Kevin White
  • Member for 7 years, 9 months
  • Last seen this week
  • Los Gatos, CA
Why do fire and security systems run at 27.6 V?
Accepted answer
83 votes

27.6V is 2 x 13.8 V. 13.8 V is a common voltage for a 12-V Sealed lead-acid battery (SLA) while under float charge. In float charging the AC power supply maintains a constant voltage across the ...

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What does a "fork" symbol mean on a schematic?
41 votes

Normally it would mean chassis ground. But it would probably mean the substrate in the case of an IC schematic. The substrate is usually connected to the ground pin or the most negative voltage (...

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What set the speed in a cassette tape recorder?
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41 votes

The hubs do not rotate at constant speed in normal tape recorders - there is what is called a capstan that is kept in contact with the tape by a rubber pressure wheel. The capstan rotates at ...

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Do power plants supply their own electricity?
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30 votes

When a power station is up and running the power it generates can be used to power its internal operation. However, many power stations do not have the ability to bring themselves back into service ...

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Power supply - purpose of the capacitor on the side of the transformer before full bridge rectifier
28 votes

The capacitor is to reduce RF interference. When a diode is conducting it will create charge carriers to carry the current - this is known as "charge storage". When the voltage reverses as part of ...

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Why is it dangerous use a coiled extension cord
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28 votes

The normal cable ratings assume that the wire can adequately disperse heat generated in the cable due to the current flowing. If you coil it up and use close to the maximum rating then it stands a ...

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Why did this device go bang?
27 votes

The bottom of C2 is not ground. It is connected to the live input by the diodes in the bridge rectifier on every AC cycle. During the negative half-cycle of the incoming AC waveform, the Neutral wire ...

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Why are the transformer windings in series with line and load in this circuit?
22 votes

That's not used as a transformer, it is referred to as a "common-mode choke". It and the capacitors connected to it are there to minimize the amount of interference that conducts from the device into ...

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Help understanding this LED lighting transistor circuit
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22 votes

This circuit is designed to provide a constant current to the LED independent of the supply voltage. The MOSFET is turned on by the voltage at the collector of Q1. As soon as the current through R1 (...

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Identifying Source of Periodic Artifact at Op-Amp Output
21 votes

Note that this is an autozero amplifier (also called chopper stabilized) - many very low offset opamps work by periodically sampling the input offset and injecting a compensating offset to counter ...

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Why is there a diode on the RX pin of the Adafruit Ultimate GPS board?
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20 votes

The pull-up resistor to 3.3V and the diode means that even if you feed the circuit with 5V logic (most Arduino boards use 5V logic) the GPS chipset will see a maximum of 3.3v. The MT3339 device may be ...

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What is this bridge like component?
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17 votes

It's just a test point for easily attaching a scope probe ground. The silk screen indicates it is connected to ground, there may be some for other signals although they are usually just small round ...

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How do professional designs compensate for offset voltage in opamps?
16 votes

Depending upon your application there may be low cost opamps that have negligible offset voltages. A common approach in modern CMOS opamps is to use an arrangement called chopper-stabilization. This ...

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Struggling to understand zero cross detecting circuit
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16 votes

D1-D4 form a bridge rectifier that will charge C1 up to about 1/20th of the peak voltage of the incoming AC power. (R1 + R2 are 440K, R3 is 22k 440/22 = 1/20). During this time that C1 is charging ...

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Rail-to-rail op-amp only reaches 90% of VCC, works sometimes, not everytime
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16 votes

You are exceeding the amplifiers input common-mode range. It has rail-to-rail output but not rail-to-rail input. The allowable input range is -0.3V to +3.8V when fed from a 5V supply. When configured ...

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Why does this small lithium ion battery have a different voltage than larger lithium ion cells?
14 votes

The voltage of any cell depends upon the particular materials used for the electrodes. The cell voltage of 2.4V indicates it is probably a lithium-titanate-oxide (LTO) cell. Although these cells have ...

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Can MOSFET exceed breakdown voltage without being permanently damaged?
14 votes

Many MOSFETs are characterized to allow overvoltage and the subsequent breakdown of drain to source provided the energy limits in avalanche are not exceeded. Provided you don't exceed these limits the ...

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Lead Acid AGM battery testing - Why am I testing it this way?
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14 votes

You could do the entire discharge at the lower rate but then it would take a lot more time. When discharging at a high rate the voltage may collapse prematurely so the battery seems discharged before ...

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What is the purpose of this NMOS transistor on an I/O pin?
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13 votes

If this inside an IC it is for ESD protection and referred to as a snap diode or ggNMOS device. It is intended to breakdown (without suffering damage itself) before the gate oxide of the IC when it is ...

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What is the purpose of the IR-diode Cover on Remote Controls?
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13 votes

The cover is made of plastic that is opaque to visible light but transparent to infra-red. It probably attenuates the infra-red a small amount but not enough to affect operation. The cover is ...

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Do you have to use an instrumentation amplifier to measure voltage across a 0.01 ohm shunt?
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12 votes

You don't have to use a differential amplifier providing you are careful about the voltage drops that occur along the grounds. If you return the bottom end of the 500 ohm resistor to the same point as ...

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What is the purpose of this extra capacitor in a notch filter?
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12 votes

You can't determine the effect of C4 without taking into account the output impedance of the previous stage. C4 is directly across the input terminals - assuming your analysis used zero output ...

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What information exactly does an instruction cache store?
12 votes

The Instruction cache stores the most recently used instructions and their addresses so that if an instruction needs to be repeated it doesn't have to be retrieved from main memory - this is much ...

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Constant current source drop
11 votes

Your load requires 15V to maintain 10mA (10mA x 1.5k). Below about 16V there is no voltage available across Q2 for it to control the current so the current has to drop. As @mkeith points out the ...

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What is the polarity of this barrel plug with a double circle?
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11 votes

The outer cylinder is negative with the inner surface being positive. The tip connects to a Maxim One-Wire memory in the power supply that is read by the computer to obtain information such as power ...

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Why does 10k ohm pull down resistor not work for pushbutton with NOT logic gate?
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11 votes

TTL logic and its descendants all source current into the inputs when low. In the case of LSTTL it will source about 0.4mA - this will cause a voltage to be developed across the 10k resistor such ...

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Why the extra transistors in this current mirror?
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11 votes

Q3 and Q4 are not "Diode connected". They are arranged as emitter followers to reduce the error that would result from the base current for Q1 and Q2 being derived from the current reference flowing ...

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P-Channel vs. N-Channel in Buck Converter
11 votes

The carriers in an N-channel FET have about 3 times the mobility of the carriers in a P-channel MOSFET. As a result for a given RdsOn an N-channel FET uses less silicon and is cheaper. However it is ...

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Does the 1x oscilloscope probe setting slow down MHz digital signals?
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10 votes

Not only does the probe have lower bandwidth when in 1:1 mode but there is much greater capacitance load on the source when in 1:1 mode. That extra capacitance will significantly lengthen the rise and ...

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White LED W/ 3V or less?
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10 votes

White LEDs are actually blue LEDs with a phosphor that converts some of the blue light into yellow light that mixes with the blue to form white. The voltage of an LED is related to the energy of the ...

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