# unijunction needed

A

#### alp soandso

Jan 1, 1970
0
Looking for a part made by ssi.

MSU 112-2

I need one. Will a few more if absolutely necessary and price is low. Not
looking to buy $200 minimum. I live in Canada. you can contact me by email by removing the first alp in the address. J #### Jamie Jan 1, 1970 0 alp said: Looking for a part made by ssi. MSU 112-2 I need one. Will a few more if absolutely necessary and price is low. Not looking to buy$200 minimum.

you can contact me by email by removing the first alp in the address.
I can't seem to find a spec on that part? Maybe if you knew some more
information on it, like the Peak Voltage, valley etc on the emitter one
could be dropped in.

I have some unijunctions in my drawers and as far as know, they didn't
make to many different types.

Jamie

A

#### alp soandso

Jan 1, 1970
0
I don't know anything about the device. I have a delay timer/relay that
is supposed to activate a cryogenic compressor after a roughing pump has
run for about one minute. It suddenly stopped working. I opened the timer/
relay.

It is a Potter Brumfield CUF-41-30120.

It has very few components wired up between 8 contacts or so. I have
replaced the capacitors (two of them) and the diode checked out fine, but
I put a new one in anyway. The resistors look fine, I have not tested
them yet nor did I plan to. I believe the coil is fine, but I will have
to unsolder a few parts to make sure I am not reading this wrong. The
prime suspect right now is the MSU 112-2, which I have been told is a
UJT, perhaps a PUJT (PUT). I wouldn't know where or how to measure the
peak voltages/current..... I would be poking around inside a relay with
240 v and several amps behind it. Not something I want to do with the
part installed on the device and the device running.

Ian, thanks for the links. If I study hard and long enough I will be
able to come up with a replacement of my own manufacture. I doubt I have
long enough to live however (I joke, but in reality my capacity to learn
new stuff has dropped off significantly over the last couple of years).
They should at least get me pointed in the right direction if I have to
try (and don't wish to pay the 200-300 bucks for a new timer/relay).

Thanks
all.

Alp

J

#### Jamie

Jan 1, 1970
0
Jon said:
If this is just a stock delay relay module, why don't you just replace
it with a new stock relay? Much easier than trying to source a part
that has been obsolete for 40 years! If there's another reason that you
have to repair this module, why not put a 555 or similar timer in there?

Jon
COST!!!!

Jamie

A

#### alp soandso

Jan 1, 1970
0
On Tue, 27 Sep 2011 14:14:00 -0500, Jon Elson wrote:

Yes, Jamie is correct, the cost of the device is ridiculous.

I would be able to build something (hack) if I could figure out how this
circuit works. With all the pdf links I got from the other newsgroup
posting I should be able to figure something out.

Thanks again
!
Alp

E

#### ehsjr

Jan 1, 1970
0
alp said:
I don't know anything about the device. I have a delay timer/relay that
is supposed to activate a cryogenic compressor after a roughing pump has
run for about one minute. It suddenly stopped working. I opened the timer/
relay.

It is a Potter Brumfield CUF-41-30120.

It has very few components wired up between 8 contacts or so. I have
replaced the capacitors (two of them) and the diode checked out fine, but
I put a new one in anyway. The resistors look fine, I have not tested
them yet nor did I plan to. I believe the coil is fine, but I will have
to unsolder a few parts to make sure I am not reading this wrong. The
prime suspect right now is the MSU 112-2, which I have been told is a
UJT, perhaps a PUJT (PUT). I wouldn't know where or how to measure the
peak voltages/current..... I would be poking around inside a relay with
240 v and several amps behind it. Not something I want to do with the
part installed on the device and the device running.

Ian, thanks for the links. If I study hard and long enough I will be
able to come up with a replacement of my own manufacture. I doubt I have
long enough to live however (I joke, but in reality my capacity to learn
new stuff has dropped off significantly over the last couple of years).
They should at least get me pointed in the right direction if I have to
try (and don't wish to pay the 200-300 bucks for a new timer/relay).

Thanks
all.

Alp

Yep, >\$200 is horrible. If you know the function(s) the relay
must deliver, you can build a circuit to do it using current
parts.

Some issues to determine concerning the function(s):
You say that the relay must operate after the roughing pump
has run for about 1 minute.

* What is the relay supposed to do if the roughing pump runs for
less than 1 minute?
* Once the relay activates, how/when is it supposed to de-activate?
* How much current at what voltage must the relay switch?
* How many poles must the relay switch?

If you have room in the machine, and are willing to re-wire a bit
if necessary, delivering the functions you need with an external
circuit is a piece of cake, provided of course that you know those
functions. OTOH, if you must create a drop-in replacement for (or
repair of) the bad part, that can be a PITA.

Ed

A

#### alp soandso

Jan 1, 1970
0
The PUT was much touted as being more stable and better for long delays
than the old style 2-base unijunction.

You stand a better chance of getting hold of a PUT these days - and if
you can't, its easy to do the same thing with a complementary pair of
transistors.

The circuit is easier to design as well, while the old style unijunction
has a set trigger point (subject to inevitable device tolerance) the PUT
is basically a thyristor with the gate at the anode end instead of the
cathode end.

Basically you set the trigger point where you want it to be (within
reason) by connecting the gate to a potential divider across the supply,
the anode goes to the C/R as did the "emitter" of the old style
unijunction.

As the C charges up to the trigger point; the anode becomes positive wrt
the gate and the PUT fires and develops a pulse across the cathode
resistor.

Thanks for taking the effort to explain. I managed to download a bunch of
files explaining this sort of thing as well, but my main background is
chemistry, not electronics. I know what the words mean, but they don't
make much sense right now.
Maybe I'll get lucky and find out it was the tantalum capacitor that went

Alp

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