24V Pump - can I connect direct?

benkay

Hi,

I've seen mixed reports on whether a 24V air pump can be wired directly to a Duet VIN Fan Header, or not.

The specs ripped from the product description are :

Rated voltage: 24V
No load current: 400MA
Recommended supply : 24V 2A (needs sourcing)

Some confirmation would be good if I can wire direct, or if I need some sort of relay/power expander.

Thanks, Ben

dc42

The main issue is likely to be the starting current. Measure the DC resistance of the pump several times, turning the rotor slightly between measurements, and take the lowest measured value. If it's less than 8 ohms than the starting current would exceed 3A, in which case you should connect a suitable thermistor in series with it to reduce the starting surge. Here's an example of a thermistor that may be suitable: http://uk.farnell.com/epcos/b57153s0330m000/thermistor-ntc-33r-1-3a/dp/1688797.

benkay

OK, I've been recommended a reprap Power expander which I understand opto-isolates the Duet board.

Presuming resistance is >8 ohms, or I put a thermistor in series, is there then no risk to the duet board? I'd prefer to wire it direct, simply to reduce the wiring mess needed for the power expander.

kraegar

I recommend an opto-isolator based on my own experience and killing a fan port.

dc42

The only risk I can see is that if the pump stalls for an extended period of time (e.g. because the pump mechanism seizes up) then the mosfet and the thermistor (f you have one) may overheat - but the motor will probably burn out too.

Hergonoway

@dc42

Based on your recommendations, is the schematic below the right one for connecting an air pump directly to the Duet ?

alt text

dc42

Yes it is. The flyback diode is optional if the Duet is revision 1.02 or later, but having one close to the motor will help to reduce any interference generated when you PWM the motor.

SM3D

@dc42 and @benkay just wanted check with both of you my wiring before connecting to my duet 1.01. I followed the post and wired the pump accordingly. Here are the pictures:

Please let me know whether it is correct or not. I surely do not want to blow my Duet.

Cheers,
Vasan

Hergonoway

@sm3d seems correctly set

SM3D

Thanks @Hergonoway will update whether berd air is effective or not

Cheers,
Vasan

benkay

Looks correct to me...although I understand that the later boards have the diode in-built so not strictly necessary to add one there.

Hergonoway

@sm3d In my case, putting the thermistor didn't closed the circuit as it should, so for now and until @dc42 finish his tests regarding what should be used I don't use the thermistor. For info I'm running the air pump MAX in 24v sold by makerhive.

SM3D

@Hergonoway I have trouble running the pump with thermistor from PWM controlled fan ports but I could run them with always on fan ports. Here is my post about it:

https://forum.duet3d.com/topic/5449/not-able-to-pwm-for-berd-air-pump/10

I even shorted the legs of thermistor and tried from PWM fan ports but no luck. If you have any ideas please post it.

Cheers,
Vasan

BenDiesel

So I have a 24v berdair hooked directly up. I usually run at 25 to 30% and have the Pam set to 255khz.
The motor get rather warm. Almost to hot to tuch. Should I be concerned?

Hergonoway

255K is way to high try 25K (F25000). In my case the sound and the temp dropped a lot.

Note that I've directly connected the 1/8" ID Tygon tube to the metal tube, it reduce pressured area and reduce heat from the pump mechanism (the white plastic part)