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Kirt Blattenberger (KB3UON)

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Littelfuse Advertisement
April 1954 Radio & Television News

April 1954 Radio & TV News
April 1954 Radio & Television News Cover - RF Cafe[Table of Contents]

Wax nostalgic about and learn from the history of early electronics. See articles from Radio & Television News, published 1919-1959. All copyrights hereby acknowledged.

Question: If a fuse is rated at 3 amps, at what value of current should it "blow?" Answer: Not at 3 amps. As this Littelfuse company advertisement from a 1954 issue of Radio & Television News magazine informs us, fuse ratings do not indicate the current value at which a fuse opens to protect a circuits, but rather the maximum current level at which it will NOT blow (open circuit). The same goes for circuit breakers. Various types of fuses are available to handle certain types of applications. For instance, a highly capacitive load will according to the well-known equation iC(t) = C * dv/dt could draw a very large amount of initial (inrush) current if there is not enough series resistance to limit it to prevent a standard fuse from blowing. The same goes for a heavy physical inertial load at startup (e.g., an air compressor motor). Highly inductive loads can cause extreme counter-emf currents to be impressed at shutdown. Both of those circuits need what is termed a slow-blow (aka slo-blo) fuse that allows a relatively high level of current to flow for a short period of time, and then settles down to the rated value of protection thereafter. At the opposite end of the spectrum are very sensitive circuits which need to be protected from overcurrents as rapidly as possible, so quick-blow (aka kwik-blo) fuses are used there. Fuses are also rated according to applied voltage, ambient temperature, vibration levels, and other operational parameters. All fuses are not the same.

Littelfuse Ad

Littelfuse Ad, April 1954 Radio & Televsion News - RF CafeThings Are Not As They Seem ... This is a perfect square. It is an optical illusion that the sides bend.

3 amps fuse will not blow at 3 amps.

Fuses are not rated by the current at which they blow. Fuses are rated by the maximum current they should carry indefinitely.

Each type of fuse blows according to the requirements of the equipment it was designed to protect.

Littelfuse has cooperated with NEC, Underwriters, Armed Forces MIL Specs Committees in establishing the characteristics of the various fuse types. Littelfuse holds more design patents on fuses than all other manufacturers combined.

3 AG "Slo-Blo"

3 AB

8 AG U/L

1 AG

4 AG Anti-Vibration


Des Plaines, Illinois



Posted April 15, 2020

Rigol DHO1000 Oscilloscope - RF Cafe

About RF Cafe

Kirt Blattenberger - RF Cafe Webmaster

1996 - 2024


Kirt Blattenberger,


RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling 2 MB. Its primary purpose was to provide me with ready access to commonly needed formulas and reference material while performing my work as an RF system and circuit design engineer. The World Wide Web (Internet) was largely an unknown entity at the time and bandwidth was a scarce commodity. Dial-up modems blazed along at 14.4 kbps while tying up your telephone line, and a nice lady's voice announced "You've Got Mail" when a new message arrived...

Copyright  1996 - 2026

All trademarks, copyrights, patents, and other rights of ownership to images and text used on the RF Cafe website are hereby acknowledged.

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