as above.
When it previously worked (2 weeks ago and on different batch of transistors), the measurements are in the 3-5mA range.
Things I've tried
this is the circuit I'm using
When it previously worked (2 weeks ago and on different batch of transistors), the measurements are in the 3-5mA range.
Things I've tried
- I've wired the circuit in a breadboard and rechecked the connections using the meter's continuity function. The circuit is simple enough, I don't expect to make any mistake.
- Vbe reading is around 0.5volts
- I tried 2 meters using the uA/mA terminal and the uA and mA function selection (my meter has separate uA/mA/A rotary selector)
- I tried a previously BC550 where I successfully measure the hFE previously and in this instance, I'm not getting the current collector current.
- I don't have a 9V battery but I used a 15V SMPS and an adjustable regulated supply (using LM3X7 part)
- I just bought 10-15 BC550 from the local electronic hobby shop and not getting any measurement.
- I tried 1M and 1.5M resistors (1M + 2x 1M in parallel). I checked the resistor values using a meter.
- I found an article on the web that says I can use the meter's diode test to see if the transistor is working. I've measured around 0.7volts from base-to-emitter and around the asme voltage from base-to-colletor.
this is the circuit I'm using
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What scale is your meter set to?
Are you using the proper "current" terminals?
Some meters have a separate set.
11.5V (net) translate into so me 7.7 uA into the Base.
Previous measurements (3 - 5 mA) translate into Hfe 390 and 650 respectively, well within spec for a BC550
There must be some wiring error.
circuit and supply are fine, no need to buy a fresh 9V battery, 9V is 9V no matter where they come from; but id you want to ......
What scale is your meter set to?
Are you using the proper "current" terminals?
Some meters have a separate set.
11.5V (net) translate into so me 7.7 uA into the Base.
Previous measurements (3 - 5 mA) translate into Hfe 390 and 650 respectively, well within spec for a BC550
There must be some wiring error.
1. I've tried uA and mA. same results.
2. I've moved the red lead of the meter to the uA/mA terminal.
3. not really sure of the wiring error. I've rechecked the wiring (it's on a breadboard) by checking for continuity and cannot find anything that stands out (+ the circuit is simple enough that I don't expect
What are the results?
Things to check which likely lead to an 'aha-moment': Vce, Vbe, Vbat at the circuit.
Jan
Things to check which likely lead to an 'aha-moment': Vce, Vbe, Vbat at the circuit.
Jan
Feed some current through the meter using a resistor and the power supply. Just to check it is working. Often there is an internal fuse protecting the low current ranges, might be blown.
Excellent idea, didn't think of the obvious.
Being a newb/beginner (no offense, just it being the case) shouldn´t be surprised ar blown fuses 🙂
Being a newb/beginner (no offense, just it being the case) shouldn´t be surprised ar blown fuses 🙂
First thought on seeing that circuit is don't use it. It is the worst possible type of example for a beginner to follow.
It is to open to 'misuse' allowing damaging currents to flow. One connection error on the transistor, measuring a voltage across B/E with the meter on ohms or diode range by mistake (turning the transistor on) and current flow could be very high.
You may have blown a fuse in the meter and if so then the transistor that was in use at that time is suspect as well.
It is to open to 'misuse' allowing damaging currents to flow. One connection error on the transistor, measuring a voltage across B/E with the meter on ohms or diode range by mistake (turning the transistor on) and current flow could be very high.
You may have blown a fuse in the meter and if so then the transistor that was in use at that time is suspect as well.
Maudio's advice is good. After you've checked your meter, if there is still a problem, then I guess you need to recheck your transistor connections. I don't know where you got your devices from, if a genuine supplier, but the various plastic devices had just about all possible lead configurations from e-b-c, b-c-e etc. so it maybe someone swapped the emitter and collector if a dubious second source. Are the diodes correct for NPN in case someone mislabelled PNP types. Measuring a transistor "upside down" (C-E swap) will show a low gain, (except, perhaps, for really old Ge or alloyed silicon devices). I presume youre meter will record a few uA, so do you get a current flow through your base resistor when the collector is open (i.e. straight through resistor- base-emitter) and when the emitter is open, (resistor to base to collector diode, collector grounded, emitter open) and if you measure resistance between collector and emitter with no base current, (make sure your resistance lead "red" is positive. Old analogue meters have black positive on resistance tests) - is it essentially o/c?
And Mooly makes a valuable point. I should have mentioned that I have a test circuit which is a simple, 4- resistor and three capacitor amplifier configuration with a transistor socket. Plugging a transistor in and connecting to 9V you can measure the voltages to see if they are correct and measure the gain using AC signals. Modern digital meters should allow AC voltage gain without heavy loading (my old AVO sadly isn't up to it, so I either use a digital meter or oscilloscope, or the AVO with an AC amplifier/buffer). I also nearly mentioned that you may have damaged your transistors, which would be unfortunate, if the test wiring had been faulty.
The Vbe in the circuit is 0.5V, which is low.
The Vbe with the diode test is 0.7V, which is ok.
Exchange the 1.5MΩ resistor for a 1.0MΩ resistor and a 500k potmeter, and measure if Vbe is changing.
Another thought: check for the contacts / leads of the components (if the wiring is ok).
Vbe is NOT a constant - it changes a tenth of a volt per decade of BASE current, plus any resistive losses. Vbe in the circuit is low because the base current is low. It must be higher on the DMM’s ’diode test‘ range. Short collector to base in the ‘diode test’ range, and vbe will decrease. This is always the second test that should be performed when doing a health check with a DMM. The higher the hFE, the more it will drop when you do this. It will never quite get as low as it would in a real circuit, since hFE is depressed when Vce is only 0.7 volts. Not as much gain, therefore more base current and more vbe.