Tech Forum request 10101 Reactive PS concens

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phillipmilks
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Tech Forum request 10101 Reactive PS concens

Post by phillipmilks »

I know that nanny-ism is all over the place - in power tools and appliances. But the use of capacitors for voltage dropping impedance is an area where caution is indeed necessary. I know of a contract controls manufacturer who ventured into this area only to experience flaming results !

As far as value is concerned, the reactance of a capacitor is defined as 1/(2*pi*f*c), where f is in Hz and C is in Farads. Given the desired maximum Zener current, the reactance value can be calculated just as if it were a resistor. Then the value can be calculated as 1/(2*pi*f*R), where are is the desired impedance.

The smoke arises when the power is applied to such a circuit when the AC line is at its peak. The cap has no charge and represents zero impedance. Pow !!

So, here's a safety moment lecture : Use AC-rated safety caps with substantial voltage over-rating; use a flameproof safety resistor in series with a value determine by the surge rating of the rectifiers.
Dean Huster
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Re: Tech Forum request 10101 Reactive PS concens

Post by Dean Huster »

Tektronix used a reactive divider for the power supply primary/battery charging circuitry in their original 200-series handheld scopes, specifically, the 211, 212 and 214 (the 213 and 221 both used teensy switchers), and I've never heard of any problems with them. Of course, in that application (the scopes were double-insulated), the current demand is pretty much constant, so there's few problems. I'd never consider using a reactive divider as part of a bench power supply!
Dean Huster, Electronics Curmudgeon
Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).

R.I.P.
Robert Reed
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Re: Tech Forum request 10101 Reactive PS concens

Post by Robert Reed »

The cap has no charge and represents zero impedance. Pow !!
How does this differ from any other cap connected to the AC line or for that matter in other power source when power is first applied? There are many circuit configurations that see similar startup scenarios which work reliably.
phillipmilks
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Re: Tech Forum request 10101 Reactive PS concens

Post by phillipmilks »

I have a Tek 2xx scope and now I'm curious - how they avoided this problem.

Most of the linear power supplies do have significant surge currents limited only by the transformer. The surge current capability of household circuits is large enough to noticeably dim lights on the same circuit, if is not limited in some way.
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haklesup
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Re: Tech Forum request 10101 Reactive PS concens

Post by haklesup »

I think the point was that if you are going to do this, recognize the hazards and likely failure modes and do select appropriate components. Just any cap won't necessarily do
Robert Reed
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Re: Tech Forum request 10101 Reactive PS concens

Post by Robert Reed »

I may have missed something on this subject, but what I have seen over a span of years in regards to AC vs. DC voltage across a capacitor is this: A cap rated at a certain DC voltage will have a decreasing rating as frequency across it goes higher and higher. For example a cap rated at 600 VDC may suffer dielectric breakdown at 400VAC @ 1 MHz. and 200 AC @ 10 MHz. This decreasing voltage rating at increasing frequency has not been openly stated in the Manu's specs but is available from them if one digs in further. Even capacitive (10:1, 100:1) scope probes will show this degradation in their specs due to their capacitive dividers. If I recall correctly, it has something to do with dielectric heating(Q) and consequent breakdown as we move upwards in frequency. But by comparison - 60 Hz is barely off the DC mark and the dielectric would seem to perform similarly. So, why would there be such a concern over this --- unless we are talking high current passing through the device. The normal range of values we might use in the above AC line situations may be 0.1 to 0.5 mfd of which would provide adequate reactances to hold current levels low(~5 - 25 ma).
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