12v/30 amp car relay+1n4148 diode?

So yeah, the title says pretty much everything...
I was thinking of speeding up my heatbed heating time and I read that a relay is the way to go. I happen to have a 12v 30a car relay and a 1n4148 diode in my spare box, and I wanted to use it as a flyback diode.
Btw, I have a ramps 1.4 and a 12v 30 amp psu.

Thanks a lot!!

The clicking may drive you mad. No need for the diode.

Using a mech. relay might work in Bang-Bang mode. But what if the contacts stick together and don't switch OFF?

By itself, a relay won't reduce heating time significantly unless you have excessive voltage drop in the electronics that switches the bed, and if that is the case then it would be better to fix the electronics.

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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].

With all the noise the printer makes, I don´t mind the clicking very much. What do you mean I don´t need the diode?

Well, I´ve checked the voltages with a multimeter, and it´s 12v or so pretty much everywhere, with the bed hot and cold. It´s taking around 40 min to get to 95c.

A relay won't fix that. You can use higher voltage if your supply is adjustable and if increasing the voltage won't damage anything else, you can put some insulation on the underside of the bed, or you can solve the real problem a get a heater that produces more heat. As a point of reference, my bed has a 0.5 W/cm^2 heater and it gets up to 105C for printing ABS in about 5 minutes.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

I guess you must be using a PCB heater if it is getting the full 12V but still isn't hot enough. If your PSU is the typical Chinese LED/CCTV one with some spare power capacity, then you can turn the voltage up to 14V to get 36% more heating power. But I don't advise this if your electronics is RAMPS with an attached LCD, because then the voltage regulator on the Arduino is likely to overheat unless you take additional steps to avoid this.

Insulating under the bed is worthwhile if you haven't already. You can also put a blanket on top of the bed during the initial heating.

Edited 1 time(s). Last edit at 02/21/2016 01:40PM 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].

Well, yes indeed, I´m using a Mk2b dual power, and also, I´m using a piece of cork under the bed, and a piece of cork on top while I wait for the thing to heat up, which, again, takes it´s time to do so.
I´ts interesting what you are saying about turning up the voltage. I do have a chinese led psu, and I tuned it to 12v. The thing is, a while ago my voltage regulator on my arduino blew up because of an affaire involving the endstops (I bet you can guess what happened ), and I ended up installing a mosfet used in this tutorial: [hassanulmakers.wordpress.com].
Now that you said it, I´m tempted to try to increase the voltage to 14v, and, I don´t know...maybe the new mosfet is up to the job?

Thanks!!

I'm guessing you mean a 7805 regulator in TO-220 packaging - that's not a mosfet. In days gone by, we treated 7805s to endless abuse and considered them almost indestructible. I don't know what they did to these new fangled surface mount regs that has made them so delicate, but it doesn't seem like progress to me.

That´s right, I meant a 7805 regulator (those three legged thingies look all the same to me ). So, do I increase the voltage on the psu, and see if the volt. regulator can take it? maybe adding a heatsink?

With a 7805 on there I'd expect everything to be good. 7805s have thermal shutdown and short circuit protection, it should be difficult to break one. If it gets too hot and shuts down then a heat sink will help, but the best approach is to remove the D1 diode on the ramps and supply the mega2560 with 8 to 9V power, then the regulator will never be working particularly hard and you can use whatever voltage you want (within reason and the limits of the capacitors and polyfuses) to power the heaters and steppers. If you want convenience you can use a buck converter to efficiently drop the ~12V supply to 9V to feed the ramps, or you can just use a small 9V psu for the 2560.

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mato84
With all the noise the printer makes, I don´t mind the clicking very much. What do you mean I don´t need the diode?

The control wires that fed the heatbed activate the relay. The extra power supply feeds the heatbed directly through the other side of the relay.

Edited 1 time(s). Last edit at 02/21/2016 04:11PM by elwood127.

Read this post. [forums.reprap.org]

I read the post, but it only talks about a solid state relay, I have an automotive relay, which, if I understand correctly, needs a diode to protect the board from emf spikes from the relay´s coil, which leads me back to the original question of this post.

Yes, the diode is a good idea to protect the MOSFET on the controller board from the back EMF created when switching current through the relay coil. I'd probably look for a diode that's intended to be used as a rectifier in a power supply, not a small signal switching diode like the 1N4148.

Some automotive relays have shunt rectifiers built in- if the relay has a polarity marking that may be why. Check the relay's spec sheet.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

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the_digital_dentist
Some automotive relays have shunt rectifiers built in- if the relay has a polarity marking that may be why. Check the relay's spec sheet.

Just checked it, no rectifiers or polarity markings, just a common mech relay.

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the_digital_dentist
I'd probably look for a diode that's intended to be used as a rectifier in a power supply, not a small signal switching diode like the 1N4148.

I have a fr157 and a fr102 diodes I desoldered from an old atx psu a while ago. Would any of those two be okay?

BTW, thanks everybody for your patience!!

I stand corrected. Thanks DD. Are caps ever used for this?

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elwood127
I stand corrected. Thanks DD. Are caps ever used for this?

A cap would make it worse IMHO. The flyback diode has an much lower resistance.

I think the FR102 would be OK. Wire it with the diode connected in parallel with the relay coil, and make sure you then connect the diode's anode to the more negative side of the connection to the controller.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

If you are going to use a relay then you should use a 120 volt heater element 1.5Kw add some oil and bread and print or fry some shrimp. Since it seems the original board is feeding the now heater with a MOSFET transistor you need to put a diode in parallel with 12 volt DC only coil of the relay measure with a meter to get the + and - wires correct the relay doesn't care how you connect but the diode does as its cathode gets hooked to the positive and the anode goes to the negative like you would with a zener. When powered current flows through the coil and not the diode then when power is removed the coil generates a voltage of opposite polarity of way more than 12 volts and the diode now conducts and brings it to zero. The 1N4148 works but you can get diodes made for the purpose. I was considering using a 120 volt 100W cartridge for my extruder

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rwt

100W in the extruder seems very high - are you printing at very high mm³/s?