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> 2N4891 Unijunction Transistor sub?, Please help us!
ronnie
Posted: January 16, 2006 12:10 am
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Hi everyone,

My son & I are building a transistor organ. The schematic is available at http://www.aaroncake.net/circuits/torgan.htm but I am having trouble finding the transistor I need, 2N4891. Does anyone have any idea where I can find this part in Toronto or any idea if another transformer part # would work? Thanks in advance for any help!

Veronica
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Geek
Posted: January 16, 2006 09:42 am
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Hi,

2N6028 is still common and available from Newark in TO for a quarter smile.gif


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ronnie
Posted: January 16, 2006 07:19 pm
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Thanks very much! So I can use 2N6028 instead of 2N4891? I guess 2N4891 is outdated?
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Geek
Posted: January 17, 2006 09:34 am
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Given that I haven't used a 2N4891 personally, I can't guarantee it will sad.gif


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awright
Posted: January 30, 2006 09:42 am
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The 2N6028 is a PROGRAMMABLE unijunction transistor (PUT), while the 2N4891 is an original type unijunction (UJT). The PUT can be programmed with two resistors to have the properties of a 2N4891 that are important to your application, but the PUT and UJT are not directly interchangeable in the absence of the programming resistors. Without the two "programming" resistors, the PUT won't do anything useful.

The difference is that the 4891 had its unijunction properties created by the impurity doping and processing of the semiconductor bar that it is made from, while the PUT is basically an integrated circuit made up of transistors interconnected so that it can simulate (very well) the properties of a UJT. In fact, a PUT is much more versatile than standard UJTs and can be programmed to have properties not feasible in a UJT. It generally takes just two resistors to program the properties you need to simulate the 4891, but that means three components for each UJT stage, rather than one. Not at all difficult, but more hassle and you would need extra mounting holes in your printed circuit board, if that is what you are using.

Don't be discouraged if you can't find UJTs to buy. If you obtain the spec sheet and application notes for the 2N6028 (or any other PUT) they will give you all the info you need to select the value of the two resistors required to program the PUT to simulate the 2N4891. However, I think the path of least resistance is to buy 2N4891s. Google "2N4891," and a bunch of suppliers of obsolete devices come up.

Have fun

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awright
Posted: January 30, 2006 10:42 am
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Ronnie, if you can only obtain a PUT, but not a UJT, post the type of PUT you obtained here and we can provide you with the values of the resistors you need to program the PUT to simulate the 2N4891.

Sorry that your introduction to electronics is turning into a hassle. It's neat that you are doing an electronics project with your son. Not enough kids are interested in electronics these days.

awright

This post has been edited by awright on January 30, 2006 10:47 am
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cparks411
Posted: May 10, 2011 08:06 pm
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I am using a 2N6027 PUT instead of the 2N4891 Unijunction transistor. What resistors will i need to add to make it similar?
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CWB
Posted: May 11, 2011 02:36 am
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uhhhh ...
you might want to start a new topic ...
this thread died 5 years ago .


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