Schematics for Spectech ST-360

This area is for discussions involving any fusion related radiation metrology issues. Neutrons are the key signature of fusion, but other radiations are of interest to the amateur fusioneer as well.
Post Reply
User avatar
Dennis P Brown
Posts: 3159
Joined: Sun May 20, 2012 10:46 am
Real name: Dennis Brown

Schematics for Spectech ST-360

Post by Dennis P Brown »

Here are the two most important schematics:
Attachments
ST360PCB-2.pdf
(64.88 KiB) Downloaded 371 times
ST360SCH-1.pdf
(267.58 KiB) Downloaded 400 times
User avatar
Richard Hull
Moderator
Posts: 14992
Joined: Fri Jun 15, 2001 9:44 am
Real name: Richard Hull

Re: Schematics for Spectech ST-360

Post by Richard Hull »

U15 does look like a possible culprit as does DA1.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
User avatar
Dennis P Brown
Posts: 3159
Joined: Sun May 20, 2012 10:46 am
Real name: Dennis Brown

Re: Schematics for Spectech ST-360

Post by Dennis P Brown »

Richard, I too think the chip U-15 is the issue; however, DA-1 could be, too (it seems to pass a diode check on a meter but not too sure that the voltage readings by the meter in diode mode make sense (short of a full failure.)) It is a strange single chip with three leads (forming two diodes in series.) No idea how to determine spec's for a replacement since the tiny chip has little to go on.

The issue for me is the schematic is absolutely wrong on how DA-1 connects in the circuit: it does, in fact, connect at the wire cross point in the drawing despite the schematic indicating it does not (no 'dot' to indicate a connection at that point.) In the circuit is does connect at that point.

I checked the scintillation cap's; they measure 0.954 nano-F; which is the additive value of 470 pF together since they are in parallel; will have to test their voltage leakage but that is for another day.
User avatar
Richard Hull
Moderator
Posts: 14992
Joined: Fri Jun 15, 2001 9:44 am
Real name: Richard Hull

Re: Schematics for Spectech ST-360

Post by Richard Hull »

DA-1 is there to theoretically, limit the voltage excursion on that line to there reverse voltage ~.7v.

Richrd Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
User avatar
Richard Hull
Moderator
Posts: 14992
Joined: Fri Jun 15, 2001 9:44 am
Real name: Richard Hull

Re: Schematics for Spectech ST-360

Post by Richard Hull »

DA-1 is there to theoretically, limit the voltage excursion on that line to their reverse voltages ~.7v. This is common to circuitry where bi-directional spikes might be encountered.

Richrd Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
Rex Allers
Posts: 570
Joined: Sun Dec 30, 2012 3:39 am
Real name:
Location: San Jose CA

Re: Schematics for Spectech ST-360

Post by Rex Allers »

On the DA-1 diode -- it could be bad. The purpose is to clamp the input signal between 0 and 5V before passing it on to the AD823. The signal is AC-coupled from WP7 through C22 and its strangely named buddy. If the pulse at the diode goes above 5V the top diode will conduct, clamping the signal into the 5V supply. If the signal goes below 0V the bottom diode will conduct clamping it at ground (0V). So the diodes are there to limit what gets to the Op Amp. If the diode saw a really big pulse it could have either blown or shorted one of the diodes.

The small package is probably an SOT-23 which is very common for transistors or diodes, either single or dual. This configuration is called series connected. Other flavors might both diode anodes on pin 3 or both cathodes.

The diodes could be simple silicon switching but quite possibly are Schottky because those have lower forward drop and will keep the clamping closer to 0 to 5V range.

Here's a possibility for schottky:
https://www.onsemi.com/pub/Collateral/BAS70-04LT1-D.PDF
and one for silicon switching:
http://www.vishay.com/docs/85718/bav99.pdf

If you can read the code printed on the top of the SOT-23 package, it might be possible to figure out exactly what the diode is. Sometimes its hard, because it's sort of like a reverse telephone lookup.

But if both diodes seem to measure OK it may be fine. Should be .7V or less in the forward direction -- but not 0, and it also should look like open in the reverse direction.

