Has anyone got one?

[Jamesrl]

The big printer has a 300 x 300mmm silicon hot bed, it draws 25a at 12v DC so to power it I need a 12v input 12v 25a output relay, so has anybody got one laying about they don't need that I could buy or have please?

[kamaangir]

Just use an ebay ssr

[Jamesrl]

Just use an ebay ssr

Ye' but it takes weeks to get one, no one seems to sell dc-dc ssr's over here

[Julian]

I had it in the back of my head that SSRs normally switched AC.

I've got some contactors I was going to throw out ... think they may be 24v coils and not sure of the contact ratings, but I'll check and post up.

[Julian]

OK, this seems to be the best one ...

24v DC coil

4 sets of contacts, so I assume 3 phases and an auxiliary (which may not be rated the same as the main contacts)

Obviously for a motor starter as it's rated 2.2 KW @ 230v AC and 4 KW @ 400v AC.

At the bottom of the rating sticker it states "Continuous current 20 amps"

Not sure if this is suitable for what you want.  Needs someone who knows what they're talking about to advise.  I do know on DC switches of any sort arcing can be a problem.

Anyway, you're welcome to it ... what's the worst that can happen?

[knighty]

just can use a normal car relay ?

40 amp is pretty normal for a car one (just the nomral every day ones in pretty much every car out there?

ebay links

40 amp normal every day one  (it's a 5 pin so it'll change over between one output and another, you can ignore that and just use one output)

this one has 100amp rateing and bolt on terminals which might be better suited to a printer?

EDIT: might as well put a little warning light on too, so you know the relay has switched ?   +ve from the relay output and -ve from whatever is handy (maybe the -ve going to the relay?)
ebay limk (pretty much first one I saw


EDIT2:  I assumed you just wanted on/off, but the printer output for the heated bed is PWM, so you'll need something that switches super fast for that to work ?

[Jamesrl]

I did look at those but not knowing enough about'm didn't go for one.

The thing is it'll be controlled by a PID so will be swithing quite rapidly, was thinking about the contacts burning out.

[knighty]

doh, made another edit while you were replying,  yeah I think you're right, printer controller will switch too fast for one of those

I'm pretty sure the printer PWM is too fast for most stuff, I think you might need to use a fet to drive it

[Jamesrl]

Wtf is a fet?

[Julian]

Not sure about the bed but with the extruder you can change the method of control from PID to on off or bang bang as the call it ... mentioned in one of my recent posts.

[knighty]

fet is mosfet, like this

(sorry for big pic, its just one from googl)


you've probably seen loads of them inside power supplies etc..

(actually, the printer-board will have one on it already for switching the heatbed on/off, it's just a smaller/low power one)

oversimplified, just imagine it's a tiny but powerful and super fast relay


you could buy a pile of them, for pennies each, maybe £1 for some good 60v 60amp ones

but, just had a brain wave (it happens now and again)  you could just use an arduino shield ?  maybe one like this?

slight problem is you need between 3v to 5v to turn it on/off,    you'll have to feed 3 to 5v into the printer boards power input  (an old phone charger?)  and then the output from the printer would would switch it fine

you also need a heat sink to keep it cool, so screw it to an old cpu heatsink etc..?


added bonus, it's opto-isolated, so on the board between your printer power and the power it's switching is a little opto-isolation chip, whcih has a tiny little led and a tiny little light sensor thingie inside it... the printer will be turning the led on/off, which the light sensor thingie (I forgot the name) picks up and turns the mosfet on/off......  makes no difference at all to the way it works, but it means if you make a mistake, or drop a screwdriver over the contacts etc... your printer control board is totally safe because there's no physical connection between that and your hotbed/psu :-)

[knighty]

edit:

I googled it, you should be able to set the ramps board to bang-bang instead of PWM for hot bed control


which means it powers the bed output either full on or full off

so then you could use a 12v car relay, which might be easier ?



mosfet would give better temperature control, but with a bigger bed it will have less of an effect, and +/- 2 or 3'C shouldn't make a difference ?

[therecklessengineer]

An SSR will work just fine with DC.

Or, as above. A MOSFET or two if you need high speed switching.

[Jamesrl]

An SSR will work just fine with DC.

Or, as above. A MOSFET or two if you need high speed switching.

So you're saying an AC/DC ssr will work ok with DC/DC, would I be right by switching the + and direct wire the -

I have a 30a 12v supply ready for it, just need a relay.

[Jamesrl]

edit:

I googled it, you should be able to set the ramps board to bang-bang instead of PWM for hot bed control


which means it powers the bed output either full on or full off

so then you could use a 12v car relay, which might be easier ?

mosfet would give better temperature control, but with a bigger bed it will have less of an effect, and +/- 2 or 3'C shouldn't make a difference ?

Cheers, I'll have a look at that.

There goes my lack of knowledge about things electrionical, bang-bang, logical but what does PWM mean?