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 Posted: Mar 13, 2012 - 06:08 AM |
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Joined: May 14, 2010
Posts: 7
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I have a little project that runs off DC (batteries or wall-wart) that ended up with some sensitive LDO regulators, so I could minimize the dropout and power a 3.3v system from a single li-ion cell. The max Vin for these is 6v.
I found a nice schematic for a crowbar circuit with a tunable cut off voltage (here). I burned a ton of brain-cycles puzzling out as much of the nuance as I could, and ordered parts for it, including some 3A slow-blow fuses. I wasn't quite sure if it would blow quick enough, but sparkfun uses PTC's in their tutorials, and those blow pretty slow.
I think I made a mistake, though. I figured on the current rising essentially without limit for the brief time until the fuse or whatever cuts it off. But the failure mode that seems most likely is someone plugging in a 9v or 12v wall wart, and I don't guess the new fangled switched mode supplies will go much beyond their rating on the way to failing. If some bozo plugs in a 200 mA wall wart, the fuse will never blow, although I suppose the wall wart will die, maybe in time to save my circuits, maybe not.
Is this right? I could start over with a "real" overvoltage approach (i.e. doesn't convert it into an overcurrent problem), maybe this, but I'd love to use all those yummy parts I ordered, and a blown fuse is a nice thing to have around when troubleshooting a nonworking gadget.
Thanks! |
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Posted: Mar 22, 2012 - 01:35 AM |
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Joined: Jul 21, 2005
Posts: 1373
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| I would guess than any UL certified wallwart will have over current protection to make it less of a fire hazard. So maybe most wallwarts will not die the death you think it might. |
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Posted: Mar 22, 2012 - 02:43 AM |
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Joined: Apr 20, 2007
Posts: 6084
Location: Long Island New York
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PTC devices do not "blow" their resistance increases as the current passing through them begins to excede their thresholds. They in turn get warm in the case of low current or hot in high current situations. The downside to PTC's is that they will continue to allow current to pass through them into the circuit. That's what keeps them "open" or "blown". THis can be a problem in sensitive circuits as there s voltage and current still being fed into the circuit.
Using your SCR as a crowbar can be done if you add a zener diode in series with the gate and connected to the power input of the device through a resistor. Since the SCR gate current/voltage is so small a 5.x volt zener will cause the scr to turn on and blow the main fuse. THis will guarantee the circuit is getting no power at all. |
_________________
Jim
I have decided that I am no longer going to plan anything in advance. In a court of law this is called Pre-Meditated, and does not look good for the defense.....
Timer function not working properly? Check CLKDIV8 Fuse first 
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Posted: Mar 22, 2012 - 07:36 AM |
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Joined: Dec 30, 2004
Posts: 8789
Location: Melbourne,Australia
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| I tend to use TVS diodes with a polyfuse is series to form a protection circuit. You want to size the diode such that if an overvoltage is supplied, the dissipation of the diode is adequately limited by the polyfuse. In low voltage circuits this isn't too much of a problem. Varistors can have higher ratings but a wider tolerance of the max voltage. Nevertheless, you need some form of current limiting otherwise a gross overload will caose the varistor to explode - literally. |
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Posted: Mar 26, 2012 - 12:35 AM |
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Joined: Mar 18, 2012
Posts: 8
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| What about a SEPIC converter to take "whatever I can get for an input voltage" and converting it to a nice regulated output? |
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