The hardest part, without fancy rework tools, will be getting the part out -- either U-15 or the diode.
Rex Allers
User avatar
Dennis P Brown
Posts: 3159
Joined: Sun May 20, 2012 10:46 am
Real name: Dennis Brown

Re: Schematics for Spectech ST-360

Post by Dennis P Brown »

Good posts! I had to do a bit of study to follow how a clamped diode system works in general and how this one does as well (the details, that is.) Makes sense; however, a bit confused how the Op-amp can then see a signal (one on top of the 1200 volt nominal charge on the cap) if the high voltage is clamped by a few orders of magnitude - does the scintillation counter's voltage drop so low that this signal/value isn't suppressed?

Will check the diodes tonight - have the pin outs from the schematic and reread on diode testing; I will say the DA-1 diode package (three pin and only 2 to 3 mm long) has the ID: "44 S" and right next to this, vertically, "44" in smaller print.) Does not mean anything to me or in my searches about diodes. Do realize that the voltage clamps at 5 volts pos and anything negative so that diode must turn on under those conditions - the ones you posted do not appear to meet those criteria or am I confused (not unlikely.)
User avatar
Richard Hull
Moderator
Posts: 14992
Joined: Fri Jun 15, 2001 9:44 am
Real name: Richard Hull

Re: Schematics for Spectech ST-360

Post by Richard Hull »

There is no way stable DC high voltage can make it through a "coupling" capacitor. Only rapidly varying voltages can pass through, (signal pulses).
The sole purpose of the capacitor is to not let voltages greater than the forward drop (junction hill) of DA-1 get into the op-amp. However snapping on the HV supply can try and push through a high voltage pulse. This is why the high voltage is to be brought up slowly. DA-1 is just a second line of guardianship on the op-amp's input line.

Given the above, capacitors and blocking diodes can fail under extreme conditions.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
Rex Allers
Posts: 570
Joined: Sun Dec 30, 2012 3:39 am
Real name:
Location: San Jose CA

Re: Schematics for Spectech ST-360

Post by Rex Allers »

Sorry Dennis but -- Ah, the electronics-foo is weak in this one. Hope that doesn't come off too negative. If you want the ability to understand circuits maybe you should spend some time with basic electronics texts or tutorials. I'm no expert but consider myself to have advanced hacker-level electronic skills. But getting back to the ST-360...

If nothing is broken, pin 3 of the diode doesn't see the high voltage. It should only see the pulses coming back from the detector. The capacitors just to the left in the schematic block the DC but pass AC.

Here's a detail capture of the circuit with some labels I added.
ST-360 Schematic detail
ST-360 Schematic detail
In that, the left side is the internal HV supply. In the top, I have outlined the end of the HV multiplier circuit. The four capacitors circled are the HV output filter. At the bottom R4 is the first resistor of a divider chain so the unit can monitor the high voltage and use this as feedabck to keep it at a steady voltage level.

So the right side of D7 is the HV supply output. For the scintillator connection this feeds through SR31 (1M) to drive the Scint at WP7. Any signal pulses from the detector will come back through W7 superimposed on top of the HV DC.

The capacitor pair C5A1 and C22 (equal to one cap of 940 pF) sees both the HV and the pulses but since the HV is steady state DC it doesn't pass through the cap to the diode. So the diode only sees the changing pulses from the detector which normally should be much less than 5 volts peak-to-peak. If there does happen to be a transition outside of the 0 to 5V range, the diodes are there to eat the excess before it gets to the Amp (U15).

One more detail is that the HV passes to WP7 through SR31 resistor. This resistor also provides a high impedance for any signal pulse coming back, so they aren't eaten up in 4 HV output filter caps.

One thing that has been mentioned is that it is a good idea to ramp up HV slowly, rather than just clicking on, because the blocking caps on the input will pass fast transitions, so very high voltage could be applied to the diodes, blowing them.

I don't remember what you said about how you were using this unit when it died. I think you said higher voltages than normal were used. Were you using an external HV supply? It doesn't look like there's any way to isolate the internal HV supply, so if you need an external HV supply, you should probably have an external blocking cap in series to the Scint input (WP7 here).

