Step 4 flyback voltage of 240v

[Denots]

Hi
Just looking for some advice for step 4 of the geotech barracuda.
I have made two coils one 300uh and one 500uh, when connected to the port with my oscilloscope connected across the damping resistor I?m getting a flyback of 240v peak to peak with both coils.
My assumption after testing the 500uh was maybe if I make a 300uh coil it?ll bring it in range but with both coils I get the same reading.

Am I right in thinking that by changing the damping resistor that will alter the voltage?

for clarification here are my results from previous steps

step 1- u7 pin 8 gives 4.9v

Step 2- u7 pin 4 gives -4.9v

step 3- oscilloscope monitoring u4 pin4 channel 1 gives a nice square wave, u5 pin 7 channel 2 gives a zigzag wave.

Step 4- oscilloscope across damping resistor gives 240v with 300uh or 500uh coils

Thanks

[waltr]

Voltages and wave forms sound reasonable.
The signal going to U5 is a square wave before D8/C21 but not at pin 7.
Are they both the Same Frequency?

The reason both coils give 240V peak is this is limited by Q1's avalanche rating.
Are you using an IRF9530 or a different MOSFET.

Is Q1 getting warm or hot? If not hot then all is good.

[Denots]

Voltages and wave forms sound reasonable.
The signal going to U5 is a square wave before D8/C21 but not at pin 7.
Are they both the Same Frequency?

The reason both coils give 240V peak is this is limited by Q1's avalanche rating.
Are you using an IRF9530 or a different MOSFET.

Is Q1 getting warm or hot? If not hot then all is good.

I see, I think the waves are ok they seem good to me, I get a square wave going into u5 and a zigzag wave coming out the other side
Using the IRF9530 that came in the pack, it is getting warm, not hot as such but mildly uncomfortable

ive ordered some more 2w resistors so will have a play and see if changing the resister to something bigger might bring the voltage down.

[waltr]

Sounds like it is ok.
You may wish to add a small heat sink on the MOSFET just to help it from getting too hot.

The Damping resistor will not have much if any affect on the fly-back Voltage. However, the exact value of the damping resistor should be tweaked to obtain 'critical' damping.

A way to decrease peak Voltage on fly-back is add some resistance in series with the MOSFET and Coil.
2 to 10 Ohms will do. I have 10 Ohms between the MOSFET and Coil on my Hammer Head. This also lowers to COIL ON current.

Please post you progress on this build here in this thread.
We all like to read about builds.

[Denots]

Ok I see that makes sense, since my last post I watched this video https://m.youtube.com/watch?v=OyS8sfv0XJ0 at 1:40 he uses a potentiometer as a damping resistor and can adjust the flyback voltage, genius I thought, then realised all my pots have 3 legs, after testing with an ohmmeter it seems I can just use two legs but haven’t had a chance to hook it up and test yet.
I realise the video is for a different circuit but wondered if similar could be applied. Or maybe he has the 3rd leg hooked up between the mosfet and the coil?

[waltr]

Yes, I and many other here use a pot (and typically another resistor is series to prevent Zero Ohms) to find the critical damping value.

To use a pot as a variable resistor tie the wiper (center lead) to one of the end leads. Use your Ohmmeter to verify it is now a variable resistor.
Now add a 100 Ohm resistor in series with the pot. This ensure there is at least 100 Ohms across the coil when you adjust the pot else if you turn the pot too far it will burn up.
Once critical damping is obtained reove the pot/resistor and measure.

Do read through all the threads here in the
Baracuda sub-forum since most of the question you will have are asked and answered.

[Denots]

Yes, I and many other here use a pot (and typically another resistor is series to prevent Zero Ohms) to find the critical damping value.

To use a pot as a variable resistor tie the wiper (center lead) to one of the end leads. Use your Ohmmeter to verify it is now a variable resistor.
Now add a 100 Ohm resistor in series with the pot. This ensure there is at least 100 Ohms across the coil when you adjust the pot else if you turn the pot too far it will burn up.
Once critical damping is obtained reove the pot/resistor and measure.

Do read through all the threads here in the
Baracuda sub-forum since most of the question you will have are asked and answered.

