EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: DCEnginerding on December 04, 2018, 08:45:20 pm
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Hey all, longtime lurker of the forums, but this is my first post so go easy on me! :D
I'm working on the AC input side design of a high capacitance power supply and would rather not incur the dissipation losses from a high power inrush limiting resistor. Is there some downside to using NTC's for inrush, followed by PTC's for current limiting? From their temperature/resistance charts there is no high resistance overlap, at the crossing point mid way both would be running around 1ohm max.
I've been searching without any luck for reference schematics (or literature... perhaps my Google foo is lacking), all I could come up with was this article which seems to indicate they work well together:
https://passive-components.eu/combined-current-control-by-ntc-and-ptc-thermistors/
There will of course be other protection circuitry, but I would be interested to hear if using both would need any other special considerations. Does anyone here have any experience using both in this manner, or know of any problems that may arise?
Thanks!
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If you use a resistor, then you would normally shunt it with a relay (etc) after soft-starting. The total energy dissipated in the resistor won't be much, even if the peak power is high momentarily. I have seen designs which add a PTC in series with the resistor so that if the relay fails to engage, the resistor won't become a problem (PTC will heat up, limiting current in both devices but preventing operation of the device as well). I have also seen designs using a NTC but shunt it after start up to prevent the wasted power wasted in the NTC, which can be significant.
I think using both NTC and PTC would work, each providing specific unique benefits. Keep in mind that the NTC needs to run hot after startup and the PTC needs to run cold, so don't place them close together!
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I think that it has an appeal to it. But how about fast power cycling? how well does the thermal time constant mesh with the electrical one associated with power cycling?
https://passive-components.eu/combined-current-control-by-ntc-and-ptc-thermistors/
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An NTC will not protect against surge currents after a fast power cycle, unless you bypass the NTC after start up so that it remains cold during normal operation.