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Old 2nd March 2010, 11:18 AM   #31
AndrewT is offline AndrewT  Scotland
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unequal current draw from the +ve halfwaves from the -ve halfwaves.
It's how all the SMPS are built.
They halfwave rectify and charge a cap to ~400Vdc. If they all came from the same manufacturer they would all draw from the same halfwave.
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Old 2nd March 2010, 12:18 PM   #32
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Originally Posted by AndrewT View Post
if capacitive reactance+ESR*I is greater than two diode Vdrops, the diodes start to bypass the capacitors during start up and maybe during periods of mild overload.
This is correct, but the soft start reduces the incidence of stress on those capacitors. During the magnetization pulse for a large toroid power transformer the current can be very, very high, and the bridge diode drops may go as high as 1.3V or more each, placing perhaps 2.6V of reverse voltage on one of the electrolytics.

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Old 2nd March 2010, 01:00 PM   #33
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Originally Posted by Bob Cordell View Post
This is correct, but the soft start reduces the incidence of stress on those capacitors. During the magnetization pulse for a large toroid power transformer the current can be very, very high, and the bridge diode drops may go as high as 1.3V or more each, placing perhaps 2.6V of reverse voltage on one of the electrolytics.

Cheers,
Bob
Are devices for 50A continuous flow enough (steady state or stedy state current - I don't know the english term) ?

Here my circuit idea for all cases, where it is possible to insert the mains plug for the wall connection also in the other way. If there are DC >800mV, the right way for plug now is clearly find out. The proposal value of 20mF (20000 uF/100V) must be a computer grade version with lowest possible ESR (e. g. RIFA/EVOX or FTcap). Together with bypass of 100uF MKP for motor run I don't expect problems. For the diodes perhaps high speed versions with more peak current could be provide additional values of realibility.
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File Type: pdf Inrush current lim + DC Filter.ckt.pdf (10.7 KB, 545 views)

Last edited by tiefbassuebertr; 2nd March 2010 at 01:06 PM.
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Old 2nd March 2010, 01:21 PM   #34
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Originally Posted by AndrewT View Post
unequal current draw from the +ve halfwaves from the -ve halfwaves.
It's how all the SMPS are built.
They halfwave rectify and charge a cap to ~400Vdc. If they all came from the same manufacturer they would all draw from the same halfwave.
SMPS all use full wave rectifiers.... They have to, to prevent the situation you are talking about, and to limit peak diode current.

You wont EVER see a SMPS with a half wave rectifier, so you are incorrect about that.

Google image search agrees with me here.
http://www.optimus-power.com/images/Smps.gif

Note the full wave rectifier.

Here is another one:
http://www.smps.us/power-supply.html


Another one:
http://freecircuitdiagram.com/wp-con...ps-circuit.gif

And the actual diagram for a standard computer power supply.
http://www.pavouk.org/hw/en_atxps.html

Last edited by Rainwulf; 2nd March 2010 at 01:28 PM.
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Old 2nd March 2010, 05:04 PM   #35
AndrewT is offline AndrewT  Scotland
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I have thrown away a few old televisions and replaced the internal fuse in a couple of others.
All of them used halfwave rectification.
Slow charge was handled by thermistor in every case.
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Old 2nd March 2010, 11:48 PM   #36
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Quote:
Originally Posted by AndrewT View Post
unequal current draw from the +ve halfwaves from the -ve halfwaves.
It's how all the SMPS are built.
They halfwave rectify and charge a cap to ~400Vdc. If they all came from the same manufacturer they would all draw from the same halfwave.
Andrew,

I certainly don't claim to be an expert on the innards of switching power supplies, but if what you say is true (and I am understanding correctly), I am shocked (aaahhh, poor choice of words). Why on Earth would the front end of a switching power supply not use a full-wave bridge to charge the input capacitor and power the supply?

Cheers,
Bob
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Old 3rd March 2010, 12:24 AM   #37
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I agree. Not finding a bridge rectifier or 4 diodes at the hot end of a mains SMPS is just wrong, and extremely bad design.

The switch on current with a bridge is bad enough, i would imagine half wave would be shocking. You wouldnt use a thermistor as a continuous surge protection, it would burn itself into a crisp.


Every single mains SMPS i have every yanked apart, mainly computer ones, use a bridge, or worst case, four discrete diodes.

Any thermistor you see in a TV smps will be for the deguass coil. There is a class of thermal device designed for the inrush current of a large smps, but its not technically a thermistor, its more a class of MOV.
Any thermister you see in a TV smps is designed to allow full current immediately, then as it warms up, limits current to a very low value, thus, creating a deguassing effect with a large aluminium or copper coil around the picture tube.
Thats the "dong" you hear when you plug in some tvs/monitors.

Finding DC on the mains would be terribly rare. Any inductive device across the mains would instantly remove the DC component, which includes any transformer or inductive motor.

