Help with best way to detect current/voltage in adjacent circuit

Thread Starter

cds333

Joined Nov 5, 2007
19
I have 3D printer with a cooling fan that by design is on constantly, however it only needs to be on when there is something to cool! I have no idea why they have it on all the time since it is loud and wastes power... So I need to build a circuit that can detect when the heating element is turned on, and then turn on the fan, and turn it back off when the heat is off. I will probably use a PIC for the brains of the circuit (since I want to have a countdown timer and some other functionality) but I am not sure the most efficient way to hook up to the heating element. It is a 24V, 40W heating element with a simple 2 wire lead. I dont want to worry about hooking into the thermocouple, it should just monitor the binary state of either on or off (ideally with a 3-5 VDC output that can hook directly to one of the pins of the microcontroller)

Any thoughts on the best way to accomplish this?

Thank you very much
 

Sensacell

Joined Jun 19, 2012
2,782
You could use an opto-isolator with the appropriate resistor wired to the heater terminals.
A 2.4 K resistor will give you around 10 ma in through the LED in the isolator, for example.

You might be able to use just a simple 2 resistor voltage divider, but you need to know if the heater is driven by a high or low side switch.
The optocoupler makes that irrelevant.
 

Hymie

Joined Mar 30, 2018
970
I would give serious consideration to replacing the current fan with one rated at 24V (if not so rated) and just wire it in parallel with the heating element.

Although you will not have whatever additional functionality you wanted, the minimal extra power drawn by the fan should not be an issue for the heater supply circuit.
 

Thread Starter

cds333

Joined Nov 5, 2007
19
That is what I meant by a countdown timer. It would run for a few minutes until the heatsink is completely cooled.

From what I know of a optocouplers they are normally used to do the opposite, that is- control high voltage with a low voltage circuit. Which type in particular would you recommend for this?

It would need to handle at least 2A on the input side

Thanks
 
Last edited:

Sensacell

Joined Jun 19, 2012
2,782
It would need to handle at least 2A on the input side

Thanks

The opto operates as a low current voltage sensor in this application, the load current does not flow through it.
I don't understand your comment?

The other thought is that what ever controls the heater might be pulsing it to vary the heating power, not just steady ON or OFF.
Your code might have to take this into account.
 

BR-549

Joined Sep 22, 2013
4,938
How old is the printer and have you documentation for it? I am not a printer guy but they usually design devices now, with efficiency at a pretty high level.

I would study the printer first and see if there is a reason for the constant fan.
 

Thread Starter

cds333

Joined Nov 5, 2007
19
The opto operates as a low current voltage sensor...
Nevermind; I now understand what you were saying originally. In parallel with a resistor to drop the current. Yes this seems like it will work.

I would study the printer first and see if there is a reason for the constant fan.
I assume it was just to save money. They need to cut costs wherever possible to get these things out around $200. There is no reason I can think of to cool the extruder when there is no heat going to it (other than a cooldown period). Ironically one of the upgrades for the "pro" model is a power supply with a fan that turns off when it is not needed.

More expensive models actually have both fans able to be controlled by the g-code, but according to the manufacturer there is no way to control this fan. But half the fun of these things is making your own upgrades!

Thanks
 
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