Insight into control board failures

[Snowy Rivers]

I have read over and over about control board failures and other failures such as fan and draft motor (exhaust) failures.

Pellets stoves have become increasingly complicated (behind the cover)
The availability of the micro controller at such low costs has made this type of controls a real favorite of manufactures.

Lets look at some of the key things that the controllers do.

Pellet feed timers__ Sets the amount of time that the auger runs at any given time.
This setting also sets the amount of time that the auger is in the off or non feeding mode.

This is accomplished by simple timing circuits on the controller board.

Now the draft inducer/exhaust fan.

The speed at which the exhaust fan runs also plays an important roll in how much air is pulled through the burn grate and thus how well the fuel (pellets or??) burns.

Adjustment to the fuel feed rate or heat output setting are usually via a multi position rotary switch or a touch pad.

Now the going gets interesting.

To control the fan speed the process implements a little device known as a TRIAC

This little semiconductor component is interesting in that it allows for voltage control without the need to disipate heat such as in the use of a resistor to drop voltage.

The triac moidifies the amount of time that each half of the line voltage cycle is on.

120 volt AC current (Household power) is an alternating current.

By chopping off a varying part of each phase the power (voltage) is controlled to allow the fan to run at whatever speed is needed to get the best burn.

Now as these components get a buttload of time on them ( and sometimes not) the triacs can fail and the motor only gets a "Half wave" instead of the full wave.

This condition locks the motor and burns it out.

I had heard from several stove techs that the cause of the locked motors was the motor failing and then taking out the board.

The real deal is that the components on the board start failing and they in turn take out the motors.

These control boards cost pennies to make compared to other methods of controlling the stoves.

The stuff is all micro electronics and is relatively cheap to produce, NOT to buy at the consumer level though.

I got off onto a search recently as I am building a custom control system for my Advantage 2T

The auger timers are easy and cheap but to control fan speed becomes clumsy and hard to do reliably.

The triac controlled by a potentiometer makes this an easy job.

The whole issue though is that the OEM stuff is a little too cheeeezy and fails far too often.


Currently I have found a company that makes Triac motor controls for small AC motors that are very rugged and low cost.


This will greatly simplify the job at hand.

A simple casablanca fan control is basically this type of device but a very low end type quality wise.

My goal is to rebuild the control systems with very solid off the shelf technology that is head and shoulders above what ws used originally.

Currently I have spoken with ST microelectronics and their sales rep has offered much hope and a very broad range of solutions.

Hope this helps others understand why things do what they do and what causes some of the failures.

Snowy

 

[smwilliamson]

This may explain why a stove may go through 2 convection motors in a season and then a control board and then never have another problem? How does one test a control board for an issue such as the "half Wave"

 

[Snowy Rivers]

Unless you have access to an OSCILLOSCOPE to monitor the voltage going to the motors its pretty much just change out parts and see what happens.

If you suspect an issue you can test the supply voltage to the fan with a VOM and this may give indications that trouble is brewing.

Unfortunatley the average home/stove owner will not be able to really know whats up until the thing takes a crap and needs service.

When the manufactures went away from quaility controls and started using all custom control boards that are made overseas the troubles really got under way.

This is not to say that a manufacture should not protect their products with some proprietary parts but, at least they should be good ones.

The quest to make pellets stoves (and everything else) idiot proof (aint gonna happen) the manufactures have added increasing amounts of complexity to their electronics.

KEEP IT SIMPLE

Safeties are a must to make sure the thing can't overheat or fill the room with smoke if a fan quits but, the added complexities of all the micro controls just assures a failure, its just when is all.

Personally I would design the stove so the exhaust fan can run at a constant speed and have the draft adjustable for the fuel level being run.


I am still thinking about doing just that with my Whitfield.

Let the exhaust fan run at full RPM and just throttle the damper back to attain the desired fire at any given fuel delivery.

Yessssss the operator will have to think a little bit but overall the amount of electronics is cut way down.

The control board takes care of the exhaust fan speed, the room air fan speed and the fuel feed rate.


Many stoves adjust the exhaust fan speed and the room air fan speed in accordance with the fuel setting and or moderate the amount of adjustment that the operator has to the room air fan based on the fuel setting.

Low fuel settings will allow a very low room air fan speed and when the fuel is increased the fan setting will automatically be increased and the levels the operator can adjust will be moved up.

This is great in theory and will keep from an overheated stove due to too low of a fan setting.


Much simpler way is to use a snap disc that puts full power to the room air fan if the blow is set too low.

If the stove reaches a certain level the fan come on high for a short time and then returns to the preset.


I am not a fan of too much electronics, it just assures trouble for the end user.


