milks
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60W load frying my relays

Fri Jun 12, 2015 11:35 am

Hi


I have a RPi switching a few devices on and off via a relay board. This has been working happily for some time now however the most powerful device has a habit of eventually breaking whichever relay it is connected to. It is a 12V/5A peltier heat pump supplied by a 60W power supply (I know this isn't ideal, I will shortly be upgrading to 120W).

The relays are rated at AC250V 10A/DC30V 10A (as seen here: http://www.ebay.co.uk/itm/5V-8-CHANNEL- ... 20fd5151b7) so should be able to deal with the load.

As an electronics noob I'm a bit puzzled as to why this is happening, surely the spike from the transformer when powering on shouldn't be enough to burn out the relay?

Since I have a number of 12v devices being operated by the relays I'm about to add a common 120W 12V supply so I don't need a separate power supply for each. The peltier cooler will be utilising this and I will ditch the over-worked 60W supply. The RPi will also be powered from this circuit using a switching regulator to step down to 5V. Max theoretical load on the new circuit will be ~90W though it's very unlikely all devices will be on simultaneously.

I'm hoping that since the 120W supply will be permanently on it will reduce any spikes occurring but thought it would be good to post here in case I'm making any glaring mistakes.

Any tips on how to debug this would be great.

Thanks 8-)

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PeterO
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Re: 60W load frying my relays

Fri Jun 12, 2015 11:46 am

In what way are the relays failing ?

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BMS Doug
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Re: 60W load frying my relays

Fri Jun 12, 2015 12:03 pm

I would be tempted to add another relay, use the 5V relay to switch a 12V relay and the 12V relay will switch your load.
Doug.
Building Management Systems Engineer.

milks
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Re: 60W load frying my relays

Fri Jun 12, 2015 12:34 pm

PeterO wrote:In what way are the relays failing ?

PeterO
The first time the relay seemed completely dead, no light and no click when toggling a high/low input. This last time the LED switches on and off and there is the click of the solenoid but the circuit remains closed in both states

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DougieLawson
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Re: 60W load frying my relays

Fri Jun 12, 2015 1:13 pm

milks wrote: The first time the relay seemed completely dead, no light and no click when toggling a high/low input. This last time the LED switches on and off and there is the click of the solenoid but the circuit remains closed in both states
Are you sure the coil of the relay is rated for 5V? Have you removed the jumper so the relay is powered separately from the opto-coupler that you're driving from the GPIO pin. Trying to drive a 5V relay from a 3V3 RPi gets that exact effect (same if you try to drive a 12V relay from 5V).
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milks
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Re: 60W load frying my relays

Fri Jun 12, 2015 1:24 pm

DougieLawson wrote:Are you sure the coil of the relay is rated for 5V? Have you removed the jumper so the relay is powered separately from the opto-coupler that you're driving from the GPIO pin. Trying to drive a 5V relay from a 3V3 RPi gets that exact effect (same if you try to drive a 12V relay from 5V).
I noticed the jumper and three pins but wasn't sure what it does so left it well alone. I'll see how it is set when I get home tonight and report back.

So am I right in thinking that with the wrong jumper setting the relay board is using the 3.3V across the input pin to actuate the solenoid rather than drawing current from it's 5V supply?

On a side note, would it be better that the (8 channel) relay board has it's own power supply rather than drawing current from the RPi's GPIO?

johndough
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Re: 60W load frying my relays

Fri Jun 12, 2015 1:37 pm

milks wrote:Hi
I have a RPi switching a few devices on and off via a relay board. This has been working happily for some time now however the most powerful device has a habit of eventually breaking whichever relay it is connected to. It is a 12V/5A peltier heat pump supplied by a 60W power supply (I know this isn't ideal, I will shortly be upgrading to 120W).
Any tips on how to debug this would be great.

Thanks 8-)

Hi

When you switch an AC3 load, you should divide by at least 3, so a 10 amp resistive(thermal) rating is 3 amp inductive.

Your 5 amp load is gonna need 16amp rated contacts, and a 10 amp inductive would warrant a 40 amp rating in my view.

http://www.schneider-electric.co.uk/sit ... irect=true

Schneider Electric et al say...

Resolution:
AC-1 - This category applies to all AC loads where the power factor is more than 0.95. These are primarily non-inductive or slightly inductive loads, such as heating. Breaking the arc remains easy with minimal arcing and contact wear.

AC-3 - This category applies to squirrel cage motors with breaking during normal running of the motor.
On closing, the contactor makes the inrush current, which is about 5 to 7 times the rated full load current of the motor.
On opening, the contactor breaks the rated full load current of the motor.

