Snub, Snubber?

[rickodetrader]

Hi All,

I have been looking at the QED PI project on another forum (http://australianelectronicgoldprospectingforum.com) and he appears to have incorporated a snub or snubber tank into the design... so I'll cut to the chase: What is it?

My best guess at this point is to reduce EMI caused by the flyback?

Am I on the right track and if so are there advantages/disadvantages (time constant wise) ie. Does it allow an earlier first sample if all other parts of the system are (for simplicity's sake) kept equal?

Do the Minelab's use this system as the SD2000 schematics would indicate they do?

rickodetrader

[Mechanic]

Hi Ricko De Trader,

A snubber is used to limit the voltage during the flyback. It is instead of just relying on the breakdown voltage of body diodes of the lowest voltage fets to limit the voltage. If there was not a snubber cct then the part that did avalanche would get hot, causing it to be less efficient, would be a source of noise and would most likely end up failing with repeated use.

Yes the 2000 uses a snubber cct, the tlc271 and related parts controls the gate on the irf710 to regulate the voltage on the 100uf 200v cap. Yes the later models also use a snubber to limit the flyback voltage. The other advantage of using a snubber to limit the flyback voltage is a nice linear current ramp.

Cheers Mick

[rickodetrader]

Hi Mick,

Thanks for the reply. Is there any time advantages that you know of? or is it mainly to preserve the components...

[Mechanic]

Hi Rick,

I think it is mostly to protect the components from over voltage and also to provide a linear turn off current ramp. It is also good engineering practice to place a snubber across a coil to prevent the switch from avalanche. I don't think it makes the decay any faster though and I don't think it slows it either. If you want faster you can raise the voltage a little as you can precisely control the voltage it clamps to.

Cheers Mick

[Sean_Goddard]

I know where BW got this snubber from as I have one in MY design too.. It came from a VERY patriotic American I know who designs PI detectors which go DEEEEEEEEP (not on the market yet) - HINT....

The main purpose is NOISE reduction. Avalanche noise kills depth on a PI (as does the FET getting red hot and falling off the PCB). there are other factors involved and looking at BW's design I see a LOT of influence from another designer in there and it's NOT anything to do with ML either!!

[ZED]

Snubber CCT'S were also used in early P.I. designs back when transistors were used to switch the coil currents,if the flyback went over the rated voltage of the transistor then the transistor would pop,the CCT incorporated a diode ,capacitor and a resistor across the capacitor to set the required voltage below that of the voltage rating of the transistor.

Zed