Mosfet and picaxe

D

Dippy

Moderator
Post your schematic.
Qu:
Are you trying to trigger a 12V coil relay with 5V?
Direct from 5V was probably beginner's luck and the extra little losses via a MOSFET were enough to stop it.

Please clarify. Whats wrong with a 5V relay?
 
N

nick12ab

Senior Member
You haven't said anything about a reverse diode across the relay coil. If this diode wasn't in the circuit then it's possible that you blew the MOSFET on first use and therefore it is no longer working. And have you used the MOSFET as a low-side driver?

Can you:
  1. Ensure that the relay does work by wiring the coil directly across the supply
  2. Ensure that the MOSFET does work by checking RDS with VGS at 12V then at 0V
  3. Post a schematic
 
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S

Simbad

New Member
Relay is working,switching from 12v.
Diode for protection is in place.
Mosfet is switching when i apply 5v to gate diretictly from my power source, but it doesnt take output pin high 5v. It seems like its not right value, or to small current?

Capture.JPG
 
N

nick12ab

Senior Member
Simbad said:
Relay is working,switching from 12v.
Diode for protection is in place.
Mosfet is switching when i apply 5v to gate diretictly from my power source, but it doesnt take output pin high 5v. It seems like its not right value, or to small current?
Click to expand...
So are you using the N-channel MOSFET as a high-side driver AND you're only powering the 12V relay with 5V?
 
premelec

premelec

Senior Member
The schematic you posted does not show where the sources of the FET go. These must go to the same V- as the PICAXE V-. Also the MOSFET is not a logic level device so could be not working at 4 volts on the gate... Have you checked what actual gate voltage turns on the relay?
 
S

Simbad

New Member
premelec said:
The schematic you posted does not show where the sources of the FET go. These must go to the same V- as the PICAXE V-. Also the MOSFET is not a logic level device so could be not working at 4 volts on the gate... Have you checked what actual gate voltage turns on the relay?
Click to expand...
Ground to the pic did the trick,thx.
You need IRLZ34, not IRFZ , and a R 100k between Gate and Ground.
Click to expand...
What is difference, logical or non?
 
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premelec

premelec

Senior Member
Lower Vgs threshhold - turns on with lower gate voltage... check the data sheets... glad you got it going
 
G

Goeytex

Senior Member
The IRFZ44N is not really a true logic level FET. It has a specified gate to source threshold voltage that can range anywhere from 2.0v to 4.0v but usually closer to 4.0volts. This FET is not a a particularly good choice for use with a Picaxe, especially if you are using 3 AAA batteries for the supply.

The gate threshold voltage is the voltage where the FET just barely starts to turn on.
 
D

Dippy

Moderator
Aha, the bypass cap. Should be your design 'default'.
Sometimes caps at the 'top end' of the relays too - it reduces surges/spikes in the power rail.

Yes, logic level MOSFET would be better, but not always needed as it depends on the App and the FET.
Often , for a tiddly relay, an appropriate bipolar is fine too.

For these small currents the IRFZ44N may be OK, but wouldn't have been my choice.
I would suggest to budding designers that they study MORE than the headlines.
Many people just look at the headline Rds value. The lower the better huh?
Study the graphs e.g. "Typical Output Characteristics"
Look at the Gate Voltage value for the tabulated Rds value quoted. i.e. the test voltage.

Check a few 'Logic' and non Logic MOSFETs and see the differences.
All these data can assist your choice - that's why they're there.:)
 
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