Question on TVS diode

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Hi Freaks,

I am trying to use the SM6T24A/CA TVS diode for one of my supplies. The supply voltage is 14V and current is 1A. Will this work to protect my circuit against transients in the supply and ESD spikes? Any other ideas of what else I can use to protect my circuit?

Thanks.

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First up, do you REALLY want a bidirectional one? For DC I'd be choosing a unidirectional one -A option not CA. You also need to consider the power disipation of the device if you hit it with overvoltage - since its a 24V device, can your device accept 24VDC? Also apply over 24Vdc the transil is going to get hot. For example : apply 20VDC -24VDC = 6V @1A = 6W. The devices specs say a temp rise of 120C/W, so the device will attempt to get to 720C. It will most likely fail before this.

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Thanks for the feedback,Kartman.

Using the -A makes sense. The only reason I chose 24V is because the ambient temp. of my board is going to be 125C and I was thinking about derating the spec but I am not sure if that makes sense here.

If my supply is 14V and the circuit can take only 15V, does that mean that I have to use a TVS diode with breakdown < 14V so it can clamp the supply below the rating of my board?

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Yes, for it to do any good.

But, you are going to be caught in a spec "bind" here. It is caused by the circuit being operated too close to the maximum input voltage limit. A 1.5KE15A will break down between 14.3V and 15.8V (at some low current) and the forward drop can be as high as 21V at 71Amps. Depending on your expected faults, I would design the input to accept at least 25V input for use with an 18V TVS.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Thanks,Jim.

So with that logic, assuming approximately linear relationship, for a 14V board rating, I will have to use a 7V TVS?

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It depends on what sort of "faults" are likely. If it is likely to be exposed to lightning, then, maybe yes. But, mostly, that is a bit extreme. If your absolute maximum input is 15V, I would use the next smaller TVS than that. For the 1.5KE devices, that is 12V.

But.... this will now conduct all the time with a 14V input.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Ambient temperature of 125'C sounds challenging.