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Hi Max
Thank you for the information, I have just took a look at it after installing it and I already like it.
Thank you for the information, I have just took a look at it after installing it and I already like it.
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Hi,
I have bad news for those who already built coils...
I made yesterday a coil with resistance = 2.5ohm like described... BUT it doesn't work on this oscillator at all.
The most of my PCB is populated so I tested it and oscillator doesn't work and checks confirms all is fine about circuit... so error is probably at coil only.
I assume who made measure had bad instrument, cause from simulation I see clear that coil series resistance must be under 1ohm, probably between 0.5ohm and 0.8ohm to work cool.
So... 1mH seems right to match 5Khz with 1uF BUT keep resistance lower... otherwise you'll get no oscillations!
I've still not tested but seems realistic to me that resistance is wrong...
If you use 35turns on 30cm diameter (94.248cm circ.) you'll get about 33meters of wire... for a resistance of (about 70ohm/1000m) 2.31 ohm total... but this just for exact 0.55mm diameter... and simulator say it will never work.
Kind regards,
MaxComment
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Ok, I've tested with an old big coil 700uH and 0.7ohm resistance... used in past for PIs... and oscillator is up and running.
Sure frequency is higher but is right using 700uH stuff... so 2.5ohm is sure a wrong value.
Kind regards,
MaxComment
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Max,
If you change the components in LTSpice to the correct models, the circuit works with 2.2 - 2.5 ohm coil (attached LTSpice output was run with coil series resistance at 2.5 ohm). The only model that I do not have that correspond with schematic is U1. I am currently btrying to find a model for a TLC074 that works wit LTSPice. The TLC074 has better noise specs that the TL074 and much much better than the 27M4.Comment
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Found SPICE models for TLC074, TLC27M4, and TL074. TLC074 causes low amplitude (4vpp) oscillations, slow oscillator startup. TL074 seemed to work best (see atachment). Coil series resistance set to 2.2 ohm in this run. With 2.2 ohm coil, peak coil current is 400mA.Comment
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Originally posted by KingJL View Post
Hi,
thanks for these informations... my problem is that I've used TL084 and it doesn't work with the 2.5ohm coil.
Troubles are still there I think. I used BC557 and BC547 but no way... if I replace the coil with one with lower series resistance it works (now I changed tx coil and get 6.8Khz at oscillator, still far from 5Khz).
I think the coil is pretty critical component here much more than in other designs... and I think changing op amp will not solve this.
Kind regards,
MaxComment
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I agree about coil being critical component. My simulations show that after 2.5 ohms, the coil current starts to decrease rapidly. Anything over 3 ohms stops oscillations. This is one coil where Litz wire may be very beneficial.Originally posted by Max View Post
The op-amp does have an effect (probably due to the slew rates), although I agree that it will not fix it. The TL074 so far works best in my simulations. All others have a slower oscillator startup or less coil current.
Regards,
J. L. KingComment
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Hi,Originally posted by KingJL View Post
I think a good solution could be using thicker wire... anything from 0.6mm to 0.85mm will be nice. The coil I've tested so far was made of 1mm copper wire!
but 1mm is too much for my taste... heavy copper and also too thick wound I don't like at all.
Litz could be an alternative... but the problem is finding so large litz wire... not so easy for me, I have the small kind but not thick ones.
I'm still wondering why I get so bad results with 0.5mm wire... but simulations show clear the effect of too resistance there, so there's nothing bad about 0.5mm but the fact oscillator stage is very critical this way.
I'm also wondering of Coinseeker schematics and data: use TL082... I've used TL084 for tests (and also LS404) and was a disaster!
With TL074 change nothing, same stuff... no oscillations on 0.5mm wire coil.
What to say... I must find some wire and wound again with thicker copper. I think also this is a case where aluminium wire, also square shaped, could be of help... but where to find some ?
Kind regards,
MaxComment
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Hi Max,Originally posted by Max View PostWhat to say... I must find some wire and wound again with thicker copper. I think also this is a case where aluminium wire, also square shaped, could be of help... but where to find some ?
Kind regards,
Max
There is sometimes aluminium wire in he degauss coil of TV sets,not all but some.Old Philips ones for example.
Regards,
Fred.Comment
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RELIC HAWK
Hello Max, Fred , Everyone , I was just wondering is the transistor BC-557 next to tx-coil correct which would be NPN. On the copied Cheap-Version of Relic-Hawk they are using BC-558 a PNP !!! Bad oscillation occuring at tx-coil because of this ? OR ,If Rev. 1.6 values are correct , then dc resistance , Coil wire gauge and number of windings are not o.k.??.............Thanks....EugeneComment
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Max,
The best stability seems to be obtained when the zener diode, connected to the base of the BC577C (the one that you had as a standard diode), is a 8.2V component. This does not solve the problem, just extends the threshold slightly.
It appears that we do not have sufficient feedback through the NPN to sustain oscillations with coil series resistance above 2.5ohm. That is why the op-amp type has effect. I think all of the oscillator components; coil, transistors, and op-amp are critical.
Regards,
J. L. KingComment
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Max,
Just to test out the theory that the op-amp has considerable impact, I substututed the op-amp for one with a very high slew rate (LT1814 quad opamp, Slew Rate = 750V/usec). Using an 8.2V zener for D1, I can use a coil with up to 3.4 ohms series resistance.
Regards,
J. L. KingComment
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Hello Max and other friends on the best forum. Regarding problems with Tx coil I have some other experience. I have original PCB of COINSEEKERP2Pro and appropriate coil DD (D = 265mm).Device has a good discrimination metals, but very little of depth. Coil Tx has 3.75ohm resistance and oscillator for a stable 4875Hz. But how much windings coil has and what is his diameter wire I don’t know. Beside that I tried on proto board to put the same configuration oscillator with the same components and with coil of COINSEEKERP2Pro, and it did not work.
Best regards
VujoComment
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Hi,Originally posted by KingJL View Post
yes you're right. All components there are critical. I think the very bad story is about transistors... cause I truly think in original RH they couldn't be BC557 or BC547 ("european" kind) but some exotic transistor type... maybe also selected about hfe and similar. About IC same stuff: probably original have nothing to do with TL0xx series but it was maybe an TS27M4 or similar stuff: BTW I tested the 27M4 there and change nothing so other stuff is involved sure.
I strongly think coil resistance is a key factor too... and if wire diameter is something 0.7 or 0.8mm it's clear that the lowering of resistance will made easy the oscillator start running properly.
I think, instead, zener value is like in original cause it's easy/cheap stuff and people who made schematic of e.g. coinseeker put it there unmodified.
Kind regards,
MaxComment
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Hi,Originally posted by VUJO View Post
thanks for informations. Who did you measure that value ? Multimeter ?
I ask cause many (also good ones) multimeters have an offset of 1-2ohm about usually.
If so... and you read 3.5 ohm there was probably 1.5 or 2.5 ohm instead , or anything between.
In severe cases the offset could be about same extent or more than real resistance being measures... so you read 3.5 but are 0.5 or 0.4 there.
I suppose the standard ML coil there is very low on resistance, but cause we don't know components used in original RH we can only guess... so is possible also that coil is really 3.5 or 2.5 but works cause of components used.
Kind regards,
MaxComment
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