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DougieLawson
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Re: 60W load frying my relays

Fri Jun 12, 2015 2:40 pm

I've run 16 5V relays from the 5V pin on a RPi (for power) and 16 GPIOs on a MCP23017 (running at 5V0, not 3V3) driven off the I2C interface.

I used a Q2W board (http://skpang.co.uk/catalog/quick2wire- ... -1171.html) to do that with the IDC connector replaced with regular header pins when I soldered it up. I took the 5V from pin#2, GND from pin#6 and I2C from pin#3 & #5. The RPi is running with a 2A supply and a wired network (no USB devices).

Next time I'd probably use http://www.modmypi.com/raspberry-pi/bre ... akout-boad as it only costs £7 (inc VAT and P&P) and it's easier to pop that on the GPIO pins on an A/B/B+/A+ or 2B
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Mortimer
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Re: 60W load frying my relays

Fri Jun 12, 2015 3:26 pm

@johndough - Peltiers are not inductive are they? I wouldn't have though derating the relay contacts for a Peltier was necessary.
--------------
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mikronauts
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Re: 60W load frying my relays

Fri Jun 12, 2015 3:35 pm

Data sheet:

https://www.ghielectronics.com/download ... elayX1.pdf

Resistive load rating: 7A @ 28VDC
Inductive load rating: 3A @ 28VDC

And the following may be an issue for you:

MAX ON/OFF SWITCHING (electrically) 30 operations per minute

Switching more often than that will basically kill the relay.

Apparently it is a very bad idea to try to drive a Peltier with PWM:

http://electronics.stackexchange.com/qu ... er-element

If you do PWM it, the lower PWM frequency the better... ie stay below 0.25Hz

When I used to overclock Athlon's and P4's like crazy, I'd leave the peltier on at full blast all the time.

You would be far better served by switching between 5VDC / 12VDC no more than every 2 seconds.
milks wrote:Hi


I have a RPi switching a few devices on and off via a relay board. This has been working happily for some time now however the most powerful device has a habit of eventually breaking whichever relay it is connected to. It is a 12V/5A peltier heat pump supplied by a 60W power supply (I know this isn't ideal, I will shortly be upgrading to 120W).

The relays are rated at AC250V 10A/DC30V 10A (as seen here: http://www.ebay.co.uk/itm/5V-8-CHANNEL- ... 20fd5151b7) so should be able to deal with the load.

As an electronics noob I'm a bit puzzled as to why this is happening, surely the spike from the transformer when powering on shouldn't be enough to burn out the relay?

Since I have a number of 12v devices being operated by the relays I'm about to add a common 120W 12V supply so I don't need a separate power supply for each. The peltier cooler will be utilising this and I will ditch the over-worked 60W supply. The RPi will also be powered from this circuit using a switching regulator to step down to 5V. Max theoretical load on the new circuit will be ~90W though it's very unlikely all devices will be on simultaneously.

I'm hoping that since the 120W supply will be permanently on it will reduce any spikes occurring but thought it would be good to post here in case I'm making any glaring mistakes.

Any tips on how to debug this would be great.

Thanks 8-)
http://Mikronauts.com - home of EZasPi, RoboPi, Pi Rtc Dio and Pi Jumper @Mikronauts on Twitter
Advanced Robotics, I/O expansion and prototyping boards for the Raspberry Pi

milks
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Re: 60W load frying my relays

Fri Jun 12, 2015 5:03 pm

Thanks everyone for the replies, though I only understand them in part :)

So the peltier was a relatively recent addition to the devices being controller by the Pi, I added around six months ago whereas the other devices have been running happily for over a year. With this in mind I'm leaning away from the suggestion of a problem with the Pi being unable to supply sufficient switching power to the relays.

mikronauts - if I've understood your post correctly, relating to climate control, it's not switching so frequently, it is on constantly through the night and just occasional bursts of a few minutes throughout the day. I'm not looking to modulate the supply voltage, it is either on at 12V or off.

Given spec on the data sheet and from a few other suggestions it seems as though BMS Doug's suggestion of having the 5V relay switch a beefier 12V relay to handle the load might be a good solution. Currently eyeing this up as a likely candidate: http://www.maplin.co.uk/p/12v-dc-16a-sp ... elay-n30aw

To give my problem some context for anyone that's interested the project provides environmental control for my collection of miniature orchids. The peltier cooler has been a bit of a thorn in my side, it's big (when decked out with heat sinks), heavy, ugly, noisy, hungry and inefficient but being able to reduce the night time temperatures, even only a little is invaluable.

