field test unit no 001 "model T"

[hdphilip]

well maybe putting a commercial coil on a "megger" would be one way to find out about it's insulation property are.

[Davor]

Avalanche limit is a natural way of arc prevention, but is not very nice regarding noise, and it stresses the components. So instead you can have a higher voltage device, but nothing to prevent arcing. Not a very good idea.

In case you have some charging control that limits the charging period when flyback goes critical - fine.

[chudster]

... or a cable, connectors, rig entrails...
How about a snubber instead? If you integrate the curve under the PI peak, you'd hardly find justification for excessive peaks.

Following rationale from the forum we want fast flyback to detect the target signal don't we. Fast flyback means higher voltage and higher breakdown doesn't it? Good question though. Do the aftermarket coil people rate the coils for voltage? I spotted these folks who advertise their flyback as a marketing thing (electromagnetic waves). http://www.metaldetection.net/Englis...raison-Eng.htm Up to 1100V. Obviously they feel its an important thing to tell customers the breakdown peak voltage. Can't say I know much about their detectors or whether that gives an edge.

Chudster

[moodz]

6666

i got them from digikey here about 16$

what i wonder is with the 1200 volt breakdown would they damage a coil with a higher flyback?

philip

... on second thoughts dont feed too much voltage into a commercial coil ... once an arc causes carbonisation you could blow your dough. What are the voltage ratings of commercial coils ??

[Davor]

... or a cable, connectors, rig entrails...
How about a snubber instead? If you integrate the curve under the PI peak, you'd hardly find justification for excessive peaks.

[hdphilip]

6666

i got them from digikey here about 16$

what i wonder is with the 1200 volt breakdown would they damage a coil with a higher flyback?

philip

[6666]

Hi Philip, they are US$124.00 here, ouch.

[hdphilip]

hello 6666

I used a cree CMF10120D, mosfet, note the package size... i had to mount it next to the pcb with jumper wires,

Philip

[6666]

Which Cree silicon carbide MOSFET did you use ?

[moodz]

Ok .... avalanche issue cleaned up ... the front end ( adaptive damping system and amplification on its own with no back end processing ) can detect the larger test targets out to nearly a meter now ... ... this puppy is going to be insane when the backend goes on.


..the usual set up ...

18 inch mono coil
0.4 ohm
100 us TX

BTW ... synchronous demod / sampling is now obsolete ....

[Davor]

Only Rx will tell you. If noisier with IRF, go for Cree, but don't forget to share your conclusions with us

[hdphilip]

Irf740 Vs, Cree Sic.....


Here's some pictures of my comparison between the Irf 740 (right) and the Cree silicon carbide MOSFET's(left). keep in mind the coil isn't the greatest(made from cat 5), neither is the coax between the coil and the front end board (4 ft RG174). the front end is simililar to carl's hammerhead, with a cree Sic diode between the coil and the mosfet.

Is it worth the extra cost? and also being twice as big.... you be the judge......

Philip

[Aziz]

Hi all,

when looking at the white trace (flyback voltage), I hope, the avalanche state behaves always the same (not varying). Otherwise, you will get more noise.

But I generally don't like avalanching the mosfet. Avoid it at all cost.
Aziz

[chudster]

Is that green with envy?

[hdphilip]

it's the green trace that's most important