Bi-Metal Thermostat values

[partyanimallighting]

Hi all, quick question. These cheaper fog machines that use bi-metal thermostats to control the heater and pump, what degree rating would be adequate or correct for individual 800 Watt, 1000 Watt and 1200 Watt models? The "normally closed" thermostat feeds the heater and, when it reaches adequate temperature, it opens cutting off the 120V supply to the heater. Then there's a "normally open" thermostat that feeds the pump and this closes at temperature (I'm assuming the same operating temperature as the heater thermostat, just "normally open" instead of "normally closed") to feed 120V to the pump. I know if the trigger temperature is too high the heater block can overheat and actually melt so we don't want this happening do we? Is there a magic formula to calculate this? Voltage ~ Watts ~ Amps ~ Degrees Celsius?

Edited March 23, 2023 by partyanimallighting

[indyld]

Don't think I've ever worked on a smoke product that didn't just use these thermal switches in a simple safety loop, rather than as control. Yikes.  

[Stuart91]

13 hours ago, partyanimallighting said:

I know if the trigger temperature is too high the heater block can overheat and actually melt so we don't want this happening do we?

The first smoke machine I ever owned (as a fairly clueless student) suffered from this problem. 

I'm guessing the thermostat failed completely because the machine overheated spectacularly, with acrid black smoke coming out of the vents on the side. 

I grabbed it to take it outside, revealing the smoke-machine sized hole that it had burnt in the carpet underneath. 

The local disco shop were quick with a replacement which lasted for a decade or so afterwards, but I always sat it on a case lid or similar. 

[Jivemaster]

Usually the best idea of temperature rating is found by reading the rating on the old one. Likely, but as a guess, it's a NC bimetalic thermal switch probably rated at 150, or 160 centigrade.

Availability depends on your local suppliers, I'd guess that there will be an electronics repairer near you. RS in the UK stock such things, so I's likely that Mouser and/or Digikey will have something.

If it's an economy Chinese smoker than it may be as cheap to buy a new one rather that doing a repair.

[KevinE]

The ultra-cheap disco-grade machines do use these as the control stat, I'd say the temperature depends on where it's mounted, some are direct, some on a little aluminium bracket. The temperature will be stamped on it somewhere, but could be either in C or F without any indication of which.

There's usually only one in series with the element and safety thermal fuse (the thermal fuse may or may not be present depending on the reputation of your supplier), the pump and switch are connected across the normally closed contact, and when it opens at temperature, the pump can effectively be operated with the heater in series, and of course it gets shunted when the block cools too much.

 

[partyanimallighting]

Hi all, indyld yes, the vast majority use these bi-metal thermostats to control overheating and in some of the cheaper models, a "normally open" is used to send 120V to the pump.

Stuart91, I remember years ago a cheapo machine melted completely and the molten aluminum poured out the vent holes in the bottom of the machine. Spectacular to say the least. This was a real cheapo model and the heater block was not in a metal case but simply wrapped in fiberglass insulation and PVC tape.

Jivemaster, the both thermostats in the unit I'm fiddling with were pretty scorched so I'm unable to decipher what's the temperature rating. I think I'll buy a range starting from around 120 degrees Celsius up to 170 degrees Celsius and test. Any and everyone correct me if I'm wrong but the lower the temperature rating of the thermostat, the faster the fog machine will cycle off once the fluid cools down the fluid block. The higher the temperature rating, the greater chance of the heater block overheating and melting.

KevinE, that's correct. The thermostats are normally mounted on a flat little plate bolted directly onto the heater and the 120V feeding the heater is connected in series across this. And I do recall there are sometimes one thermostat and sometimes two. My question is, how will the pump be triggered with only one thermostat? This thermostat will normally cut 120V supply to the heater when it reaches operating temperature but how does the pump receive 120V via the same feed when this thermostat cuts power? If it's wired before the thermostat the pump can be triggered at anytime via the switch/remote which is not normal operating procedure. If it's after the thermostat it won't receive 120V to be triggered. How is it wired? This here is why I would install the "normally open" thermostat (of the same value) parallel with the 120V heater feed to trigger the pump. Does this seem correct?

Edited March 23, 2023 by partyanimallighting

[Jivemaster]

In the least expensive smoker I'd expect there to be a single bimetalic thermal switch. When the (NC) switch is cool it switches the heater on, when the switch operates and goes off the (live side) pump is enabled and switched by the remote or timer.  In the better engineered there will be safety features like a thermal fuse to prevent thermal meltdown.

As a guess the temperature is chosen that the fluid base (water) vapourises rapidly  but the smoke component (glycol???) doesn't decompose (much)! The duration of the "Smoke available " period will be controlled by the initial turn off temperature and the (lower) reset temperature and the thermal mass of the heater block. It's important that the reset temperature for the thermal switch is hot enough that liquid never settles in the heater. 

Good smokers will have MUCH better design of heat control and fluid control. For a basic thermal switch the temperature differential is typically 30 - 40 degrees Centigrade.

[Jivemaster]

Always compare the cost of time and money to do a repair against the cost of a new machine.

Remember that the cheapest smokers had connectors for the remote control with total disregard for the connector rating or intended use, sometimes involving live mains on one pin on a connector normally intended for audio. 

[KevinE]

 

Quote

My question is, how will the pump be triggered with only one thermostat? This thermostat will normally cut 120V supply to the heater when it reaches operating temperature but how does the pump receive 120V via the same feed when this thermostat cuts power? If it's wired before the thermostat the pump can be triggered at anytime via the switch/remote which is not normal operating procedure. If it's after the thermostat it won't receive 120V to be triggered. How is it wired?

as per my last post. The pump circuit is wired across the thermostat. When the thermostat is closed, the heater is on and you can't pump. When it's open, the pump (and switch) has power and can be operated. 

[KevinE]

pardon my graphical skills but I'm sure you get the gist