24

That "transformer" is a common mode choke. It's used to suppress EMI (either being induced onto the line and affecting the circuit or being transmitted from the circuit out over the line). It's called "common mode" because it's very effective in suppressing HF currents that are common to both lines.


21

Yes, it's for a sort of clearance. Depending on the voltage and the environment (eg. dust, humidity) for safety (and safety agency approvals) you may need clearance across surfaces that is in the 8mm or even greater range. That's inconvenient in a 2-dimensional design like a PCB (on a barrier strip or switch, you can have a 3D barrier between terminals ...


21

It will not unduly compromise the galvanic isolation or safety because it is is an XY rated safety capacitor. It's also low capacitance (less than 1 nF) so the impedance at mains frequency is rather high and the resulting current will be low, even if L and N were accidentally reversed and a path was present from -DC OUT to earth. Although the working ...


14

Kapton tape is a widely available industry standard product for this scenario. You can view the specs here - from table 3 you can see that it has a dielectric strength of over 100kV/mm, plenty strong enough for your purposes. It comes in different thicknesses but any standard 'off the shelf' kapton tape will suffice. I'd recommend it over electrical tape as ...


14

Why are there so few bench osilloscopes with dedicated isolated channels? Is it a design issue, a cost issue, a business issue or is it just better with differential probes? It is a cost (price) and demand issue. Most users don't need isolated inputs. Most users have ground-referenced signals so a scope with non-isolated inputs is OK. Cost is an issue as ...


12

Galvanic isolation means that no (significant) current can flow between two parts of the circuit (here, the input side and the output side). It doesn't mean the output voltage of a circuit has to be lower than the input voltage. Galvanic isolation is very important in mains power supplies, because a failure to galvanically isolate the two sides may put the ...


11

The magnetic ones (and some other non-optical types) transfer only changes to the state. Although that page you linked says that they work to DC, that's only because there is a latch on the other side that maintains the last known state. Consider the NVE IL710 In some cases, this could be a deal killer. Secondly, another disadvantage is cost. You'll never ...


11

If you want to call it PoE, you need to stick to IEEE 802.3af-2003 or IEEE 802.3at-2009. These standards call for isolation. This answer could end here: no, you cannot build a PoE device without isolation. This is also assuming that the data lines are already isolated, so the only potentials without isolation is the power lines. You also will have a ...


11

That looks like a good safe project to me. You might consider checking the voltage between the 9.9 V terminals and earth. You will probably find some random voltage perhaps even 115 volts. Connect a 115 volt light incandescent bulb between the 9.9 V terminal and earth. With that connection, the voltage across the bulb should drop to nearly zero. That should ...


10

You can if you only need functional isolation. If safety isolation is required you need a transformer designed for that. Since you are only powering half the primary you can only put in half of the primary current so you only get half the VA rating of the transformer, so compared to a real isolating transformer you end up with twice as much iron and an ...


10

The behaviour is described by Paschen's Law. Electrons are accelerated in the electric field between two electrodes. If there are gas molecules in the path of the electron it will collide with them with a certain probability and it transfers some of the energy to the molecule. The longer the statistical distance between two collisions is, the longer the ...


10

From your description, it sounds like you've made a safe isolated source of 9 VAC. To verify that the output is isolated from the AC Mains, you could measure (with the thing unplugged!) the resistance between the mains terminals and your output terminals - there should be no continuity (inifinite resistance) between the mains pins and the 9 VAC jacks.


9

If the insulation on one fails, the remaining one will cop 4,500 VDC which will definitely make it fail. There's also no guarantee the voltage will spread evenly across them - the insulation on the devices might be measured in MΩ, but if one device is 1 MΩ and the other is 3 MΩ the voltages across each will be 1,125V and 3,375V and you are in for a bad day. ...


9

I don't agree that you need to be an expert in medical device design to accomplish this task, but you do need to do some research. Get a copy of IEC60601 and study it. There is a lot more to this type of project than just isolating the patient from the computer. You're going to need to treat your USB connection as if it were mains. Find a transformer that ...


9

Sure they have transformers: - The transformer looks different to a regular AC type but it's still the largest single component on the PCB but a whole lot smaller than it would be for 50/60 Hz operation. The transformer is the big yellow taped thing in front of the heat sink at the back/left. SMPSs also use opto-isolators for feeding back a measure of the ...


