Heated relay overheating

I have the Persia I3v 12 inch printer and I keep burning out heatbed relays. I have went through 3 of the green ones, the type that comes with the build. I then converted to the 12v 30/40amp Auto type relay and they are failing also. Now it not Imediatly but it seems to happen most often when I try a larger print

So it's a mechanical relay? Which bit burns out? The coil side from the controller or the high current side to bed?

What power is your bed? I use a mechanical 40 amp auto relay for a 300 x 200 pcb heater at 12.5amps/24v no issues so far the relay has a built in fuse so offers some protection.

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Simon Khoury

Co-founder of [www.precisionpiezo.co.uk] Accurate, repeatable, versatile Z-Probes
Published:Inventions

A mechanical relay is a poor solution for driving a bed heater. Either use electronics that is capable of driving the bed heater directly - which for a 12" printer probably means using 24V power - or use a solid state relay (SSR). For SSR recommendations, see [duet3d.com]. That page was written for Duet electronics, but the SSR choice applies to any printer 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].

If you switch to an SSR make sure you get the matching heatsink. I use a 40 x 40 x 20mm fan hooked up to the control side of the SSR and have had no overheating issues.

Quote
elwood127
If you switch to an SSR make sure you get the matching heatsink. I use a 40 x 40 x 20mm fan hooked up to the control side of the SSR and have had no overheating issues.
[attachment 87100 DSC00939.jpg]

If you need a heatsink and fan like that, then I think you must have picked the wrong sort of SSR, such as the SSR-40DD that I advise against on the page I linked to.

Edited 1 time(s). Last edit at 12/07/2016 06:56PM 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].

Quote
dc42
A mechanical relay is a poor solution for driving a bed heater. Either use electronics that is capable of driving the bed heater directly - which for a 12" printer probably means using 24V power - or use a solid state relay (SSR). For SSR recommendations, see [duet3d.com]. That page was written for Duet electronics, but the SSR choice applies to any printer electronics.

I've got over a thousand hours of printing time on my printers, that both use automotive relays... no issues yet

An automotive relay is a great solution, especially when compared to a cheap SSR. The on-resistance of a relay is miles better than an SSR, hence why the cheap SSR's burn out or require ridiculous cooling apparatus. If you want to use a heat bed driver that costs more than the rest of your electronics, then fine, go ahead with an expensive SSR. But a cheap 30 amp automotive relay is a great solution for a RAMPS setup.

ok bed is 300 by 300 and its the bed side the overheats

Can you measure the current your bed is drawing?

All I can suggest is simply a relay rated for higher current, make sure you are using bang bang control, not PWM. Install an automotive fuse rated for something between the current draw of your bed and the rating of the relay.

If you are using 12v power consider switching to 24v, you effectively halve the current by doing so. If you are using a relay then you could use a second 24v power supply just for the bed, this means not changing hot end heaters/fans etc...

Edited 1 time(s). Last edit at 12/08/2016 06:01AM by DjDemonD.

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Simon Khoury

Co-founder of [www.precisionpiezo.co.uk] Accurate, repeatable, versatile Z-Probes
Published:Inventions

ok if I checke it right it shows 7.85 amps and right now I have a 12v 30 amp relay connected

Something odd must be going on then, I run 12.5amps through a 40amp relay, never had any issue from the relay, slightly melted fuse holder at one point. Without more information I have nothing more to add.

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Simon Khoury

Co-founder of [www.precisionpiezo.co.uk] Accurate, repeatable, versatile Z-Probes
Published:Inventions

Quote
DjDemonD
Something odd must be going on then, I run 12.5amps through a 40amp relay, never had any issue from the relay, slightly melted fuse holder at one point. Without more information I have nothing more to add.

