unijunction equivalent circuit?

So is there any simple circuit using conventional NPN and PNP transistors?

I've seen this circuit
http://sub.allaboutcircuits.com/images/quiz/02139x02.png in _Sourcebook of
Electronic Circuits_, but it doesn't really work- it latches but does not
release when used in a relaxation oscillator.  Perhaps it did work with
lower gain transistors.

-- 
/*  jhallen@world.std.com AB1GO */                        /* Joseph H. Allen */
int a[1817];main(z,p,q,r){for(p=80;q+p-80;p-=2*a[p])for(z=9;z--;)q=3&(r=time(0)
+r*57)/7,q=q?q-1?q-2?1-p%79?-1:0:p%79-77?1:0:p<1659?79:0:p>158?-79:0,q?!a[p+q*2
]?a[p+=a[p+=q]=q]=q:0:0;for(;q++-1817;)printf(q%79?"%c":"%c\n"," #"[!a[q-1]]);}

On Thu, 29 Aug 2013 22:34:19 +0000 (UTC), jhallen@TheWorld.com (Joseph
H Allen) wrote:

>
>So is there any simple circuit using conventional NPN and PNP transistors?
>
>I've seen this circuit
>http://sub.allaboutcircuits.com/images/quiz/02139x02.png in _Sourcebook of
>Electronic Circuits_, but it doesn't really work- it latches but does not
>release when used in a relaxation oscillator.  Perhaps it did work with
>lower gain transistors.

You need at least one Vbe resistor (on the NPN) to establish a holding
current trip point.
		
                                        ...Jim Thompson
-- 
| James E.Thompson                                 |    mens     |
| Analog Innovations                               |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| San Tan Valley, AZ 85142   Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.

On Thu, 29 Aug 2013 15:49:18 -0700, Jim Thompson
<To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote:

>On Thu, 29 Aug 2013 22:34:19 +0000 (UTC), jhallen@TheWorld.com (Joseph
>H Allen) wrote:
>
>>
>>So is there any simple circuit using conventional NPN and PNP transistors?
>>
>>I've seen this circuit
>>http://sub.allaboutcircuits.com/images/quiz/02139x02.png in _Sourcebook of
>>Electronic Circuits_, but it doesn't really work- it latches but does not
>>release when used in a relaxation oscillator.  Perhaps it did work with
>>lower gain transistors.
>
>You need at least one Vbe resistor (on the NPN) to establish a holding
>current trip point.
>		
>                                        ...Jim Thompson

A quick surf produces this....

* http://www.duncanamps.com/spice/miscsemi/2n2646.sub
*
*Default N-Channel Unijunction Transistor
.SUBCKT XUJT 1 2 3
DE 1 4 EMITTER
VE 4 5 DC 0
HVE 6 0 VE 1K
RVE 0 6 1MEG
BBB 5 7 I=0.00028*V(5,7)+0.00575*V(5,7)*V(6)
CBB 5 7 35P
***RB1 7 2 38.15 RMOD
***RB2 3 5 2.518K RMOD
RB1 7 2 RMOD 38.15
RB2 3 5 RMOD 2.518K
.MODEL RMOD R TC1=.01
.MODEL EMITTER D (IS=21.3P N=1.8)
.ENDS XUJT

* Motorola IP=.5U IV=6M VB1(sat)=3 Rbb=6.1K Vob1=3.6: E, B1, B2
.SUBCKT X2N2646 1 2 3
DE 1 4 EMITTER
VE 4 5 DC 0
HVE 6 0 VE 1K
RVE 0 6 1MEG
BBB 5 7 I=0.00028*V(5,7)+0.00575*V(5,7)*V(6)
CBB 5 7 35P
***RB1 7 2 38.15 RMOD
***RB2 3 5 2.518K RMOD
RB1 7 2 RMOD 38.15
RB2 3 5  RMOD 2.518K
.MODEL RMOD R TC1=.01
.MODEL EMITTER D (IS=21.3P N=1.8)
.ENDS X2N2646

This is in B-source notation.  In your Spice variant you may need to
replace BBB with GVALUE notation.
		
                                        ...Jim Thompson
-- 
| James E.Thompson                                 |    mens     |
| Analog Innovations                               |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| San Tan Valley, AZ 85142   Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.

Can you use a pseudo PUT or do you need a UJT?

If a PUT works, try:   http://www.4qdtec.com/putpr.html

Steve 

On a sunny day (Thu, 29 Aug 2013 22:34:19 +0000 (UTC)) it happened
jhallen@TheWorld.com (Joseph H Allen) wrote in <kvoi9b$r1n$1@pcls7.std.com>:

>
>So is there any simple circuit using conventional NPN and PNP transistors?
>

I dunno but I have a bunch of 2N2646 UJTs from ebay for cheap.
Those work OK.

On Thu, 29 Aug 2013 22:34:19 +0000 (UTC), the renowned
jhallen@TheWorld.com (Joseph H Allen) wrote:

>
>So is there any simple circuit using conventional NPN and PNP transistors?
>
>I've seen this circuit
>http://sub.allaboutcircuits.com/images/quiz/02139x02.png in _Sourcebook of
>Electronic Circuits_, but it doesn't really work- it latches but does not
>release when used in a relaxation oscillator.  Perhaps it did work with
>lower gain transistors.

You need to keep the value of (RB1 || RB2) << the timing resistor, and
maybe a high-value B-E resistor on the NPN. 

A synthetic PUT made with discrete transistors won't have the "gate"
to "anode" voltage capability of a real PUT (typically st like 40V)
due to the B-E breakdown voltage. 

Flipping the PNP to get a PNP with high B-E breakdown and miserably
low beta might work, but the peak current will be increased (reducing
the maximum value of the timing resistor). 


Best regards, 
Spehro Pefhany
-- 
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
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