MiniPuls3 Problems

[johnyp]

Hello. Some days ago i built the minipuls3 project from pulsdetector.de website and have one problem. The detector works fine except that detection depth is really low. It can detect a metal jar at max 10cm depth. The only think in the design i did not manage to find and did not solder is the 5.0 Volts surpressor diode D4. Is it possible the absence of this diode to give me those problems ? If anyone have built this detector please tell me your experiences. Thank you and sorry for my bad english.

[johnyp]

Please if anyone has any ideas let's here them cause i really need to fix it really soon. Thank you in advance!

[Qiaozhi]

Please if anyone has any ideas let's here them cause i really need to fix it really soon. Thank you in advance!

Have you tried posting your question on their forum?
http://www.pulsdetektor.de/apboard/main.php

[johnyp]

No. I think that most of people there speaks German and i don't. I will btw post there also. Anyone here any ideas ?

[johnyp]

And now i did post there also but no reply yet... Here what is heppening? Will anyone reply to me or no one knows what the problem may be ?

[WM6]

And now i did post there also but no reply yet... Here what is heppening? Will anyone reply to me or no one knows what the problem may be ?

Put here schematic you use, mark on it your solution, and probably you get something in return. Try to replace D4 with two anti-parallel diode type like 1N4148.

[johnyp]

But i did at my first post. Schematics and everything is here and i followed all of them. The only thing i did not solder is a supressor diode named D4 at the circuit cause i could not find it. Thank you.

[johnyp]

Thank you for your reply but my question is : Do you think that this diode supressor is necessary and that is why i don't get high penetration depth ?

[WM6]

Thank you for your reply but my question is : Do you think that this diode supressor is necessary and that is why i don't get high penetration depth ?

I don't think that this diode is related to your depth results. Try first to make some changes with R1 (use R with 30% smaller value and pot /500E or 1000E/ in serial, instead of R1) and test with different value your target again. Put you coil on clean place with no metallic parts 80cm around.

[porkluvr]

Thank you for your reply but my question is : Do you think that this diode supressor is necessary and that is why i don't get high penetration depth ?

Yes, and yes. You have probably damaged U3 so get another one of those too.

[WM6]

Yes, and yes. You have probably damaged U3 so get another one of those too.

They can detect but not as sensitive as expected. How can they detect if U3 is broken?

[porkluvr]

They can detect but not as sensitive as expected. How can they detect if U3 is broken?

All is not black and white. If you over-stress an opamp input, the device may still amplify but with higher noise levels than what is expected, or else with lower amplification factor.
Or both.

I do not recall ever seeing a PI detector that does not have over-voltage protection at the preamp input.
It's there for a reason. Just because the OP could not find the originally specified protection diode and decided to omit that component out of convenience, would that make it OK? No, it's very foolish.


edit: Maybe U3 will still function adequately (?!), but D4 should still be installed to clamp the preamp input voltage and reduce time to recover from saturation.

[johnyp]

Thank you all for taking time to reply to me. I measured the voltage in the input of the opamp and it is 15volts. I think that it should be 05volts with D4 placed, am i wrong ? All pi designs use two 1n4148 anti-parallel like WM6 sayed. But if i use this configuration don't i limit the input voltage to diodes break voltage ( about 1,5 volts ) ? What you think about low depth detection? The coil is exactly what they are saying closed to a plexy glass case.

[porkluvr]

Johnnyp,

Please let me illustrate an analogy to your situation:

I went bungee jumping the other day. The weather was nice, and I found a scenic bridge. Just, one thing: I could not find a suitable bungee cord, so I decided to jump without one.

I thought it would be fun, but I woke up in the hospital in a body cast from head to toe. Won't somebody please tell me what went wrong because I can't figure it out?
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You measure 15V at U3-2 because you have a short duration 300V~400V peak that is getting integrated with 0V, the average value being displayed on your multimeter.

If you have an oscilloscope, why not probe U3-2 and then you will gain a better understanding of the consequence of having omitted one very important component.

By the way, I would not be surprised if there were more components besides the LF357 that are damaged... that's just the nature of the beast. Sometimes electronics can be very un-forgiving of carelessness (especially when you are generating high-voltages). Forget what I said in post 12 about "maybe U3 will still function..."). I believe that is very unlikely.

[johnyp]

Yes i think you are all right. I shouldn't have fired it up without the supressor. I bought a new LF357 but i can not find anywhere the 5volts supressor so maybe i will use a 1Watt zenner and see. Thank you all for make me admit my mistake. See you.