49 votes

How does the switch on the back of older computer power supplies work to vary between 110 and 220 volt input?

It is a very simple trick: simulate this circuit – Schematic created using CircuitLab If SW1 is open, the 4 diodes work as a standard bridge rectifier. If V1 is 220 VAC you can expect a ...
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  • 2,191
10 votes

What does EE10 or EE15 mean for transformer cores?

EE cores are named with their dimensions. The following is from EE10's datasheet from TDK: As you can see, "10" here indicates the width in millimetres of a half core. Correct Full name ...
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6 votes

What are the limits of transformer turn ratios?

Nothing stops one from using a transformer to generate high voltages, it's done all the time. A couple of common applications: Neon signs Strobe lights Ignition coils Cathode ray tubes in televisions ...
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  • 4,565
5 votes

Help identifying possible faulty transformer

It’s a custom transformer. If it is indeed burnt up, then the simplest if tedious way to fix it would be to measure wire diameter and number of turns for each winding while you unwind. Then rewind it ...
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5 votes
Accepted

What are the differences between these ferrite cores?

EC and ETD are almost identical. ER is slightly different. EC and ETD (ETD means economic transformer design) have both circular centre leg and circular winding window. ER's winding window is not ...
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5 votes

Ethernet transformer

Ethernet is a digital signaling standard. Pulses must be sent and received, over a modest distance (~100m), without consuming excessive power, avoiding random errors, and tolerating ambient noise (...
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  • 3,017
5 votes

What are the limits of transformer turn ratios?

Specifically for pulse operation, it's winding wire length. Roughly speaking, a wire is a transmission line (TL), whether it's wrapped in a shield like coax cable, or wound around itself like a big ...
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  • 3,017
4 votes
Accepted

System blocks of switching power supplies

I am aiming for a 12V/10A (120 Watts) DC switching power supply. OK What I think the system block should look like 1- 220V AC to some DC value this will be mainly a transformer, plus usual filter ...
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  • 384k
4 votes
Accepted

LC meter fails to measure inductance of a transformer primary with the presence of opened secondary

As some pieces of information are not known, I guess only this. You are measuring the inductor at a frequency (10 kHz or more, probably at a "high" resonant frequency) as pointed out in the ...
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  • 4,982
4 votes

What are the limits of transformer turn ratios?

1000:1 turns ratio also implies 1 million to 1 impedance ratio. To drive a 1 Megohm load you need to drive the primary from something competent to drive 1 ohm. Then you find the leakage inductance and ...
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4 votes

Doesn't the law of conservation of energy invalidate Ohm's law?

That's not how it works. If you step up voltage to 48V, then that same 1 ohm resistor draws 48A current at 48V, not 3A. The resistor will get 2.3 kilowatts, and you need 192 amps at 12V to provide it, ...
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  • 89.9k
3 votes

Ethernet transformer

Can someone tell me what is the meaning of Open circuit inductance That's the inductance of a winding when all the other winding(s) are left open (Similarly, the inductance measured when all the ...
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3 votes

Best way to make a system which is both 120 and 220 Volt compatible

It's a bit hard to answer without some idea of the power involved. A two stage 1/3 HP vacuum pump is going to lead to different thinking than a small sampling pump. If it's closer to the latter, I ...
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3 votes
Accepted

Best way to make a system which is both 120 and 220 Volt compatible

The easiest way is to make sure all your components are able to run on low voltage DC power, and then, use a listed switching power supply with a wide input range. This will also make it easier to ...
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3 votes

In a DC power-supply design, is it safe to connect the GND of the secondary side of the transformer to the AC input part?

The mains transformer input MUST NOT be connected to GNDA. The mains transformer current should circulate between live and neutral. The mains side safety ground is just that, for safety. It should not ...
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  • 142k
3 votes

Why is the LTspice transformer failing to step up?

The primary coil is 4uH with 1ohm parasitic resistance Here's the problem. Your input excitation voltage has a frequency of 1 Hz hence, at this frequency, 4 μH has a reactance of 25 μΩ AND, the ...
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  • 384k
2 votes
Accepted

Voltage mode Ethernet PHY and Current mode Ethernet PHY

Can anyone tell me the reason why the center taps of magnetics are shorted in case of connection to a current mode PHY They aren't shorted but connected to Vcc. This is because the PHY driver ports ...
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  • 384k
2 votes

Transformer copper loss

I believe this is from Electric Machiney Fundamentals, 4th Edition. I see what is going on: the short circuit test is done by shorting the secondary coil of the transformer, not by applying a full ...
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2 votes
Accepted

Can transformer secondary side noise couple back to the primary side?

Transformer coupling between primary and secondary windings is mutual and given by a coupling constant (between 0 and 1), which tells you how much of the flux going through one winding will also go ...
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  • 11.1k
2 votes

Can transformer secondary side noise couple back to the primary side?

The leakage flux doesn't magnetically couple the windings. The main flux does. And the noise in either winding will be reflected to the other winding via the main flux. So yes, if the transformer ...
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2 votes
Accepted

Single phase voltage (230V) without using a transformer?

Even if neutral and earth/ground are connected somewhere upstream in the electrical system, under normal conditions, the neutral wire is the conductor intended for carrying return currents and the ...
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  • 89.9k
2 votes
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Selection of voltage transformer and current transformer for protection in substation

We typically size CTs so at bank full load they have less than 5A in secondary (for 5A CTs like used in U.S.). Full load current on 66 kV side of your 10 MVA bank is, $$I_H= 1000\frac{10}{\sqrt{3}\...
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  • 3,300
2 votes

Is this a "gapped" ferrite transformer/choke?

Impossible to tell by inspection. Given some more context (pinout, surrounding circuit), or measurements (windings and pinout, inductance, estimated number of turns), the internal gap can be ...
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  • 3,017
2 votes
Accepted

Help identifying possible faulty transformer

There is nothing to identify. It is a custom transformer for the charger and likely it is not even broken. The charger has a switching mode power supply and this is the main switching transformer. It ...
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  • 89.9k
2 votes

LC meter fails to measure inductance of a transformer primary with the presence of opened secondary

It’s not the best to use a “random multimeter” when doing transformer prototyping. Get a proper LC meter. Doesn’t have to be expensive, just decent enough to do L/C/R/ESR measurements at various ...
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2 votes
Accepted

Phase Shifted Full bridge converter transformer design

Don't concern about the magnetizing current. Those formulas are for inductors (coupled as the case may be for flyback). Transformers for forward converters use high inductance, maybe a thin air gap ...
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  • 3,017
2 votes

Why is a short circuit in the secondary winding of a transformer reflected on the primary side?

First a small back step. When Vp is applied to the primary winding, a primary current starts to flow and this causes a magnetic flux in the core, refer to the 'right hand rule' in physics texts for ...
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  • 515
2 votes

Ethernet transformer

In transformers, we distinguish between excitation current and magnetizing current. The excitation current represents energy passed from primary to secondary. The magnetizing current represents energy ...
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  • 1,611
2 votes

Do I need a resistor for my doorbell wiring?

Stepping up your voltage is a mistake. It won't do what you want, because you misunderstand the problem. When a smart doorbell is wired up with 2 wires, it powers itself (recharges its battery) by ...
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2 votes
Accepted

Common base amplifier - no gain

The voltage gain in CB amplifiers relies on the impedance ratio of Collector to Emitter just like a negative feedback Op-Amp as the common base provides the bias current. This means the emitter is ...
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