Vintage germanium circuit

[Ian - G4JQT]

I have recently resurrected an old school project of 40 years ago. It's a magnetometer that we never got around to building.

A nearby changing magnetic field is supposed to trigger the relay, which latches on until reset. One gauss moving at one cm/second is supposed to be sufficient.

I know there are much better designs now, but for nostalgic reasons I'd like to build it in its original form. I've started, but not got very far...

The circuit uses (unconventionally drawn!) PNP germanium transistors and I have them in my workshop - all tested and working.

TR4 appears to be drawn in error with the collector/emitter reversed. (Corrected in my construction.) The pick-up coil I'm using at present is an old Post Office 2000Ω solenoid.

I've replaced the 50Ω relay with a 50Ω resistor for test purposes and running it from a 9 volt battery. (There is no back EMF protection diode across the relay on the circuit.)

The first stage appears to be working more or less like a conventional amp, but that's about all that's happening. At the Darlington pair TR2/3 everything seems to stop! No current flows through the 50 ohm temporary 'relay resistor', even with a volt or so tapped onto C1.

I can't really fathom how it's supposed to work. Is there something seriously wrong with the circuit? Is this worth perusing?

If I can get it going I'd like to try it with Si NPN transistors with decent gain.

Regards

Ian

[TrevorG3VLF]

I can't see how it works either. The relay is driven with AC which surely cannot be right or is the relay pulled in by the first edge of the signal. If so, then a complementary ouyput will not be necessary.

[G6Tanuki]

It does look odd .

Perhaps if the collector of TR2 were connected to the 9V rail? Then TR2/3/4/5 would be more like a classic "totem pole" class-B push pull audio amp.

Though even then R6 looks rather high-value: I'm more familiar with the resistor in that position being something like 680Ohm or 1K when using the old AC128/176 type Germanium transistors.

But it still doesn't explain how the relay's supposed to be pulled in when fed with AC.

[ukcol]

Would it make more sense if TR4 were NPN? An AC127 might suit.

Originally Posted by G6Tanuki

But it still doesn't explain how the relay's supposed to be pulled in when fed with AC.

Perhaps the relay has a latching contact enabling one pulse to latch it.

[Ian - G4JQT]

Thanks for the comments so far.

I've redrawn it in a more conventional way, and indeed it would make more sense to have TR4 as NPN with its emitter connected to TR5e and C5.

If contact RL/2 was placed in series with R6 (and the buzzer, RL/1 and 'reset' dispensed with for now) it might be possible that a pulse at C2:

1) switches on TR4 (NPN),
2) earths C5 which
3) momentarily activates the relay which
4) closes RL/2 putting
5) Vcc onto TR5 base via R6, but... then TR5 can't conduct because there's an NPN transistor in its emitter!

There's also some ambiguity around the buzzer earth. Perhaps buzzer -9V should be earth. Then when RL/1 closes the buzzer sounds.

But then what does the diode do once the relay has been activated?

All very odd.

Ian

[mhennessy]

Looks like contacts RL2 is to latch on the relay. The reset switch is presumable normally-closed.

I agree that it looks like TR4 ought to be an NPN, acting as an EF.

TR2 and TR3 are connected as a Darlington pair, and are both switched off until a large enough signal comes from pre-amp TR1. When that happens, the voltage at the collector of TR2/3 will fall from -9V to ground. C5 will energise the relay.

R6 is quite a high value, but then it just needs to provide operating current to the TR2/3 pair. It doesn't have to provide much in the way of base current to TR5, which of course it would need to in an audio amplifier. All TR5 is doing is providing a means to discharge C5 when the relay is switched off.

What's interesting is how the error with TR4 crept in. Incorrectly drawn as PNP, and incorrectly shown in the parts list

Cheers,

Mark

[John_BS]

Mark:

That OC71 darlington pair with 56k collector load: don't you think the leakage in TR2 would have TR3 switched on hard as the DC quiescent state? It only requires c 150uA total collector current. Certainly at elevated temperatures......

John

[mhennessy]

Hi John,

It's a good question - I hadn't considered that as a possibility. But if that's the case, how would the circuit work? Does reverse-biasing the BE junction reduce the leakage current, meaning the signal is triggered by the opposite part of the cycle? In which case, we need TR5 to saturate, taking the negative end of C5 "above" the 9V rail. If so, isn't R6 a bit on the high side?