BMS Doug
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Re: 60W load frying my relays

Fri Jun 12, 2015 7:23 pm

milks wrote:Thanks everyone for the replies, though I only understand them in part :)

So the peltier was a relatively recent addition to the devices being controller by the Pi, I added around six months ago whereas the other devices have been running happily for over a year. With this in mind I'm leaning away from the suggestion of a problem with the Pi being unable to supply sufficient switching power to the relays.

mikronauts - if I've understood your post correctly, relating to climate control, it's not switching so frequently, it is on constantly through the night and just occasional bursts of a few minutes throughout the day. I'm not looking to modulate the supply voltage, it is either on at 12V or off.

Given spec on the data sheet and from a few other suggestions it seems as though BMS Doug's suggestion of having the 5V relay switch a beefier 12V relay to handle the load might be a good solution. Currently eyeing this up as a likely candidate: http://www.maplin.co.uk/p/12v-dc-16a-sp ... elay-n30aw

To give my problem some context for anyone that's interested the project provides environmental control for my collection of miniature orchids. The peltier cooler has been a bit of a thorn in my side, it's big (when decked out with heat sinks), heavy, ugly, noisy, hungry and inefficient but being able to reduce the night time temperatures, even only a little is invaluable.
Whole your selected relay has better specifications I feel that this one would be a better choice.

Those mini-pcb mount type of relays can suffer from contact arcing, the wider seperation available from the larger relay (and the beefier contact pads) should reduce the arcing and increase the lifespan of the relay (and as it is dual pole you will have a spare set of contacts to use if one side does burn out).
Doug.
Building Management Systems Engineer.

Moe
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Re: 60W load frying my relays

Sat Jun 13, 2015 12:21 am

Well, we're talking DC here so you don't need to worry about inductance or de-rating or anything like that. You are operating the relays well within spec.

I don't think it would be a control problem; the Pi is sourcing the current from the the 5V supply and sinking it to ground, not trying to energise it with 3.3V from a GPIO pin. The fact you can hear it clicking means the coil is working - if the current isn't getting through it simply means the contacts aren't making contact.

In this case, I suspect the contacts have just worn out. Switching inevitably results in arcing and corrosion; a higher load just makes it worse and results in a shorter lifespan. If the contact is a bit rubbish to start with then even a steady load will cause them to heat up and oxidise, and let's face it - these things are not exactly expensive.

The solution, as suggested above, is to use a better relay. I use a bog-standard automotive one to power the motors on my robot. They work at 12V, they're cheap, they can handle big amps, and they last.

http://www.ebay.co.uk/itm/AUTOMOTIVE-12 ... 256aa5b9b5

You would still need to use the smaller relay to drive it though.
Submarine communication systems engineer and amateur robot enthusiast.

trevatxtal
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Re: 60W load frying my relays

Sat Jun 13, 2015 6:00 am

A pump start up load is Three or more times running load.
Inductive load creates high fly back voltages so contact voltage should be at least twice normal voltage.
Speed of opening contacts is a vital factor faster the better.
Ratings on all imported/manufactured electronic equipment should be suspect, they do not have to comply with UK EU or US law.
Trev

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rpdom
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Re: 60W load frying my relays

Sat Jun 13, 2015 7:19 am

trevatxtal wrote:A pump start up load is Three or more times running load.
Inductive load creates high fly back voltages so contact voltage should be at least twice normal voltage.
Speed of opening contacts is a vital factor faster the better.
Ratings on all imported/manufactured electronic equipment should be suspect, they do not have to comply with UK EU or US law.
Trev
A "Peltier heat pump" is not an actual "pump" with a motor, it is a solid state device. It isn't inductive and doesn't have a large start up load, just a fairly constant current draw that (iirc) depends on temperature differential on the two side.

boyoh
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Re: 60W load frying my relays

Sat Jun 13, 2015 7:54 am

milks wrote:Hi


I have a RPi switching a few devices on and off via a relay board. This has been working happily for some time now however the most powerful device has a habit of eventually breaking whichever relay it is connected to. It is a 12V/5A peltier heat pump supplied by a 60W power supply (I know this isn't ideal, I will shortly be upgrading to 120W).

The relays are rated at AC250V 10A/DC30V 10A (as seen here: http://www.ebay.co.uk/itm/5V-8-CHANNEL- ... 20fd5151b7) so should be able to deal with the load.

As an electronics noob I'm a bit puzzled as to why this is happening, surely the spike from the transformer when powering on shouldn't be enough to burn out the relay?

