Tag
hardware
16 articles.
- Crystal, oscillator or ceramic resonator: which clock?
A crystal is ±120 ppm over ten years, a ceramic resonator ±1.1 %, an RC oscillator ±2 %, and each is right for something. What UART, USB and a radio need.
- Oscilloscope probes: 10:1, and why the ground lead rings
A probe is a circuit, not a wire: 10 MΩ and 11 pF, a 117 nH ground lead, and a bandwidth of its own. An 11 pF probe on a 6-inch clip rings at 140 MHz.
- Pull-up vs pull-down resistors: which one, and what value
Which direction, and what value: top set by leakage and bottom by the driver's sink current, 1.8 kΩ to 727 kΩ in TI's case, which is why 10 kΩ works.
- Dropout voltage: why the datasheet number is not yours
Dropout is a resistance times your load, not a figure: TI's 160 mV at 200 mA is 0.8 Ω, so 100 mA drops 80 mV. Where it comes from, and how to check headroom.
- Switch debounce: the RC, the Schmitt trigger and the numbers
A switch bounces up to 10 ms; logic answers in nanoseconds. Würth's RC and a Schmitt input fix it, but the 1 µF crosses the SN74HC14's threshold mid-bounce.
- Why a relay has a flyback diode, and why it slows release
A flyback diode gives the coil current a path, so the switch sees 12.7 V instead of hundreds. TE's note shows the cost: slow release, and welded contacts.
- MOSFET gate resistor: what value, and what it actually sets
The gate resistor sets one thing, how fast the drain slews during the Miller plateau, and costs one thing in return. TI's design: 10.5 Ω for a 2.3 kV/µs edge.
- Reverse polarity protection: diode, MOSFET or ideal diode?
A Schottky costs 0.4 V and 4.65 W at 10 A; a P-channel MOSFET costs 35 mV but lets holdup capacitors drain backwards; an ideal-diode controller does neither.
- AWG wire gauge and current: what actually sets the limit
Why a wire's current rating belongs to its insulation rather than its copper, what the ampacity charts really measure, and why voltage drop usually binds first.
- Resistor codes: colour bands, SMD digits and EIA-96
How to read every resistor marking, why the E-series values look arbitrary, and which codes are genuinely ambiguous — with the tolerance derived, not looked up.
- How long will the battery last? Estimating it properly
Capacity over average current is the easy part. Here is what the cell's rated drain, its internal resistance and the cutoff voltage do to that answer.
- UART baud rate error: how much you can get away with
Why 2 % per end is the limit, where 8.5 % errors come from, what the odd crystal values buy, and what a fractional baud divider costs you in return.
- LED series resistor: the value, and the current you get
Compute the resistor from headroom, then find why one part number gives a 2:1 current spread on 3.3 V, what an MCU pin really delivers, and when to stop.
- 555 astable design: frequency, duty cycle and error
Size RA, RB and C for a 555 oscillator, find out why the duty cycle cannot reach 50 %, and see which of the datasheet's error terms actually dominates.
- I2C pull-up resistors: the value is a window, not a number
Size an I²C pull-up from both ends: Rp(min) from the 3 mA sink limit, Rp(max) from rise time and bus capacitance — with the worked window at 3.3 V, 200 pF.
- Bringing up a board that does nothing
A first-power-on procedure in the order that finds faults fastest: current before voltage, rails before reset, reset before clock, clock before code.