MOSFET specs for RAMPS

I want to buy a MOSFET in the UK to switch an additional 12v fan under control of one of the GPIO pins, and I'll get a couple of spares in case I ever need to replace the ones in my RAMPS. I shall probably use the 7switch circuit design by Traumflug, but I can't find the IRLB8743 available locally.

Yes, I've seen the recommendations ( IRLB3813, IRLB8743, FDP8870, AOT240L, CSD18502KCS) but they aren't exactly abundant here (when you just want 2 or 3). Can anyone tell me what the important specs are if I'm looking at other models?

I think I've worked out:

Vgs < 3.3v
Vds > whatever you plan to switch (plus some overhead)
Id > 50A (less than that gets too hot when using PWM)

Any other values I need to be concerned about (Rds)?

Or am I unlikely to find any viable alternatives?

Edited 2 time(s). Last edit at 01/31/2015 06:05PM by plankton.

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Here's one in the UK for just 2 pounds, no shipping >> Ebay UK

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Quote
plankton
Can anyone tell me what the important specs are if I'm looking at other models?

See [reprap.org]

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Generation 7 Electronics	Teacup Firmware	RepRap DIY

Okay, probably my fault, I should have been more specific.

I need to order from this company if possible (in order to make up a minimum order value), I also like to try to understand what I build, and the components selected (rather than blindly copy someones design). I'm not an electronics engineer, so maybe overlooking some factors, but as far as I can see the requirements for a RAMPS mosfet are:

Vgs < 5v (yes, I know I said 3.3v in my OP, that was copied from something I saw in another post without realising that the logic voltage on the Mega is 5v)
Vds > whatever you plan to switch (plus some overhead)
Id > 50A (less than that gets too hot when using PWM)

Let's forget using this for heatbed or extruders, this will be used in a 7switch design to switch fans, lighting, or similar (comparatively low current devices), but it may need to do PWM. So I can see the FDP7030BL which seems to easily meet those requirements (Vgs 1.9v, Vds 30v, Id 60A), are there any problems with that choice?

One last question, I assume I need to use a voltage divider on the gate input in order to drop the 5v arduino logic level to circa 1.9v (Vgs), from my calculations a 100R and 68R resistor should produce a gate voltage of 2.02v (close enough). The reason I question this is the SevenSwitch schematic shows values of 10R and 1M (which doesn't really divide the voltage at all), so what am I missing?

Apart from the parameters you mentioned, the other critical one is Rds(on) measured at Vgs= 4.5V. Multiply this by the square of the maximum current that the mosfet will be required to pass to get the static power dissipation. Try to keep this down to 1W or less, then you won't need a heatsink.

You don't need a voltage divider on the gate. You need a gate series resistor to moderate the load on the Arduino output and slightly slow down the switching time, to avoid ringing, avalanching and excessive EMI. A suitable value is 100 to 180 ohms. A high value resistor to ground is also recommended so that the mosfet is held off when the Arduino output pin has not been initialised.

From memory, I think Farnell sells the IRLU8743, which is the same as the IRLB8743 but in a slightly different package, still suitable for through hole mounting but less suitable for attaching a heatsink.

Edited 3 time(s). Last edit at 02/01/2015 02:53PM by dc42.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

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Thanks, that makes much more sense now.

So I can see that the mosfet I suggested would be ok for low current devices, but not for switching a heatbed, and the gate series resistor explains the SevenSwitch design (I was confusing the gate threshold voltage with the max gate voltage, which is why I thought a voltage divider was needed).

Now I can build the circuit, AND understand it.

An IRLB/U 8743 easily switches 20 amps without even needing a heatsink and this resistor on the SevenSwitch (10 ohms) ist just a protection measure.

Edited 1 time(s). Last edit at 02/02/2015 04:45AM by Traumflug.

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Generation 7 Electronics	Teacup Firmware	RepRap DIY

Actually I found IRF2804 which I think should be a good substitute

VDSS = 40V
RDS(on) = 2.0mΩ (at Vgs 10v)
ID = 75A

Thr IRF2804 may not be a good choice due to the Vgs(thr) rating (look at Fig 3 for 5V). Most heatbed drivers are controlled by the MCU directly so you need to select what is commonly referred to as a logicalLevel MOSFET or subLogicLevel. It's a good idea to check what the RDSon for 4.5V is if they don't claim to be Logic Level driven.

I've always recommened IRLB8743 or IRLB3034/IRLB3036 as good ones to use. IRLB3034/IRLB3036 are usually more expensive but have better RDSon (specially the IRLB3034) and sometimes can be found when the IRLB8743 isn't available.

Edited 1 time(s). Last edit at 02/02/2015 10:58AM by Learner.

Hi,
If I replace the MOSFET on RAMPS with IRLB8743, will the 300x300 heatbed work??

Quote
prashantdshinde
Hi,
If I replace the MOSFET on RAMPS with IRLB8743, will the 300x300 heatbed work??

How much current does the 300x300 heatbed draw? The RAMPS can only deliver 11A because of the fuse. Even if you replace the fuse, I don't think it would be wise to put much more through the RAMPS.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Quote
Learner
Thr IRF2804 may not be a good choice due to the Vgs(thr) rating (look at Fig 3 for 5V). Most heatbed drivers are controlled by the MCU directly so you need to select what is commonly referred to as a logicalLevel MOSFET or subLogicLevel. It's a good idea to check what the RDSon for 4.5V is if they don't claim to be Logic Level driven.

I've always recommened IRLB8743 or IRLB3034/IRLB3036 as good ones to use. IRLB3034/IRLB3036 are usually more expensive but have better RDSon (specially the IRLB3034) and sometimes can be found when the IRLB8743 isn't available.

IRLB3034 shows VGS = 4.5V, ID = 172A @max 2mΩ whereas the IRLB8743 shows VGS = 4.5V, ID = 32A @max 4.2mΩ both of those are for Rds(on) so which of those is making one better over the other?

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The IRLB3034 is better based on just those specs, but you probably want to dig a little deeper just to be sure.

Given the recommendations here for that FET tho, I think it'd be a safe choice.

Edited 1 time(s). Last edit at 12/25/2015 02:14PM by Cefiar.