dc suppression using TVS diodes?

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goinl
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dc suppression using TVS diodes?

Post by goinl »

I know TVS diodes are intended for transient suppression but can they also be used for dc suppression where Vdc = Vpeak?
"If it aint make sense, then it aint."
rshayes
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Re: dc suppression using TVS diodes?

Post by rshayes »

A transient suppressor will also try to clip DC levels above its threshold value. The average power capability of these devices is limited. Normally transients are so short (microseconds) that they do not heat the device significantly. Continuous operation is limited by the device dissipation, and this may be quite low.
goinl
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Re: dc suppression using TVS diodes?

Post by goinl »

I am working with the SA5V0CA TVS diode: http://www.fairchildsemi.com/ds/SA/SA5V0CA.pdf
I would like to be able to use this were a 400V dc voltage is possible. Refer to the schematic at this web site: http://eet.etec.wwu.edu/goinl/project/s ... meter2.pdf
where TVS1 is across a 10Mohm load. Max voltage: 400V<p>No, screw that, go to http://eet.etec.wwu.edu/goinl/project/s ... caling.pdf instead (simplified version).<p>For example, on a .4V range it is possible to have as high as 400V at V2. Can the TVS handle 400Vdc?<p>[ May 05, 2005: Message edited by: goinl ]</p>
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rshayes
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Re: dc suppression using TVS diodes?

Post by rshayes »

If you apply 400 volts to that transient supressor, it will act almost as if it were a short circuit. What happens depends on the impedance of the 400 volt source. If it is low, the transient suppressor will draw high current until it overheats.<p>You need something to limit the current to the transient suppressor. This could be a simple series resistor. It has to be a high enough value to limit its own dissipation to a safe value, and must be capable of withstanding 400 volts (Possibly 200K at 1 watt). This will require that the impedance of the voltmeter is either much higher than this or that this resistance is incorporated into the circuit.
cato
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Re: dc suppression using TVS diodes?

Post by cato »

Also, even if you have just 100 V unclamped across TVS, that means you have 100V on the input to R5. Assuming a virtual ground at the inverting input, you are asking for 100ma to flow through R5. The way the virtual ground works is that the current really flows through R6. But that's only gonna happen if the OpAmp can swing the output to - 500V. Which, since you don't have a negative power supply and even if you did it would be -5V not -500, ain't gonna happen. So, what you will end up with is about 100 V on the inverting input and a magic smoke release just about everywhere.<p>Be Careful
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Chris Smith
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Re: dc suppression using TVS diodes?

Post by Chris Smith »

A MOV might be better, even for Dc. <p>It doesn’t react until the stated value, and then it clamps large transient peaks into the mega joule range to the other lead, and to ground. <p>But still a balance over all must be met as in the total wattage, amperage, voltage and heat build up and dissipation for any device. <p>There are plenty of suppressors that are voltage sensitive way up into the thousand volt range including the Zener.
goinl
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Re: dc suppression using TVS diodes?

Post by goinl »

How about this, Stephen: http://eet.etec.wwu.edu/goinl/project/s ... aling2.pdf<p>(switches: 400V, 5A relays)<p>[ May 06, 2005: Message edited by: goinl ]</p>
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cato
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Re: dc suppression using TVS diodes?

Post by cato »

Well.... that looks safer. However, I suspect it will also be noisey, might tend to oscillate and have offest voltage issues.<p>Its really just that straight through, no resistor in series switch position thats troubling....
rshayes
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Re: dc suppression using TVS diodes?

Post by rshayes »

The protection part is OK. Even if the attenuator isn't there, the series resistor and transient suppressor will still operate.<p>The offset current of the OP747 is specified to be less than 2 nanoamps. At the 1 meg impedance level this can create an offset as high as 2 millivolts. This can be reduced by lowering the impedance level if it is really necessary. Lowering the resistor values will require a higher wattage in the series resistor. Actually, the typical offset current is .1 nanaoamps, so your chances are pretty good that the offset won't be intolerable.<p>The 5 meg feedback resistor combined with the capacitance of the transient suppressor may cause stability problems. You may need to add a small amount of capacity across the 5 megohm feedback resistor to insure stability. I would try about 10 picofarads to start, but a larger value may be necessary if the capacitance of the transient suppressor is high.
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