Heatedbed 12V 300W - Help

Hello Folks

I bought a 12V 300W silicone heater from Robotdigg and im trying to make it work but im getting some trouble with it.

First of all i bought 2 SSR relay's, a fotek 25dd and 40dd , DC DC SSR.

I have burned down both relays.
Now i have changed the Mosfets inside to a P55NF06 like the ones mounted on the extra Ramps 1.4 i had laying around.

Now i have then attached a big alu heatsink but it doesnt do the job well, because the relay works and i can get to the desired temperature at 95-100 degrees.
But thats only possible if i wait 40 mins for it to reach it. And the heatsink is untouchable.

So my question is, how come i meet this problem it doesnt seem others need a heatsink that big. (See this link: [imgur.com] )

Thanks in advance.

How long would it take to heat up to 100C with a direct connection to the PSU ?

Did you check the wiring and PSU for voltage drop?
Is the wire gauge sufficient?
Did you verify the thermistor with a second ( known ) one?
-Olaf

First, cheap DC-DC SSRs are no good for this application. If you measure the voltage drop of your SSR you will find it is about 2V. That is why it gets so hot, and also why the bed power is lower than it should be. For example, if 10V is reaching the bed then the heating power is only 70% of what it would be at 12V.

Second, for any printer with a bed size greater than about 200 x 200mm and a heated bed that isn't driven from AC mains, it's much better to use 24V power than 12V. This halves the current to the bed, so that a Duet or other good electronics board can drive a 300W heater directly, without needing an SSR.

The easy but expensive solution to your problem is to replace the SSR by a decent one with low voltage drop, e.g. Crydom DC100D40 costing about $100.

A less expensive solution that may work is to use one of these [reprap.me]. It's essentially a 5-terminal SSR. The 25A current that your heated bed draws is a little more than I would be comfortable with; it will dissipate about 5W of power and get quite hot. You could connect two in parallel if need be.

Edited 1 time(s). Last edit at 12/04/2015 05:33AM by dc42.

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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].

Thank you guys for the reply.

I have now tested and measured a bit.
First off, i have a Fluke 179 and measured with the thermocouple i got with it. Seems to be in order with the thermistor in side the silicone heater.

There are 2.3V over the SSR relay (MOSFET)
And 10.2V over the heated bed.
12.5V on the PSU.

So yes im not getting full 12V on the heated bed. The bed i have measured to 0.5Ohm

I have talked to robotdigg and they are willing to replace my heater for an AC bed. But it costs quite alot to ship it to China

But how come others can måske it work and not me :/?

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TRoager
But how come others can måske it work and not me :/?

They don't, unless they are using good SSRs with low voltage drop. There are several other posts on these forums reporting exactly the same problem.

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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].

I've seen a self made MOSFET power switch with several TO-220 FETs in parallel.
Don't remember the name, but this will for sure be cheaper than a high end SSR and has low voltage drop.

Can someone jump in with details/links
-Olaf

I've used one of these on each of my 2 printers (one driven by a RAMPS board, the other by a Duet):

reprap_me

Admittedly my printers' heatbeds are 140W each, not 300W, but these boards do have a considerable current-carrying capacity (do read the spec for more details). Even when taking a heatbed to 110C the board's components barely get above 'warm to the touch'. Note that I didn't use the optional screw connectors - I soldered the cables directly to the solder pads (recommended for higher currents).

The supplier is based in Copenhagen, Denmark so may not be convenient - but they are pretty good at delivering overseas.

I forgot where I found my setup. I use a IRF4905 mounted on a used microprocessor with fan on a 12 x 12 silicon heat bed with a 10k resistor. Heats to 100 in about 3 to 4 min. Got 10 for about 1.40 US.

Edited 1 time(s). Last edit at 12/07/2015 09:50AM by Putzer.

I have thought about mounting mosfets in parallel but i was seaching for a solution that wasnt a work around or something like using a compatible controller. ( I'm designing my own controller and cant design it if i can't deside how it should work.)

I do like the Reprap.me's power extender but i do not intend to buy from them.

So now im wondering if i just should buy the AC Heater and try out a SSR AC.

But thanks guys for the answars