Snowy

 

[pete324rocket]

Snowy Rivers,do you have a schematic for the drolet eco-45 that you would share.I took a couple years of industrial electronics a ways back and would like to be well equipped when this thing takes a hot lunch. Plus I still have too much draft vs. pellet feed,wondering if there is a finer adj. on the board.

 

[pete324rocket]

bttt.

 

[vvvv]

i added a light dimmer [pwm] to my 25pdvc & have monitored the exhaust blower temp with IR gun- never reaches 100*f. i'm assuming the motor will "burn out" from actual overheating? i cut the wattage ~1/2 of the [coil] exhaust motor. I think the combustion air is designed to accomodate the biggest flame of the burn cycle as the pellets burn & any smaller flame has excess combustion air- the Europa gassifying burner has the steadiest flame i've been told as well as a close to ideal stochiometric air/gas ratio= ~ 10/1
i used to constrict the combustion air intake physically so to sustain a smaller fire until i realized that the vacumn created might be relieved from another point such as a leak in the stove or around the glass

 

[Snowy Rivers]

Sorry for the late response.

Static discharge could be a contributing factor.

You shuffle across the carpet on a dry winter day and touch the control knob/buttons on the panel and ZAAAAAAAP you have just hit the micro electronics with 20,000 volts.

Yup that same little gag that we used to try on the cat when we were kids "And each other"

This will fry these micro electronics right quick.

Even with plastic knobs and buttons it can still happen.

The issue has been thrashed around by several stove manufactures over the years.

The older stoves with the mechanical relay controls were not nearly as sensative.

With todays "All on the board" controls, with touch pads and such, the issue can be troublesome.

The secret is grounding of the controls in such a way that any static discharge is run to ground rather than into the components of the control board.

sadly, once a high voltage charge is unleashed, there is really no way of knowing where its going to travel.

Best preventive you can do is to always touch the stoves metal case before touching any of the controls.

This technique will allow an static buildup in you as you walk across the floor to be released harmlessly to ground right through the metal case and not through the electronics first.


Spoke with an electonics supplier yesterday on some solid state timers for my stove rewire project and he was relating a situation they had some years back where static was taking out electronics right and left.

Their customer was spitting nails and ready to head for another supplier. They figured out the issue and added grounding to the controls to eliminate the static problem.

I won't say that this is all of the issues but, it certainly could be a contributing factor.


Food for thought.

Snowy

 

[tjnamtiw]

That's some great info, Snowy, for we 'tinkerers'. Have you looked at Comfile Technologies with their CuBloc system. It uses both Basic programming and Ladder Logic (that I am used to using) together, which is highly unusual. You can do a lot with ladder logic but some things are more easily handled in Basic and then referenced back to the ladder logic. Most of the stuff in the stove's logic can be handled by ladder logic except the thermocouple readings and, like you said, the motor speeds, although the latter could be handled by relay closures for each speed feeding different control voltages.

Another option that we used at work for our control systems is EZAutomation.com. A little more pricey but it really works well.

 

[tjnamtiw]

For instance,

www.pensandcalls.com/quadrafirelogic.doc

)

 

[odlaw]

I was seriously thinking of adding a PLC (Programable Controller) to my York insert. I do not want to use a thermostat to ignite as I believe the constant starting and stopping would kill the igniter and there is usually a bit of a clinker when shutting down the stove. I have a situation in pre winter and spring seasons that I want the stove to run only part of the day and then shut down in the afternoon as the sun heats up the house. A simple AC timer will not work because you need to have the exhaust keep working for a while after you stop feeding fuel. I could program different feed rates using selectable inputs which would correspond to the way different fuels burn. This would be far easier than opening up the cover and adjusting on and off time pots.
Sorry for the rant, just some ideas.
Odlaw

 

[Snowy Rivers]

The whitfield used a standard on time of about 1.5 - 2 seconds and this was only adjustable with a scewdriver on a pot that is on the board.

The off time is the burn rate and that is the adjustment knob. 1 being the longest time at 10 seconds and the number setting being the highest heat setting with the time at 1.5 seconds off time.

The way in which the on off time and other controls can be handled with many different electrical devices.

Some are expensive and some cheap.

The manufacturers have all gone away from the "Off the shelf" stuff in favor of custom build controls.

The proprietary boards are cheaper for the OEM as they get the stuiff made over seas and in large enough quantity that its a real cost savings item.

Unfortunately this does not bode well for the end user "US"

Luckily these systems can easily be replaced with far simpler parts that are available off the shelf.

Even an auto lighter can be set up to work with some fairly simple electronic controls.


Snowy