I followed the RAMPS wiki and hooked up my Prusa PCB heated bed according to instructions "powering the heated bed directly from a separate PSU, grounding on the RAMPS". With the bed temperature set to 80C, the power MOSFET (D8) on the RAMPS board is already too hot to touch. I don't think I can go up any higher without putting a heatsink and a fan. I would like to reach 120 C. Does anyone have their RAMPS set up this way with a heated bed running at 120C? I'm thinking that I have to use a relay. https://reprap.org/forum/read.php?219,87291,87291#msg-87291 Wed, 14 Oct 2026 00:28:43 -0400 Phorum 5.2.23 https://reprap.org/forum/read.php?219,87291,88155#msg-88155 https://reprap.org/forum/read.php?219,87291,88155#msg-88155 brnrd RAMPS Electronics Thu, 23 Jun 2011 13:23:48 -0400 https://reprap.org/forum/read.php?219,87291,88152#msg-88152 https://reprap.org/forum/read.php?219,87291,88152#msg-88152 jcabrer RAMPS Electronics Thu, 23 Jun 2011 13:13:52 -0400 https://reprap.org/forum/read.php?219,87291,88105#msg-88105 https://reprap.org/forum/read.php?219,87291,88105#msg-88105 -------------------------------------------------------
> That's what I have in my RAMPS too. You don't have
> a heatsink on it? How hot does it get when you're
> bed is at 120C?

better question, how hot do they get when the bed is at ~65c on its way to 120c? At that point they've probably been on for enough time to reach equilibrium without any breaks for temperature control...]]> Triffid_Hunter RAMPS Electronics Thu, 23 Jun 2011 04:50:42 -0400 https://reprap.org/forum/read.php?219,87291,88077#msg-88077 https://reprap.org/forum/read.php?219,87291,88077#msg-88077 brnrd RAMPS Electronics Wed, 22 Jun 2011 17:39:18 -0400 https://reprap.org/forum/read.php?219,87291,88053#msg-88053 https://reprap.org/forum/read.php?219,87291,88053#msg-88053 jcabrer RAMPS Electronics Wed, 22 Jun 2011 15:06:34 -0400 https://reprap.org/forum/read.php?219,87291,87677#msg-87677 https://reprap.org/forum/read.php?219,87291,87677#msg-87677 -------------------------------------------------------
> Are you sure you followed MY setup? One wire from
> the bed goes to the GND on D8, the other wire goes
> to +12vdc on the RAMPS power connector. Check the
> resistance on the bed. Maybe you have a short.

There's no short, his math is good, I checked it myself. simple result of P=I^2.R, Rds(on)@Vgs=5v, Tj(max) and Tj-amb shows that that particular mosfet isn't capable of supplying more than 10A without exceeding its maximum die temperature unless heatsinked. (I always assume that ambient is 50c because it gets that hot in several parts of the world)

The bed resistance does need to be around an ohm to put out the power that it does from 12v. P=V^2/R, so 12v into 1 ohm will give 144 watts.]]> Triffid_Hunter RAMPS Electronics Sun, 19 Jun 2011 23:47:17 -0400 https://reprap.org/forum/read.php?219,87291,87676#msg-87676 https://reprap.org/forum/read.php?219,87291,87676#msg-87676
Also, I don't see how you can run 5 (RAMPS+extruder) + 12 A (bed) = 17 A through one ground pin in the power connector on the RAMPS board.]]> brnrd RAMPS Electronics Sun, 19 Jun 2011 23:33:13 -0400 https://reprap.org/forum/read.php?219,87291,87672#msg-87672 https://reprap.org/forum/read.php?219,87291,87672#msg-87672 jcabrer RAMPS Electronics Sun, 19 Jun 2011 21:55:25 -0400 https://reprap.org/forum/read.php?219,87291,87523#msg-87523 https://reprap.org/forum/read.php?219,87291,87523#msg-87523 brnrd RAMPS Electronics Sat, 18 Jun 2011 06:49:53 -0400 https://reprap.org/forum/read.php?219,87291,87503#msg-87503 https://reprap.org/forum/read.php?219,87291,87503#msg-87503 -------------------------------------------------------
> According to the specs for the power MOSFET on
> RAMPS 1.2 (stp55nf06l), the Static drain-source on
> resistance is typically 0.016 ohm and maximum
> 0.020 ohm with Vgs of 5V. Assuming that the
> resistance of the PCB heater is 1 ohms, the power
> being dissipated by the MOSFET should be less than
> 2.8 Watts with a 12V power supply.

The TO-220 package can dissipate up to 2 watts (175c die-50c ambient=125c dT / 66c/w = ~2w) with no heatsink. Above that and you will definitely need either a better mosfet or a heatsink. This mosfet can carry up to sqrt(2W/20mR)=10A with no heatsink.