Excellent advice thanks for clearing that up for me, not been able to get on here all day due to technical reasons, have to give this a try tomorrow though if I get time.
I see what your saying though, by using the pot in that way you are able to find the exact sweet spot and measure the resistance to get the final resister value in a 2w version.
I have been reading as much as I can but there is so much to learn, just started an electronics crash course on Udemy so a lot more things are starting to make sense now, plus I run a business so that takes a lot of my time, then to top it off we got a puppy so as you can imagine it’s hard to find the time to relax and learn and juggle time between all these different things. Like building a coil and a housing is a whole nother subject and takes a lot of learning as well as trial and error.

Glad I found metal detector circuits though, there’s something about finding treasure with something you built with your own hands

[waltr]

You are catching on and this does take time.

Building and learning how these detects work is quite interesting and there is much to do.

I also went with designing (I work in electronics) and building my own detector to learn and have to satisfaction of finding treasures with something I built.
Took me about 4 months to have a detector working and after nearly 2 years still learning a improving the design. I still read and re-read many of this forums threads to pick out ideas.

[Denots]

Oh dear, I slipped with my probe when measuring u5 and shorted together pins 7 & 8, now my waves aren?t square anymore, the rising & falling edges are straight but the peak is a slight upward ramp now, going to have to replace some parts it seems

edit: it?s not the adjuster on the scope, checked that already

[Denots]

Over the last few days parts have been arriving so I can carry on with different bits and pieces, this is how it?s looking now.





Managed to mount the on/off switch in the handle and the dials in the conduit access plate. The circuit box mounts nicely on the shaft and tucks out of the way quite nicely.
Coil housing is made of two plastic picnic plates concealing a 350uh coil, eventually the housing will be sealed but for now ive left it easy to unbolt while I perfect my coil making skills.
Ultimately I?d like to fit a dial on top of the coil housing that lets me switch between 300, 400 & 500uh but that can wait till the circuit is complete and working.


To join the coil and circuit box I?ve gone for 1.5m DMX lighting cable with matching panel mounts, the components have little latches that lock the plugs into the sockets so should be quite nice and secure.


Here is a picture of u4 (40106)pin 4, it used to be a nice square wave before the short



Now have a look at u4 pin 1, pretty sure it shouldn?t be be like this?



I?ve replaced the hex Schmidt at u4 and also the LT1054chip at u5 to no avail, waiting on some 2n222 transistors to turn up now.
Also waiting on some assorted low ohm resistors to run in series with the coil and drop voltage slightly (q1 is getting a bit warm with 240v passing through it).

Then after all that I can set the flyback resistor and carry on to the next step

[Denots]

Just replaced u1 transistor 2n2222 and still getting the same off shaped wave as in the pictures in last post
Its so strange, the power supply side of the circuit everything is looking good, getting a nice square wave all the way across r23 and up to u1, it’s at that point the signal is getting mangled, time to start replacing capacitors and resistors it seems

[6666]

You have to be very careful with getting the correct legs in the correct holes with those 2222's

[Denots]

You have to be very careful with getting the correct legs in the correct holes with those 2222's

Im pretty sure it’s the right way round, I ordered exactly the same part that came out and put it exactly the same way round, tbh I don’t think there was anything wrong with the old one, just a process of elimination till I find the part that’s causing the issue.
Got all the parts on order to replace everything I have soldered to the left hand side of the board that I haven’t already replaced, so 1% resistors, mosfet, those little box caps, just hoping after all this it doesn’t turn out to be theboard that I’ve damaged lol

[jeanhubert]

Hi,

I also bought geotech baracuda board and I am building it. I do like you design. It's really nice.
I just measured for U4 pin1 and 4. They really look like yours. (I don't know why I can't upload my screenshots, I can only add images from url)
So maybe you or we are good to go.
Here I have very short depths of detection, I am guessing my damping resistor isn't right.
I read previous posts and also inside the metal detector second edition.
And now I am just waiting for more 2W resitor values. I'll let you know if I can make it work as expected

[kt315]

you can upload your pics just you done first post. try now.