Why a resistor between the coil and front amp ( i.e.- R12 in the HH-D1)?

[GT Blocker]

Hey all,

This is probably a stupid question, but I must...

The 1K (HHD1) between the coil and front end op-amp is needed in what part of the pulse/sample? In other words, is the resistance there to protect the amp during flyback and really un-necessary after a quantifiable time?

GTB

[Qiaozhi]

Hey all,

This is probably a stupid question, but I must...

The 1K (HHD1) between the coil and front end op-amp is needed in what part of the pulse/sample? In other words, is the resistance there to protect the amp during flyback and really un-necessary after a quantifiable time?

GTB

The preamp (IC6) is configured as an inverting amplifier with a gain determined by the R13/R12. i.e. 1000x (60dB). The diodes are there to protect the inverting input of IC6 from the flyback voltage generated by the coil. If R12 was replaced by a short-circuit, then two things would happen. Firstly the preamp gain would effectively be in open loop, and secondly the diodes would not let the coil voltage rise above 0.7V.

In some designs, a MOSFET is used to disconnect the coil from the preamp input during the high voltage flyback, only reconnecting once the voltage has dropped to a safe level. In that case the need for the resistor (R12 in this case) would depend on the opamp configuration.