I have an espresso machine that turns on a AC pump by sending 120V to it. I want to replace that with a 24V DC pump, which will turn on or off whenever that AC signal is being sent. How can I safely remove the AC pump from the circuit? And also how can I detect that AC signal is being sent?

One thing to note is I was considering just letting that AC signal control the DC power supply for the new DC pump, however I've heard it's not smart to power on and off DC power supplies like that.

 

What is the point of replacing the AC pump with a DC one...is it dead?

Ken

 

More info would be helpful. Is it just a brushed DC motor? And is there any phase angle control going on with the ac motor (is there any sort of power control)? What kind of snubber do they have? The worst thing that can happen when the adapter is receiving intermittent power is that it just doesn't supply power. As long as the DC motor has a snubber, you'll be fine. So try connecting it directly.

 

More info would be helpful. Is it just a brushed DC motor? And is there any phase angle control going on with the ac motor (is there any sort of power control)? What kind of snubber do they have? The worst thing that can happen when the adapter is receiving intermittent power is that it just doesn't supply power. As long as the DC motor has a snubber, you'll be fine. So try connecting it directly.

It's brushless. I don't have a controller for it yet.

 

Why are you replacing it though?

 

It's an upgraded, more reliable pump that's used on nicer machines.

But that's kind of beside the point. All I really need to know is how to safely remove an element from a AC circuit while still being able to sense when the AC would be there, with something like a hall sensor or whatever y'all might recommend. At that point I could design a controller that would turn the pump when needed.

 

I think a small mains transformer makes the most sense. Add a rectifier on the output, a small capacitor with resistor to discharge it. Make a comparator circuit that requires that to be at least a few volts to turn high. Have this drive the controller. BLDC controllers often require a PWM to work, so you may need an additional 555 circuit or something.

 

If you just need to turn on the DC pump when AC is present, then you could use an AC (coil) relay with its NO contacts in series with the motor.

 

I would use a DPST AC coil relay, and use that to switch your 24V motor on, you can use the other set of contacts as a voltage monitor or use an opto-coupler across the coil.

 

The solid state option may be a little more economical though. Are these AC relays really so cheap? And they would also make an annoying clicking sound.

 

The solid state option may be a little more economical though.

SSR's are not particularly cheap.
And they will require added circuitry to convert the 120Vac to the low voltage DC voltage required by the SSR control input.
But they would be quiet and likely more reliable.

 

I mean salvaging a mains transformer and using a rectifier and capacitor, then using a comparator circuit and mosfet. I bet you could get the other components from mouser for under $2.

 

One thing to note is I was considering just letting that AC signal control the DC power supply for the new DC pump, however I've heard it's not smart to power on and off DC power supplies like that.

Turning the supply on and off should not be a problem.
How much DC current does the motor require?
Did you have a particular DC supply in mind for the job?

 

In #4 he said he has a BLDC motor, but no controller. It would be convenient if a controlled has an ENABLE input. Interesting going from a straight switched AC motor to a BLDC one plus a controller plus a AC to DC power supply plus an AC detection circuit.

Ken

 

Turning the supply on and off should not be a problem.
How much DC current does the motor require?
Did you have a particular DC supply in mind for the job?

Unfortunately I haven't been able to find the current value anywhere. I'll have to check on a power supply later.
I want to be able to vary the voltage electronically somehow (probably just with an arduino) so that I can control the pressure live. Not sure what kind of power supply that would be.

 

You have a lot of stuff going on. The standard way of interfacing is to use an I/O module. See https://www.mouser.com/datasheet/2/626/IO_Mod_AC_Input-335391.pdf for an example. These provide an isolated output for a micro to use.

Although, the simplest approach, but not the best approach, is just use a 120 V relay. The contacts with a pull up can provide a logic signal. The signal may bounce since it's a mechanical contact.

 

Unfortunately I haven't been able to find the current value anywhere. I'll have to check on a power supply later.
I want to be able to vary the voltage electronically somehow (probably just with an arduino) so that I can control the pressure live. Not sure what kind of power supply that would be.

Hi Tate, sorry to drag up an old thread, did you ever manage to do this, I am doing the same with a Fluid O Tech DC Rotary Pump and wondering if I can just power the DC PSU from the AC wires that powered the vibration pump. Relay size is ok.