Current amplifier for variable speed motor?

[hotponyshoes]

I have a piece of vetinary equipment, this has an output to run an external peristaltic pump.

It has a small pump at the moment and that needs changing for a bigger one.

I'd guess the small pump is pulling 2 amp (probably quite a bit more at start up) and the bigger pump maybe 6 amps.

Pumps are 24v dc.
The machine outputs 0-25v dc. 0v is off, then it starts at about 11v and varies the speed between 11v-25v.

If I connect the bigger pump to the machine directly I get a 'pump failure' error when I try to start it and it cuts power to the pump.
If I remove the tubing from the bigger pump (so no load) it works fine.
There are only 2 wires that run directly to the motor so there is no speed sensor or anything at the pump end.
I guess there is some sort of load/current sensing inside the machine giving the error as it thinks the pump is blocked/faulty.

I don't really want to open the machine up and start fiddling with things to see if I can bypass the error somehow and I certainly don't want to burn out anything by overloading it.

So, is there some sort of 'amplifier' module I can use externally?
Something that will take an input of 0-24v and give a higher current output at the same voltage?

 

[6ply]

Mechanical relay would be easy.

 

[hotponyshoes]

Mechanical relay would be easy.

That won't vary the speed though will it?

 

[6ply]

Oh sorry, I missed that bit. I assumed it was on/off.

 

[6ply]

Are you sure that it is actually varying the DC voltage and not outputting pwm signal that when you measure with a meter looks like a variable voltage?

 

[hotponyshoes]

Are you sure that it is actually varying the DC voltage and not outputting pwm signal that when you measure with a meter looks like a variable voltage?

It might be pwm.
If I can remember how to use my scope I'll check.

But does that make any difference?
When you said pwm I thought, ah I can just use the output to feed a solid state relay and feed the relay from a bigger 24v supply.
But I don't think that will control the speed will it?
If I use a ssr with a 11v 'coil' it will be on all the time, if I use one with a 24v coil it will only come on at full speed?

 

[6ply]

I would check using a scope before you go any further.

If the motor speed is controlled by a variable (non PWM) DC voltage, you could consider using a power transistor and the new motor in emitter-follower configuration.

 

[sako243]

Even if it's not you could have a circuit that converts analogue voltage to a PWM output or similar. Not sure of anything off the shelf but buildable for sure.

 

[Ross365]

I once ( a long time ago) constructed a Pulse Width controller which had the added feature that it could sense the load and automatically increase the pulse width to maintain the torque/speed. It was very effective on a small 12V mini drill when used with attachments like sanders. I found the design on the net; think it was 2 or 3 IC's and not too demanding to construct.

Don't think I ever seen a commercially made one.

 

[daedalusminos]

Firstly determine whether it's linear or PWM.

Then check how intelligent the error detection is - it could just be an over current, an over and under current (does it error if no pump connected) or it could track the voltage set for over and/or under. If you're going to add an external driver it needs to trick the error detection into think nothing is wrong.

External driver with PWM should be straight forward, a linear drive would need a bit more thought to ensure it maintains stability over load conditions.

 

[voipio]

Does the machine throw an error if the motor is disconnected? If it does, then using some sort of amplifier system isn't going to work, as the low current drawn by the amplifier input would also probably also throw an error.

 

[hotponyshoes]

Does the machine throw an error if the motor is disconnected? If it does, then using some sort of amplifier system isn't going to work, as the low current drawn by the amplifier input would also probably also throw an error.

No. Or at least it didn't when I just had my voltmeter connected to the output.

 

[voipio]

It might be possible to put a controller like this into service:
www.ebay.co.uk/itm/267128750378
Together with a 24V, 10A power supply.
The supply voltage from the equipment could be applied between bottom and top ends of the control potentiometer, with the top end of the pot disconnected from the controller circuitry. The pot could initially be set to minimum, and then carefully adjusted to set a suitable voltage divider ratio, to provide the expected motor speed range, and then internalised and "sealed" to stop tampering. This, of course, assumes that the potentiometer is supplied with DC from the controller, which might only be 5V.

 

[voipio]

It might be possible to put a controller like this into service:
www.ebay.co.uk/itm/267128750378
Together with a 24V, 10A power supply.
The supply voltage from the equipment to the motor could be applied between bottom and top ends of the control potentiometer, with the top end of the pot disconnected from the controller circuitry. The pot could initially be set to minimum, and then carefully adjusted to set a suitable voltage divider ratio, to provide the expected motor speed range, and then internalised and "sealed" to stop tampering. This, of course, assumes that the potentiometer is supplied with DC from the controller, which might only be 5V.

Corrected for clarity.

 

[hotponyshoes]

Looks like pwm

 

[sako243]

Directional or just speed control?

If just speed hook it up to a suitable MOSFET with a bit of protection circuitry (flyback diodes, gate resistors), bigger power supply and job done.

 

[hotponyshoes]

Directional or just speed control?

If just speed hook it up to a suitable MOSFET with a bit of protection circuitry (flyback diodes, gate resistors), bigger power supply and job done.

Just speed.

The above is a bit beyond my level of understanding!
Is there an off-the-shelf version?
Or a diagram and parts list

 

[voipio]

Being as the motor supply is simple PWM at 20kHz, a simple MOSFET Source Follower, or Emitter Follower, if using an NPN bipolar transistor, instead of a MOSFET. It's actually a very simple circuit. In the MOSFET diagram below, V+ comes from an external 24V supply. For an NPN bipolar transistor-based circuit, the Collector terminal goes to 24V, Base terminal is the input, via a suitable resistor, and the Emitter goes to the output. In choosing the MOSFET, look for N-channel devices with a Vgs(th) (gate source threshold voltage) close to 0V, and a Vgs (gate source breakdown voltage) well above 24V. Also, take note of @sako243 's comments regarding internal protection circuitry.

The module that I suggested, in posting #13, would probably require the motor control output from the equipment to be filtered by a simple RC filter, having a time constant of around 0.1 to 1 second, before being passed to the module's input, else there's a chance that the motor speed might not run very smoothly.

 

[rich_c]

There definitely will be off the shelf drivers that take PWM input too but cheaper ones will probably expect a digital logic level 0 - 3V (or 5V) as input so you'll still need to wave a soldering iron at some components either way. To drop the voltage you've got down to logic level you'd want a resistor divider, or zener, or opto coupler etc.

If you want off the shelf for handling the power switching side searching for an H bridge module rather than speed controller will probably net better results. Here's just one example https://www.ebay.co.uk/itm/175729369055

 

[mike 109444]

You might want to put an ameter in line because the sort of current you mentioned seams very high !
I work on kit that uses a lot of 24v dc peristaltic pumps and they are each fuse protected by a 630ma fuse!