UJT circuit design problems - UK Vintage Radio Repair and Restoration Discussion Forum

Oldcodger · 4th May 2015, 10:50 pm

Looked like the best place to post.

I was helped with a circuit for an electronic organ on another topic. I can’t remember any theory on UJT from my studies, so looked up an on line tutorial on UJT, to find that the circuit worked as I suspected. Looking a bit further, I found the intrinsic stand of ratio(ITR) of a 2n2646 was .65, and that the time constant of the wave form was

T=R*C*Ln (1/1-ƞ)---( where ƞ is the ITR.

From the text on the design the frequencies available are centred on middle C, so top frequency is 493.88 HZ. (B4).
This equates to T of .002 secs. And from circuit (using half the value of the pot in the adjust circuit), this gives lowest resistance as 12.5k.
1/(1-.65) = Ln of 1.05082

This gives a T of .0006 = freq of 1666hz.

Anyone know what/why the results are not consistent ?

kalee20 · 5th May 2015, 12:32 am

External resistors in series with the bases will modify the frequency considerably - does the circuit feature such resistors?

Skywave · 5th May 2015, 12:47 am

Quote:

Originally Posted by Oldcodger

[snip] using half the value of the pot in the adjust circuit [snip]

And you stated 12.5 kΩ, that being half of 25 kΩ. But isn't there a fixed value R in series with that pot. on the cct. diagram? If so, then the 12.5 kΩ value is incorrect.

However, design maths is fine for certain applications, but when it comes to designing oscillators with UJTs, I've always used the practical approach: it's simply less hassle!

I start with a constant-current generator - say 10 mA - and pass that current through a fixed 4k7 resistor + 25 kΩ pot., wired in series. Now the µF value of the charging capacitor will obviously depend on the required centre freq., so I can't be too specific here, since I am talking about my method as a 'design approach'. Let's say I have chosen 1 µF. A quick check with the 'scope will tell me if I am near to the required freq. If I'm not, then I change the value of C accordingly, bearing in mind that the required µF value of C will be inversely proportional to the required freq., (For a fixed mid-value of the series R and a given UJT.)
Finally - continuing the 'practical theme' - if the resultant waveform is taken across the capacitor (which will be a linear ramp in my example), I always use a FET as a source-follower to buffer the oscillator waveform to the next stage.

Al.

Karen O · 5th May 2015, 9:41 am

I have been playing with a UJT myself recently. The thing to watch (in my opinion) is that the current into the emitter is allowed to fall below the holding current, otherwise the UJT will never switch off. In practise this means the charging resistor cannot be too small.

I've seen circuits proposed for UJTs which I'm certain won't work for this very reason.

camtechman · 5th May 2015, 10:10 am

Have you taken a look at the "original" Stylophone circuit, the version that uses a UJT? Perhaps that may give some information ?

Here 'tis:

Oldcodger · 5th May 2015, 10:13 pm

Camtech- that's what started my interest. It's stated on write up that note range is centred around middle C, so I'd take that to mean octave up ( top note =B4) which is around 493hz. In that circuit( from #10 on -https://www.vintage-radio.net/forum/showthread.php?t=115892 ) that's what started my interest.
It's stated on write up that note range is centred around middle C, so I'd take that to mean octave up ( top note =B4) which is around 493hz. In that circuit ,the last resistor is a 10Kresistor with a 5k pot in series, so I was looking at an average resistance of 12.5k( 10k +half of pot). Lowest R from formula would mean highest note. ,the last resistor is a 10Kresistor with a 5k pot in series, so I was looking at an average resistance of 12.5k. Lowest R from formula would mean highest note.
I'm working from the premise that I know the circuit I'm working from does work ( I built one from same circuit many years ago ,and I know it works ) and that the equation I'm working from is correct. I'm not ( at moment ) interested in the practicalities, merely if I've made an error in my calculations (although I can't see where) , or whether there is a problem in the equation .

Oldcodger · 6th May 2015, 10:05 pm

gents- thanks for the thoughts and advice. Knew there was something wrong in the maths, hence my request. I'd been using W7 calculator and a copy of Basin ( Speccy basic programmer) for my calculatons, and relying on the value returned by the W7 calculator for the capacitor ( hence the factor of 10 error

). After a lot of years, I managed to get son to remember that the calculator I lent him years ago to do his OND ,was mine. After a couple of pound of coal to get a head of steam up

, I punched in the numbers and with a value of 10k for the last resistor, and half the series pot ( 2.5k ), making 12.5k, I got a frequency of approx 560hz, close enough to satisfy me. Working out the value of resistance to get 493.88hz, gave a value of approx 14k ohm. Many thanks for letting me bounce this off you ,something I found many times at work solved problems . Looking at last post, I consider my reply slightly tetchy, if so ,I apologise profusely.
Mods, I'd consider this now closed.

