4 Channel Optocoupler Isolation MOSFET Module review.

Other Hardware General Electronics
1

I needed an MOSFET board to drive high current relays. I found this board,

The specifications look pretty good, input voltage, 3 to 24V, power output voltage 3 to 55V. So I purchased it. At the time I did not have the above link. After finding the above link, I scrolled down and found a partial schematic on the unit, and i became very concerned, knowing that most N channel MOSFET's have a 20V gate to (-) limit. Now that I have it, I decided to check it out for myself.

I decided to do a basic profile on it.

Power input for the MOSFET was set at 27.5 V. This is half of the rated voltage.

Optocoupling input. Gate.

1.2V 2.28V this is when the MOSFET first triggered.

2.0V 15.60V

2.5V 25.20V this is well above the gate voltage limit.

3.0V 25.92V

4.0V 25.955V

5.0V 25.97V

10.0V 25.999V

15.0V 26.012V

20.0 V 26.019V

24.0V 26.020V that is a 1.3V drop.

at MOSFET power = 30V.

Optocoupling input. Gate.

24.0V 28.169V

And at MOSFET power = 50V.

Optocoupling input. Gate.

5.0V 45.589V

Remember this is a MOSFET. It only needs a static charge on the gate to turn it on. There is no actual current flow through the gate.

In my opinion, In order to make sure you do not exceed the gate threshold limit of the MOSFET, this board should not be used with a voltage higher than 20V power output.

This board needs to be redesigned. A Zener need to be added to the gate. Along with the better voltage divider.

Fortunately my project is limited to 12V. But if your project uses more than 20V, you may want to look for a different board.

2

Hi,
Can you post a copy of the schematic you are using to get the results in your post, please?

Thanks.. Tom.. :slight_smile:

3

Here is a link to the product page.

scroll down to find a basic schematic.

Let me know what you think.

4

Hi,
Circuit from links page

703×334 236 KB

Yes, I can see what you mean, it is possible if you use a supply voltage of higher than 20V, you can get >20Vgs that is the spec Max Vgs.

A 18V or 15V zener from gate to source would provide the protection needed,

Where abouts on the circuit are you measuring your voltages?

Tom... :slight_smile:

5

I measured the voltage from the gate of the transistor to the source pin of the transistor, (-), gnd.

If they were using this circuit to switch the (+) side, It would probably be fine. But since they are switching (-) side, In my opinion, the voltage is way too high.

It seems that many of these circuits designed for Hobby use is not designed very well. they seem to think they can throw something together, and no one will notice the errors. I have found many examples of poor designs from supposedly highly educated electronic engineers. I posted this as an example of what to look for. Maybe if enough of us post this kind of poor engineering, they will critique there designs a little better.

We need well-designed, safe products. Especially for the .hobbyist. Someone who is new to electronics may not catch the errors, and in some cases dangerous errors. Especially when using the specifications that they give. Some are shock hazards, others are fire hazards.

All products need to be safe for their intended specified use.

6

Clearly that 10k resistor needs a 12V zener across it. Most MOSFETs are designed for 12V drive. The
2k2 would need to be 1W for the full voltage range.