# Tag Info

33

The power supply you've found in this device is of a type known as a capacitive dropper. (More information in the Wikipedia article "Capacitive power supply".) The primary reason why you don't see this type of power supply often is simple: it is unsafe. This is because one leg of the AC power supply must, by necessity, be connected directly to the circuit. ...

27

Ground means whatever is attached to this symbol in the schematic: Everything that touches this symbol in the schematic is actually connected to everything else that touches the symbol. Since so many things connect to it, this makes the schematic easier to read. Usually the negative side of a battery is attached to that. But, there are many circuits that ...

25

Capacitive power supply circuits have been used for decades in things like alarm clocks, small appliances (coffee makers) and the like. They work pretty well for small constant loads without the bulk of a heavy transformer. The example circuit uses for C1 a capacitor labeled 225K, which refers to 2.2µF with a tolerance of 10% (see capacitor notation). ...

25

Modern AC-DC power supplies do the voltage conversion in three steps. Roughly speaking, the process is as follows. First, they rectify the AC into DC, so 100 V AC gets into about 140 V DC, and 240 V AC results in about 340 V DC. This is a first step. This is the range of voltages that the second stage of converter is dealing with. And this voltage has ...

24

Will a 500mA fuse after the 12-to-5V linear voltage regulator make the design safe against electric shock since the buck converter is non-isolated? No. A fuse does not protect against electric shock. Fuses are for limiting current in a fault condition to protect equipment, wiring and prevent fire. What you are proposing is potentially lethal. You must ...

24

As the law $P = U * I$ , to acheive the same power at lower voltage, you need to increase the current. In resistive components, like wires, pcb traces, transformer wire (green), losses increase to the square of the current, as $P(loss) = R * I ^ 2$. In switching components and other diodes/rectifiers, (Green) the losses equal to \\$ P(loss) = V(...

20

Can a 50 Hz, 220 VAC transformer work on 40 Hz, 180VAC? Yes it probably can - the initial worry is saturation problems due to operating at the lower frequency but, with the voltage dropping to 180 V at 40 Hz, this produces virtually the same magnetization current as 220 V at 50 Hz. Strictly speaking, if the transformer is nominally rated for 220 V at 50 Hz,...

19

As Oskar Skog proposed, the power factor corrector (PFC) is the main suspect. The PFC is usually a precisely controlled boost converter that converts the pulsating rectified mains to something like 350-400 V. A boost converter's efficiency depends on the difference between the input and the output voltage - the more the input, the less it has to ...

17

...can I use that (polyfilm (myler) capacitor) instead of an X2? No. There's a reason for that and that reason is safety. Especially in a (mains connected) power supply you want to take safety seriously. If you don't you risk harm from smoke/fire/explosion etc. You should read this article on all about circuits why using an X-class capacitor is needed. ...

15

Well, you're not really minimizing power dissipation here with the pre-regulator. Your pre-regulator is a very simple linear regulator that is designed to provide a lower input voltage to your LM317 regulator. The Zener diode and R3 will provide a relatively stable 14 V. Transistor Q1 is in an emitter-follower configuration, so you get a diode drop down ...

14

The answer is "yes" - but it is dangerous because there is no isolation and component failure can lead to the full voltage and current from the mains appearing at the output. Here is a description You need to adjust values to get your 5VDC. If you do not know how to do that, stay away from it.

13

The main difference is that the volume of production for board-mounted AC-DC converters is very low, so you pay a boutique price. Decent quality fully approved (safety and EMC) wall-wart adapters are made in huge volume so the costs are relatively low. When was the last time you saw an AC-DC converter module in a consumer product? Usually it's either a ...

13

Can't believe I wrote all that crap about diodes... MUR860 will indeed sound better, but the explanation is a bit subtle: Silicon diodes do not turn off instantly. As the voltage across the diode goes negative, current still flows in the reverse direction for a short time, until the charges stored inside the diode are cleared out. When this is done, the ...

12

A Bit of History The suggestions behind this topic go against what a lot of electrical engineers have been taught since their first circuits course - that AC is better for power transmission. After all, in the "war of currents" in the late 1800's, it was Tesla who helped Westinghouse fight for AC, eventually defeating Edison's dreams of a DC empire. The ...