5th May 2015, 12:32 am	#2
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,088	Re: UJT circuit design problems External resistors in series with the bases will modify the frequency considerably - does the circuit feature such resistors?

5th May 2015, 9:41 am	#4
Karen O Rest in Peace Join Date: Jul 2011 Location: Bridgnorth, Shropshire, UK. Posts: 787	Re: UJT circuit design problems I have been playing with a UJT myself recently. The thing to watch (in my opinion) is that the current into the emitter is allowed to fall below the holding current, otherwise the UJT will never switch off. In practise this means the charging resistor cannot be too small. I've seen circuits proposed for UJTs which I'm certain won't work for this very reason.

5th May 2015, 10:10 am	#5
camtechman Nonode Join Date: Jan 2008 Location: Milton Keynes, Bucks. UK. Posts: 2,552	Re: UJT circuit design problems Have you taken a look at the "original" Stylophone circuit, the version that uses a UJT? Perhaps that may give some information ? Here 'tis: Attached Thumbnails __________________ When I die, please don't let my Wife sell my collection for the amount I told her I paid for it!

5th May 2015, 10:13 pm	#6
Oldcodger Nonode Join Date: Oct 2014 Location: West Midlands, UK. Posts: 2,181	Re: UJT circuit design problems Camtech- that's what started my interest. It's stated on write up that note range is centred around middle C, so I'd take that to mean octave up ( top note =B4) which is around 493hz. In that circuit( from #10 on -https://www.vintage-radio.net/forum/showthread.php?t=115892 ) that's what started my interest. It's stated on write up that note range is centred around middle C, so I'd take that to mean octave up ( top note =B4) which is around 493hz. In that circuit ,the last resistor is a 10Kresistor with a 5k pot in series, so I was looking at an average resistance of 12.5k( 10k +half of pot). Lowest R from formula would mean highest note. ,the last resistor is a 10Kresistor with a 5k pot in series, so I was looking at an average resistance of 12.5k. Lowest R from formula would mean highest note. I'm working from the premise that I know the circuit I'm working from does work ( I built one from same circuit many years ago ,and I know it works ) and that the equation I'm working from is correct. I'm not ( at moment ) interested in the practicalities, merely if I've made an error in my calculations (although I can't see where) , or whether there is a problem in the equation .

6th May 2015, 10:05 pm	#7
Oldcodger Nonode Join Date: Oct 2014 Location: West Midlands, UK. Posts: 2,181	Re: UJT circuit design problems gents- thanks for the thoughts and advice. Knew there was something wrong in the maths, hence my request. I'd been using W7 calculator and a copy of Basin ( Speccy basic programmer) for my calculatons, and relying on the value returned by the W7 calculator for the capacitor ( hence the factor of 10 error ). After a lot of years, I managed to get son to remember that the calculator I lent him years ago to do his OND ,was mine. After a couple of pound of coal to get a head of steam up , I punched in the numbers and with a value of 10k for the last resistor, and half the series pot ( 2.5k ), making 12.5k, I got a frequency of approx 560hz, close enough to satisfy me. Working out the value of resistance to get 493.88hz, gave a value of approx 14k ohm. Many thanks for letting me bounce this off you ,something I found many times at work solved problems . Looking at last post, I consider my reply slightly tetchy, if so ,I apologise profusely. Mods, I'd consider this now closed.

4th May 2015, 10:50 pm	#1
Oldcodger Nonode Join Date: Oct 2014 Location: West Midlands, UK. Posts: 2,181	UJT circuit design problems Looked like the best place to post. I was helped with a circuit for an electronic organ on another topic. I can’t remember any theory on UJT from my studies, so looked up an on line tutorial on UJT, to find that the circuit worked as I suspected. Looking a bit further, I found the intrinsic stand of ratio(ITR) of a 2n2646 was .65, and that the time constant of the wave form was T=RCLn (1/1-ƞ)---( where ƞ is the ITR. From the text on the design the frequencies available are centred on middle C, so top frequency is 493.88 HZ. (B4). This equates to T of .002 secs. And from circuit (using half the value of the pot in the adjust circuit), this gives lowest resistance as 12.5k. 1/(1-.65) = Ln of 1.05082 This gives a T of .0006 = freq of 1666hz. Anyone know what/why the results are not consistent ?