| LoL Muntzing. Is this about my post about me figuring out that | i can use a pic to drive a FET? | I hope i didn't offend anybody with my self discovery, | i am just learning and i was ecstatic when i made a boost | power supply with out even looking at a single schematic.
Also, sometimes different people, have different meanings, for the same word. For me, Muntzing is a bad thing. Basically engineering by hit-n-miss method, and not knowing how the damn thing works. Its okay to simplify a design, if you know what you're doing. "That", I don't consider Muntzing. As an example, take the anode driver circuit again. It usually has two transistors: (1) the PNP on the high side (MPSA92), and a (2) NPN on the low side (MPSA42). The low side NPN, is driven from the uC. The resistor in "series" from the uC IO bit thru the resistor, to the NPN's base (on the other end) is needed. But sometimes I see a resistor from base to emitter on the NPN, just like the on the high- side PNP. This one is not needed, and can be removed. Why ? The output port of a uC (at least AVRs and PICs, but not 8051s) are push-pull. That is they have two transistors on the output, that either "source" the output to a logic-1 (hi, Vdd, ... +5V), or "sink" the output to a logic-0 (low, Vss, ... 0V). Any charge on the BE junction has a low impedance (close to zero ohms plus that series resistor) either high or low (depending which way output state is). You don't have this push- pull path, on the high side, hence the need for the resistor there. In short, high side=yes on BE resistor, low side=no on similar resistor. Another example: pull-up resistors on CMOS. Not needed. Also, CMOS parts draw the least power, if the high and low signals, at their inputs, are either all the way up at Vdd, or all the way down at Vss. If you know how the circuit works, then you know what's needed or not. Sometimes we take a calculated risk. Using a simpler circuit, that does the job, but might be either, less durable (failure=smoke), or less reliable (failure=behaves funny, sometimes). That is, you use one circuit, if you're designing a weapons system, that might stay idle for 20-years, then needs to function perfectly, on a moments notice. Or, a consumer item, that will be trashed in 5-years, so there's no need to design it to last forever. In that case, use lower rated parts (less margin), and/or leave out protection circuits. This, too, is not Muntzing, if we design for a certain market. Oh, on FET driving directly from you're uC. Its a definite maybe. There are FETs out there, that are called "logic-level" power FETs. Most FETs won't realize their lowest ON resistance, until they see close to 10V at the gate (relative to source). A logic level FET is designed to be at (or very close) to that low Ron, with only 5V at the gate. So shop around for such a FET. Nick mentioned that capacitance from drain to gate, which may (or may not) transfer a HV pulse to the gate. The gate (to source) voltage is rated at 20V max for almost all FETs. Usually when the HV pulse on the drain is generated, the gate is yanked down to zero V by a pull-down (or "sinking") transistor on your FET driver circuit, so that coupled pulse (to the gate) current is also sunk down to GND (0 Volts). But, what if you turned switched that driver back to high, while that pulse is climbing. Or even worse, what if that output goes Hi-z (tristate, because of an unexpected reset). Then that small capacitance WILL apply a HV pulse to the uC output bit. From experience, this usually means a very dead uC. A reset on most uCs, causes all the IO bits to reset as inputs (Hi-Z). This is why, its usually recommended to stick a FET driver in between the uC and the FET. There are many dedicated chips out there that do this function, or it can be made from discrete components. But, if its a quick-n-dirty toy, you only intend to use on Halloween, then its okay to drive it directly, with no intermediate driver. Beer will probably short it out anyways. Damn waste of beer ! Get me a Dos Equis Dark, or a Guinness Stout. After I return from the mens room, I'll give you a Bud Light. Its yellow and has a head anyways. -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. To post to this group, send an email to [email protected]. To unsubscribe from this group, send email to [email protected]. For more options, visit this group at http://groups.google.com/group/neonixie-l?hl=en-GB.
