N Mosfet problem

I have N Mosfet and i tested it using diode position the +ve terminal on drain and -ve on source and it give short when i move the +ve to the gate and keep -ve on source ,it give open .
Is that mean mosfet failure ?

There is a (internal) parasitic diode between the drain and source. In the case of an N-channel MOSFET, the cathode is connected to the drain and the anode is connected to the source. Make sure that you aren't just seeing the 0.7 V or so voltage drop by using the diode test position on your ohmmeter.

If it really shows up a dead short (0 volts between drain and source while current is flowing), it is defective.

DickCappels said:

There is a (internal) parasitic diode between the drain and source. In the case of an N-channel MOSFET, the cathode is connected to the drain and the anode is connected to the source. Make sure that you aren't just seeing the 0.7 V or so voltage drop by using the diode test position on your ohmmeter.

If it really shows up a dead short (0 volts between drain and source while current is flowing), it is defective.

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If the cathode of the diode is connected to the drain , it will give short when the +ve terminal on source and -ve on drain , but it shorted when +ve is connected to drain

If the NFET's gate is floating while you are using an Ohmmeter connected + to Drain and - to Source, you can read anything from open to a short depending on the stray charge that is on the Gate.

Use a clip lead to connect the Gate to the Source while making this measurement. That will show only reverse diode leakage (<1uA) if the NFET is ok.

MikeML said:

If the NFET's gate is floating while you are using an Ohmmeter connected + to Drain and - to Source, you can read anything from open to a short depending on the stray charge that is on the Gate.

Use a clip lead to connect the Gate to the Source while making this measurement. That will show only reverse diode leakage (<1uA) if the NFET is ok.

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I made your circuit in this post http://forum.allaboutcircuits.com/showthread.php?t=90959&page=1which is four resistor 7 watt and four Mosfets , but the whole circuit heat raised up . the four Mosfets and the op amp.
I don't know what is the reason . I connected the ground of the op amp supply with ground of the pv panel . the readings of the current is hesitating up and down .when I split the ground of the opamp and the pv panel and turning the potentiometer there was no output response from the opamp .

How big a heat sink did you bolt the FETs to????

Post a picture of the circuit.

i used the heat sink of the computer power supply like this one in the pic http://www.google.com.eg/imgres?sa=...bnh=171&tbnw=219&start=18&ndsp=24&tx=84&ty=60 , could that be a reason ?

As I remember it, you needed to dissipate in excess of 100W, so the heat sink would have to be TWENTY times bigger than your picture. Even so, expect the temperature to rise to >40degC, which will seem very hot to touch...

would you please suggest a suitable heat sink ?
another question, it is written in datasheet that Vgs = 10 v for Rds on and the potentiometer and 12 volt supply didn't give that level of voltage should i decrease the resistor value?
what about the oscillation of current up and down at the starting before it got hot ?

You will need a heat sink big enough such that when it is dissipating ~120W, its temperature is less than 60deg C above ambient temperature. This means its thermal resistance is ~0.5degC/W. This requires an aluminum shape with a surface area > 1200cm^2 and a fan forcing air over it. Here is a good article about how to calculate heatsink requirements.
Go back and read (and take to heart) what I said about the size of the heatsink in post #30 in the original thread.

The NFETs are used as an electronically-controlled variable resistors, so the Vgs will be just above Vt, maybe ~4 V. With a 12V power supply, the opamp I showed in Post #27 in the original thread is more than adequate. What are your powering the opamp with?

Obviously, the negative side of the opamp supply must be connected to the negative side of the panel under test.

Show me a link to the resistors you are putting between the Source of the NFETs and common.

the power supply is 12 v and it is a digital supply when i connect the circuit and the pv panel the displayed value on the power supply is changed i don't know why .the last trial today the pv panel output was 25 volt and the displayed value on power supply was 55 volt when turning the pot. .
this is a pic for the resistors 3.3 Ω 7 watt http://www.ebay.com/itm/10pcs-Ceramic-Cement-Power-Resistor-7W-3-3-ohm-/161092220362

and i used heat sink with dimensions 17mm x60mmx30mm its form like this one http://www.auberins.com/index.php?main_page=product_info&cPath=2_30&products_id=348

the displayed value on the power supply is changed i don't know why .the last trial today the pv panel output was 25 volt and the displayed value on power supply was 55

Click to expand...

Perhaps the circuit is oscillating and causing weird voltage measurements?

Alec_t said:

Perhaps the circuit is oscillating and causing weird voltage measurements?

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what is the reason for the oscillation?

The circuit I posted called for 1.5Ω 5W resistors. Did you parallel two 3.3Ω resistors to get 1.65Ω?

No i didn't, why should i use 1.5 Ω ?? is that a condition ? i got that i control the current so the resistance value won't make great effect except on the voltage drop on the resistor consequently the mosfet .

Look at the circuit I posted again. To sink 8A from the solar panel, each NFET and Source resistor sinks 2A each. The power dissipation in each resistor is RI^2. With 1.5Ω, the dissipation in each resistor is 2*2*1.5 = 6W. If you use 3.3Ω, the dissipation is 2*2*3.3 = 13.2W. You have overloaded the resistors by more than a factor of two!

The fact that the output voltage of the 12V lab supply is jumping all over makes me think that you damaged the opamp, or possibly you have damaged NFETs. Also a possibility, the pot is the one you burned up, and the output is jumping all over the place as you change the setting.

What OpAmp are you using? Do you have a Vdd to Vss bypass capacitor on it?

the maximum output current from this cell is 4.35 A and 34.6 volt that will sink four Fets and that mean 1 A in each branch which will make 1 *1*3.3 = 3.3 watt in each resistor .and voltage drop 3.3 volt so max voltage drop on fet is 31.3 volt consequently 31.3 watt. the resistors have never got hot with me.
I am using lm 358 op amp and i don't use bypass capacitor
the most confusing here that the max output of power supply is 25 volt

what is the reason for the oscillation?

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One possible cause is stray wiring inductance coupled with FET gate capacitance. Do you have low-ohm (e.g. 33-100 ohm) resistors in series with each gate and immediately adjacent to the FET?

Alec_t said:

One possible cause is stray wiring inductance coupled with FET gate capacitance. Do you have low-ohm (e.g. 33-100 ohm) resistors in series with each gate and immediately adjacent to the FET?

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I just put 5 KΩ before the four mosfet gates to protect them .

I used this power supplyhttp://www.amazon.com/Mastech-HY3005D-Variable-Linear-Supply/dp/B000E14F56 to simulate the pv panel and adjust it to source 7 volt and it did well . i could change the current from 0-2 A when i tried to increase it upper than 7 v it went crazy . the led indicator called C.C changed to C.V every thing current and voltage go upside down ,and that what was happening with the pv panel. i don't know why . may be i should increase the number of mosfets.