I've placed 22-ohm 0805 resistors in series with the HV supply lines in the
latest version of my easy-to-build AMP-62 high-voltage amplifier board. This
version is often made with aggressive output current-limit values, e.g., 150mA.
But it's intended that such currents last only a short time, e.g., to rapidly
charge a capacitive load (during the rapid output slewing, the 22-ohms only
drops 3.3 volts). By comparison, the original version of this board, as
described in our x-Chapter book, has a wimpy 5mA current limit. But we assume
that users of this amplifier version who need its extreme slewing capability
understand the risks, when they select the 150mA current-limiting resistor
values.
If the output were to be shorted, and draw 150mA from the supply, firstly it's
hoped the supply voltage will soon fall dramatically, due to its own say 20mA
current limit. Meanwhile the 22-ohm 0805 resistor would dissipate 0.5 watts, or
4x its rated power, and hopefully after a while it would fail open. But if it
doesn't fail fast enough, one of the high-voltage MOSFETs, dissipating 40 watts,
might short. At that point the current would increase further, and the 22-ohms
would certainly fail, stopping the high power dissipation and removing the load
from the supply.
OK, I see I'm going to get roasted for this one.
--
Thanks,
- Win
using an smt resistor as a fuse
Started by ●January 9, 2019
Reply by ●January 9, 20192019-01-09
onsdag den 9. januar 2019 kl. 22.44.00 UTC+1 skrev Winfield Hill:> I've placed 22-ohm 0805 resistors in series with the HV supply lines in the > latest version of my easy-to-build AMP-62 high-voltage amplifier board. This > version is often made with aggressive output current-limit values, e.g., 150mA. > But it's intended that such currents last only a short time, e.g., to rapidly > charge a capacitive load (during the rapid output slewing, the 22-ohms only > drops 3.3 volts). By comparison, the original version of this board, as > described in our x-Chapter book, has a wimpy 5mA current limit. But we assume > that users of this amplifier version who need its extreme slewing capability > understand the risks, when they select the 150mA current-limiting resistor > values. > > If the output were to be shorted, and draw 150mA from the supply, firstly it's > hoped the supply voltage will soon fall dramatically, due to its own say 20mA > current limit. Meanwhile the 22-ohm 0805 resistor would dissipate 0.5 watts, or > 4x its rated power, and hopefully after a while it would fail open. But if it > doesn't fail fast enough, one of the high-voltage MOSFETs, dissipating 40 watts, > might short. At that point the current would increase further, and the 22-ohms > would certainly fail, stopping the high power dissipation and removing the load > from the supply. > > OK, I see I'm going to get roasted for this one. >https://www.te.com/commerce/DocumentDelivery/DDEController?Action=showdoc&DocId=Data+Sheet%7F1773216%7FD%7Fpdf%7FEnglish%7FENG_DS_1773216_D.pdf%7F1879229-4 0805 fusible resistor fusing power 2.5W !
Reply by ●January 9, 20192019-01-09
On 9 Jan 2019 13:43:46 -0800, Winfield Hill <hill@rowland.harvard.edu> wrote:>I've placed 22-ohm 0805 resistors in series with the HV supply lines in the >latest version of my easy-to-build AMP-62 high-voltage amplifier board. This >version is often made with aggressive output current-limit values, e.g., 150mA. >But it's intended that such currents last only a short time, e.g., to rapidly >charge a capacitive load (during the rapid output slewing, the 22-ohms only >drops 3.3 volts). By comparison, the original version of this board, as >described in our x-Chapter book, has a wimpy 5mA current limit. But we assume >that users of this amplifier version who need its extreme slewing capability >understand the risks, when they select the 150mA current-limiting resistor >values. > >If the output were to be shorted, and draw 150mA from the supply, firstly it's >hoped the supply voltage will soon fall dramatically, due to its own say 20mA >current limit. Meanwhile the 22-ohm 0805 resistor would dissipate 0.5 watts, or >4x its rated power, and hopefully after a while it would fail open. But if it >doesn't fail fast enough, one of the high-voltage MOSFETs, dissipating 40 watts, >might short. At that point the current would increase further, and the 22-ohms >would certainly fail, stopping the high power dissipation and removing the load >from the supply. > >OK, I see I'm going to get roasted for this one.An 0805 will probably survive 0.5 watts for a long time. If the end caps are heat sunk to pretty big pads or traces, it will usually survive forever. Your resistor will act like a fuse, at (guessing) several watts or so. (Didn't somebody do some tests like that