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Old 5th Mar 2010, 11:02 pm   #1
jimmc101
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Default Some thoughts on diode probes

There is a thread running in another section about a Taylor Model 22 electronic fault finder. Rather than hijack that thread I've started this one.

Looking at the original circuit of the diode probe I noticed that there is no DC path on the anode of the diode, the circuit must rely on the reverse leakage current of the diode for any rectification/detection to take place.
(Without any leakage C1 would charge to the peak voltage of the input and then no further current could flow.)
I wonder why this was done, leakage current is not a well defined parameter and in any case the rectification efficiency / sensitivity is going to be poor.

Another thing, if the probe is connected to a high voltage, say 350v and then connected to ground the diode is going to be reversed biased by the 350v stored in C1. OK it's not much energy but the again it is a point contact diode. Maybe this is what 'improves' the leakage.

Adding a resistor (second Fig) helps but it would be more efficient to raise the load resistor (R2) and lower the new one (R3) and change C2 to suit.

Adding a second diode (third Fig) improves efficiency and prevents breakdown of D1. (D2 is protected by D1 since the output is connected direct to the grid of a valve (not shown) and thus clamped at a small positive voltage.)

As an aside, to use a thermionic diode a shunt circuit would have to be used to avoid problems heater - cathode capacity. (last Fig)

Taylor must of had a good reason to do what they did, but what was it?

Jim
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Old 5th Mar 2010, 11:16 pm   #2
kalee20
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Default Re: Some thoughts on diode probes

I've seen many circuits where a germanium diode is used in series with a coupling capacitor. Like you, I wonder that it's obviously intended to make use of diode leakage, which is a bit inelegant insofar as it's highly temperature dependent.

Your Fig 3 is what I would use, if constrained to use s*m*c*nd*ct*rs as it is easily the best (although given a free hand Fig 4 would be my preferred option).
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Old 5th Mar 2010, 11:23 pm   #3
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Question Re: Some thoughts on diode probes

Hi Jim,

I follow the point your making about a d.c. path for the anode of the diode not being present, but is one really needed? With a sine-wave input at the probe tip, C2 will charge up to the peak +ve value of that waveform. When the waveform is -ve, the diode fails to conduct and C2 discharges through R2. Therefore, the +ve peak value of the input sine-wave is passed on to the measuring circuit. C1 is there to provide a d.c. block; this is an RF probe, so the measurement of d.c. voltages with it is not relevant.

Something adrift with my thinking, perhaps?

Al. / Skywave.
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Old 5th Mar 2010, 11:44 pm   #4
kalee20
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Default Re: Some thoughts on diode probes

Well, C1 will charge up also and with a 'good' diode (ie negligible reverse leakage), once C1 is so charged, then the diode will never conduct again, C2 will discharge to zero via R2, and the measuring circuit thus reads zero!
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Old 6th Mar 2010, 12:07 am   #5
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Default Re: Some thoughts on diode probes

I think the original circuit just recognises that the diodes leakage resistance is quite low, so why add any more to it.

By the way, the valve probe (the CT54 uses something like the above) has the advantage over the semiconductor that it works down to a much smaller voltage since there isn't a minimum forward voltage drop (because the heater supplies the necessary energy).
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Old 6th Mar 2010, 8:30 pm   #6
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Default Re: Some thoughts on diode probes

Yes, but how much leakage is required?

As I see it the maximum value for the mean DC current trough R2 is equal to the reverse leakage current of D1, any greater and C1 must alter its charge.

Take for example a 10v pk-pk input voltage, ideally this would give a mean output voltage of 5v which would require a minimum leakage current of 50uA! (This ignores the effect of R1 and the diode's forward voltage drop.)

I have no data for the GEX13 other than VR = 30v max, IF = 30mA max. The closest device I have data for is the OA90 (30v, 45mA).

For the OA90 the leakage current (at 25'C) is given as 20uA typ at VR = 10v giving a typical maximum mean output of 2v.
(At 1.5v the leakage is given as 2.4uA typ so things don't get much better at lower voltages.)
Of course lower than typical leakage would give even lower output.

For the OA91 the typical production spread of leakage current (at 25'C) is given as 0.5uA min to 11uA max at VR = 10v
This implies that for the input of 10v pk-pk the mean voltage across R2 could be as low as 50mV.

Jim
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Old 6th Mar 2010, 9:46 pm   #7
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Default Re: Some thoughts on diode probes

At Thorns we used diode probes we made in the lunch break. Picture 3 on Jimmc101 is close but C1 was 220pf and R1 was missing (cap to diodes) then connected to a AVO8. This was used on all sorts of circuits to aid fault finding and worked very well indeed.
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Old 7th Mar 2010, 1:09 am   #8
MichaelR
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Default Re: Some thoughts on diode probes

Quote:
Originally Posted by jimmc101 View Post
Taylor must of had a good reason to do what they did, but what was it?

Jim
Jim, this kit must have appeared in the very early 60's , semiconductors were not as "cheap as chips "then. Moreover the probe design is being used for no more than a signal tracer than an accurate measuring instrument hence maybe it was all about costs.

I must say the thread is highlighting that even the most basic of circuit can have a lot to it.

Mike
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