Protection of 1uF capacitor against short circuit

Shorting a charged electrolytic cap won't damage the cap. I used to build huge banks of caps, charge them, and short them, and make impressive explosions.

A few-turn coil of heavy wire would magnetize anything.

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John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  
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Reply to
John Larkin
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Ripple current?

Reply to
krw

There are electrolytic caps rated for shorted outputs (so-called photoflash capacitors), but that rating isn't seen on smaller capacitors. Since the resistive nature of the capacitor isn't known, the possibility of local heating (and physical damage) can be limited by putting a few ohms in series with the capacitor. 5 ohms isn't going to impede normal operation, nor would a small inductor (5 ohms at 1 kHz would come to 27uH).

The modern affinity for low-ESR in capacitors is useless in this application.

Reply to
whit3rd

The application here, though, is an (ASR33?) teletype mechanism; those operated at 110 baud, or 120 Hz. The typical drive was intended to be a high impedance (current source), and the receiver was a short.

Reply to
whit3rd

I don't get why the cap is needed anyway. Seems to me the inductance of the load makes the cap pretty pointless. If the boost circuit were turned on and off, the voltage would ramp up pretty quickly since the load inductance will prevent an initial high current.

Protection from over current could be done by using the opto you don't need anymore to sense the current. Parallel the LED with a 10 or 12 ohm resistor and if the current reaches 100 mA or so the output of the opto is used to shut down the boost circuit. No large electrolytic caps to worry about. A 0.1 uF ceramic could be added to smooth the 100 kHz ripple to reduce EMI. Otherwise I don't see the need for a large output cap. What am I missing?

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Rick C
Reply to
rickman

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