Inrush current-limiting

Bitrex can't even handle Ohm's law. ...Jim Thompson

The full-wave case was done by a German guy in the '40s. See e.g. any year's ARRL Handbook between then and my latest one (2004).

Cheers

Phil Hobbs

There are numerous variables that affect the current peak so no simple calc is possible.

I have done *actual measurements* with low value electros, a bridge rectifier and an 8amp domestic breaker while monitoring line current with a wide band Hall effect transducer.

Spikes well over 100amps were observed in about 20% of switch-ons. If I had used film caps, the number would be significantly higher.

.... Phil

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I once had to deal with an SCR circuit that drove a solenoid controlling th e pinch roller on a Tandberg tape recorder. The solenoid would pull in and immediately fall back again. Couldn't see a darn thing wrong with the SCR s ystem but it certainly did not pass enough current to hold the solenoid in place.

Then another tech had a look and pronounced that the SCR circuit was meant to oscillate at a high frequency holding the solenoid that way. I pointed o ut there was a diode across the coil that would prevent any oscillation.

The real answer was nothing to do with electronics.

Hidden at the back of the solenoid was a small magnet that was meant to hol d the plunger in so avoiding the need for any DC current. A short pulse of current in the reverse direction released the plunger.

The position of the magnet needed adjusting.

.... Phil

.... Phil

No, voltage is unpredictable, if the supply is weak it'll never reach 90% if the supply is high it'll reach 90% when there's still another 20% to go, measure voltage across the resistor, when that's low it's safe to close the relay.

you want a fuse - perhaps the resistor is one!

that won't work as a limit unless you also disable the full-wave mode of the bridge, else it'll just run as a full wave bridge augmented by that capacitor.

Nah, it's an inductor, it soaks up enegry in the peaks and disburses it when the rectifier output is below the capacitor charge,

=== ___mmm_____ +| | /\ | ~ < > ~ === \/ | -|_______|___

probably saturates during inrush, but after that it smooths the ripple.

I've seen them done on a non-interleaved E-I core with a paper gap.

the pinch roller on a Tandberg tape recorder. The solenoid would pull in an d immediately fall back again. Couldn't see a darn thing wrong with the SCR system but it certainly did not pass enough current to hold the solenoid i n place.

t to oscillate at a high frequency holding the solenoid that way. I pointed out there was a diode across the coil that would prevent any oscillation.

old the plunger in so avoiding the need for any DC current. A short pulse o f current in the reverse direction released the plunger.

Ha! Great story. Thanks.

Cheers, James Arthur

Wouldn't it be wonderful if there were a circuit that had its threshold dependent on a voltage ratio, instead of a fixed reference? :-)

Current through the resistor would work just as well, which... could have interesting implications for how the control circuit is powered, too.

Hmm...

The fuse is a given! :-)

This works fine, in smaller applications. You often see 4.7 ohm 2W resistors in wall adapters, for seemingly no good reason, until you realize they are the flameproof fusible type, and no, there's no fuse (proper) in there.

I wouldn't suggest relying on a power resistor as a fusible link, though: ceramic ones can sit there glowing for ages, before finally wheezing out -- and the metal-case ones blow out like a cannon!

Tim