in-rush limiter

[halfa32]

Does anyone here use a in-rush limiter on there PS. The ones I have seen are being used on the Ajax CNC cap kit. Any help wpuld be great.
Thanks,
Halfa32

[sendkeys]

hehe i have no idea what ajax is talking about because on there cap kit i see nothing to say it has any type of limiter.

looks like ac cap., bridge and a resistor used for bleed down.

[unterhaus]

Originally Posted by sendkeys hehe i have no idea what ajax is talking about because on there cap kit i see nothing to say it has any type of limiter. looks like ac cap., bridge and a resistor used for bleed down.

The black thing I think you are saying looks like a cap is no doubt a thermistor. That's how they limit inrush current on your computer because having directly line connected capacitors, inrush protection is critical. There are positive and negative types of thermistors. For inrush, you want the kind that reduces resistance with temperature.

[sendkeys]

ahh i see.When ever i think im getting better at electronics i always get to show my ignorance haha I should have knowed because i have used thermistor befor but they were very small but i never used them in this way. This will work out nice for future projects:P thanks unterhaus for the heads up

http://www.mouser.com/catalog/620/374.pdf

you know after you start adding up the parts.
4 for the bridge,
2 for the thermistor
10+- board, wire clamps, resistor

(10-15 hour or so of time to build it?)

not really to bad of a deal for 30 bucks for people who dont have parts laying around. :P

[unterhaus]

Originally Posted by sendkeys i have used thermistor befor but they were very small but i never used them in this way. .... ...snip by unterhaus... .... not really to bad of a deal for 30 bucks for people who dont have parts laying around. :P

There's something to be said for a nice neat board. It would definitely be difficult to buy a pcb and the parts for that price.

It's hard to remember that thermistors are used as sensors and for inrush. At work, the thermometers are little electronic things with thermistors in them. At Christmas, they turn the heat down to 55F by computer. One of the guys wanted to work on his thesis, but they wouldn't turn up the heat just for him -- they only do that for lab animals or if you can prove your experiment needs higher temperatures. We were going to modify the thermistor circuit so we could adjust the resistance, but he came up with an easier plan and hung a bag of snow on the thermostat. The modification would have been useful though, they are really bad about setting the temperature, and there is a very limited range that you can set the temp yourself.

[halfa32]

Thanks for the info. Does anyone have a easy way figure the size of thermistor needed for a cnc type PS. Also is it wired in series or par?
Thanks again,
Halfa32

[Mat-C]

If you mean capacitor charging inrush (not sure you do), then a thermister solution might not be the ideal way forward (too much resistance all the time, too slow to react, etc).
I'd think a current limiter is more the way forward. (Easy enough to make with a mosfet and a bit of analog control circuitry, doesn't need to be 0.001%).

[bunalmis]

Add series resistor to PS AC line and remove (bypass) by relay after 1sec.

[Mat-C]

> Add series resistor to PS AC line and remove (bypass) by relay after 1sec.

It's a good and simple idea, the problem is keeping everything in the correct state when dealing with power on/off transients (eg machine powered up for half a second, then turned quickly off and on). This may or may not be important to your application.

[ViperTX]

Why do you need an in-rush current limiter?

[GrahamIT]

Originally Posted by ViperTX Why do you need an in-rush current limiter?

At initial turn on of a high current toroidal power supply, the magnetic flux within the coils has not yet stabalised and the large discharged smoothing caps need to be charged. They charge so quickly, that to the as yet unstable transformer, they will appear as a virtual short circuit. Drawing at least twice if not more than three times the normal run current (for a very short duration), hence the need for a slow blow (anti-surge) type fuse in the supply line.

The inrush current limiter allows the caps to charge at a slower more relaxed rate, giving the transformers flux time to stabilise and reducing the high initial current draw to a less fuse blowing, light dimming level and reducing the stress on your power supply componants. Should help them last a little longer. Some good circuits for this can be found here.

http://sound.westhost.com/project39.htm