Electronics > Beginners
Understanding circuit for inrush current limiting
T3sl4co1l:
The zener and diode aren't necessary for a 12V supply, but they are helpful for a switched source. It's an appnote with other apparent errors, don't read into it too deeply. :-//
Tim
soldar:
Yeah, it seems like they extracted that from somewhere and did not adapt it correctly. I would ignore it as there are simpler solutions.
For 300 Watt probably a NTC would suffice. Most computer PSU are of higher wattage and just use an NTC.
If you want to go the relay way you can just power the relay from the filter caps. Suppose the input is 230Vac. Once rectified you will have in the order of 300Vdc. You could use a relay for 200~300 Vdc, maybe with an appropriate resistor in series. The relay will be open until the caps voltage reaches a certain point and will then close.
A relay solution is more intellectually "better" but a relay can give you problems much more probably than a NTC.
In any case you need a good filter to block noise and transients in both directions.
If cost and space are not at a premium you can look into fitting a filter with some serious inductance which would, not only seriously filter the regular current but also the inrush. I have occasionally used the primary or secondary of a transformer for this purpose.
AngraMelo:
thank you guys for helping.
What I would like to know is, if I use a NTC or the resistor/relay on the primary side of the transformer will I effectively be limiting the inrush current for the transformer as well as the capacitor bank?
It is way easier to find a NTC (and relay) to the primary side currents then the secondary.
soldar:
--- Quote from: AngraMelo on December 22, 2018, 01:50:03 am --- It is way easier to find a NTC (and relay) to the primary side currents then the secondary.
--- End quote ---
I am confused by your post. What do you mean?
spec:
--- Quote from: AngraMelo on December 22, 2018, 01:50:03 am ---thank you guys for helping.
What I would like to know is, if I use a NTC or the resistor/relay on the primary side of the transformer will I effectively be limiting the inrush current for the transformer as well as the capacitor bank?
It is way easier to find a NTC (and relay) to the primary side currents then the secondary.
--- End quote ---
Hi AngraMelo,
Yes, in most cases the NTC thermistor is placed on the mains side of the PSU transformer and it limits the power to all circuit elements after the NTC thermistor.
Bear in mind that the NTC thermistor will run very hot and should be mounted off the PCB and with a length of self-lead so that the NTC thermistor heat does not crystallize the solder joint or discolor the PCB. You can get stand-off ceramic beads for this purpose. Sometimes high melting point solder is used for soldering thermistors and other power devices to a PCB.
Also bear in mind that the NTC thermistor, even when at a high operating temperature, will add resistance to the primary winding which will contribute to the overall resistance/impedance of the secondary winding.
There are a whole range of NTC thermistors available and there will be one to suit your application just fine. But unless you are using a torodial transformer, in-rush current control may not be required.
https://www.digikey.co.uk/products/en/sensors-transducers/temperature-sensors-ntc-thermistors/508?FV=fff803f7,fff80044
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