EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: pspeirs on August 22, 2013, 08:47:26 am
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hi All,
I'm trying to get a crowbar circuit working that will provide protection when using a plug pack. I'm using a 12V 1.4A plugpack and want to protect against a situation where I have a breakdown of insulation in the transformer and I end up with 240VAC on the circuit.
I tried a simulation which seemed to work fine, however in real life it doesn't seem to be happening.
Basically, we come in through a 2A fuse. A zener is in series with a 10k resistor between + and gnd. From the junction of the zener and resistor I go to the trigger of a triac which is again between + and gnd.
Hope someone can steer me in the right direction here.
Cheers,
Paul
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You might try something like the attached circuit. The cap is to filter out transients. I didn't model that part so it might need to be tweaked. It won't need to be nearly as big as what you used in your schematic.
You didn't say so I assumed the input voltage is AC.
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I'm trying to get a crowbar circuit working that will provide protection when using a plug pack. I'm using a 12V 1.4A plugpack and want to protect against a situation where I have a breakdown of insulation in the transformer and I end up with 240VAC on the circuit.
If you're using a half decent quality plug pack, that's not a very likely scenario. On an off-line transformer you have three layers of insulation to get through, the enamel coating on the primary, on the secondary and a tape barrier between the two. On a properly made transformer the only real way this can happen is through catastrophic over-heating (i.e. smoke etc.) Any decently made plug-pack will have protection against conditions that could cause this, in the case of the old iron core transformer they typically have a thermal fuse and for SMPS's there'll be current limiting etc.
Assuming you still want to protect against the scenario of insulation breakdown, I don't see that simply crow-baring the output is going to be effective. Your typical off-line plug-pack is generally fully isolated from input to output, meaning if you have a breakdown in insulation at one point in the transformer, then you could end up with a high potential relative to earth not across your output terminals. In this, which I see as the most likely case, the output could still be within normal range yet be sitting on top of 240VAC relative to earth. But this is where having a RCD (GFI in US) will hopefully save you.
My 2c is just buy quality plug-packs and have a working RCD (GFI).
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Agreed, the scenario would be rare, but for what its worth for a couple of components, worth the effort blowing fuses up on the breadboard. That kept me busy for 30 minutes or so. Still have some thinking to do with the whole idea, but as you've said any half decent plugpack will have protection built in.
Cheers,
Paul