Electronics > Projects, Designs, and Technical Stuff
Simple DIY Linear PSU
xavier60:
Because there is no drive current flowing through Q1, there must be a problem in the output block.
If a BJT seems ok on diode test, also check for junction leakage on resistance range, mainly C-B.
Are you still using undersized transistors?
panoss:
--- Quote from: xavier60 on April 11, 2020, 07:17:51 pm ---If a BJT seems ok on diode test, also check for junction leakage on resistance range, mainly C-B.
--- End quote ---
So I measure resistance between C-B? Which measurement is ok and which implies a damaged transistor?
--- Quote from: xavier60 on April 11, 2020, 07:17:51 pm ---Are you still using undersized transistors?
--- End quote ---
I went back to D209L ;D (for Q3 and Q4).
Q2 is a BC557.
They worked fine (before short circuiting the PSU 's output).
xavier60:
--- Quote from: panoss on April 11, 2020, 07:25:28 pm ---
--- Quote from: xavier60 on April 11, 2020, 07:17:51 pm ---If a BJT seems ok on diode test, also check for junction leakage on resistance range, mainly C-B.
--- End quote ---
So I measure resistance between C-B? Which measurement is ok and which implies a damaged transistor?
--- Quote from: xavier60 on April 11, 2020, 07:17:51 pm ---Are you still using undersized transistors?
--- End quote ---
I went back to D209L ;D (for Q3 and Q4).
Q2 is a BC557.
They worked fine (before short circuting them).
--- End quote ---
Any good silicon PN junction should measure totally open circuit, no leakage at all with the probes connected in the direction that will reverse bias the junction.
You need to get suitable parts some day.
panoss:
I measured Q2, Q3 & Q3.
Q2: C-E, resistance: 32 ohms! Both directions...
So it 's like a resistor of 32Ω, it 's bypassing Q3 & Q4, so this is why it 's outputing the input voltage!
Btw, in diode mode it looks fine!
--- Quote from: xavier60 on April 11, 2020, 07:35:23 pm ---You need to get suitable parts some day.
--- End quote ---
This will happen some day... ;D
WattsThat:
--- Quote from: panoss on April 11, 2020, 08:00:34 pm ---I measured Q2, Q3 & Q3.
Q2: C-E, resistance: 32 ohms! Both directions...
So it 's like a resistor of 32Ω, it 's bypassing Q3 & Q4, so this is why it 's outputing the input voltage!
Btw, in diode mode it looks fine!
--- Quote from: xavier60 on April 11, 2020, 07:35:23 pm ---You need to get suitable parts some day.
--- End quote ---
This will happen some day... ;D
--- End quote ---
I suspect meter operator error. Chances are about 99.999% that Q2 transistor is shorted collector to emitter. Checking on DMM diode setting should show less than 0.1 volt C to E regardless of lead polarity. A good transistor should show INF.
Measure a 27 ohm resistor on your meters diode check function - what does it show?
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