Fast MOSFET Test


Often in online discussion I find myself having to explain how a mosfet is head, the test with a multimeter is the fastest but, not from the certainty that the component functions properly in impulsive regime.

Rummaging through the drawers where support temporarily purchases made at trade shows I found my hands an envelope containing 50 pieces of CD4049 probably purchased in bulk among the 5 Hey 20 cents per piece, frankly I do not remember, definitely for more figures I would not have taken them at a trade show.

I decide that it's time overstating least one for something that is useful and simple, a mosfet test in an alternating regime that give me the response on the integrity, on the polarity and whether or not this is the parasitic diode between drain and surce.

Given that should not be a cumbersome expensive thing we try to limit the bone while maintaining the components that I prefix functions.

First thing is a mosfet, without going too far in the explanations is a three-terminal semiconductor element.

Between D and S there is a resistive channel which is modulated by the voltage present in G, sometimes between D and S it is also present a parasitic diode which is usually reverse-biased and therefore not significant in normal operation.

What you need to be tested

It should first be proved the efficiency of the aforementioned diode, subsequently the efficiency of the resistive channel, and in the third abutment when subjected to the proper pilot channel is modulated resistive.

That way it seems complex, but it automates the whole is very easy to use.


To get started you need a simple oscillator, does not require too many components and can be included in the integrated, I decided to use the CD4049

The most obvious choice is to provide a square wave oscillator achievable with only two external components, sufficiently stable for the use in question and at a frequency higher than the permanence of the retina in a manner not notice any flicker.

Made to the scheme uses the remaining 4 inverter as a current buffer for driving and display of results.

Scheme finale

It came out this

U1.1 and U1.2 constitute the oscillator whose output drives the remaining four elements connected to the D and S terminals of the MOSFET in question, R2 and simultaneously via the S1 button will pilot the gate of test element, R4 has the sole function of discharging the gate capacitance to turn off the MOSFET in the absence of piloting, In Q1 and Q2 are complementary scheme mosfet in parallel, in fact there are not physically, represent the elements to be tested.


Initially, once positioned the component and date to the circuit voltage between D and S will have an alternating voltage with a positive and a negative time.

It may occur 4 conditions in the two LEDs in antiparallel

1 LED D1 D2 turns on and remains switched off indicates that it is a mosfet P and the parasitic diode works

2 LED D1 D2 turns on and remains switched off indicates that it is a mosfet N and the parasitic diode works

3 no LED lights and or no parasitic diodes or MOSFETs is damaged

4 Both LEDs light up and the component is in short

Second phase

It determined that only one LED will now light up is known to work in alternate pressing the button, and here we have only two conditions

1 both LEDs are turned on with the demonstration that the channel is driven correctly

2 only one LED remains lit indicates that the channel is not driven and the component is broken

With just two more LEDs you could not get, Furthermore the test had to be fast and the test here is done in three seconds, positions the component from the voltage, look at the state of the LEDs, press the button and see if you both LEDs switch, simpler than that I do not know what to invent.

In the description is left out but R3 is intuitive cge only serves to limit the current flowing in the LEDs.

Below a possible solution with single-sided printed and under a bridge to the integrated

side components

copper side

3D View

although I think that given the small number of components for a bit of matrix board can be used without making the printout.

Amilcare Greetings

2 replies
    • Amilcare
      Amilcare says:

      Thank you information. I just explained how to make a simple and useful circuit. If something is valid because it does not make it known. Given the simplicity I would be surprised that someone had not already had the same idea. Probably I had seen him a few years ago and my mind has revived him without realizing that I had already seen it somewhere. Not bad in the end I do not think he did some wrong to anyone proposing such a simple circuit. If you let me have the source to mention in the article I do not need to take possession of something other.


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