---
On the subject of IDing the diode(s), I did a search on 'smd marking 44s diode'. It is a BAS40-04. That is Schottky, as I had guessed. Here's a datasheet linked from one of the returned sites...
http://www.s-manuals.com/pdf/datasheet/ ... diotec.pdf

They should be easy to find on Mouser, Digikey, etc.
Rex Allers
User avatar
Dennis P Brown
Posts: 3159
Joined: Sun May 20, 2012 10:46 am
Real name: Dennis Brown

Re: Schematics for Spectech ST-360

Post by Dennis P Brown »

First off, thank you all and especially Alex Allers for the time and effort you all have put into answering my questions; I've learned more digital electronics in this post then I have in all my previous efforts (wish I was joking.) Most of what you have both explained I now understand (been studying individual parts and how they work in more detail; even did some original reading (1970's!) on design/use of Op-amps) and have a working concept relative to this circuit; more importantly, what you all have explained applies to many other types of circuits - I have looked at a few other schematics (for both a GM and scintillation systems) and now see many of these details in those circuits, as well.

I now far more clearly understand how the signal is passed through the caps and why the HV is blocked. A bit dense at times but in time I do catch on ... . That info on the resistor (SR-31) was extremely helpful (and critical) since that device in that location had really confused me at first. That extra clarification really makes how the circuit functions far more understandable for me, and frankly, it was definitely not something that would ever have crossed my mind - especially relative to those HV caps filtering out the signal one wants to 'see' - what you said makes such good sense. The clamper also amazes me (now that I understand its function and how they operate; so simple but so very useful and ingenious) and these "little" details that are needed besides the Op-amp really helps explain what the various components do and how their special functions make this circuit actually function properly.

I am starting to better understand the "sensing" circuit for the HV supply; your additional comment about the sensing HV resistor really clarified that part of the circuit, as well.

This was not just a very good example in signal amplification design for a detector but also an outstanding explanation in the details that are required to make a circuit to behave in a manner that achieves what is needed.

Aside: Yes, I used an external HV supply but the ST-360 allows one to turn off its internal high voltage (and when used, the internal HV can be ramped up in 20 volt steps. For me, the issue was the unit only goes to 1250 volts and I needed 1950 volts for the neutron tube.) However, I did apply my high voltage too fast and that, I suspect, is the likely cause of the damage as you have pointed out.

How you found that diode you linked to is amazing; I did a lot of searches and came up even more confused. I now understand what that strange "S" like marking on the diode was (had no idea originally.) The data sheet shows the company's logo and it is that symbol!) So, the part you located is the correct one as in the circuit (and also, they show two other diode arrangements that are also used in the ST-360 circuit.) I will order all the parts to keep on hand.

You are absolutely correct that I should have had a blocking cap at WP-7 (I had intended to do that but in dealing with many issues (besides the HV build) had forgotten to incorporate that in my system - that was a very stupid over sight.)
User avatar
Dennis P Brown
Posts: 3159
Joined: Sun May 20, 2012 10:46 am
Real name: Dennis Brown

Re: Schematics for Spectech ST-360

Post by Dennis P Brown »

Well, one leg of the diode chip is bad (reads open for a fraction of a second then starts to conduct and show a 0.7 volt when it should read open.) So, if the diode took a hit, then it is very likely U-15 is bad (just received new U-15 chips but the diodes will be awhile before they arrive.)

The photo-multiplier does not detect neutrons significantly above the noise so, it isn't suitable.

The water cooling of my fusor chamber works great and keeps it fairly cool!
User avatar
Dennis P Brown
Posts: 3159
Joined: Sun May 20, 2012 10:46 am
Real name: Dennis Brown

Re: Schematics for Spectech ST-360

Post by Dennis P Brown »

Replaced the diode voltage clamp (micro-soldering while work under a microscope!) and the ST-360 works well with a scintillation detector (through, maybe a bit more noisy now.)

However, can't get it to work with my neutron tube - while I have a cap between the ST-360 (built into the voltage divider) and the applied power for the neutron tube (the voltage divider lowers the total voltage the ST-360 cap see's to around 1200 volts) my ST-360 signals a system failure when I go above 1750 volts on the neutron tube.

Will have to do more work to see if it is tube related or the ST-360.
Post Reply

Return to “Neutrons, Radiation, and Detection (& FAQs)”