Mosfet IRLB8721 Pin Connections

Is there supposed to be a connection between the Drain and the Source of a mosfet?

Im doing continuity test with my multimeter and I get a connection between those while the mosfet is closed or off.

The gate of a mosfet is so sensitive that touching it will turn the mosfet on or off.
And it could stay on or off.

Clip a testlead to the source FIRST, and the other end to the gate.
Now you should measure nothing (open circuit) between source and drain (+lead to drain).
FIRST unclip the gate, then the source.
Leo..

You could also be seeing a source to drain diode. Try switching the polarity.

ok im on continuity and Im touching drain to source and im getting about 100 ohms. But when it was soldered onto the perf board I was getting a continuity beep

We have no idea what is on your perfboard so that observation is only useful to you. You didn't mention whether you tried swapping leads, as I suggested.

Hi,
What is your multimeter?

Can you please post a copy of your circuit, in CAD or a picture of a hand drawn circuit in jpg, png?

A picture of your project would also help.

Thanks.. Tom.. :slight_smile:

Yes I tried swapping leads and now that it’s off the board, there is no continuity.

I was sure I checked there was no connection due to soldering so I could almost rule that out.

Here is the image of the diagram and board.

FullSizeRender (48).jpg

IMG_8617.JPG

My multimeter is a Klein cl1000 and I also have a centech 7 function meter.

Marciokoko:
Is there supposed to be a connection between the Drain and the Source of a mosfet?

Im doing continuity test with my multimeter and I get a connection between those while the mosfet is closed or off.

Unless you have gate shorted to source, how do you know its off?
Are you measuring the right direction (MOSFETs have a body diode that will conduct if
reverse voltage is applied).

Ok let me explain it better. When I posted it was because I had moved my diagram to a perf board but I had left out something that I decided to add, so I moved it to a new perf board. The circuit was working fine in the original board.

Upon moving it to the new board and adding the new component (a manual override switch), I decided to check connections with continuity. I checked that the button was closing the circuit properly when pressed. I guess I learned that continuity doesn't work over a diode - probably due to the pn junction. I also learned that my multimeter's continuity/resistance setting will show me a resistor's resistance! That was sweet, no more painful color code reading for me :slight_smile:

Those were pretty much all the connections I wanted to test. The only component missing was the mosfet because its so big, I always put it in last. So I put it in and decided to test the its legs for continuity. I was surprised to find a connection (beep) between Drain and Source. I quickly googled and found that it could mean a bad fet that had been shorted. So I decided to take it out and replace it. As I was taking it out I made an even bigger mess (i haven't mastered the art of solder wick desoldering)...ended up removing a few solder pads by accident on those 3 terminals but I got it out. Anyway, I tested the fet by itself because of course the connection could have meant a faulty soldering job even though it didn't seem to be. It turns out that Drain and Source show NO continuity. It actually showed about 100 Ohms with the leads one way and 140Ohms with the leads in the other direction.

So I guess my fet works?

Hi
Where in your picture are you placing the MOSFET?
Where G, D, S ?

Tom...

Sorry, I'm trying to upload the image but it's failing. Posting from an iPhone is a pain.

Gate = F17
Drain = G17
Source = H17

Hi,
Not being able to see under the board, I would assume F17 is connected to E17, D17, C17, B17 and A17?

Tom.. :slight_smile:

Yes, here is the diagram and board.

You’ll probably be able to see it better in the diagram as the soldering is kind of messy.

image.png

Hi,
What have you got the source connected to. it should be gnd.

Tom... :slight_smile:

Yes, Source is at H17, but the paper is a bit burnt. As I was soldering I was marking which connections I had made by touching the iron to the paper.

Source at H17 goes down towards H18 and connects to both the Ground pin for the 12V power source at J18 and stretches over to join to Ground from Arduino A18

Marciokoko:
So I decided to take it out and replace it. As I was taking it out I made an even bigger mess (i haven't mastered the art of solder wick desoldering)...ended up removing a few solder pads by accident on those 3 terminals but I got it out.

Get a solder sucker, some "no clean gel flux" and make sure your soldering iron is temp controlled (700F is good. 800F almost guarantees you'll lift pads - I did a test of this once in response to a customer complaint)

Ok I've got everything set up. Tested continuity and everything seems right. Ran the project but its not working anymore. So I tested voltage across the 2 paths towards the gate.

I managed to read 4.47V out of the Arduino GROUND pin and after the diode at the D13 pin exactly when the pin is supposed to go high (every 5 seconds in my sketch). So every 5 seconds I read 4.47V between GROUND from Arduino at the perf board and D13 connection at the perf board after the diode. I guess this is because of the 0.6V drop of the diode. I also tested the manual override and I also get 4.47V.

Then I moved the lead downstream after the 220O resistor and got 4.37V. After that R comes the GATE and I want to know if the Gate is working properly. How can I measure this?

Marciokoko:
Anyway, I tested the fet by itself because of course the connection could have meant a faulty soldering job even though it didn't seem to be. It turns out that Drain and Source show NO continuity. It actually showed about 100 Ohms with the leads one way and 140Ohms with the leads in the other direction.

So I guess my fet works?

No useful information in those measurements I'm afraid - the MOSFET is sensitive to the voltage on
the gate w.r.t. the source, and you didn't define that voltage.

You want to measure the resistance between drain and source in two different circumstances:

  1. with gate connected to source - resitance should be extremely high (10's or 100's of megaohms)
  2. with gate at +5V with respect to source - resistance as specified for Rds(on) in datasheet for 5V, milliohms.

You also want to check that the resistance between gate and source is extremely high (gigaohms!)

Without all these measurements you have only a poor idea of the state of the device.

Common failure modes include leakage between gate and source, between source and drain (and typically
gate too), open-circuit source (melted lead wires), or completely fused internally (low resistance between
all terminals). Another failure mode is burst open (exploded!), but that's easy to spot visually :slight_smile:

Failure can be caused by static electricity, MOSFETs have no inbuilt protection against this so proper
static handling precautions must be followed.

I removed the mosfet and tried another and it didnt work. So I put the mosfets on a breadboard and tried them like this and they seem to work. Here is the 2nd mosfet I tried(IRLB4132): June 22, 2016 - YouTube.

Here is the original mosfet I had tried, (IRLB8721): June 22, 2016 - YouTube

The only difference is that with the 8721, the LED lights up a bit even on LOW at the gate. The connection is simply a 220Ohm resistor between Drain-LED and 3.3V from the Arduino and a 9.6kOhm resistor between Gate and Pin D13 from Arduino.

WOuldnt this mean I have an issue with my perf board?

Hi,
Can we have a picture of the other side of the board please.

Thanks.... Tom... :slight_smile: