how do i reduce startup current in AC/DC converter

Thread Starter

Carl Luppino

Joined Mar 27, 2019
3
Hi Everyone,
I have a small problem with a high turn on /startup current caused by an small AC/DC converter.
I am using an Simple converter to convert 24vac to 12vdc, this is being energized by an irrigation controller.
the problem is, that when the circuit is switched on the controller gives an over current error and shuts down.
the converter only draws .3mA and with the load at full only draws 45mA, so I'm assuming the problem is startup current.

is there a simple way to reduce the startup current the controller is sensing?
i was looking at a thermister but unsure if this is a good solution

the controller can handle up to 500mA
the ACDC converter is rated to 1000mA

thanks for any help
 

Picbuster

Joined Dec 2, 2013
990
Input caps start with an extreme low impedance resulting in a high current.(rush-in current).
You need place a component in serial with a high impedance to start with running to a low in N seconds over delta Temp.
A negative thermal resistor.
Things to look at.
a: power dissipation NTC
b: resistance at low impedance and the power accumulated.

Please look at https://www.electronics-tutorials.ws/io/thermistors.html

Picbuster
 

Thread Starter

Carl Luppino

Joined Mar 27, 2019
3
thanks, picbuster
Ill have a read in a little while..
Would i simply place the NTC in series with the AC input to the AC/DC converter?
I was thinking 15 degrees would be a suitable temp change...is that reasonable?
 

ian field

Joined Oct 27, 2012
6,539
thanks, picbuster
Ill have a read in a little while..
Would i simply place the NTC in series with the AC input to the AC/DC converter?
I was thinking 15 degrees would be a suitable temp change...is that reasonable?
NTC inrush thermistors are common for mains voltage stuff, but might be harder to find for 24V systems.

Soft start modifications to SMPSU control circuits - had their moment of fame - but they don't really do much for reservoir cap inrush.

My thinking arrives at adding an active PFC front end, which makes soft start fairly easy. The essential point is that the reservoir capacitor is AFTER the PFC front end instead of before it.
 

Thread Starter

Carl Luppino

Joined Mar 27, 2019
3
thanks Ian,
After many hours of homework and trying to remember my circuit anaylsis lectures from 30 years ago, I decided to cheat and isolate the circuit from the controller and use a simple 24VAC relay, powered by the main power supply and triggered by the controller.
the controller sees the coil and deals with that nicely and the associated problems with inrush current are avoided.
its not as nice as i would have liked but it all neatly fits in the housing so all is good.

I am still very interested in a simple solution to this project.
I started out with a simple bridge rectifier,cap and voltage divider, thinking this would be simple, but ended up making it more complicated that it needed.
thanks for the help.
 

ian field

Joined Oct 27, 2012
6,539
thanks Ian,
After many hours of homework and trying to remember my circuit anaylsis lectures from 30 years ago, I decided to cheat and isolate the circuit from the controller and use a simple 24VAC relay, powered by the main power supply and triggered by the controller.
the controller sees the coil and deals with that nicely and the associated problems with inrush current are avoided.
its not as nice as i would have liked but it all neatly fits in the housing so all is good.

I am still very interested in a simple solution to this project.
I started out with a simple bridge rectifier,cap and voltage divider, thinking this would be simple, but ended up making it more complicated that it needed.
thanks for the help.
NTC inrush thermistors are less popular on audio PAs because transient response takes a hit from a less rigid power source.

A common solution is a low value series resistor to take the edge off inrush, a relay shorts the resistor out after a delay.
 
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