Frank Huges transistor tester

[kseastrand]

hi to all. have any of you built the Frank Huges transistor tester? I have built one and am having trouble with it. I cannot seem to keep the leds flashing. if I probe with logic probe on the out put of c and d trigger sometimes it will work or at least flash the leds alternately . I even built a second one on a bread board with different components still same thing what am I missing here? thanks for any help, here is a link to the circuit http://na7kr.us/in-circuit_transistor_tester.htm
Ken

 

[audioguru2]

The link works when it is not crammed together with "newbielink". This link works:  http://na7kr.us/in-circuit_transistor_tester.htm .

I think the tester slowly destroys a transistor when it makes the base-emitter reverse biased higher than its 5V maximum allowed rating which causes the fragile base-emitter junction to have avalanche breakdown like a zener diode. Then the LED that is supposed to be turned off glows a little.

I have used thousands of transistors and all of them have a part number printed on them so I knew if they were NPN or PNP. Also I have never found a defective new transistor so I never needed to tested them. 

 

[kseastrand]

Sorry about that link, what I was actually talking about is that with no transistor hooked up it does not do the alt. flash to show open or no device connected sort of self test and I was more interested in this tester for testing in circuit.
thanks
Ken

 

[audioguru2]

When there is no transistor connected and the switches are set to "Tr" then the CD4093 IC must have plenty of output current or the LED load stops it from oscillating.
But with a 9V supply two paralleled gates of a CD4093 have a minimum output current that is too low to work in this circuit so it is a bad design.

Try increasing the value of the 470 ohm resistor at the bottom of the schematic.

A hex Schmitt Trigger inverters IC (74C14 or MC14584) should have been used. Then the oscillator can be separate from the LED drivers, and the oscillator will not be loaded down by the LEDs.
But I still think the circuit destroys the transistors it is testing by causing avalanche breakdown when it reverse-biases the base-emitter junction with a voltage that is too high.

 

[kseastrand]

Thank you so much audioguru for the help, I will try that but I think you are right about being a bad design. if you know of a better one let me know. I have been just using my multimeter set to diode test to check transistors by isolating one led. and I do have transistor test on my lcr meter but I have to remove it from circuit I was looking for the easy way to test in circuit thanks again for the help.
Ken

 

[Hero999]

According to the text. The circuit was originally designed for TTL and has been modified to use CMOS. Try using 74HC ICs and a low drop out 5V regulator, such as the LM2936-5.0.

 

[kseastrand]

hey thanks hero999, I will try that and also audioguru's idea too I will post back with results soon.
thanks again guys
Ken