I need to switch a diesel heater that is the same type as Webasto or Eberspacher diesel powered air-heater for heating a small greenhouse.

The heater has a warm-up phase and a shut-down phase which is normally controlled by a single push-button being pressed to start or stop. It cannot be controlled simply by opening and closing the 12V power supply to the heater unit.

I can design the upper and lower thermostat switching but I need to have a circuit that gives a single pulse or shot to the button circuit on temperature rise and temperature fall.

So if I have a 12V circuit that "makes on rise" and another circuit that "makes on fall" I need an interposed circuit for each switching operation that when given a 12V signal will close a switch momentarily for, say 1 second, that then simulates a switch press. The same logic for the " make on fall" operation.

Anyone? If this topic has already been covered then please accept my newbie apologies.

Zircon

 

Combining the two is pretty trivial (unless I'm missing something in the fine print here).

The bigger problem is ensuring that something doesn't get skipped so that the system is out of phase.

For instance, what happens if one of the trigger events is missed so that your control system thinks that it shut things off, but in fact the heater is still running? Or the opposite? There are ways to deal with this, but there are some potential demons in those details.

 

If a button press is somehow missed then the heater has an upper limit stat but it only senses the air throughput and not the ambient temperature of the heated space. Alternatively if a low temperature trigger does not activate the heater then maybe frost will hurt sensitive plants but so be it.
If the non-latching trigger circuit is active for, say, 2 seconds, then the on and off processes should be reasonably reliable. Does this help?

 

A 2-Second-long-Pulse doesn't guarantee anything.

When Power is first applied, what condition will the machine be in ?
If You can't answer that question like as if your life depended on it, then, You really don't know.

It is not possible to reliably Thermostatically-Control the Heater using only an alternate-action-Push-Button-Switch,
unless the alternate-action is created mechanically inside the Switch it's self,
( which is highly-unlikely, but it certainly is a remote possibility ).

One possibility is to locate the Over-Temp-Safety-Switch-Circuit.
Try repeatedly breaking this Circuit, and then re-connecting it.
If the Heater will reliably cycle On and Off by doing this,
You "may" be able to control this Circuit with a Thermostat.
If the Heater will not resume normal-operation after re-connecting this Circuit,
then it is NOT going to be Thermostatically-Controllable with the factory-installed Control-Board.
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When power is first applied the machine is in standby and waiting for an instruction. One shgort press on the "on" button will initiate a warm-up cycle and then the heater will begin heating when the igniter is hot enough to cause the fuel to burn. Pressing the "on" button again once the heater is in "burn" mode will cause the fuel pump to stop working and then a shut down sequence begins which runs the fan untill the heat-exchanger is cool and then it goes back to "standby mode" This cycle can be repeated until there is no fuel left or the 12V supply is disconnected. However disconnecting the power when the heater is in full operation causes unburnt fuel to remain in the combustion chamber and this leads to rapid soot build-up if this happens more than once or twice.
I can reliably control an upper and lower temperature limit with sufficient hysteresis between 10C and 4C with programmable thermostat circuits. My issue is having a circuit close momentarily when an upper limit 12V command is issued by either of the thermostats.

 

"" One possibility is to locate the Over-Temp-Safety-Switch-Circuit.
Try repeatedly breaking this Circuit, and then re-connecting it.
If the Heater will reliably cycle On and Off by doing this,
You "may" be able to control this Circuit with a Thermostat.
If the Heater will not resume normal-operation after re-connecting this Circuit,
then it is NOT going to be Thermostatically-Controllable with the factory-installed Control-Board. ""
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I think it would be pretty easy to find a wire or two that has power when the unit it on and maybe a separate for the burner to differentiate between states. I think it could easily be done with a microcontroller if you are willing to learn a little programming. You could also monitor the system to be sure it turns on and off as it should and "press" the button again should things get out of sync. I'm sure there are other ways of connecting other logic together to make it work at the expense of a more complex circuit.

Edit...

Nevermind. Without actually knowing the construction of the "button" what I said is more or less not worth anything. If it's just a button then yes... if it's some form of controller then there's probably a lot more to it.

