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| Understanding circuit for inrush current limiting |
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| Jwillis:
Inrush current is more damaging to the rectifier and can shorten the life of the filter capacitors.Can also be damaging to the transformer because the coil acts like a dead short until the field is created.Generally it's best to have inrush current limiting on the primary side. In your circuit place your transformer between C104 and the bridge rectifier. This picture will generally outline the operation of your circuit. The limiting resistor may be a ceramic or NTC .Ceramic will work just a well in this circuit .NTC Thermistors are not necessary in this circuit if you don't have any. The charge time of filter capacitors can be slowed by increasing the resistance of limiting resistor .Wattage can be increased with parallel resistors . For example two 20 ohm 10W ceramic resistors in parallel will be 10 Ohm 20W. If you are limited to component supply you can parallel lower wattage resistors to get the power rating you require. |
| spec:
--- Quote from: Jwillis on December 24, 2018, 01:50:36 am ---Inrush current is more damaging to the rectifier and can shorten the life of the filter capacitors.Can also be damaging to the transformer because the coil acts like a dead short until the field is created. --- End quote --- Perhaps you are thinking of a mains transformer's magnetizing current. AFAIK, most DIY and stock transformer/rectifier inputs, as opposed to rectifier/capacitor inputs, do not have any inrush current limiting per se. Don't forget that the input voltage is a sine wave of 50Hz or 60Hz, depending on your local. It does not have an infinitely fast dv/dt. In fact, the waveform is quite slow. The primary of a mains transformer has a significant resistance, not to mention the source resistance and inductance of your mains supply and the unit's fuse, wiring and mains switch resistance. So there can never be a dead short with an implied infinite current flowing, although a torodial transformer's primary resistance can be quite low. If no field is generated, there will be no transformer action, and thus no induced voltage in the secondary so, thus, no current through the rectifier or into the reservoir capacitor. Note that mains transformers, diodes, and reservoir capacitors normally handle huge currents every half cycle of the input sine wave, during the conduction angle. Rectifier diodes especially are designed to handle massive peak currents for this reason. As an example, take a look at the datasheet for the humble 1N500x family of rectifier diodes: only 3 amp average current rating, but able to handle a 200 amp peak: https://www.vishay.com/docs/88516/1n5400.pdf |
| Jwillis:
Your right spec.In most electronics the transformer is quite small,which is to say a low VA .So the current draw will be quite small.The inrush current becomes a problem on large VA and very large VA transformers.A transformer draws inrush current that can exceed saturation current at power up.These current transients can exceed 10 times the rated current of the transformer and 50 times the rating on a toroidal. The inrush current is most damaging in the first microseconds of start up when the field is null to the weakest and the magnitude of the inrush current depends on the point on the AC wave the transformer is switched on. At the AC peak voltage theirs no inrush on start up.Its when the AC voltage is zero on start up you get the highest inrush current that can exceed the maximum saturation current of the transformer.This all starts in the first cycle of the AC wave .1/60 of a second for 60Hz and 1/50 of a second for 50Hz and go several cycles until the transients die away.On large transformers it can take several seconds for the transients to die off and the field balances out. For rectifiers the current load of the bulk capacitors may not exceed the pulse peak for any period of time but can exceed the average power rating of the rectifier for an extended period of time until the bulk capacitors are charged.This is what can shorten the life of rectifiers. Inrush current becomes more of an issue as the power rating of the transformer increases and the size of the bulk capacitors increase.Normally transformers under 300 VA won't pose to much of a problem.Over 300 VA with bulk caps exceeding 20000uF, inrush current limiting may be something to consider.When they get as high as 800 to 1200 VA with capacitors over 50000uF then it gets really exciting. But theirs really no reason not to put current limiting in smaller power systems aside from the potential cost increase. |
| soldar:
--- Quote from: spec on December 23, 2018, 10:08:06 pm --- Computer PSUs do not have a transformer input- they have a rectifier/ capacitor input which has quite different inrush characteristics to the OPs circuit. --- End quote --- I would say most PSUs today are switchers and have no mains transformer. I have several DELL laptops powered by external brick-type PSUs and they have no inrush protection and will blow fuses and trip breakers like crazy. About a year ago, in a hotel in London, it blew the fuse of the plug/adapter and I had to repair the fuse with a strand of wire. All this with no tools to speak of. So now all my bricks are fitted with a cord which is as thin as I can get it. This adds a certain resistance and the heat is dissipated away from the PSU. This is a very good way of limiting inrush current and also even out the spikes of current which go through the diodes in normal use. And a very light cable is a plus when traveling. I also add a NTC in the cable itself. It looks like a tiny wart. So I solve the inrush problem without even touching the PSU itself, only by replacing the main power cable. --- Quote from: spec on December 23, 2018, 10:08:06 pm ---Yes, it is amazing the components you can get from junk, and computer PSUs are a rich source. One of my favorite sources of components are the bases of failed EL ceiling lights: capacitors solid and electrolytic, transistors, inverter transformer. --- End quote --- I have crates and crates of electronic junk which I use as source for simple basic components. A basic line filter and inrush limiter is something I would not even consider designing or buying as I can get it from my junk pile. Heat sinks, diodes, transistors, caps, etc. And when I feel like killing some time and relieving some stress I will spend a while removing components from boards. Sometimes slowly and carefully with a soldering iron. Sometimes I will hold a board over an open flame and remove the components much faster. I have crates with dozens of transformers and have built countless linear power supplies pretty much with recycled junk. I really need to stop collecting junk. :) |
| spec:
--- Quote from: soldar on December 24, 2018, 09:42:03 am ---And when I feel like killing some time and relieving some stress I will spend a while removing components from boards. Sometimes slowly and carefully with a soldering iron. Sometimes I will hold a board over an open flame and remove the components much faster. I have crates with dozens of transformers and have built countless linear power supplies pretty much with recycled junk. I really need to stop collecting junk. :) --- End quote --- ;D Ditto But also you find that some so called junk only has a minor fault, like a corroded solder joint, or even just a blown fuse. I got hold of a load of PAT/ground drop units at one time, which were only scrapped because they no longer complied with some new standard or another. They were in a bulletproof metal case and had a very nice 10A mains transformer. I built a Rolls Royce battery charger from one. They had no surge limiter and they made a great 'boing' sound when you switched them on, but my battery charger was still going strong after 30 years when it was crushed when a building collapsed on it. |
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