[SOLVED] Motor mosfet problem

[Dogbreath97]

I'm having some problems with my mosfet (IRF840 Vds 500V). Im trying to get a 230V motor working with a 3.3V signal from a raspberry pi. I've included the schematic. The GPIO.OUT is the output of the raspberry pi. When the 100K resistor is in place the mosfet seems to be turned 'off'. When I remove the 100K resistor the mosfet is turned 'on'.

When I have the 100K resistor in place and an output of 3.3V of the GPIO the Fet is still turned 'off', I was wondering if someone knows what I might be doing wrong...
My circuit:



Also, which program should I use for making schematics? I used paint for the one above (not great as you might see)

 

[Audioguru]

Your Mosfet needs an input voltage much higher than 3.3V for it to turn on.
The datasheet shows that some IRF840 Mosfets conduct only 0.00025A (250uA) which is almost nothing when the input is 4V. Most of them need an input of 10V for them to completely turn on.
Some Mosfets are rated as "logic-level" and need an input of 4.5V to completely turn on. But you need a level-shifter circuit to do either one.

 

[larry_walker]

Hi.

If you allow me a suggestion, I think you may obtain a better result with an opto(de)cupler MOC3021, followed by a triac BTA20 600B (if the opto(de)cupler current does not suffice).

You can buy those on E-bay or Aliexpress for less than $5.

Here you have the links for the data-sheets.
Within the data-sheet of the opto(de)cupler you'll find an example of a circuit with an opto(de)cupler and a triac.

https://pdf1.alldatasheet.com/datasheet-pdf/view/27236/TI/MOC3021.html
https://pdf1.alldatasheet.com/datasheet-pdf/view/22034/STMICROELECTRONICS/BTA20-600BW.html

 

[KlausST]

Hi,

Safety first!

With your circuit the whole raspberry carries mains potential.

THIS IS DANGEROUS!

I strongly recommend to use some galvanic isolation.

******

My question:
Is your motor for AC or for DC supply?

With your circuit the motor runs on DC.

*****
I agree with larry_walker. This is easy and ensures isolation.

Besides this there are mechanical relays, solid state relays, photo-mos relays, (all with galvanic isolation)
Solid state relays usually come with two options
* immediate switching (speed control possible)
* zero cross switching (On/off)

Klaus

 

[Dogbreath97]

Hmm, I have a small transformer lying around which outputs about 12 V. I think I might use that as a level shifter...

My motor needs a DC supply, its a very weird motor (I scavanged it from a bread baking machine). I was wondering, will I still need galvanic isolation with a level shifter (different transformer and diode)?

I will read up about optodeculpers and triac circuits, I have never heard of them before but they might come in handy in the future.

I was initially looking for mechanical relays but my local hardware store did not have them, and ordering them takes ages where I'm from. This is why I got the mosfet instead. I was hinting/asking the guy there if I would not need a level shifter for activating the Fet from a raspberry pi and he was like: no, it needs less then a volt for activation. This is why I was confused when it was not working. I guess I'll just not go back there as they kinda rip me off every time I go there...

 

[KlausST]

Hi,

Transormer: is for AC only, you can't isolate the DC gate drive voltage with it.

(different transformer and diode)?

I don't understand this.

and ordering them takes ages where I'm from.

In Europe ... there are distributors like farnell, digikey, mouser and a lot others...they usually deliver winthin a day or two..
In many other regions of the world it will be similar.

For a beginner I recommend to use ready to buy solutions, especially when safety is a concern.

Klaus

 

[Dogbreath97]

Yeah, but then I would use a diode for rectifying the ac output...

That way I can use a transistor as a switch to turn the mosfet on...

I haven't got any experience with farnell. But most other online electronic sites have really high shipping fees... Except for eBay which takes about a month to arrive. I was planning on doing this with the things I have and not spending loads of money on shipping costs etc.

 

[hobbyckts]

It will be better to try it out with isolated circuit. So go for opto coupler.

As far as making the schematics there are so many free tools available for making. My suggestion is to go with kicad (free) or Orcad (if you willing to purchase license)

 

[Dogbreath97]

Ok, I'll order an opto coupler. I found a MOC3201 on eBay...

Thanks for the help and the suggestions!

 

[c_mitra]

Yes, opto-coupler is a very good suggestion for your purpose. It will allow 12V to be applied to the FET and also provide isolation as a very welcome bonus. If I need one, I usually order 2 or 3.

Even if it is not needed, it is good to have some simple diode capacitor protection for the mosfet and /or at the motor. They will save some headache later.