recently?) Given good end cap heat sinking, most resistors (from 0603 to 1206) will have about the same central hot-spot temperature at a given power. It's a time race between the 20 mA supply current limit (plus any capacitive energy storage) and the SOAR of the fet. The dominant thermal time constant of a fet is usually in the 100 ms ballpark. Here's a 1206: https://www.dropbox.com/s/rmgio9oagh88e90/DSC02057.JPG?dl=0 https://www.dropbox.com/s/viez4tcl6zch6in/DSC02064.JPG?dl=0 https://www.dropbox.com/s/qms6olt3zh1b5vz/DSC02053.JPG?dl=0 https://www.dropbox.com/s/sv56ee1d1p9oksh/DSC02062.JPG?dl=0 At 30 volts, 18 watts, it lasts about 60 ms with no conductive cooling, and about 300 ms soldered to huge copper pours. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●January 9, 20192019-01-09
What are you fusing? To what end? "High voltage" is particularly onerous because said fuse will continue arcing the whole time the supply is draining down. The transistor protects the fuse, not the other way around!! Tim -- Seven Transistor Labs, LLC Electrical Engineering Consultation and Design Website: https://www.seventransistorlabs.com/ "Winfield Hill" <hill@rowland.harvard.edu> wrote in message news:q15pui0gsl@drn.newsguy.com...> I've placed 22-ohm 0805 resistors in series with the HV supply lines in > the > latest version of my easy-to-build AMP-62 high-voltage amplifier board. > This > version is often made with aggressive output current-limit values, e.g., > 150mA. > But it's intended that such currents last only a short time, e.g., to > rapidly > charge a capacitive load (during the rapid output slewing, the 22-ohms > only > drops 3.3 volts). By comparison, the original version of this board, as > described in our x-Chapter book, has a wimpy 5mA current limit. But we > assume > that users of this amplifier version who need its extreme slewing > capability > understand the risks, when they select the 150mA current-limiting resistor > values. > > If the output were to be shorted, and draw 150mA from the supply, firstly > it's > hoped the supply voltage will soon fall dramatically, due to its own say > 20mA > current limit. Meanwhile the 22-ohm 0805 resistor would dissipate 0.5 > watts, or > 4x its rated power, and hopefully after a while it would fail open. But > if it > doesn't fail fast enough, one of the high-voltage MOSFETs, dissipating 40 > watts, > might short. At that point the current would increase further, and the > 22-ohms > would certainly fail, stopping the high power dissipation and removing the > load > from the supply. > > OK, I see I'm going to get roasted for this one. > > > -- > Thanks, > - Win
Reply by ●January 9, 20192019-01-09
On Wednesday, January 9, 2019 at 4:44:00 PM UTC-5, Winfield Hill wrote:> I've placed 22-ohm 0805 resistors in series with the HV supply lines in the > latest version of my easy-to-build AMP-62 high-voltage amplifier board. This > version is often made with aggressive output current-limit values, e.g., 150mA. > But it's intended that such currents last only a short time, e.g., to rapidly > charge a capacitive load (during the rapid output slewing, the 22-ohms only > drops 3.3 volts). By comparison, the original version of this board, as > described in our x-Chapter book, has a wimpy 5mA current limit.OK no more x-chapter teases, unless I get to proof early copies*, or it's being published soon. But we assume> that users of this amplifier version who need its extreme slewing capability > understand the risks, when they select the 150mA current-limiting resistor > values. > > If the output were to be shorted, and draw 150mA from the supply, firstly it's > hoped the supply voltage will soon fall dramatically, due to its own say 20mA > current limit. Meanwhile the 22-ohm 0805 resistor would dissipate 0.5 watts, or > 4x its rated power, and hopefully after a while it would fail open. But if it > doesn't fail fast enough, one of the high-voltage MOSFETs, dissipating 40 watts, > might short. At that point the current would increase further, and the 22-ohms > would certainly fail, stopping the high power dissipation and removing the load > from the supply. >Hmm well my first idea would be to try and fry some, with a bigger fet if necessary. (It doesn't have to be fast.) How about one of those resettable polyfuses? The ones I've tested fail earlier (lower currents) with each fail. George H. *I'd most likely make a terrible reviewer of circuits, with way more questions than 'real' errors.> OK, I see I'm going to get roasted for this one.> > > -- > Thanks, > - Win
Reply by ●January 9, 20192019-01-09
