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As some of you have been following the other threads, I was intending on upgrading my power supply to 24V. And when i opened up my power supply i encountered an unpleasant surprise. The connector for my heat bed had melted. It was still working but luckily i caught it in time.

From now on i shall be using the heated bed with 24V exclusively. So i did not remove the whole connector assembly, or make any preparations to install high amperage wire. Instead I installed a JST connector on to it. I will be using the PWM output only as a signal for my SSR (or poor man's SSR).
As recommended by other people on the forum, i had ordered some FOTEK 40A SSR's. However I was in a hurry to finish some parts for my good friend Mokk. When he heard of my misfortune he conjured up the poor man's SSR. He built it and sent it, the next day it was in my possession.
It is based on a mosfet rated for 70A.


Today i had time, and I installed the 24V power supply and the Mosfet circuit on my printer. I soldered some extension wires to the existing ones coming from the heated bed. I will upgrade those to 8AWG silicone wires when they arrive.
It seems to be working fine so far. Very happy with the result.
Constructive criticism welcome.

That heat sink must get super hot. At about 20C/W and 25A it will be almost 200C, well beyond the rating of the MOSFET. I don't know what kind of connector that is but it probably can't do those currents either. With those dinky wires it's probably not 25A yet but if your supply can do it, it will be when you put in the 8AWG.

Thank you for the feedback.

Which wires are you referring to?

The red and black ones? Those only convey a signal. They do not carry any power. The other wires are not heating up yet, but they will be upgraded soon.

The mosfet does get hot, but i have a fan blowing on it. While in operation it is cold to the touch. While heating the bed, it does get warm but nothing you could not hold on your had for extended periods of time. (Without the fan it is impossible to hold).

I was looking at the wires from your burnt up connector, I assume those are the ones that still go to the bed. Those should be much larger, but they are probably reducing the overall current because of their resistance.

Ok, with a fan the heat sink C/W will be much less (forced convection). But if the fan fails then it could start on fire, something to consider. Still I would do something about that connector. Also make sure the RAMBo is using 12V for that output, otherwise you'll be violating the 20V Vgs of the MOSFET.

Still not sure which ones you mean.

But i did cut the old wires transmitting power from the rambo to the bed and soldered the yellow and black wires on to those. These will get replaced at the next overhaul.
The red and black wires soldered on to the rambo go to trigger the mosfet so no real current there and they are still powered by the Rambo's internal mosfet so 12V.

I am happy with the setup. I can get it up to heat in a matter of minutes. Did not count yet as i have been printing non stop.


NOTE TO OTHERS: IF YOUR BED OR HOTEND USED TO HEAT UP BUT LATER REFUSES TO HEAT UP TO THE SAME TEMP, CHECK YOUR WIRING.

I got some fake SSR's and they did not last at all.

This module also failed in the end (and it failed in the closed circuit position). But while it worked it did perform better than the fake SSRs.

I will be rebuilding this again with 3 mosfets in parallel attached to a proper heatsink and good wiring.

Please be CAREFUL! I had a cheapo SSR go up in flames - literally. If I had been printing unattended it could have been a disaster. I RARELY print unattended BTW for that exact reason.

Indeed.

I have learned not to trust electronics.


I am sharing my experiences so others are also careful.


I have acquired a big heatsink and 2 mossfets in parallel, if all goes well i should build v2 tonight.

I will be designing it so it is ventilated by the 24v psu fan exhaust and it will reside on a glass shelf.

I just completed V2 and i must say it is a massive improvement. The previous one was always intended to be temporary to be replaced once i got my ssrs which turned out to be crap.

The old one would heat up to the point of being untouchable. V2 only gets slightly warm. I should still tidy up the wires going to the bed at some point.

The heatsink was intended for a fotek SSR. And the screws to hold down the mossfets were salvaged off the crap ssrs.



The heatsink:

[img]https://www.surplussales.com/Images/Hea ... 5DA_lg.jpg[/img]
http://www.banggood.com/Wholesale-Alumi ... 48066.html

On a lighting dimmer we make I used to use really nice VNP devices, MOSFET with over volt, over temp, and over current safety circuits built in. I always ran these without a heat sink for up to 10 Amps each. Probably could have gone higher as they ran barely above room temp. Unfortunately they no longer make them in the larger current ratings so I've had to redesign my board to use regular MOSFETs without as much protection. The new board is in two parts, the controller and an 8-channel SSR. The nice thing is the SSR board handles a total of 80 Amps without heat sinks, is isolated, and fits in a standard USA electrical box (it's the size of a residential dimmer). The bad thing is to include a controller board (sequencer for signs, dimmer for lights) it takes another slot so it needs a double-wide box.

Are you at liberty to share a link to the 8 channel ssr?

I am in the process of designing a hoverlamp for myself. And i shall require 10A for the lamp that i have to switch from 1.8V.

We actually started selling these, then our shopping cart crashed during an update and at the same time we got swamped with orders for one part (review like happened with the fishing reels). I switched back to the old cart with that missing since at the moment we don't need any more orders. Probably be starving for work next month.

I just checked the cart catalog folder and it seems it lost the picture of the board. I'll have to get that fixed.

Here's a picture of the driver board and a silkscreen of the SSR board.

[img]http://interstellar.com/products_files/ ... RS-232.jpg[/img]

@Teoman,

What were the part numbers for the two MOSFETs you used on your V2 SSR?

Ill get back to you when i reach my printer.

Can anyone confirm that the mossfet on the rambo responsible for switching the power to the heated bed is the one right next to the capacitor?

And will adding a heatsink right under it help remove some of the heat it generates?