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Using an Oscope Probe on my DVM meter, external power supply to test transistors

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lewisP

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I have a BNC to banana adapter, Can I use an Oscilloscope probe on my DVM meter because the oscope probe is shielded instead of using my fluke DVM probes?

To Increase the Test voltage output of my Fluke 87 DVM meter. Can I use an external variable power supply set to +30volts dc and use it in series with the red fluke probe to apply a higher test voltage when checking transistors junctions?
The fluke internally has a current limiting resistors with diode mode when checking transistors. If I put a power supply in series with the red probe to increase the test voltage i can test the transistors junctions at a high test voltage?
 

My DMM produces its own voltage when testing a diode/transistor. It is calibrated to perform that function. I do not recommend applying an external voltage. It could ruin your Fluke instrument. Is it worth the risk?

Do you wish to test the B-E and B-C junctions? The reading won't depart much from 0.6 V.
 

but I'm trying to test a 2N2904A transistor at a higher test voltage compared to the low test voltage from a DMM.

Why would it ruin my fluke meter?

I can put my DMM meter to measure DC volts and use an external power supply as my external test voltage, but what resistor value do I use for a current limiting resistor. Do I put the current limiting resistor on the base or collector? Do I apply the external across the collector and emitter directly?

Do you wish to test the B-E and B-C junctions?

Yes to test at a higher test voltage

My fluke 87 is testing the junctions at a very low voltage

In diode mode the fluke should have a current limiting resistor built inside the fluke with a low test voltage outputting the probe leads

The reading won't depart much from 0.6 V.

Each Junction should read 0.7vdc with a test voltage up to 60volts DC the datasheets says

It will measure 0.7volts dc. Testing the transistor at a high test voltage to much sure its working.
 

I can put my DMM meter to measure DC volts and use an external power supply as my external test voltage

This method is okay. It is normal use of the meter.

In diode mode the fluke should have a current limiting resistor built inside the fluke with a low test voltage outputting the probe leads

The DMM contains an ADC converter. It's the heart of the meter.
By attaching resistive dividers, it is able to measure different ranges. This makes it safe to connect it to 30 V.

However the ADC will not give a sensible reading if exposed directly to more than a fraction of a volt (200mV or 400mV, I believe). It probably has some degree of overvoltage protection built in.

Normally it will not be subjected directly to 30V. However when you use the diode test function, it accesses the internal battery. From the description you gave, I thought that you plan to inject 30V into the leads while using diode test. This could ruin something inside the meter.
 

For transistor and diode testing, forget DMMs and use and old analog mA meter. To measure current use shunt resistor, fo r voltage use drop resistor. With a mA meter you can use any DC power supply, best use a variable-voltage regulated one.

Analog meters indicate small current variations you cannot see on a DMM. With some care such meter is not so easy to damage as an expensive DMM.

- - - Updated - - -

An oscilloscope probe typically is a 10:1 voltage divider, so you can use it with a DMM for DC and AC voltage indication.
The division ratio is based on 1 MOHm oscilloscope input impedance while a DMM typically has 10 MOhms. So you must calibrate DMM reading with such probe.
 

Just set up a jig for your transistor, +30V feeding the base via a 2M, and feeding the collector via 10K, negative to emitter. The base current will be 30/2 micro amps = 15. So collector current will be Hfe times this and the voltage drop across the 10K will be 10K X 15 X Hfe X 10 ^-6. Simples!
Frank
 

However when you use the diode test function, it accesses the internal battery. From the description you gave, I thought that you plan to inject 30V into the leads while using diode test. This could ruin something inside the meter.

Yes you remove the red probe from the DMM meter and plug it into the positive input into a power supply set at 30volts. The negative input of the power supply plugs into the positive input of the DVM meter. This will inject +30vdc plus the DMM internal battery voltage into the transistors junction. The DMM will have internally a built in current limiting resistor in the diode mode but not in the DC voltage mode. The DMM will have a ADC , current limiting resistor and internal battery. The external power supply is adding to the internal battery only on the red probe. Batteries in series add, just add the external power supply only to the red probe it will add the test voltage higher. This way you can test the transistors junctions at a higher test voltage.

If this doesn't work, how would you do this type of test?
 

Code:
Parameters  Test conditions        Min Max Unit 
BVCEO  ,  IC = 10.0 mA, IB = 0,     60   ---  V dc
BVCBO  ,  IC = 10 µA, IE = 0,       60   ---  V dc
BVEBO  ,  IE = 10 µA, IC = 0  ,     5.0  ---  V dc
ICBO    ,  VCB = 50 V, IE = 0       ---  10   nA
hFE  ,  IC = 150 mA dc, VCE = 10 V, 40  120   ---

Electrical Characteristics    TA = 25°C

[B]Due to limitations of probe testing, only dc parameters are tested. 
This must be done with pulse width less than 300 µs, duty cycle less than 2%[/B]

If you do not understand the datasheet, ask a detailed question.


THis is clearly not compatible with your proposed setup.

The Diode Test in ALL DMM's uses a low CC source ~<1mA which measures threshold voltage, rather than Vf at rated current.

How do you propose to put a current source in series with a voltage source? No way.

But you can use your Fluke 87 to measure Vceo with a 100Vdc source limited to 10uA, which may be slightly lower than rated level at 10mA using a pulse.

Power dissipation in the transistor at 60V ~@10uA is 600uW, If using 100Vdc use a (100-60)=40V/10uA=4MOhm Resistor. If you dont have one 1MOhm will be ok, then measure using 200V scale

I am assuming there are no other loads in circuit, if so, then define for proper ICT setup mode.

If you wanted to use your scope probe it must be 1:1 with BNC>Banana Adaptor
 

I was trying the diode test function on my DMM. I connected it to various led's.

I have used it on ordinary diodes. For them it gives readings in the 600-700 mV area (as we expect).

The meter delivers small current through the device under test. With led's, it illuminates all types slightly. This is because it contains a 9V battery.

However it does not give a volt reading for any color except red led's. Red is 1.8V or so.

Other colors are greater than 2V. For those, the DMM shows '1' which means overrange.
 

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