Detection distance for a US nickel and quarter

[6666]

A simple test is to use a a bench test meter set to a range of 0 - 200ohms. short the gate to the source and take to the -ve probe, and connect the drain to the positive probe. I checked what the voltage was between probes on the 200ohm range and it was 300mV. Data states that the conditions for measuring rDS(on) are = or < than 100mV. However, 300mV is not too far removed, so I was happy with that. I then selected two J113's that measured 2.05V VGS(off) and measured the resistance on the testmeter. One was 53ohms and the other 54ohms. This is a good enough match considering that the jfet has a resistor in series with the drain, usually of a few kilohms. Latterly, I have used a NPD5566 which is a dual matched J113. Hard to get nowadays though.

Eric.

Thanks Eric I have tried your method and it works nicely, I should be able to rip through my bag of 50 J113's cheers

[Ferric Toes]

Hello Eric, thanks for the reply, thats an interesting approach using VGS(off), most of the testers I have seen used IDSS for matching, I have a digital transistor tester and looking at the display it gives three readings, one is Zero current @ XX volts, one j113 tests at 2.7 volts so I guess that is VGS off, I dont have a handbook for it.
But IDSS is all over the place.
I will build this manual simple tester , I think it will give a better indication of VGS, we are getting down to zero temperatures over night now , hard to get into work shop

A simple test is to use a a bench test meter set to a range of 0 - 200ohms. short the gate to the source and take to the -ve probe, and connect the drain to the positive probe. I checked what the voltage was between probes on the 200ohm range and it was 300mV. Data states that the conditions for measuring rDS(on) are = or < than 100mV. However, 300mV is not too far removed, so I was happy with that. I then selected two J113's that measured 2.05V VGS(off) and measured the resistance on the testmeter. One was 53ohms and the other 54ohms. This is a good enough match considering that the jfet has a resistor in series with the drain, usually of a few kilohms. Latterly, I have used a NPD5566 which is a dual matched J113. Hard to get nowadays though.

Eric.

[dbanner]

Is there a reason it should be done?

Clean switch off, no leakage. Not sure at these switching rates, just curious to know if there was any benefit to be had. Perhaps it obviates the need for matching pairs also, since the line is pulled to ground at the instant of the pulse step. Don't know, just speculation, ideas.

[green]

Eric, what is your opinion on pulling the line to ground during the off cycle by using a second jfet driven by the inverse pulse . Nonsense or is there another reason why this should'nt be done.

Is there a reason it should be done?

[dbanner]

What I measure is VGS(off) which is the gate source cutoff voltage. This appears to have a direct relationship to the drain source on resistance, which is what we are really interested in. So if we have two devices which have the same VGS(off) then the on resistance should be a reasonable match. The National data book I have only gives a maximum value for rDS (on) of 100 ohms. No minimum or typical. Data gives VGS(off) as between -0.5V and -3V. I have just measured one of my J113's and got a reading of -2.6V.

Eric.

Eric, what is your opinion on pulling the line to ground during the off cycle by using a second jfet driven by the inverse pulse . Nonsense or is there another reason why this should'nt be done.

[dbanner]

There is no issue with the rise time of the gate drive pulse. So Vgs pinch off and on resistance are the parameters for matching. I saw on Cscope schematic that FETs should be matched to within +/- 0.25V pinch off. FETs with close Vgs off having similar on resistance, well you could measure that by monitoring the Idss.

[6666]

What I measure is VGS(off) which is the gate source cutoff voltage. This appears to have a direct relationship to the drain source on resistance, which is what we are really interested in. So if we have two devices which have the same VGS(off) then the on resistance should be a reasonable match. The National data book I have only gives a maximum value for rDS (on) of 100 ohms. No minimum or typical. Data gives VGS(off) as between -0.5V and -3V. I have just measured one of my J113's and got a reading of -2.6V.

Eric.

Hello Eric, thanks for the reply, thats an interesting approach using VGS(off), most of the testers I have seen used IDSS for matching, I have a digital transistor tester and looking at the display it gives three readings, one is Zero current @ XX volts, one j113 tests at 2.7 volts so I guess that is VGS off, I dont have a handbook for it.
But IDSS is all over the place.
I will build this manual simple tester , I think it will give a better indication of VGS, we are getting down to zero temperatures over night now , hard to get into work shop

[dbanner]

[Ferric Toes]

Hello Eric, just received a bunch of J113 fets, I have a transistor tester that tests jfets, what is the most important parameter to look for when trying to match them IDss ? thanks.

What I measure is VGS(off) which is the gate source cutoff voltage. This appears to have a direct relationship to the drain source on resistance, which is what we are really interested in. So if we have two devices which have the same VGS(off) then the on resistance should be a reasonable match. The National data book I have only gives a maximum value for rDS (on) of 100 ohms. No minimum or typical. Data gives VGS(off) as between -0.5V and -3V. I have just measured one of my J113's and got a reading of -2.6V.

Eric.

[6666]

Hello Eric, just received a bunch of J113 fets, I have a transistor tester that tests jfets, what is the most important parameter to look for when trying to match them IDss ? thanks.

[6666]

As long as they are equal width.

Thanks, should have known.

[Carl-NC]

Does coil size effect indoor testing if coil is stationary and coin is swept over coil?

Static metal can deflect the decay curve enough to overload the demods. You can compensate with a manual offset adjust. Larger coils also couple in more EMI.

Do the long samples still eliminate EFE ?

As long as they are equal width.

[6666]

Do the long samples still eliminate EFE ? I am avoiding people as much as possible, damn virus

[green]

By the end of today I will have a detector using long sample pulses ready to test in the field outdoors. Timing is 350uS TX pulse; 2A peak current; p.r.f. 520; first delay 15uS, sample 1 & 2, 150uS; intersample delay 500uS (end of 1 to start of 2). Workshop range test on quarter looks good. Equivalent size cupro-nickel shilling a bit less range, but still good. Tests so far on 8in coil, but need to use 10inch or 12inch which is not possible indoors due to surrounding metal. Hope to test on the beach in due course but too many people there at the moment disregarding the safe distancing rule to get out in the sun. I have to be especially careful as in the very high risk category.

Eric.

Does coil size effect indoor testing if coil is stationary and coin is swept over coil? Interested in your test results.

[Ferric Toes]

By the end of today I will have a detector using long sample pulses ready to test in the field outdoors. Timing is 350uS TX pulse; 2A peak current; p.r.f. 520; first delay 15uS, sample 1 & 2, 150uS; intersample delay 500uS (end of 1 to start of 2). Workshop range test on quarter looks good. Equivalent size cupro-nickel shilling a bit less range, but still good. Tests so far on 8in coil, but need to use 10inch or 12inch which is not possible indoors due to surrounding metal. Hope to test on the beach in due course but too many people there at the moment disregarding the safe distancing rule to get out in the sun. I have to be especially careful as in the very high risk category.

Eric.