Need help driving mosfet using ramps D8 output
Posted by bryanbdp
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Re: Need help driving mosfet using ramps D8 output January 04, 2016 09:37PM |
Ok you can do the trick with two small signal transistors and 5 resistors.
You dont need to get the mosfet out of the ramps, you can use the outpot connector.
From the right side: Inside the Ramps is the D9 microcontroller output.
Then comes the Mosfet on Ramps board. This is like "open collector", you need a pull-up-resistor to get highlevel-signals too.
The pullup resistor is the vertical one.
To limit the current on the fist transistor basis, you need to have a resistor. 10k limit the current at about 1mA, this is enough. Little transistors have high gain around 200 to 550x.
This first transistor do invert your signal only.
Now the next one, you know, limit the current..
Use a little signal type transistor here too, it doesnt matter what exact transistor, nearly every little one does the job.
The load resistor here is 470 Ohm, but u can use a 10k here too. It discharges the gate.
We have on heated bed only slow frequencys, so its not time critical.
Your mosfet irf 3205 will have around 0.35V voltage loss (drain to source) at your 40 ampere power from heated bed.
So you get 14 Watt of heat here.
You will need a cooler for this, at least 8 K/W for free-air-cooling or your mosfet will burn down. (Smaller K/W values are better)
You can use a smaller one if you place this in the air from a little computer fan.
Edited 1 time(s). Last edit at 01/04/2016 09:38PM by gafu.
CTC 3D Drucker, fabrikator II mini, ultimaker-diy,
Modifizierter Prusa I3 Boxed.
Baubericht im Blog: [blog.gafu.de]
You dont need to get the mosfet out of the ramps, you can use the outpot connector.
From the right side: Inside the Ramps is the D9 microcontroller output.
Then comes the Mosfet on Ramps board. This is like "open collector", you need a pull-up-resistor to get highlevel-signals too.
The pullup resistor is the vertical one.
To limit the current on the fist transistor basis, you need to have a resistor. 10k limit the current at about 1mA, this is enough. Little transistors have high gain around 200 to 550x.
This first transistor do invert your signal only.
Now the next one, you know, limit the current..
Use a little signal type transistor here too, it doesnt matter what exact transistor, nearly every little one does the job.
The load resistor here is 470 Ohm, but u can use a 10k here too. It discharges the gate.
We have on heated bed only slow frequencys, so its not time critical.
Your mosfet irf 3205 will have around 0.35V voltage loss (drain to source) at your 40 ampere power from heated bed.
So you get 14 Watt of heat here.
You will need a cooler for this, at least 8 K/W for free-air-cooling or your mosfet will burn down. (Smaller K/W values are better)
You can use a smaller one if you place this in the air from a little computer fan.
Edited 1 time(s). Last edit at 01/04/2016 09:38PM by gafu.
CTC 3D Drucker, fabrikator II mini, ultimaker-diy,
Modifizierter Prusa I3 Boxed.
Baubericht im Blog: [blog.gafu.de]
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