Question regarding R values on SMT vsn of HH

[malacoda]

Hi Carl,

I need a small bit of confusion cleared up before I can proceed with assembling the timing circuit; I hope you can help.

The parts list shows the following:
Resistors: R46 - 10k, R46a 330
Pots: R46 10k

And the paragraph under the 'substitutions' section regarding pulse timing (top center of page 12) states that for a potentially more practical range of 3-20 us, make R46 2k and R46a 220.

Thing is, on the schematic, the R46 pot is depicted along with R46a but there is no 'second' R46 depicted for the application of the R46 10k resistor listed in the parts list... nor any pad available for it on the SMT PCB.

I'm just trying to determine if the part listing of an 10k R46 resistor was an error so that, if I decide to make the substitution, I can be sure I should be replacing the 10k pot with a 2k pot rather than a surface mount 10k resistor that isn't identified (or is there but I'm just not seeing...) that should be replaced with a 2k SMT resistor.

regards,
Malac

[MojaveRed]

Hello All,

The same situation exists for Option 1. R46 is a pot on the schematic -- that's clear. The fixed resistor, R46 @ 10K, seen in the parts list must be spurious.

Another point of possible confusion are R37 and C11 which provide a synch signal to the 7660. Although they can be omitted, they appear in the Option 1 schematic but are absent from the parts list. They might ought to be added just for consistency.

[Carl-NC]

R46 is a 10k pot... it should not be listed as a resistor. Make it a 2k pot for a lower delay range. Sorry.

I'll add R37 & C11 to the parts list, sorry again.

Thanks for the feedback!

[MojaveRed]

Hi Carl,

No 'sorry' necessary. That's a tremendous ptoject you've got going there.

[malacoda]

You said it MojaveRed

It is a great project. Thanks Carl.

[MojaveRed]

Carl,

Here's another, again with reference to Option 1: R14 appears in the Parts List but nowhere on the Option 1 schematic, the full schematic or on the board. I'm assuming, unless I've overlooked it, that it doesn't exist and should be ignored.

[Carl-NC]

Yes, R14 was deleted for the Rev C board.

[MojaveRed]

Carl,

While we're at it, I understand that R15 can be shorted but I'm simply curious as to what your original intent (sounds like a supreme court case) was for a "default" Option 1, regarding R15.

I ask because R15 appears on the PC board and Option 1 Schematic but is absent from the full Parts List. I suspect you meant to have it there and it just needs to be added to the Parts List.

Unrelated: R17 and R19 appear on the board and Parts Placement but are absent from the Parts List. Need to be added?

Is it correct that essentially all avalanche current from Q3 goes through R10?

One last: on your own build of the HH coil as described, how close did your product come to 500 uH?

By the way, I'm just finishing up the "transmit circuitry and receiver preamp" section of an Option 1 board. Looking forward to finishing it off in the next week. Having a good time with it, thanks.

[MojaveRed]

Carl,

I think my post above about R15 got missed in the shuffle. I'm still interested.

[Carl-NC]

I added R15 so that when Q2 is used as pull-down switch (Q3-gate connected to Q2-collector, R8/R9 shorted) Q1 won't overdrive Q2 and burn it up. Otherwise, it can be shorted. You're right, I didn't put it in the parts list, nor did I put R22 in... I should add them both with a "see text" note.

Yes, R17 & R19 need to be added, too.

You hope that all the switched current from Q3 Comes from C10, and not through R10. R10/C10 form a charge reservoir that is supposed to supply the transient current for the coil.

On my coils, I've typically gotten to within 5% or so of 500uH.

- Carl

[MojaveRed]

Thanks for making that clear (R15).

Your term "switched current from Q3" then refers to what I called 'avalanche current'[?].

Is it correct then to think of the MOSFET body diode as breaking down, when the flyback voltage exceeds a certain value, to allow current to flow from the R10/C10 tank and through the coil in response to the collapsing field's attempt to maintain (i.e. oppose change in) the pulse current that had been flowing just prior to switch-off?

[Carl-NC]

OK, I misunderstood. I use a FET with a high enough breakdown (400v) so it doesn't avalanche during flyback. If you allow it to avalanche, it will heat up, though it really shouldn't hurt anything, providing it doesn't overheat. Not sure if it has much effect on detection depth... the region of clipping is long before the sample is taken, but the breakdown current does flow through the coil and briefly generates a magnetic field.

The R10/C10 tank is to supply current during the normal coil turn-on pulse.

- Carl

[Unregistered]

Current

Does anybody know what is the current for the hammerhead?100mA?200mA?

[Carl-NC]

I recall 80mA or so... not really sure.