On Sunday, January 29, 2023 at 6:20:44 PM UTC-5, John Walliker wrote:> On Sunday, 29 January 2023 at 20:11:31 UTC, Fred Bloggs wrote: > > > > That's a really weird circuit. > > I'm pretty sure the BAVs are there for ESD protection. Now the hunt is on for the rationale behind BC850 clamp. > The BC850 clamps in conjunction with the 22k input resistors are almost certainly there > to protect the patient from excessive dc under single fault conditions. Direct current > flowing through electrodes attached to the skin can cause severe burns if it is sustained > for a long time - which could easily happen in some medical situations. > Nobody would put 22k resistors in series with the input of a low-noise amplifier if > they didn't need to. Their purpose is to limit the current that can flow when driven by the clamping > voltage. Without the clamps the fault current through the electrodes would be around > 10 times greater if, for example, one of the BAV99s developed a short to Vcc. > > JohnThe circuit runs off 1.8V battery power.
Not OT: BC850 as a diode clamp?
Started by ●January 29, 2023
Reply by ●January 30, 20232023-01-30
Reply by ●January 30, 20232023-01-30
On Monday, 30 January 2023 at 17:58:07 UTC, Fred Bloggs wrote:> On Sunday, January 29, 2023 at 6:20:44 PM UTC-5, John Walliker wrote: > > On Sunday, 29 January 2023 at 20:11:31 UTC, Fred Bloggs wrote: > > > > > > That's a really weird circuit. > > > I'm pretty sure the BAVs are there for ESD protection. Now the hunt is on for the rationale behind BC850 clamp. > > The BC850 clamps in conjunction with the 22k input resistors are almost certainly there > > to protect the patient from excessive dc under single fault conditions. Direct current > > flowing through electrodes attached to the skin can cause severe burns if it is sustained > > for a long time - which could easily happen in some medical situations. > > Nobody would put 22k resistors in series with the input of a low-noise amplifier if > > they didn't need to. Their purpose is to limit the current that can flow when driven by the clamping > > voltage. Without the clamps the fault current through the electrodes would be around > > 10 times greater if, for example, one of the BAV99s developed a short to Vcc. > > > > John > The circuit runs off 1.8V battery power.Where is that stated on the schematic that Ricky originally posted? John
Reply by ●January 30, 20232023-01-30
On Monday, January 30, 2023 at 1:34:09 PM UTC-5, John Walliker wrote:> On Monday, 30 January 2023 at 17:58:07 UTC, Fred Bloggs wrote: > > On Sunday, January 29, 2023 at 6:20:44 PM UTC-5, John Walliker wrote: > > > On Sunday, 29 January 2023 at 20:11:31 UTC, Fred Bloggs wrote: > > > > > > > > That's a really weird circuit. > > > > I'm pretty sure the BAVs are there for ESD protection. Now the hunt is on for the rationale behind BC850 clamp. > > > The BC850 clamps in conjunction with the 22k input resistors are almost certainly there > > > to protect the patient from excessive dc under single fault conditions. Direct current > > > flowing through electrodes attached to the skin can cause severe burns if it is sustained > > > for a long time - which could easily happen in some medical situations. > > > Nobody would put 22k resistors in series with the input of a low-noise amplifier if > > > they didn't need to. Their purpose is to limit the current that can flow when driven by the clamping > > > voltage. Without the clamps the fault current through the electrodes would be around > > > 10 times greater if, for example, one of the BAV99s developed a short to Vcc. > > > > > > John > > The circuit runs off 1.8V battery power. > Where is that stated on the schematic that Ricky originally posted? > > JohnWhere on that schematic does it say it was anything else? Or even line powered?
Reply by ●January 30, 20232023-01-30
On Monday, 30 January 2023 at 18:50:26 UTC, Fred Bloggs wrote:> On Monday, January 30, 2023 at 1:34:09 PM UTC-5, John Walliker wrote: > > On Monday, 30 January 2023 at 17:58:07 UTC, Fred Bloggs wrote: > > > On Sunday, January 29, 2023 at 6:20:44 PM UTC-5, John Walliker wrote: > > > > On Sunday, 29 January 2023 at 20:11:31 UTC, Fred Bloggs wrote: > > > > > > > > > > That's a really weird circuit. > > > > > I'm pretty sure the BAVs are there for ESD protection. Now the hunt is on for the rationale behind BC850 clamp. > > > > The BC850 clamps in conjunction with the 22k input resistors are almost certainly there > > > > to protect the patient from excessive dc under single fault conditions. Direct current > > > > flowing through electrodes attached to the skin can cause severe burns if it is sustained > > > > for a long time - which could easily happen in some medical situations. > > > > Nobody would put 22k resistors in series with the input of a low-noise amplifier if > > > > they didn't need to. Their purpose is to limit the current that can flow when driven by the clamping > > > > voltage. Without the clamps the fault current through the electrodes would be around > > > > 10 times greater if, for example, one of the BAV99s developed a short to Vcc. > > > > > > > > John > > > The circuit runs off 1.8V battery power. > > Where is that stated on the schematic that Ricky originally posted? > > > > John > Where on that schematic does it say it was anything else? Or even line powered?So we are all trying to guess the answer based on insufficient information. Regardless of that problem, there is a requirement to limit the maximum dc (and ac, depending on frequency) currents that can flow through electrodes connected to people or animals both under normal operating conditions and in single fault conditions. See IEC 60601 for chapter and verse. John