On Wed, 9 Jan 2019 18:27:30 -0600, "Tim Williams" <tiwill@seventransistorlabs.com> wrote:>What are you fusing? To what end? > >"High voltage" is particularly onerous because said fuse will continue >arcing the whole time the supply is draining down. > >The transistor protects the fuse, not the other way around!! > >TimI've connected 0805 resistors across the AC line. Try it. They usually just go PIP. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●January 9, 20192019-01-09
On 9 Jan 2019 13:43:46 -0800, Winfield Hill <hill@rowland.harvard.edu> wrote:>I've placed 22-ohm 0805 resistors in series with the HV supply lines in the >latest version of my easy-to-build AMP-62 high-voltage amplifier board. This >version is often made with aggressive output current-limit values, e.g., 150mA. >But it's intended that such currents last only a short time, e.g., to rapidly >charge a capacitive load (during the rapid output slewing, the 22-ohms only >drops 3.3 volts). By comparison, the original version of this board, as >described in our x-Chapter book, has a wimpy 5mA current limit. But we assume >that users of this amplifier version who need its extreme slewing capability >understand the risks, when they select the 150mA current-limiting resistor >values. > >If the output were to be shorted, and draw 150mA from the supply, firstly it's >hoped the supply voltage will soon fall dramatically, due to its own say 20mA >current limit. Meanwhile the 22-ohm 0805 resistor would dissipate 0.5 watts, or >4x its rated power, and hopefully after a while it would fail open. But if it >doesn't fail fast enough, one of the high-voltage MOSFETs, dissipating 40 watts, >might short. At that point the current would increase further, and the 22-ohms >would certainly fail, stopping the high power dissipation and removing the load >from the supply. > >OK, I see I'm going to get roasted for this one.We usually use the output resistors to limit the load current eg protect the load. I would put current protection on the MOSFET to protect the MOSFETs. Why let the driver burn up if the resistor fails closed, which there is good chance it arc's over and turns into a carbon blob. But thats just me. Cheers
Reply by ●January 9, 20192019-01-09
On Wed, 09 Jan 2019 21:36:58 -0500, Martin Riddle <martin_ridd@verizon.net> wrote:>On 9 Jan 2019 13:43:46 -0800, Winfield Hill <hill@rowland.harvard.edu> >wrote: > >>I've placed 22-ohm 0805 resistors in series with the HV supply lines in the >>latest version of my easy-to-build AMP-62 high-voltage amplifier board. This >>version is often made with aggressive output current-limit values, e.g., 150mA. >>But it's intended that such currents last only a short time, e.g., to rapidly >>charge a capacitive load (during the rapid output slewing, the 22-ohms only >>drops 3.3 volts). By comparison, the original version of this board, as >>described in our x-Chapter book, has a wimpy 5mA current limit. But we assume >>that users of this amplifier version who need its extreme slewing capability >>understand the risks, when they select the 150mA current-limiting resistor >>values. >> >>If the output were to be shorted, and draw 150mA from the supply, firstly it's >>hoped the supply voltage will soon fall dramatically, due to its own say 20mA >>current limit. Meanwhile the 22-ohm 0805 resistor would dissipate 0.5 watts, or >>4x its rated power, and hopefully after a while it would fail open. But if it >>doesn't fail fast enough, one of the high-voltage MOSFETs, dissipating 40 watts, >>might short. At that point the current would increase further, and the 22-ohms >>would certainly fail, stopping the high power dissipation and removing the load >>from the supply. >> >>OK, I see I'm going to get roasted for this one. > > >We usually use the output resistors to limit the load current eg >protect the load. >I would put current protection on the MOSFET to protect the MOSFETs. >Why let the driver burn up if the resistor fails closed, which there >is good chance it arc's over and turns into a carbon blob. > >But thats just me. > >CheersI like to compute fet power dissipation, rather than current limiting. You can safely push a fet a lot harder that way. -- John Larkin Highland Technology, Inc lunatic fringe electronics
Reply by ●January 9, 20192019-01-09
George Herold wrote...> > OK no more x-chapter teases, unless I get to proof early copies*, > or it's being published soon.I'm happy to send a draft of the HV amplifier section to anyone who wants to look at it. It's a good example of the advanced material in the x-Chapters. (We're aiming to be out this year.) -- Thanks, - Win
Reply by ●January 10, 20192019-01-10
Tim Williams wrote:> The transistor protects the fuse, not the other way around!!If the transistor is not properly selected for fusing, one may also try to augment it with the Soviet LED technique: https://imgur.com/gallery/YegLM Best regards, Piotr