 

What exactly are you working with and are there any wiring diagrams / schematics available anywhere?

 

Since the push-button signal from the thermostat could conceivable get out-of-phase and switch it on with from a high temperature (such as if the sun heats the greenhouse), then I would add a fail-safe relay that will cut off the 12V power if the temperature gets too high.

Otherwise, the pulses could be generated by a 555 timer circuit configured as a 1-shot astable.

 

"" One possibility is to locate the Over-Temp-Safety-Switch-Circuit.
Try repeatedly breaking this Circuit, and then re-connecting it.
If the Heater will reliably cycle On and Off by doing this,
You "may" be able to control this Circuit with a Thermostat.
If the Heater will not resume normal-operation after re-connecting this Circuit,
then it is NOT going to be Thermostatically-Controllable with the factory-installed Control-Board. ""
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Please ignore the fact that it is a heater involved as this seems to be running away with the message.
I seek a simple circuit the when given a 12V signal will initiate a brief closing of zero volts contact nrelay for 2 seconds which then opens again rather than latching closed. Is this possible ?
Is there a circuit diagram for a 555 timer as a one-shot a stable device as suggested by Crutschow?

 

Is there a circuit diagram for a 555 timer as a one-shot a stable device as suggested by Crutschow?

Yes.
Below is the LTspice sim of an example circuit:
It generates a >1s pulse (yellow trace) to activate the relay (red trace) for either of the inputs going to 12V (as long as both aren't high at the same time).

(The relay coil doesn't need a protection diode since the 555 has an emitter follower output stage which will absorb any negative spike from the coil inductance)

Does that do what you want?

 

It will certainly do what he wants .................
It might even work for years ................
until, by some fluke accident, it doesn't.
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It will certainly do what he wants .................
It might even work for years ................
until, by some fluke accident, it doesn't.
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But the same can be said of just about anything -- it might work until it doesn't. Pretty much everything fails sooner or later.

 

Some things can be predicted,
and may even be likely to occur,
others can't be predicted.

It's always a good plan to eliminate as many known points of failure as possible.
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Some things can be predicted,
and may even be likely to occur,
others can't be predicted.

It's always a good plan to eliminate as many known points of failure as possible.
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Agreed -- or at least as many as reasonable. In this case, he has stated that failure in either direction is a tolerable, albeit not desirable, outcome. This sets a limit on how much effort is reasonable to prevent even known points of failure from happening.

One way to deal with an out-of-sync situation is to force a synchronization each cycle by removing power completely each time the heater is turned off by first activating the button and then, after a sufficient delay let the heater's controls to complete the normal shutdown process, remove power completely. Then either cycle power back on or just wait until the thermostat calls for the heater and first apply power and wait long enough to be comfortable that it is ready to be turned on and then cycle the button. That would be simple to incorporate and would almost certainly eliminate the out-of-sync potential. The next thing that might be considered is either additional temperate trip points -- or a timeout after a normal trip point -- that cycles the heater's power and moves it to the desired state if it is determined that the greenhouse is either not warming up or cooling down as expected, possibly with some kind of alarm if this is detected (or detected and not recovered from). Each of these improves the robustness of the system, but comes at greater cost and complexity which might not be warranted.

 

Cycling the Power is a great idea, I don't know why that didn't occur to me earlier.


Thermostat makes Main-Power-Supply "Hot", wait ~30-Seconds, send Push-Button-Pulse,
When Thermostat is satisfied, send Push-Button-Pulse, then wait ~30-Seconds, then kill the Power-Supply.


This would make the Heater completely Automatic,
even if a Temporary-Power-Outage occurs, it will start back up, by it's self.

Should be easy to make with a couple of Dual 555-Chips,
2 - 12V-Relays, and a LM-35 Temperature-Sensor-Chip.

But it does require some experience with building an Electronic-Circuit-Board.
And, if a Circuit-Board is going to be built anyway, why not incorporate an Electronic-Thermostat !!!

Now we need to find-out if the Thread-Starter is up to the challenge, or knows someone who is.
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