Multimeter Input Stage

Input Impedance Voltage Measurement Most multimeters will have an input impedance of 10Mohm. This means it will effectively apply a resistance across what you measure of 10Mohm, potentially affecting the measurement if the source has a high impedance Current Measurement Will depend on the meter, but it might be say 1ohm for the mA range […]

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Collector-Emitter Sustaining Voltage

Transistors are often specified with two collector-emitter max voltages: (Vces) collector-emitter voltage (Vbe=0V) (Vceo) collector-emitter voltage (Ib=0A) Vceo(sus) is the Collector-Emitter Sustaining Voltage and its confusing. It’s basically the the collector-emitter breakdown voltage with the base open circuit, which is the most vulnerable state for a transistor. This diagram from Electrical Ratings and Characteristics of […]

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Mosfet Design Checklist

The basic key checks for using a Mosfet in a circuit design: Drain-source voltage Absolute max rating is well above the max voltage being switched.Allow for maximum voltage peeks and pulses that may be present. Gate-source voltage The voltage you will apply to the gate is within this absolute max rating. Drain current (continuous and […]

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AC vs DC

AC RMS AC is typically quoted as the AC RMS Voltage. E.g. UK mains voltage of 230V AC is 230V RMS. Peek AC Voltage Calculating the peek voltage of an AC RMS voltage: VAC * sqrt (2) Example: 230 V AC RMS * sqrt (2) = 325 V peek Example: 1500 V AC RMS * […]

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Screened Cables

Braided Screen Good strength and flex life (needed in applications with repeated cable flexing) Does work well at all frequency types but performs well and is recommended at low frequency to medium frequencies. Disadvantages Braided shield does not guarantee 100% coverage, typical 70% to 95% coverage (depends on how tight the branded formation is woven) […]

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Flex PCB’s

General design tips Designing a flex PCB is no different to designing a regular PCB in terms of how you go about designing it. Corners Avoid using right angle corners, use curves, to reduce strain on the copper that could lead to it cracking Planes Avoid using solid copper regions, use cross hatch instead. Components […]

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PCB Manufacture Options

Pads finish Standard tinned finish can be a bit uneven. Not an issue for lots of components, but can cause problems for some more specialist packages Gold plated finish option is really good when dealing with BGA, LGA, etc packages, where you really want a perfectly flat high quality finish to ensure the BGA package […]

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Mosfet diode replacement

Mosfet providing polarity protection without diode voltage drop For a P channel mosfet, whenever the Gate voltage is lower than the (Source Voltage – Gate Threshold voltage) the MOSFET conducts. If the gate voltage is higher than this, it does not conduct. The greater the voltage difference from the Source the more the MOSFET can […]

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Supressing back EMF

A simple way to think about inductive effects is to remember that the current in an inductor acts similar to the inertia of a mass, it wants to keep flowing, even after the voltage (force) across it it removed. It will keep flowing until all the inductive energy (1/2 LIĀ²) is dissipated, which can generate […]

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Propagation delay and PCB layout

Basic questions When do you need to start caring about matched net lengths etc? 100Mz to 200Mz is a good handwavy guide (but not necessarily!) How much propagation delay is added by PCB traces? 1nS of propagation delay is added to a signal for every 150mm / 6″ of PCB trace. Dielectric constant Varies between […]

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