Author Topic: Basic level shifter going very wrong  (Read 1665 times)

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Offline Simon

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Basic level shifter going very wrong
« on: August 14, 2015, 06:55:52 pm »
I have made this circuit, it should take a 10V input and provide a 24ish V output.

for some reason all I get is a constant 24V output. I have loaded the output with 100K but nothing, I can try a base to emitter resistor on the input and did put one on the PNP but to no avail. Odly I only get 7.5V tops on the collector of the NPN transistor.

Any ideas ? do i actually have to start using mosfets ?
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Offline Simon

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Re: Basic level shifter going very wrong
« Reply #1 on: August 14, 2015, 07:14:46 pm »
I have done a quick test on a breadboard with just the NPN transistor and pullup resistor and base resistor, I will only get 7.5V peak on the collector, but when the transistor is off it should rise to the 29V supply...
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Offline Andy Watson

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Re: Basic level shifter going very wrong
« Reply #2 on: August 14, 2015, 07:28:56 pm »
What is driving the input? I would be tempted to put a pull-down resistor on the base - to be sure.
Have you got the Emitter and Collector the right way around?
 

Offline Simon

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Re: Basic level shifter going very wrong
« Reply #3 on: August 14, 2015, 07:37:59 pm »
I tried a base resistor, will try turning it over but following datasheet pinout.
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Offline Simon

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Re: Basic level shifter going very wrong
« Reply #4 on: August 14, 2015, 07:40:03 pm »
Yep turning the transitor over worked, suppose i better breadboard the whole thing to make sure the PNP one is up the right way too
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Offline SeanB

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Re: Basic level shifter going very wrong
« Reply #5 on: August 14, 2015, 07:56:47 pm »
Load the output with 1k, it should pull down then. Your base emitter resistors will help with leakage, 10k on both transistors will work. It will stay at 24V just from leakage, you will need a pull down in any case.
 

Offline Simon

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Re: Basic level shifter going very wrong
« Reply #6 on: August 14, 2015, 08:03:52 pm »
Yea i've loaded output and put BE resistors on both, works now it was just the upside down NPN
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Offline c4757p

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Re: Basic level shifter going very wrong
« Reply #7 on: August 14, 2015, 08:06:53 pm »
Yep turning the transitor over worked, suppose i better breadboard the whole thing to make sure the PNP one is up the right way too

I will only get 7.5V peak on the collector

This is a big clue - try to remember it for the future! VEBO - the emitter-to-base breakdown voltage - is around 7V, and flipping it upside down typically causes that breakdown. If you see that voltage on a "collector" when it should be much higher, check the orientation! :-+
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