flyback voltage

[authere]

Hi All,
I'm having trouble working out how to assess how much flyback voltage i have so that i can apply the right sized resistor to prevent burning things out. I have already burnt out one GP3000 and my oscilloscope, i tried fitting across the RX coil a variable resistor but forgot to check as to where it was set to and it glowed white hot before i'd realised what was going on.I used a 1000 ohm var. res. as all that was req'd was about 660ohms, i'm not sure but i think i was on the right track.
The latest project is a Mono coil which is usually around 255-300uH for a minelab coil, but as someone has said the recieve curcuit prefers to have more inductance to get a better return signal, so i've wound a Mono coil with 450uH and tapped into the coil at 300uH for the TX curcuit, RX being 450uH.With this coil i have no idea as to the flyback requirements so if anyone can help, please do!!
Regards Ron

[WM6]

Try to get the same signals.

You burn scope too, CRT or digital one?

[Carl-NC]

Generally, the value of the damping resistor is chosen to get a critical (or slightly overdamped) response. You would need an oscope to determine this.

The wattage is chosen based on the time-averaged power dissipated. This can be due to not only the flyback voltage but also the turn-on time. For most any classic PI design, a 1-watt R is sufficient. I don't know about the GP3000. If your pot went white-hot then initially it may have been set at way too low a value. I would not use a pot all alone, rather connect it in series with a minimum-value R like 470 ohms.

- Carl