs, Data Sheet Specs, Practical Considerations

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rjagodowski
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Week #14: (12/4-8) Triacs, Data Sheet Specs, Practical Considerations

Post by rjagodowski »

We'll begin this week finishing up the discussion of Triacs presented back in Week #13.

Before we discuss Data Sheet specs, here's a quick note about the differences between classical linear power supplies and switched mode power supplies (SMPS) aka switching power supplies. Difference between linear & switching power supplies. Note the use of the term "pulse width modulation". It is a concept used in many aspects of power control.

Then we will revisit data sheets about devices we've discussed over the semester.

Some of the specs we will be looking at will deal with temperature & power related parameters.

Here's a tutorial from Analog Devices on Thermal Design Basics. This applies mostly to ICs, but the thermal concepts are similar for all electronic devices.

This link presents How to Select a Heat Sink. The thermal analysis for proper heat sink selection is rather complex and usually reserved for engineering-level analysis. Therefore, we won't really attempt to work through the process. However, by looking at the various charts & graphs, you should be able to get a sense for some of the considerations designers must use when specifying a system to remove heat, including convection, ducting, heatsink, thermal compounds and liquid cooling systems. For a technician, the moral of the story is that any heat dissipating equipment removed as part of a service must be replaced. Thermal compounds should be re-applied with fresh compound of the proper type.

1N4000 Series Rectifier Diodes from Vishay Semiconductor.

Using some typical 1N400X values, if there is 0.75 volts across a forward biased 1N400X diode with an average current of 1.0 Amps, the diode will be dissipating 0.75 Watts. Using the 50 ohms thermal resistance from Junction to Ambient, gives (50 *C/W) x (0.75W) = 37.5 *C temperature difference. That means if the ambient temperature is 25 *C, the junction temperature will be about 62.5 *C (25 *C + 37.5 *C). (In *F, ambient is about 77 *F and the junction temp would be about 144.5 *F) From the spec sheet, the maximum junction temperature is spec'd at 150*C, so this device should be able to handle this current continuously. These are all approximate calculations, but it does give a designer a rough idea as to whether or not the device will be subjected to excessive junction temps.

1n47xx Series Zener Diodes from Vishay Semiconductor.

Another example: Look up a 1N4739 to find it's zener voltage rating. How much power will this device dissipate with 100 mA through it at it's zener voltage? (910 mW). From the data sheet, how much above ambient temperature will the junction be with 100 mA flowing through the diode? (100.1 *C). If the ambient temperature is 40 *C, what is the approximate junction temp with this current? (140.1*C) Can this zener diode survive with this current flowing through it at 40 *C ambient temp? (Most likely, max. junction temp is spec'd at 175 *C, and it should remain below that. But it IS getting close.)

2N3904 NPN Transistor Data Sheet We'll look at a variety of specs, but one graph to note is Fig. 15, which shows how the (normalized) DC Current gain varies with temperature.
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