PC power soft turnon

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If some of the computers in my home
are switched on with the power-switch
sometimes the fuse (10A,230V) trips, probably
due to inrush current.

I plan to built a soft turnon switch.
First idea is to use a bridge rectifier
together with a MOSFET or IGBT and limit the
current by measuring it via a current transformer.

Are there better / simpler / similar methods ?
(Ok, there is the old NTC method).
What ideas come to your mind ?

Additional info is gien below!

Last Edited: Mon. Jan 3, 2011 - 07:35 PM
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ossi wrote:
What ideas come to your mind ?

I would start talking to an electrician about the wiring in your house, the existing circuit breakers (esp. if they are old, some types wear out), and what it takes to update the system.

If that isn't feasible, the old Motorola app note AN1542 has some good treatment.

PS: Do I need to tell you that m*llw**d is talking shit, again?

Stealing Proteus doesn't make you an engineer.

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I've opened some PC PSU's that got in my hands. Very crappy-lousy chinese ones that failed in overtemperature (fan's rotors stuck) and found that the PSU pcb's had complete protection (which included an NTC thermistor) and EMI/RFI attenuation sections placed on the PCB but unpopulated/jumped away.

I don't know if that guys do this for countries like mine, in which nobody cares about inter-equipment interference, but, just in case, at least the design of that cheapy PSU's included at least some form of "startup protection" altough not enabled.

Maybe your PSU was for our country and ended up in the "Europe shipping bay" and hence in you hands!

Nachus

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Second to ArnoldB: tripping 10A fuse is suspicious. Is the fuse You're talking about a real circuit breaker? What type and specs, if yes?
Argument for this is that PC PSU have to be fitted with a fuse with lower trip current.
Well if PC does not mean CRAY.

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With a switch mode supply, the input current is HIGH when the input voltage is low. These are, after all, "constant power" converters in which the input POWER is (nominally) the same as the output power. Unlike linear regulators with input CURRENT (nominally) same as output current.

Thus, when the turn-on current is high as it tries to charge all the caps, if you try to soft-start it, the input current will be even higher. So, that is NOT what you want to do.

I would check that the AC input voltage stays at the level it should be while the supply is starting. If it is too low because of small wiring gauge or due to poor electrical connections, it can cause exactly the problem you report. So, even though the result is a "popped" circuit breaker in your unit, the cause might really be poor wiring in the line power circuit leading up to your computer.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Ok, I did some measurements. I used a standard probe for voltage, A TEC current-probe for curren and a
TEK TDS3032 as storage scope.

Steady state input current of PC supply is a rather
distorted sine, but it's clear that th PC supply has
a power factor correction. I could not measure excessive input currents during turnon.
So the PC is probably not the problem.

So I measured the input current of the ne big
flatscreen. Steady state input current shows
the typical current only flowing in the voltage
maxima. About 1 Amp peak current and only
.25 A RMS current.

If turnon hits the voltage maximum the input current
exceeds 25 Amps (see pictures). Thats probably too much. But a simple zero voltage switching could cure
that problem.

Attachment(s): 

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The simplest solution is to replace the fuse with a slow trip fuse that used for older air condition power supply.These air conditions had not a slow startup circuit for the motor and for this reason they caused high currents at startup.

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I'm still curious about where exactly this 10A fuse is, and is it a fast or slow blow type.

Tom Pappano
Tulsa, Oklahoma