Tunnel diodes

[WME_bill]

These rare components are often discussed. Either under Test Equipment, as currently the thread upon Tektronix453A, or under Sets & Parts wanted, currently Tektronix 284 Tunnel diode.
For those new to these, I attach a summary of the types from GE (USA), who were the main manufacturer, with notes of the comparable RCA, Mullard, STC, Siemens and TSF versions. Very specialist microwave types were also made by Microwave Associates, Marconi and others, but at a price and not easily adapted to the usual trigger generator.
Use of an ohmmeter to test these devices is the quickest way to destroy them. I show a static method, and also that using a scope which is delightfully easy to set up and use. Just always be careful to start the current at minimum and increase it slowly until you see the inverted V slope. Your concern is to measure the first peak current, and see that there is a drop in voltage to the dip after, usually about 300mV.
wme_bill

[Bazz4CQJ]

Thanks for that info Bill; it inspired me to dig out some components that have been tucked away in my treasure for 40 years, some 1N3862's.

I couldn't recall if these are tunnels, but I found this http://www.google.co.uk/url?sa=t&rct...57799294,d.bGQ which suggests that they are tunnel rectifiers rather than diodes, but I couldn't understand the distinction? What can I usefully do with them; any good for a the timebase of a wobulator?

B

[Radio Wrangler]

There is a subgroup of tunnel diodes called 'Backward diodes' they have the usual characteristic in the forwards direction, but the reverse direction is more interesting in that they can be used as rectifiers, well 'detectors' is more likely a name, for signals down into the sub milliwatt region. They can be found in the most sensitive RF heads for some HP and R&S RF power meters. I think the forwards characteristics are somewhat compromised to get the strange zero-threshold reverse capability. Too specialised to do anything but their intended purpose. Also, very fragile.

DAvid

[Herald1360]

Originally Posted by Bazz4CQJ

I couldn't recall if these are tunnels, but I found this http://www.google.co.uk/url?sa=t&rct...57799294,d.bGQ which suggests that they are tunnel rectifiers rather than diodes, but I couldn't understand the distinction?

The info in that book seems to be a bit odd. Rectifier and diode are terms used usually in the same sort of way as rope and string (a sort of vague indication of size). If anything, the "rectifiers" seem to be lower current devices than the "diodes".

In any case if it looks like a duck, floats like a duck and quacks like a duck there's a good chance it is a duck!

[Bazz4CQJ]

Taking a second look, I found that most of the stache are 1N3862's, but there are others like a 3858, 3713 and a couple of BD7 back diodes. These were all picked up (used) around 1973 and might well be all duds - no idea!

The "high point" of the use of tunnel diodes for amateurs could have been the Dip oscillator (TDO !) Heathkit brought out, and which many still survive, as judged by comments in forums. It was jinxed, however, as the composite resistors used all drifted well away from their nominal values and the TD was a special made by GE for that one application and may not have been good device. It was claimed to run up to 250MHz, even though the kit was not optimised for that in terms of construction.

I will set up a little test rig and find out if my collect is wheat or chaff.

[PETERg0rsq]

An even simpler, but less informative (as in OK or Not OK) way of testing tunnel diodes can be found in the HP 3310A service manual.

One component, a signal generator, and an oscilloscope.

I will be trying WME_Bill's circuit, as may give more useful information on diode performance than the simple OK, NOT OK test i use.

Peter

[cmjones01]

What is it about testing tunnel diodes with a multimeter that kills them? Excess voltage? Too much current? Surely not all meters are born equal in terms of their ability to kill tunnel diodes...

[Herald1360]

Sounds a bit like not testing 50mA fuses with an AVO on ohms/100 range!

(They all check out as open circuit )

[Bazz4CQJ]

Some of the ones I have been looking at today begin to conduct at 50mV, so I suspect that 1.5V may not suit them much.

[cmjones01]

Fair enough - but the HP test circuit that Peter posted uses 3.5V p-p through 1kOhm, which isn't going to be any gentler than most multimeters, I'd have thought.

Chris

[Radio Wrangler]

The normal barrier for Germanium is around 300mV, so any tunnelling through the energy barrier, has to take place at lower voltages. If testing a diode is going to damage it, then the problem could either be too high a voltage on a high ohms range applied checking the reverse direction, or too high a current on a low ohms range checking the forwards direction.