12

That is a capacitive dropper supply as others have said, but I am going to take a slightly different view of the things safety..... Iff it is built right into the torch such that no part of the torch or charging circuit is accessible without the use of a tool (So, Battery, LEDS, switch, whatever else) is all sealed inside a plastic box with a suitable mains ...

10

You have your multimeter leads plugged into the amps socket. It will act like a short circuit. If you are trying to measure voltage then plug the red lead into the red socket marked "VmA" (in the middle).

9

Symbology These symbols are used in the US (at least). Appliance classes http://en.wikipedia.org/wiki/Appliance_classes Isolation diagrams Isolation diagrams are block diagrams, which show the isolation features, such as: transformers, opto-couplers, mains connections, safety capacitors. Example of a well-detailed isolation diagram Example of a ...

9

Let's start with all your options, so you can see why you really, really want a transformer. You want to convert a high voltage to a low voltage. You can do that with a linear power supply, or a switching power supply. A linear supply at the voltages you're talking about would be horribly inefficient. 220 VAC rectified gives ~310 VDC. 7.2/310 = 2.3% ...

9

is your friend... (Well maybe that's debateable.. but 10S search found me this.) e-Bay

8

There are three issues at work here: the voltage ripple on the output, peak currents through the rectifier, and the reverse-recovery losses of the diodes. Within the context you specified (household) the latter two are unlikely to matter, but I'll include them anyway for completeness. First, the ripple voltage. I=C dV/dt. If you know the current load (I), ...

8

You need to compare apples to apples. The board mount converter you've listed has several key specifications (just to name a few): 2% load regulation 0.5% max line regulation 2% initial accuracy 0.2%/C tempco 0-70 degrees C operating conditions 100mV ripple/noise OVP, overload protection, and short circuit protection ~80% efficiency That wall wart ...

8

Yes the yellow thing is a transformer, since it is used at a much higher frequency than 50 or 60 Hz mains, it can be much smaller. Isolation is indeed very important. A user must be able to safely touch the secondary (5V) side without feeling a thing. The optocoupler senses the output voltage and feeds back a signal that tells the input (mains voltage) ...

8

If you want to identify one 'component' that's responsible for the constant output voltage, then it's the 'feedback'. The forward path which includes the flyback transformer pushes a controllable amount of power to the output. The voltage on the output is measured, and the feedback requests a smaller or larger amount of power moment by moment, to keep the ...

7

EN61000-3-2 is a European standard which dictates PFC requirements. Most power supply manufacturers design in PFC so that there aren't any problems marketing the product worldwide. PFC is also helpful if you want to operate with "universal" AC input (85-264VAC) as the down converter will see a constant input voltage (usually 400VDC) regardless of the input ...

7

You do NOT want to try to power the drill with circuity that is attached direct to the AC powerline without the isolation that a transformer provides. Such method of powering the tool will likely be a serious safety hazard as the tool design is sure to not be designed with the proper double insulated characteristics that a proper mains attached tool would ...

7

This will work. Wall warts typically have a floating ground so that you can use them for purposes like this.

7

Resistors have voltage ratings, as well as power ratings, and in SMT parts these can be surprisingly low. Part of the reason for series chains is UL requirements, part is just seeking a high enough voltage rating. Network 3 is a rather over complicated snubber to limit primary side voltage excursion as the mosfet turns off (Transformer leakage inductance ...

7

Most countries require that devices not conduct any significant amount of current between either of the mains supply leads and any exposed metal surface, even when a significant potential difference (e.g. 1000 volts) is applied between the supply leads and that surface. There are three ways devices can meet this requirement: Don't have any connection ...

7

You'll find hundreds of internal, print-mount, closed frame, open frame, ... power supplies on distributors like digikey. So, as usual, a single property, in this case supply size, is seldom the only thing you look at when choosing a device. The one you found is not a complete supply. Please at least open the datasheet from the website and look at the ...

7

If you study closely the application note, where I suppose you found this schematic, it says clearly that this circuit outputs 12V on J2.1 (pin 1 of J2). See also Figure 1 on its first page. So, you have the solution already there. No modifications are necessary. This means the U1 generates directly 12V at its output. The application note, last paragraph on ...

Only top voted, non community-wiki answers of a minimum length are eligible