Bleeder resistor to dissipate static
in [Design]
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From: KJ on 5 Feb 2010 23:27 How viable is it to use a bleeder resistor rather than a direct galvanic connection to try to cut down the transient induced by static? In our system we have moving paper that will generate occasional discharges to a large metal plate which supports the mechanics. Underneath that plate (~1 inch) there is a circuit board that has a single connection to a post coming down from that plate. A flex circuit is plugged into a connector which runs back up through that plate up a tower to an LCD. Occasionally the discharge can disrupt the LCD temporarily, basically resetting it. One thought was to try to control this discharge by inserting some series resistance between the post and where it would otherwise connect to the circuit board to provide the 'earth' connection to the circuit board. Obviously this would need to be a resistor capable of withstanding high volts and impulse type events as well as PCB layout rules to support this. Does anyone have any references to such a technique? The alternative solution is a 27 cent ferrite around the flex circuit, and cost is a concern with the product so exotic/expensive approaches are out. Also investigating alternatives up near the LCD as well, but just wondering if there is a good way to dissipate the static shock as a bit of heat rather than disrupting the ground and everything at the LCD that is sitting up on a perch..and do it for ~$0.25 USD or less. KJ
From: Ban on 5 Feb 2010 23:56 "KJ" <kkjennings(a)sbcglobal.net> schrieb im Newsbeitrag news:b527773b-c8e9-4a4e-be29-dca4b2fdb53a(a)u9g2000yqb.googlegroups.com... > How viable is it to use a bleeder resistor rather than a direct > galvanic connection to try to cut down the transient induced by > static? > > In our system we have moving paper that will generate occasional > discharges to a large metal plate which supports the mechanics. > Underneath that plate (~1 inch) there is a circuit board that has a > single connection to a post coming down from that plate. A flex > circuit is plugged into a connector which runs back up through that > plate up a tower to an LCD. Occasionally the discharge can disrupt > the LCD temporarily, basically resetting it. One thought was to try > to control this discharge by inserting some series resistance between > the post and where it would otherwise connect to the circuit board to > provide the 'earth' connection to the circuit board. Obviously this > would need to be a resistor capable of withstanding high volts and > impulse type events as well as PCB layout rules to support this. Does > anyone have any references to such a technique? > > The alternative solution is a 27 cent ferrite around the flex circuit, > and cost is a concern with the product so exotic/expensive approaches > are out. Also investigating alternatives up near the LCD as well, but > just wondering if there is a good way to dissipate the static shock as > a bit of heat rather than disrupting the ground and everything at the > LCD that is sitting up on a perch..and do it for ~$0.25 USD or less. > > KJ Better to use a separate ground connection with an extra wire to the grounded frame or another low impedance point and isolate the bolt from your circuit. Try it temporary first if it helps. ciao Ban
From: Robert Baer on 6 Feb 2010 01:22 KJ wrote: > How viable is it to use a bleeder resistor rather than a direct > galvanic connection to try to cut down the transient induced by > static? > > In our system we have moving paper that will generate occasional > discharges to a large metal plate which supports the mechanics. > Underneath that plate (~1 inch) there is a circuit board that has a > single connection to a post coming down from that plate. A flex > circuit is plugged into a connector which runs back up through that > plate up a tower to an LCD. Occasionally the discharge can disrupt > the LCD temporarily, basically resetting it. One thought was to try > to control this discharge by inserting some series resistance between > the post and where it would otherwise connect to the circuit board to > provide the 'earth' connection to the circuit board. Obviously this > would need to be a resistor capable of withstanding high volts and > impulse type events as well as PCB layout rules to support this. Does > anyone have any references to such a technique? > > The alternative solution is a 27 cent ferrite around the flex circuit, > and cost is a concern with the product so exotic/expensive approaches > are out. Also investigating alternatives up near the LCD as well, but > just wondering if there is a good way to dissipate the static shock as > a bit of heat rather than disrupting the ground and everything at the > LCD that is sitting up on a perch..and do it for ~$0.25 USD or less. > > KJ Well...a resistor (normally) conducts current. Since you are talking about electrostatic voltage / current, the current is low and a resistor would bleed off the charge meaning little or low net voltage. I would use something between 1K and 10Meg depending on the severity of generation. I also would add a metallic or other conductive shield between the generator and sensitive PC boards and directly connect it to the PCB ground system, NO other connection (it can have insulation around it if needed).
From: whit3rd on 6 Feb 2010 14:13 On Feb 5, 8:27 pm, KJ <kkjenni...(a)sbcglobal.net> wrote: > How viable is it to use a bleeder resistor rather than a direct > galvanic connection to try to cut down the transient induced by > static? > > In our system we have moving paper that will generate occasional > discharges to a large metal plate which supports the mechanics. > Underneath that plate (~1 inch) there is a circuit board that has a > single connection to a post coming down from that plate. A flex > circuit is plugged into a connector which runs back up through that > plate up a tower to an LCD. Occasionally the discharge can disrupt > the LCD temporarily, basically resetting it. A bleeder resistor won't have any place to connect, will it? What you really want, is a Faraday cage around the sensitive LCD (but it may be awkward to put metal window-screen material over the face). A frequently seen solution is a set of corona points (basically, tiny little lightning rods) next to the paper which carries the charge. Another might be some UV LEDs, which may create enough ionization to discharge the paper. Radioactive strips in the old record-cleaner brushes performed the same function.
From: legg on 7 Feb 2010 15:42
On Fri, 5 Feb 2010 20:27:11 -0800 (PST), KJ <kkjennings(a)sbcglobal.net> wrote: >How viable is it to use a bleeder resistor rather than a direct >galvanic connection to try to cut down the transient induced by >static? > >In our system we have moving paper that will generate occasional >discharges to a large metal plate which supports the mechanics. >Underneath that plate (~1 inch) there is a circuit board that has a >single connection to a post coming down from that plate. A flex >circuit is plugged into a connector which runs back up through that >plate up a tower to an LCD. Occasionally the discharge can disrupt >the LCD temporarily, basically resetting it. One thought was to try >to control this discharge by inserting some series resistance between >the post and where it would otherwise connect to the circuit board to >provide the 'earth' connection to the circuit board. Obviously this >would need to be a resistor capable of withstanding high volts and >impulse type events as well as PCB layout rules to support this. Does >anyone have any references to such a technique? > >The alternative solution is a 27 cent ferrite around the flex circuit, >and cost is a concern with the product so exotic/expensive approaches >are out. Also investigating alternatives up near the LCD as well, but >just wondering if there is a good way to dissipate the static shock as >a bit of heat rather than disrupting the ground and everything at the >LCD that is sitting up on a perch..and do it for ~$0.25 USD or less. > >KJ The reset is likely coincident with the discharge from the paper, which you can't attach a resistor to - all you can do is provide a more controlled discharge path that does not involve the victim circuit, or prevent the charge from accumulating in the first place. Otherwise, resistive bleeders across isolation barriers are safety listed commodity components with values between 1/2 and 12 Meg. Pilkor PPSR37 , PWSR37 and Phoenix HVR types come to mind. If this isn't a safety barrier being crossed, then simply using parts of similar construction would probably serve the purpose. Frankly, US$0.27 does not strike me as a barrier in the cost of the 'system'. RL |