Transformer hum isnt caused by a dc component, its caused by a non sine non symmetrical wave being placed across the mains lines, or a loose wire/lamination. The most common cause of that is very large industrial equipment.


The absolute best way to help remove unwanted signals from the mains is with a power conditioner, which really is just a 1:1 transformer, usually with taps at say 1:1.1, 1:1.05, 1:1, 1:0.95 and 1:0.9 with a automatic switching using triacs to maintain line voltage.
The transformer in power conditioners is usually massive and designed to be resonant at the mains frequency, perfectly idea for remove any possible DC from the line, as well as cleaning up the waveform and helping remove a lot of noise from the line.

As a bonus, a basic 1:1 isolation transformer is also the perfect way to remove earth loops.

Putting any form of non linear device in your mains line, be it bridges or even large electrolytic capacitors is a recipe for actually ADDING noise and non linearity to your mains supply for your amplifer.

In this forum, its a popular opinion to remove caps from paths to help increase the quality of sound, and here in this thread, there is a consensus for adding non linear devices like diodes and caps TO the design in the name of increasing quality of sound.
Bit of a logical hole there somewhere.

You want your toroid/transformer to be fed with the cleanest sine wave possible, with no DC offset, which is a rather rare occurance anyway, especially if you have a fridge or flourescent light, feed it with a nice clean sine wave from a ferro resonant power conditioner.

Last edited by Rainwulf; 3rd March 2010 at 12:37 AM.
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Old 3rd March 2010, 03:12 AM   #38
Eva is offline Eva  Spain
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Quote:
Originally Posted by AndrewT View Post
unequal current draw from the +ve halfwaves from the -ve halfwaves.
It's how all the SMPS are built.
They halfwave rectify and charge a cap to ~400Vdc. If they all came from the same manufacturer they would all draw from the same halfwave.
You are not right, SMPS are built the opposite, for symmetrical current draw. Now I can be considered a commercial SMPS designer, so I suppose I have some authority to say that. I have only seen half wave rectification in ultra low cost switchmode battery chargers or small wall warts, all below 5W.

Conventional SMPS full wave rectify 220V-240V AC to 310-340V DC, or use a voltage doubler on 120V AC to get 340V DC. Newer SMPS use active Power Factor Correction, which simulates a resistive load on mains line through a full bridge rectifier, sometimes with as low as 1% THD, and outputs approx. 400V DC with slow regulation. You probably understood the circuit the wrong way when you looked inside the SMPS appliances you mention.

However, I have seen half wave rectification used to allow half power operation in old appliances like space heaters, hair driers, soldering irons, etc... I own a 1500W space heater from early 1980s that also does 750W through a 1N5407 diode. When I turn it on at half power, toroids start to buzz.

I believe this is no longer allowed by safety regulations.

Note that in most parts of the world mains plugs have no polarity (unlike in UK), including Europe, so when using those troublesome appliances, chances from drawing more current in one direction or in the other are mostly random and the DC offset should ideally tend to be cancelled.


To Rainwulf:

I did many tests some years ago concerning DC offset on mains line and buzzing toroids. Distorted waveforms can make a transformer produce some noise when they have strong high frequency components, but DC on mains line results in far more noise due to transformer saturation, which manifests itself as sharp high current peaks just before voltage crosses zero.
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Last edited by Eva; 3rd March 2010 at 03:19 AM.
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Old 3rd March 2010, 03:31 AM   #39
Eva is offline Eva  Spain
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First image is current (red) and voltage (blue) in a 750VA toroid with the usual DC offset at my place, .2A/div and 100V/div. Little audible noise.

Second image is current and voltage when I turn on the 1500W heater in half power mode, 2A/div and 100V/div. Note the huge current peaks due to saturation. Big audible noise.

Third image is current and voltage when a simple DC filter is added (1000uF and 2 back to back diodes), .02A/div and 100V/div. Transformer becomes completely quiet, even with the heater in half power mode.
Attached Images
File Type: gif osci0.gif (9.6 KB, 1372 views)
File Type: gif osci1.gif (9.6 KB, 1089 views)
File Type: gif osci2.gif (9.9 KB, 1093 views)
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Old 3rd March 2010, 03:40 AM   #40
Eva is offline Eva  Spain
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And these waveforms are from a 3KW active PFC prototype that I did a long time ago. They were obtained while simulating low line conditions, blue trace is input voltage at 50V/div, red trace is input current at approx. 10A/div (working at over 2KW).

Note how well the current waveform tracks the voltage waveform. The slight phase lead in current waveform is because several microfarads are required in parallel with the input to filter EMI properly.
Attached Images
File Type: gif PFC3K_0.gif (28.1 KB, 1072 views)
File Type: jpg PFC3K_1.jpg (81.2 KB, 1083 views)
File Type: jpg PFC3K_3.jpg (75.3 KB, 407 views)
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