I agree completely. I'm using 30 amp automotive relays, with around 12 amps current flowing through them. No problems on these at all. The only things that I can think of are:

1) Switching too fast. If you're using PWM and your relay is buzzing, then this will cause problems. The relay should click every couple of seconds when the bed is at temperature.
2) Non resistive load. If you have a whopping great capacitor across the bed terminals then this might cause issues.
3) Relay getting too hot. This would only happen if it's mounted basically ON the heatbed. They don't create much heat at all.
4) Power supply cranked up too high. I bump my power supplies up to around 14 volts, but no higher. Relays are fine at that voltage.


That's all I've got

Edited 1 time(s). Last edit at 12/08/2016 09:10PM by nebbian.

1) ok dont know if PWM is on or not, could you link something to show me how to check/fix that.
2) Im using a standard 12v 30 amp power supply, how do you adjust that. I just tried again using a Dorman 12v 30 amp relay and it failed again. checked current again before fail and and it was 7.89 during print
3) now the dumb question, does it matter which side of heatbed relay is hooked to because right now its hooked to pos side and neg goes direct to power supply

1) go in marlin configuration.h and look for this line

//#define PIDTEMPBED

it needs to be commented out as it is written here. This enables bang-bang i.e. heater on full/heater off control, classic themrostat if you prefer. If it is enabled then the controller pulses power to your relay at 10hz+ which will damage it, it can't switch that fast. You'd know if this was happening it would make a nasty noise instead of just going click, click every few seconds.

2) standard chinese LED supplies (metal brick things) have a small potentiometer next to their DC output you can nudge the voltage up or down. Going up a little can help, but this changes the voltage for your entire system so its sometimes okay and sometimes not.

3) most people ground the bed via the relay and send +ve to it directly, I don't I send the +ve through the relay, in terms of current carried its the same either way.

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Simon Khoury

Co-founder of [www.precisionpiezo.co.uk] Accurate, repeatable, versatile Z-Probes
Published:Inventions

ok this is what configuration H says at the moment

//#define PIDTEMPBED
//
//#define BED_LIMIT_SWITCHING

// This sets the max power delived to the bed, and replaces the HEATER_BED_DUTY_CYCLE_DIVIDER option.
// all forms of bed control obey this (PID, bang-bang, bang-bang with hysteresis)
// setting this to anything other than 256 enables a form of PWM to the bed just like HEATER_BED_DUTY_CYCLE_DIVIDER did,
// so you shouldn't use it unless you are OK with PWM on your bed. (see the comment on enabling PIDTEMPBED)
#define MAX_BED_POWER 256 // limits duty cycle to bed; 256=full current

#ifdef PIDTEMPBED
//120v 250W silicone heater into 4mm borosilicate (MendelMax 1.5+)
//from FOPDT model - kp=.39 Tp=405 Tdead=66, Tc set to 79.2, argressive factor of .15 (vs .1, 1, 10)
#define DEFAULT_bedKp 10.00
#define DEFAULT_bedKi .023
#define DEFAULT_bedKd 305.4

//120v 250W silicone heater into 4mm borosilicate (MendelMax 1.5+)
//from pidautotune
// #define DEFAULT_bedKp 97.1
// #define DEFAULT_bedKi 1.41
// #define DEFAULT_bedKd 1675.16

// FIND YOUR OWN: "M303 E-1 C8 S90" to run autotune on the bed at 90 degreesC for 8 cycles.
#endif // PIDTEMPBED

so it looks as bang bang is enabled, see any other things there that might be a problem

Can't see anything wrong with that.

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Simon Khoury

Co-founder of [www.precisionpiezo.co.uk] Accurate, repeatable, versatile Z-Probes
Published:Inventions

probably wont make a difference but I switched relay from pos side before hotbed to neg side after hotbed and will give it one more try with another 12v 40 amp mech relay. Ok being its a 12v 30 amp power supply if I get an SSR and heat sink what would be the ones to get if wire switch doent work

would this be overkill ,,,,,Baomain SSR-100DD 100A DC-DC 100A Solid State Relay Module 3-32VDC/5-80VDC + Heat Sink only 18.00 US

Quote
shsims
would this be overkill ,,,,,Baomain SSR-100DD 100A DC-DC 100A Solid State Relay Module 3-32VDC/5-80VDC + Heat Sink only 18.00 US

I can't find any specification of the maximum voltage drop of that SSR, so I suspect it is very high like the so-called Fotek SSR-40DD and I strongly recommend against using it.

