Low Capacitance Tranzorb in the 10-30V range
in [Design]
|
Prev: motor driver boards
Next: Soundcard with 3rd octave eq
From: Archimedes' Lever on 29 Jan 2010 03:21 On Thu, 28 Jan 2010 21:52:20 -0600, mook Johnson <mook(a)mook.net> wrote: >On 1/26/2010 8:34 PM, Joerg wrote: >> mook johnson wrote: >>> Gents. >>> >>> I'm looking for a tranzorb or equivilent that can discharge a 45mH >>> inductor with 1A DC flowing through it if one end gets disconnected. >>> This tranzorb must have less than 100pF of capacitance. The tranzorb >>> will be in parallel with the inductor. >>> >>> any thoughts? All if the 1.5KE like parts have 800pF and up. >>> >> >> Not sure what you want to do but if the inductor is pulled down, held >> and then flies back until it leans into the TVS maybe the trick in >> figure 1b works: >> >> http://www.microsemi.com/micnotes/122.pdf >> >> But keep in mind that TVS devices are meant to snuff the occasional >> spike. Regular load dumps out of 45mH might be responded to with a loud >> *PHOOMP* and some debris flying about. The last time I did a >> (transistorized) load dump for a roughly 20mH coil at a few amps the >> dump part needed to be heat-sinked. Without that it would have soldered >> itself right off the board. >> > >Found some of the SA18 parts at digikey. Looks like the trick they use >is to put a diode junction in series with the high capacitance zener >junction. As long as the zener is not conducting the diode junction >looks like a small capacitance in series with a large one so the net is >a small capacitance. > >The downside of course is you now need two devices do bidirectional. > >no free lunch again but this will work since I only need to do this in >one place. :) > > >Thanks for the input guys. > > > You can buy the dual device in a single package.
From: Fred Bartoli on 29 Jan 2010 08:59 mook Johnson a �crit : > On 1/26/2010 8:34 PM, Joerg wrote: >> mook johnson wrote: >>> Gents. >>> >>> I'm looking for a tranzorb or equivilent that can discharge a 45mH >>> inductor with 1A DC flowing through it if one end gets disconnected. >>> This tranzorb must have less than 100pF of capacitance. The tranzorb >>> will be in parallel with the inductor. >>> >>> any thoughts? All if the 1.5KE like parts have 800pF and up. >>> >> >> Not sure what you want to do but if the inductor is pulled down, held >> and then flies back until it leans into the TVS maybe the trick in >> figure 1b works: >> >> http://www.microsemi.com/micnotes/122.pdf >> >> But keep in mind that TVS devices are meant to snuff the occasional >> spike. Regular load dumps out of 45mH might be responded to with a loud >> *PHOOMP* and some debris flying about. The last time I did a >> (transistorized) load dump for a roughly 20mH coil at a few amps the >> dump part needed to be heat-sinked. Without that it would have soldered >> itself right off the board. >> > > Found some of the SA18 parts at digikey. Looks like the trick they use > is to put a diode junction in series with the high capacitance zener > junction. As long as the zener is not conducting the diode junction > looks like a small capacitance in series with a large one so the net is > a small capacitance. > > The downside of course is you now need two devices do bidirectional. > > no free lunch again but this will work since I only need to do this in > one place. :) > > > Thanks for the input guys. > > > > Well, this works... depending on what you're doing. If you have occasional large voltage swings, then it doesn't help at all. OTOH, if you have small AC voltages, or large voltage swings, but at such a frequency that the TVS parasitics have no time to discharge, then it'll work fine. I see you want under 100pF. You might want to try several lower voltage TVSs in series if the above remark applies. -- Thanks, Fred.
From: Joerg on 29 Jan 2010 17:22 mook Johnson wrote: > On 1/26/2010 8:34 PM, Joerg wrote: >> mook johnson wrote: >>> Gents. >>> >>> I'm looking for a tranzorb or equivilent that can discharge a 45mH >>> inductor with 1A DC flowing through it if one end gets disconnected. >>> This tranzorb must have less than 100pF of capacitance. The tranzorb >>> will be in parallel with the inductor. >>> >>> any thoughts? All if the 1.5KE like parts have 800pF and up. >>> >> >> Not sure what you want to do but if the inductor is pulled down, held >> and then flies back until it leans into the TVS maybe the trick in >> figure 1b works: >> >> http://www.microsemi.com/micnotes/122.pdf >> >> But keep in mind that TVS devices are meant to snuff the occasional >> spike. Regular load dumps out of 45mH might be responded to with a loud >> *PHOOMP* and some debris flying about. The last time I did a >> (transistorized) load dump for a roughly 20mH coil at a few amps the >> dump part needed to be heat-sinked. Without that it would have soldered >> itself right off the board. >> > > Found some of the SA18 parts at digikey. Looks like the trick they use > is to put a diode junction in series with the high capacitance zener > junction. As long as the zener is not conducting the diode junction > looks like a small capacitance in series with a large one so the net is > a small capacitance. > The capacitance looks huge, figure 4: http://www.fairchildsemi.com/ds/SA%2FSA170CA.pdf [...] -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Nemo on 30 Jan 2010 09:24 Keep in mind the C varies with applied V across the tranzorb. They sometimes quote C at the breakdown voltage, where it's much lower. -- Nemo
From: mook Johnson on 30 Jan 2010 19:03
On 1/29/2010 4:22 PM, Joerg wrote: > mook Johnson wrote: >> On 1/26/2010 8:34 PM, Joerg wrote: >>> mook johnson wrote: >>>> Gents. >>>> >>>> I'm looking for a tranzorb or equivilent that can discharge a 45mH >>>> inductor with 1A DC flowing through it if one end gets disconnected. >>>> This tranzorb must have less than 100pF of capacitance. The tranzorb >>>> will be in parallel with the inductor. >>>> >>>> any thoughts? All if the 1.5KE like parts have 800pF and up. >>>> >>> >>> Not sure what you want to do but if the inductor is pulled down, held >>> and then flies back until it leans into the TVS maybe the trick in >>> figure 1b works: >>> >>> http://www.microsemi.com/micnotes/122.pdf >>> >>> But keep in mind that TVS devices are meant to snuff the occasional >>> spike. Regular load dumps out of 45mH might be responded to with a loud >>> *PHOOMP* and some debris flying about. The last time I did a >>> (transistorized) load dump for a roughly 20mH coil at a few amps the >>> dump part needed to be heat-sinked. Without that it would have soldered >>> itself right off the board. >>> >> >> Found some of the SA18 parts at digikey. Looks like the trick they use >> is to put a diode junction in series with the high capacitance zener >> junction. As long as the zener is not conducting the diode junction >> looks like a small capacitance in series with a large one so the net >> is a small capacitance. >> > > The capacitance looks huge, figure 4: > > http://www.fairchildsemi.com/ds/SA%2FSA170CA.pdf > > [...] > My bad its a SAC15 from litelfuse. |