MD in Radio-Plans magazine

[Esteban]

Need value of this termistor in project MD:

Un détecteur de métaux avec compensation de l’effect de sol.

Radio-Plans N° 448, 449, 450 et 451

And:

1. Des correctifs aux schémas originaux sont parus dans Radio-Plans n° 454 de septembre 1985. by J. Lassus:

2. Radio-Plans N° 479 (octobre 1987) et 481 (décembre 1987): réalisation de Mr. Lassus différant du précèdent par le système de correction des effets de sol).

[chemelec]

Need value of this termistor in project MD:

Un détecteur de métaux avec compensation de l’effect de sol.

Radio-Plans N° 448, 449, 450 et 451

And:

1. Des correctifs aux schémas originaux sont parus dans Radio-Plans n° 454 de septembre 1985. by J. Lassus:

2. Radio-Plans N° 479 (octobre 1987) et 481 (décembre 1987): réalisation de Mr. Lassus différant du précèdent par le système de correction des effets de sol).

You Need more than Just the Value.
There are Thousands of different Temperature Curves for Every Value. That is Also Important.

[Esteban]

Hi Gary.

Thanks. I have the Spanish reprinted version. Hope in French publication can find more explanation about the curves. The autor said that this detector works in the temperature range of -10ºC to 60ºC.

Regards

Esteban

[Max]

Hi Gary.

Thanks. I have the Spanish reprinted version. Hope in French publication can find more explanation about the curves. The autor said that this detector works in the temperature range of -10ºC to 60ºC.

Regards

Esteban

Hi Esteban,
I've used also a thermistor in a VLF some years ago. But assure you that when someone uses this stuff...something is wrong in the project : these NTC/PTC resistors are used to compensate drift from temepature changes...
but constructive process are not so accurate so you probably end-up with a continuos drift here.
I think that such a component in a MD mean only that big problem of drifting are present and the detector is anything but STABLE.

So I suggest you strongly don't waste your time with it.

Best regards,
Max

[Max]

Need value of this termistor in project MD:

Un détecteur de métaux avec compensation de l’effect de sol.

Radio-Plans N° 448, 449, 450 et 451

And:

1. Des correctifs aux schémas originaux sont parus dans Radio-Plans n° 454 de septembre 1985. by J. Lassus:

2. Radio-Plans N° 479 (octobre 1987) et 481 (décembre 1987): réalisation de Mr. Lassus différant du précèdent par le système de correction des effets de sol).

Hi Esteban,
just a question:
is the fet I see connected as constant current regulator that feeds the second fet that is used in hartley oscillator ???
If so: give up with this stuff man --> I had the same and doesn't work well in any case...not ground signal correction as they said but strong drift here!
Problem here is that fets drift a lot even with small changes in temperature (I've realized also 0.1C of variation is a huge problem on what I had!!! ::crazy: and that's because of minority carriers that came in place and huge amplification required in this kind of circuit.
If you want to solve the problems there adbandon fets at all and shift back to some micro-tube (e.g. RAYTHEON) hartley schema.

Best regards,
Max

[Esteban]

Yes, and just the author said that this occurs with 2N3819 but no with BF245 B or BF244 fets in the epoch he design this detector.

[Max]

Yes, and just the author said that this occurs with 2N3819 but no with BF245 B or BF244 fets in the epoch he design this detector.

Hi Esteban,
when I was in truble with my old VLF I've tested every fet types (couples) I found but never solved the problem that way. I've tested "matched pairs" and thing were good...but I think it are difficault to find now ! So I think only way to solve at now is redesigning the osc. section using tubes. Tubes haven't minority carriers ! So they are REALLY stable in temperature. Most of these fet circuits are lightweight version of old tube-oscillators (hartley mainly) but one must keep in mind that fet aren't tubes...so drifting occours quite easy and it's not so controllable with NTC/PTC.
The idea was to find a "resistor" curve that matched fet curve thus annulling drift...from a serious approach it's a MESS only!
Real , commercial and seriour MDs never used this stuff.
Don't belive that a BF244/245 could solve this kind of problems: they need to justify with hobbists when things go worse... and the md drift everytime !

Best regards,
Max

[Qiaozhi]

Need value of this termistor in project MD:

Un détecteur de métaux avec compensation de l’effect de sol.

Radio-Plans N° 448, 449, 450 et 451

And:

1. Des correctifs aux schémas originaux sont parus dans Radio-Plans n° 454 de septembre 1985. by J. Lassus:

2. Radio-Plans N° 479 (octobre 1987) et 481 (décembre 1987): réalisation de Mr. Lassus différant du précèdent par le système de correction des effets de sol).

I suspect this MD design is a non-motion type. One of the advantages of using motion detection is that this drift problem is effectively ignored.

[Max]

I suspect this MD design is a non-motion type. One of the advantages of using motion detection is that this drift problem is effectively ignored.

Hi Qiaozhi,
I totally agree. It's a non-motion for sure. Because small temperature changes and drifts can be cancelled easy by motion mod there. I did that way and this solved ANY problem. But also tube-mod is effective mantaining non-motion design.

Best regards,
Max