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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].

Ok I don't see why but now that I switched wires the relay is warm but not failing. I've got through two prints now and it looks like another problem, 1st print came out good, 2nd print of same part came out ruff and undersize in height

If you want heat to stop burning out an electrical component, fit a busbar. Solder in between the wire connections to the component.

Old school but they work - cone shaped lumps of copper do the trick (pointy ends at the connections that get hot, not the relay). Aluminium also good, but not as good as copper or brass. For one thing, they don't transmit electricity as well, for another, soldering onto aluminium is hellishly tricky.

[en.wikipedia.org]

Edited 4 time(s). Last edit at 12/17/2016 09:27PM by DragonFire.

The relay burnouts could also have been caused by a loose wire. The power cable for the bed should be as sort as possible, 60-80cm max to avoid any issues.

I should note that switching from the RAMPs to even a cheap DC SSR may provide improvements even with the huge voltage drop. Just keep the switched portion of power cable supplying the heated bed is as short as possible.

You should always try to use switching when powering a bed through an SSR. I have mine set to 50% duty cycle and it stays luke warm mounted 1cm away from the heated bed with no heatsink, with no slowdown in heating time. I use a China High Frequency 25A relay, sadly they are often sold unbranded.

is it possible to show me your set up. It seems strange that after all this time I cant find the problem, Even just changed the thermister on bed to see if it was wrong. Something Im missing somewhere. it was fine for about 6 months. just tried another relay and it didnt even finish print before it failed

heres an interesting observation, when I put my meter on to test the amps which 8.02 and to check the relay the wires got very warm. thats with no extra load to heatbed. could PS be putting out to much power

The heated bed will only draw a certain maximum amount of current proportional to its resistance for any given voltage.

Assume a mk2 pcb heater with 1.2ohms resistance at 12v it will draw 10A if your PSU can supply 10A. If your PSU can only supply 5A then it only gets 5A. It doesn't matter how big your PSU is if it can supply more than 10A. Think about the mains electricity - its basically a massive power supply, limited only by very large fuses, but a heater only draws the current its resistance allows it to.

If your wires are too small, they will heat up. In the OP's case what's odd is that the bit getting hottest is the relay, which it shouldn't be if its adequately specified.

I am not sure what you mean by no extra load? The load is determined by the resistance of the heatbed.

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Simon Khoury

Co-founder of [www.precisionpiezo.co.uk] Accurate, repeatable, versatile Z-Probes
Published:Inventions

ok just checked PS output, it was only showed 10.6, could low voltage be causing the problem.

Is that under load or with everything off?

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Simon Khoury

Co-founder of [www.precisionpiezo.co.uk] Accurate, repeatable, versatile Z-Probes
Published:Inventions

thats with PS hooked up and test + to -, nothing running

Yeah that's low, if its a chinese LED brick style PSU adjust the voltage trim pot next to the DC connectors to increase the voltage to 12v. Other types of PSU's may vary.

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Simon Khoury

Co-founder of [www.precisionpiezo.co.uk] Accurate, repeatable, versatile Z-Probes
Published:Inventions

would that cause bed relay relay to overheat

I don't know but it is possibly related. As a general rule if you want to have to handle less current you use higher voltage. So a 200w heater running at 6v will need much more current than a 200w heater running at 24v.

Turn it up if you can and try again, if you have the spare relays to burn.

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Simon Khoury

Co-founder of [www.precisionpiezo.co.uk] Accurate, repeatable, versatile Z-Probes
Published:Inventions