There are also some RF Schottky diodes and some old point-contact diodes not up to handling an ohmmeter with a 15v battery behind it. Not to mention nearly all 5v, 3.3v etc logic.

David

[cmjones01]

That makes sense. I've just experimented with one of my bench Fluke 8600A multimeters, and its open-circuit voltage on all ohms ranges is 4V. Short-circuit current is a maximum of 1mA even on the 200 ohm range. Since a tunnel diode conducts in both directions, isn't it going to be tough to damage it with excessive voltage unless there's a lot of current available? I've got a couple of tunnel diodes here with Tektronix part numbers on them, I think, and I want to look after them...

Chris

[Radio Wrangler]

Fluke's 4v is very artfully chosen. Some transistors will start 'pining for the fjords' if the reverse base-emitter voltage exceeds about 5v. This might not be immediate destruction, but progressive reduction in gain, Ft and worsening of noise fig.

David

[WME_bill]

Tunnel diodes current rating.
The absolute maximum current quoted in GE Tunnel Diode Manual of 1961 is 5x the peak current forward, and 10x in reverse. They warn that the Germanium active element is very small, and easily burnt out.
As the diode is very low impedance both forward and back, it is the current flow that does the damage.
When testing, I will just exceed the peak current. That is quite sufficient to show the voltage swing from peak to valley, usually about 350mv.
Backward diodes are a specialist microwave detector, and so doped that there is virtually no tunnel diode negative resistance region. They conduct at around 200mV in reverse and 950mV forward. Confusingly called Tunnel Rectifiers, and still available. wme_Bill.

[woodchips]

On the principle that any information is better than none, I include a couple of pages I have just found out of an ancient Jermyn catalogue.

[jimmc101]

Those prices certainly show why they never caught on; pity, they were useful devices.

The full 'GE Tunnel Diode Manual of 1961' is available online.

(To quote from the archive... Copyright 1961 General Electric Co. Theory and applications of tunnel diodes.
Copyright not renewed in 1989, hence in public domain.)

Jim

[stuarth]

On the subject of damaging Tunnel Diodes or other components with a multimeter, you can work out the voltage and current for conventional analogue meters quite easily.

The equivalent circuit seen by the device under test is the ohms range battery in series with a resistance equal to the center scale resistance of the chosen range.

So for an AVO 8, the maximum voltage with the leads open circuit is the battery voltage, maximum currents into a short circuit (or diode with a low forward voltage) are
Ohms/100, 1.5V battery, 20Ohms center scale, 1.5/20=75mA
Ohms, 1.5V battery, 2kOhms center scale, 1.5/2k=750uA
Ohmsx100, 15V battery, 200kOhms center scale, 15/200k=75uA

I've always thought the Ohms/100 range is much more dangerous then the Ohmsx100 range, despite the latter's 15V battery. The 200k series resistor will limit the current to safe values for most devices.

DMVs and other electronic meters need more careful analysis, for example the Ohms circuit might include capacitors which could deliver too much peak energy to the device under test.

Stuart

[Humber888]

For owners of the Peak Atlas semiconductor tester, I've recently tried seeing if this will plot the forward I/V curve of a tunnel diode and it sort of works. First start up the DCA Pro software and couple up the tester via USB. Then select Graphs -> PN Junction and set the upper limit of Vs initially low (say 2V). If you know the Ip of the diode is, say, 10mA you can set the Vs higher (to 10V). The aim here is not to exceed the Imax of the diode, which is often about 2-5 times the Ip figure (the DCA75 can provide up to about 12V via 1kOhm). Only now connect the diode and press start. If nothing useful is plotted, ramp up Vs a bit. I have tried both 1mA and 10mA diodes and the tester does plot a peak current at about 60mV forward bias, decreasing to a minimum around 400mV, and then rising rapidly.

My DCA75 gives an Ip figure of about half the actual in all cases I tried (diodes also tested with one of the more conventional circuits described before in this thread), and the Imin is higher than actual, giving a flattened negative resistance plot. However, for a quick go-nogo test this seems acceptable. I doubt that the DCA75 will plot diode curves for those with Ip above about 10mA due to the maximum current available from the tester. Hope this is vaguely useful.

Mike