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I have a system to drive two peristaltic pumps (labelled 12V, that draw 300mA) in a microscope in the lab in order to refresh the media of my cells for long term experiments. In the current setting I'm pumping 40mL/min, which is too much. I need to be able to control the flow-rate (lets say, in a perfect world, between 0.5ml/min to 10mL/min).

I am using a 2A 12V DC power source.

Pumps are in parallel. I have two 8 ohms resistors right before each pump (in each parallel branch). (it was my first try to reduce the speed of the pumping that's why they are there)

I also, now, have 4 paralleled 4ohm resistors right after the power source (I needed to drop the current for today's experiment and it was all I had nearby and I didn't want them to fume even though they are higher than 1.2watts, 6 I believe).

So my question. I want to have a way to regulate the flow. Potentiometers usually come in high resistance forms and low wattage (I found a few 100 \$ \Omega \$, and a 10 \$ \Omega \$ - 5W online. Should I use the 10 \$ \Omega \$ potentiometer?

Can I connect a potentiometer in parallel with the pumps trying to create a resistive voltage divider? However I don't know how to calculate the value to that potentiometer. Is this the best way to do it? I also can't to spend a lot of money on this.

I am concerned about temperature since the whole system (except the 12V power supply) is inside of a 37ºC enclosure to keep the microscope warm and the cells happy.

I saw this related post, but I have no idea on what values and pieces to use. Slowing Down a 12 volt pump with a potentiometer?

This is the state of the thing right now:

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ I think I'd restrict the flow using a valve or other fluid control device, rather than trying to control it by slowing the pumps. Do you really need two pumps? I'd think that using only one pump would reduce the flow to half. \$\endgroup\$ – Peter Bennett Oct 30 '15 at 2:35
  • \$\begingroup\$ Hello, I need two pumps, one to put liquid from a container to the experiment and another to remove liquid form the experiment to a container (the culture plate has to always have the same volume or it will leak to the microscope),they flow at same rate, I checked. I can try to make it with only one pump (I could put a second tube that would go to through the peristaltic pump). Other problem is the vibration the pumps are doing, I need to keep focus on the sample and I noticed a bit of vibration going through the connecting tubes,reducing the rotation would help with that.Thanks for the input. \$\endgroup\$ – João Mamede Oct 30 '15 at 2:56
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As a first step, since you want to control the speed of the motors, you want to control voltage, not current. I would suggest replacing the resistors with a variable voltage power supply, standard lab equipment. (You might leave a low value variable resistor in series with each motor, normally turned to minimum R, so you can slow down one motor if it's slightly faster than the other to compensate for mechanical differences). Then the power supply can vary the speed of both motors together.

However, that won't work over a huge range. I'd guess it'll work down to 10-20% of rated speed, i.e. 4-8mL/min whereas you need to get down to about 1% of rated speed. It may be better than this, depending on the design of the motors themselves (friction etc) but controlling speed reliably down to 1% is optimistic.

I think you may need to consider interposing a 4:1 gearbox between motor and pump. If you're lucky, something suitable will be available from stock from a motor supplier such as Maxon, otherwise some mechanical engineering is required.

Alternatively, for greater precision at more complexity, replacing the motors with stepper motors and suitable drive circuitry would allow you to control the pump speed down to individual steps (1/200 revolution) with any precision you need.

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  • \$\begingroup\$ Okay, thanks. Something I can also do is to change the tubing to a different gauge. I don't need to have precision but I need to reduce it as needed. I will pull an old power supply from electrophoresis bench to test that. \$\endgroup\$ – João Mamede Oct 30 '15 at 15:48
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It is difficult to estimate what voltage will provide the flow that you want because the efficiency of the pump and motor have a significant effect. What you really need is a variable voltage power supply. A fixed voltage supply of a lower voltage might help a lot. A 5 volt, 1 amp supply would be good to try. You could even get 3 or 4 1.5 volt batteries and see how many you need in series to get the speed that you need. Once you cut the voltage down closer to what you need, the 10 ohm, 5 watt rheostat would be more useful. You won't get good results trying to make an adjustable voltage divider. You need a variable resistance in series, a rheostat or a rheostat connected potentiometer. This is a pretty crude solution, but it may work pretty well for your purpose.

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