9

That is what's known as a common-mode choke. It presents a high impedance to any common-mode currents which might flow in the + and - pairs. Any common-mode currents which flow in the attached ethernet cable will have a strong tendency to radiate at levels in excess of legal EMC limits. An ideal common-mode choke will present no impedance to differential ...


9

Looks like a common mode voltage problem to me. Although differential signalling, it still needs a common GND. (source) Replace your 1nf 500v capacitor with a jumper. Or wire signal ground to all nodes. Shield still stays connected to earth. The shield is connected to the Chassis ground on the various devices in the loop - and not at the 'master' end - ...


9

Is there any reason this wouldn't work the way I think it would? You cannot expect that circuit to work because the guitar signal ground connection has to connect into the op-amp circuit ground node and without that connection you are just going to get noise. The impact of this is that you are not therefore isolating the two receiver circuits as you ...


8

By not connecting earth mains to neutral, you're allowing the wall's line voltage to "float". I.e., although the hot and neutral line will always be 120VAC with respect to each other, there's nothing stopping them from rising above ground potential. The stick figure on the right will get a nasty shock if the line is floating far above earth ground--which is ...


8

The vast majority of people only get in contact with electricity via an appliance, tool or other device. If you connect the neutral to the earth and earth all the devices a circuit breaker will likely trip if the earth resistance is low enough. If you'd isolate the neutral you would not know unless you touch the neutral or the neutral makes contact with the ...


8

Here is how I have dealt with AC line detection in an efficient manner using an isolated opto-coupler. Key attributes of the circuit: The selected opto-coupler is one with a low forward current requirement The circuit works on either 120 or 240VAC systems The KSP44 transistor is high voltage type The transistor function is to cut off drive to the opto-...


8

There are two keys to making such a concentrically wound design work safely at mains voltage: For most insulating materials used for the interwinding film, mylar included, DTI (distance through insulation) does not determine how much insulation is needed, and a single layer of mylar tape suffices as a result. Creepage (i.e. distance along the insulation ...


8

The spikes seem to have a few common heights that are roughly related by factors of 2, which strongly suggests that they are noise-induced single-bit errors in the binary data. One good way to address this is to take the standard deviation of all the data, and then simply throw away any samples that are more than, say, 2σ from the mean. Another ...


8

No, there is no good reason to use an expensive and large optocoupler if you're sharing ground. The optocoupler's prime advantage is isolation. The common use of this appears to be cargo cult adoption from cheap expansion kits for development boards. You can protect the pin with cheaper discrete components, or multiple "discrete components" ...


7

Clearance is about the distance between live and null or about the distance between two isolated circuits. Not so much about the pins on a triac'like device as electrical safety is not an issue in this case. The worst that could happen is that your load stays powered, but fuses will not blow. If you are concerned about the distance between the pins, you can ...


7

The STP12PF06 datasheet (first google hit for 'to-220p to-220fp') shows that the f version has a metal tab, and the fp version has only plastic on the outside (at least at the front, but I assume also on the back). This makes it easier to mount the package galvanically isolated, but the plastic layer seems to add some thermal resistance: the rthj-case of ...


7

Making a non-inverting optocoupler from a phototransistor is easy, and you can do it in several ways. First, just do as Spero Pefhany suggested, and drive the input as an active-low device. Second, drive the input as you consider normal, but use a pull-down rather than a pull-up output: simulate this circuit – Schematic created using CircuitLab In ...


7

The whole separation of grounds thing is a common source of misunderstanding and cargo cult engineering. Except for devices that are specifically designed to provide isolation (IE, digital isolators, isolating amplifiers), all of the ground pins on an IC, regardless of what they're labeled, are really the same ground. They are labeled differently because ...


7

Looks like too much circuitry, which leads to more cost, complexity, failures. There is nothing in the question that indicates anything more than series resistors are required. Adding components, like isolated switching power supplies, adds components with much higher failure rates than a few resistors and diodes. The circuit below is well protected, ...


7

The MOSFET turns on when a voltage is applied between its Gate and Source. In your circuit there is no path from the negative side of V1 to the MOSFET Source, so it won't turn on. All that will happen is the entire circuit (M1/Rload/V2) going up and down in time with V1. You don't need an actual ground, but you do need to get M1's Gate and Source to the ...


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