greencardigan
Senior Member
Can I connect the 08m pwm output pin directly to the MOSFET gate? Or do I need a resistor in between? I have some BUZ71 mosfets.
Connecting to a mosfet
- Thread starter greencardigan
- Start date
desulfator
Senior Member
No!
Unless you want to risk your PICAXE.
Often when NFETs fail, they fail to a shorted condition from drain (positive supply) to gate.
Which means that your poor little PICAXE will attempt to sink the current that can be developed by the positive supply. Usually this is a tad more than PICAXE can handle.
A current limiting resistor is needed to protect the PICAXE in the event this happens.
The NFET will run a little cooler if you parallel a ceramic cap to the current limiting resistor.
While blocking DC the cap will apply the leading edge of a signal to the gate which reduces the time the NFET is exposed to a transitional state between shutoff and turned on with heavy loads.
This will preserve the switching speed.
Edited by - desulfator on 21/11/2006 23:00:48
Unless you want to risk your PICAXE.
Often when NFETs fail, they fail to a shorted condition from drain (positive supply) to gate.
Which means that your poor little PICAXE will attempt to sink the current that can be developed by the positive supply. Usually this is a tad more than PICAXE can handle.
A current limiting resistor is needed to protect the PICAXE in the event this happens.
The NFET will run a little cooler if you parallel a ceramic cap to the current limiting resistor.
While blocking DC the cap will apply the leading edge of a signal to the gate which reduces the time the NFET is exposed to a transitional state between shutoff and turned on with heavy loads.
This will preserve the switching speed.
Edited by - desulfator on 21/11/2006 23:00:48
greencardigan
Senior Member
Can you give me an idea of a suitable resitor value?
I'm switching 12v @ 1.75A.
I'm switching 12v @ 1.75A.
premelec
Senior Member
If you can deal with a slightly delayed on and off time 1000 ohms should be fine limiting the 5 v peak current to 5ma. A power MOSFET has a large gate capacitance to charge and discharge - see spec sheet - typically it might be 2 nF so the RC time constant would be 10^3 x 2x10^-9 = 2micro seconds... The gate _current_ is very low - the main thing is to get the MOSFET fully turned on - that has to do with threshold voltage q.v. - otherwise a lot of voltage will appear across the MOSFET - heat!
Don't forget to put a reverse diode across any inductive load and if you are worried about drain-gate short you could put a zener diode from gate to source [5v] and fuse etc... there are a variety of ways to protect the PICAXE from harm... [20K resistor drive would be easy..].
Edited by - premelec on 22/11/2006 05:33:18
Edited by - premelec on 22/11/2006 05:39:25
Don't forget to put a reverse diode across any inductive load and if you are worried about drain-gate short you could put a zener diode from gate to source [5v] and fuse etc... there are a variety of ways to protect the PICAXE from harm... [20K resistor drive would be easy..].
Edited by - premelec on 22/11/2006 05:33:18
Edited by - premelec on 22/11/2006 05:39:25
1: BUZ71 does not have logic level gate threshold. You'll prolly be okay at 2 Amps but be warned you are not turning the mosfet on very hard.
2: The series resistor is to protect the driver (Picaxe) in normal operation. In addition you also need a 5v1 zenor/tranzorb across the Gate-Source pins of the mosfet. As premelec has said the Gate is a capacitor and it is effective to both the Drain and Source so any sharp transitions on either are reflected straight back into the Gate.
Evan
2: The series resistor is to protect the driver (Picaxe) in normal operation. In addition you also need a 5v1 zenor/tranzorb across the Gate-Source pins of the mosfet. As premelec has said the Gate is a capacitor and it is effective to both the Drain and Source so any sharp transitions on either are reflected straight back into the Gate.
Evan
Jeremy Leach
Senior Member
I can see what you're saying about gate surge current and that it's a good idea in general. I've used a ZVP2106 mosfet directly driven from a PICAXE output with no problems, but it isn't a power mosfet and just checked the spec sheet and it's 0.1nF gate capacitance. Perhaps I should have added a resistor too though, just in case.
All these little circuit tweaks (and whether to connect direct or not) depend a lot on FET type and your circuit.
For newbies that are reading these forums (some of whom don't intend to learn about electronics /transistor theory or simply can't be bothered) its safe practice to insert a lowish value resistor to a MOSFET. But note: the value you choose may affect the switching speed. And, of course, a resistor is a useful 'link' for single sided pcbs!
A few FET/MOSFETs have a zener in them I seem to remember and the little Zetex FETs as mentioned by Jemery are excellent for low pwoer high/low sided switching. ZVP and ZVN 2106 and 4306 etc.
Usually in MOSFETs you want to dissipate that Gate charge asap for fast switching. But there are various techniques for improving this depending on the device. Get yourself a Tektronix and have a look. And for pwming you want to get transistor full on / full off as fast as possible even if you interfere with the neighbours baby alarm.(A joke).
For newbies that are reading these forums (some of whom don't intend to learn about electronics /transistor theory or simply can't be bothered) its safe practice to insert a lowish value resistor to a MOSFET. But note: the value you choose may affect the switching speed. And, of course, a resistor is a useful 'link' for single sided pcbs!
A few FET/MOSFETs have a zener in them I seem to remember and the little Zetex FETs as mentioned by Jemery are excellent for low pwoer high/low sided switching. ZVP and ZVN 2106 and 4306 etc.
Usually in MOSFETs you want to dissipate that Gate charge asap for fast switching. But there are various techniques for improving this depending on the device. Get yourself a Tektronix and have a look. And for pwming you want to get transistor full on / full off as fast as possible even if you interfere with the neighbours baby alarm.(A joke).
<i>And, of course, a resistor is a useful 'link' for single sided pcbs! </i>
Yeah, I still use a lot of through hole components for that very reason.
<i>A few FET/MOSFETs have a zener in them I seem to remember ... </i>
Lots of power fets have a zenor across the Source-Drain but not the Source-Gate.
Evan
Yeah, I still use a lot of through hole components for that very reason.
<i>A few FET/MOSFETs have a zener in them I seem to remember ... </i>
Lots of power fets have a zenor across the Source-Drain but not the Source-Gate.
Evan