I was brought up with silicon, so I've never really explored the leakage aspects of germanium

Mark

[orbanp1]

As for TR4's polarity in the circuit, bjt transistors can be used in the inverse mode in switching circuits.
The advantage of the inverse mode is that the C-E saturation voltage is a lot smaller than that of a "regularly" used transistor. The disadvantage is that the beta is also smaller.
When switching small currents, the small beta might not be a problem.

If this was the original intention of the designer, that is not clear.

As for myself, I have never seen a circuit with a transistor used in the inverse mode, but I have never worked on specialty switching circuits.
I did read about this in a text book on solid state switching circuits.

Regards, Peter

[Herald1360]

Originally Posted by mhennessy

Does reverse-biasing the BE junction reduce the leakage current, meaning the signal is triggered by the opposite part of the cycle?

Shorting base to emitter on a leaky germanium transistor reduces the C-E leakage noticeably compared to the level with the base open circuit. I believe that reverse biassing the B-E junction will reduce this further. Typically Ge transistors can stand higher B-E reverse voltages than Si ones, which is useful for a simple multivibrator running on a 9V supply.

Circuit wise, assuming that the various resistor values do make sense in practice, with no changing magnetic field, TR1 collector will be hard on, so its collector is near 0V. TR2/3 darlington should be cut off by the 0V on TR2 base. The centre point of TR4/5 will be somewhere near -9V as TR5 will be turned on via RL and R6. I think the blob at the bottom of the buzzer circuit should be at 0V.

If a changing field induces a rising positive voltage at PU coil/C1 junction, TR1 will turn off, the increasing negative voltage at TR1 will turn on TR2/3 via C2 which will turn on TR4 and off TR5 pulling the bottom of C6 towards 0V and turning on the relay. RL2 then latches the relay on and RL1 turns on the buzzer.

The whole thing relies on the field changing fast enough to turn off TR1 and far enough to hold it off for long enough for the relay to latch. Hopefully the various time constants associated with C1, C2 and C5 have been chosen to let it all work.

I'm sure it would be less device dependent with Si devices but it looks like apart from the missing 0V and wrong polarity TR4 it should work.

[John_BS]

The trouble is we only have the Icbo figure specified (12uA max, 4.5uA typ @ room temp).

As Mark says, if the pair was supposed to run conducting, the relay drive would require a -ve going pulse sufficient for it to latch, and then the 56k pull-down looks wrong. Also, with a 50 ohm relay coil, does the simple shunt 9v zener supply droop to something much lower when the relay latches? I think the circuit may have been derived empirically to work with a particular bunch of components! (assuming it ever did work, and that the schematic is correct )

[mhennessy]

I've done some more thinking about this...

Cutting a long story short, if TR4 was PNP, then its C-B junction will be forward-biased, and the base could never rise "above" ground. This is enough to convince me that the transistor must be NPN.

I suppose the next step might be to bread-board it. I have enough Ge transistors, but am pretty busy with other things today.

Oh, just a small correction re. TR1: it's not saturated; the collector voltage will be somewhere in the 5.5V region, depending on hfe. The voltage gain will be quite reasonable - perhaps 15-20 with the specified detector coil...

All the best,

Mark

[roffe]

I wonder...suppose that the circuit is correct after all(well,apart from some minor flaws)?
Then if we accept that all transistors are correct according to the parts list and if we turn TR4 upside down,then what do we have?A perfectly normal audio quasi-complementary amplifier.Complete with output capacitor and all!

R6 delivers a relatively low current base current for TR4 and TR5.

When the relay pulls,it connects to earth through the reset button,latches and simultaneously removes the base current from the output transistors.

The question is-is a 300uF cap(not charged) enough to pull down a 50 ohm relay?If it is,the circuit should work as a sort of class C amp that responds only to strong pulses and draws the output down towards earth.

I would have preferred a more sensitive relay-50 ohms is rather low.

And something is wrong with R7 in the power unit.It simply cannot deliver enough current for the relay and the rest of the circuit.It's value is too high.

[mhennessy]

With TR4 turned upside down, then its base won't be able to rise above 0.2-0.3 volts. If you want to make it into a quasi-complimentary stage, you'd need an NPN driver.

I agree that the relay does seem to be rather low impedance, and would draw 180mA at 9V. Of course, it won't need the full 9V to keep it pulled in, but even so...

No specification is given for the mains transformer, but I tend to agree about R7. Suppose we have 180mA flowing through an 820 ohm resistor - Ohm's law tells us there should be nearly 150 volts across it! Clearly something wrong there

I assume that 50 ohms is a mistake - perhaps 500 ohms is more realistic. Even so, R7 might still be on the high side, but again, we don't know anything about the mains transformer.