I use IRL3803 which has only 9mR@4.5v so can handle 15A with no heatsink.]]> Triffid_Hunter RAMPS Electronics Fri, 17 Jun 2011 23:03:44 -0400 https://reprap.org/forum/read.php?219,87291,87373#msg-87373 https://reprap.org/forum/read.php?219,87291,87373#msg-87373 about to start in progress.

Thanks everyone for the help.]]> brnrd RAMPS Electronics Thu, 16 Jun 2011 17:02:09 -0400 https://reprap.org/forum/read.php?219,87291,87341#msg-87341 https://reprap.org/forum/read.php?219,87291,87341#msg-87341
There's a thick and short trace for ground going from the power connections to the D8 powerMOSFET. You can see it in the lower left corner in this screen capture of the board. The power plug terminals are in the corner and just above it, there are 3 terminals that also connect to ground that I intend to connect to.

To be safe, I have reinforced the ground trace by soldering a thick wire between the MOSFET and the 3 terminals.]]> brnrd RAMPS Electronics Thu, 16 Jun 2011 09:52:55 -0400 https://reprap.org/forum/read.php?219,87291,87338#msg-87338 https://reprap.org/forum/read.php?219,87291,87338#msg-87338
> Would a heatsink like this AAVID THERMALLOY with a
> thermal resistance of 24 C/W be sufficient?

It should be with a fan, but I would go for something a bit bigger to be on the safe side as without a fan it would get to about 90C. Also bear in mind what temperature RDS is quoted at. It is often 25C, which implies an infinite heatsink and it gets higher as junction temperature increases.

>
> The other concern that I have is the amount of
> current that will be going through the ground leg
> of the power supply terminal of the RAMPS board
> which should be around 12 A. I think this might be
> too much in addition to the current drawn by the
> extruder heater. So, I plan to connect the ground
> leg of the 12V supply to the heated bed to the 3
> ground terminals on the board next to the power
> connector. Is this good enough?

I am not familiar with the board layout, but you should be able to reinforce the ground track with a thick copper wire on the back and then it is only the connector rating you have to worry about.]]> nophead RAMPS Electronics Thu, 16 Jun 2011 09:37:09 -0400 https://reprap.org/forum/read.php?219,87291,87332#msg-87332 https://reprap.org/forum/read.php?219,87291,87332#msg-87332
Would a heatsink like this AAVID THERMALLOY with a thermal resistance of 24 C/W be sufficient?

The other concern that I have is the amount of current that will be going through the ground leg of the power supply terminal of the RAMPS board which should be around 12 A. I think this might be too much in addition to the current drawn by the extruder heater. So, I plan to connect the ground leg of the 12V supply to the heated bed to the 3 ground terminals on the board next to the power connector. Is this good enough?]]> brnrd RAMPS Electronics Thu, 16 Jun 2011 07:43:13 -0400 https://reprap.org/forum/read.php?219,87291,87318#msg-87318 https://reprap.org/forum/read.php?219,87291,87318#msg-87318 VDX RAMPS Electronics Thu, 16 Jun 2011 04:05:29 -0400 https://reprap.org/forum/read.php?219,87291,87317#msg-87317 https://reprap.org/forum/read.php?219,87291,87317#msg-87317
Or add a level changer to increase the gate drive from 5V to 12V. You can see how I did that here: [hydraraptor.blogspot.com] with Q1 increasing the gate drive to Q5 from 3.3V to 12V to get lower on resistance. The extra components only cost pennies.

A relay won't last very long if you look at the spec for how many times they will operate under load and assume it will toggle every few seconds when controlling a bed.]]> nophead RAMPS Electronics Thu, 16 Jun 2011 03:59:58 -0400 https://reprap.org/forum/read.php?219,87291,87302#msg-87302 https://reprap.org/forum/read.php?219,87291,87302#msg-87302 brnrd RAMPS Electronics Wed, 15 Jun 2011 20:44:47 -0400 https://reprap.org/forum/read.php?219,87291,87301#msg-87301 https://reprap.org/forum/read.php?219,87291,87301#msg-87301 jcabrer RAMPS Electronics Wed, 15 Jun 2011 20:34:34 -0400 https://reprap.org/forum/read.php?219,87291,87291#msg-87291 https://reprap.org/forum/read.php?219,87291,87291#msg-87291 RAMPS wiki and hooked up my Prusa PCB heated bed according to instructions "powering the heated bed directly from a separate PSU, grounding on the RAMPS". With the bed temperature set to 80C, the power MOSFET (D8) on the RAMPS board is already too hot to touch. I don't think I can go up any higher without putting a heatsink and a fan. I would like to reach 120 C.

Does anyone have their RAMPS set up this way with a heated bed running at 120C? I'm thinking that I have to use a relay.]]> brnrd RAMPS Electronics Wed, 15 Jun 2011 18:39:07 -0400