Since I have a number of 12v devices being operated by the relays I'm about to add a common 120W 12V supply so I don't need a separate power supply for each. The peltier cooler will be utilising this and I will ditch the over-worked 60W supply. The RPi will also be powered from this circuit using a switching regulator to step down to 5V. Max theoretical load on the new circuit will be ~90W though it's very unlikely all devices will be on simultaneously.

I'm hoping that since the 120W supply will be permanently on it will reduce any spikes occurring but thought it would be good to post here in case I'm making any glaring mistakes.

Any tips on how to debug this would be great.

Thanks 8-)
Have you looked into the possibility of using
Thyristors ( SCR) , In your project . At least
it will eliminate contact bounce when rapid
Switching and noise spikes
BoyOh ( Selby, North Yorkshire.UK)
Some Times Right Some Times Wrong

trevatxtal
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Re: 60W load frying my relays

Sat Jun 13, 2015 8:46 am

Many thanks rpdom
You are of course right about A "Peltier heat pump" (non inductive load)
The "Peltier plates" I have ( China imports) are rated 5amp at 13 volt but draw nearly six amp.
The point I should have made more clear is that all power supply's should be over rated and for some items many times.
Also ratings are not to be trusted, until tested and confirmed.
Thank you for pointing it out I would not like to give wrong information.
Trev

johndough
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Re: 60W load frying my relays

Sat Jun 13, 2015 10:12 am

Mortimer wrote:@johndough - Peltiers are not inductive are they? I wouldn't have though derating the relay contacts for a Peltier was necessary.
Hi

No, not at all.

However if you are switching/interrupting the power supply side, you could be encountering transients etc.

Moe
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Re: 60W load frying my relays

Sun Jun 14, 2015 12:29 am

Every load that is switched causes transients, and everything electrical is switched at some point. The bigger the load, the bigger the surge. It's just physics. PSUs, circuit breakers and slow-blow (T) fuses are all designed to cope with this. So long as the steady state current doesn't exceed the PSU's rating, there should be no problem; there is no need to over-spec the PSU for the surge current. In fact a lot of PSUs run more efficiently at their rated output.

If you're really worried about transients, add some capacitors to take the edge off. If you're using a 'wall-wart' phone charger type PSU then extra capacitors might actually increase the relay's lifespan.

Also, product ratings only mean the product is safe and tested to fulfil its stated purpose; they do not guarantee it will never fail.
Submarine communication systems engineer and amateur robot enthusiast.

danjperron
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Re: 60W load frying my relays

Sun Jun 14, 2015 1:48 am

Well I did design a sensor with a peltier. I was using PWM but it was with a power mosfet.

If I remember well it was a TC1410 driver with a IFR540. Never had problem with it but the peltier had a very good heatsink with a fan to dissipate the heat and It was to keep a humidity sensor at a constant temperature which was 25 celsius.

I think you will have less problem with a power mosfet on heatsink and it doesn't need to be a big heatsink since the Rdson is very low.

boyoh
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Re: 60W load frying my relays

Tue Jun 16, 2015 7:05 pm

milks wrote:Hi


I have a RPi switching a few devices on and off via a relay board. This has been working happily for some time now however the most powerful device has a habit of eventually breaking whichever relay it is connected to. It is a 12V/5A peltier heat pump supplied by a 60W power supply (I know this isn't ideal, I will shortly be upgrading to 120W).

The relays are rated at AC250V 10A/DC30V 10A (as seen here: http://www.ebay.co.uk/itm/5V-8-CHANNEL- ... 20fd5151b7) so should be able to deal with the load.

As an electronics noob I'm a bit puzzled as to why this is happening, surely the spike from the transformer when powering on shouldn't be enough to burn out the relay?

Since I have a number of 12v devices being operated by the relays I'm about to add a common 120W 12V supply so I don't need a separate power supply for each. The peltier cooler will be utilising this and I will ditch the over-worked 60W supply. The RPi will also be powered from this circuit using a switching regulator to step down to 5V. Max theoretical load on the new circuit will be ~90W though it's very unlikely all devices will be on simultaneously.

I'm hoping that since the 120W supply will be permanently on it will reduce any spikes occurring but thought it would be good to post here in case I'm making any glaring mistakes.

Any tips on how to debug this would be great.

Thanks 8-)
What is the outlay and running cost of the Peltier Heat Pump
To extract or what ever you want to do.
What will your gain be, cost wise
BoyOh ( Selby, North Yorkshire.UK)
Some Times Right Some Times Wrong

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