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DC12 400 undervolt

J

jwimsett

Gawd
Joined
Jul 16, 2012
Messages
638
I'm trying to find a easy way to undervolt my dc12 400 pump. It's rated for 8-12V 1.8A 18W. It runs off of 12v 3 wire fan connector. There are 12 - 7 v harness adapters, but I have a feeling the pump would burn them up. Any suggestions?
 
jwimsett said:
I'm trying to find a easy way to undervolt my dc12 400 pump. It's rated for 8-12V 1.8A 18W. It runs off of 12v 3 wire fan connector. There are 12 - 7 v harness adapters, but I have a feeling the pump would burn them up. Any suggestions?
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You could just wire it yourself.

Instead of connecting 12+ and ground, connect 12+ and 5+, to give you 7v
 
Zarathustra[H] said:
You could just wire it yourself.

Instead of connecting 12+ and ground, connect 12+ and 5+, to give you 7v
Click to expand...

No, that'd give you 17V. You need to connect the +12 to the +5 (+12+5=17), but connect the +5 backwards! (+12-5=7) It's simple math.
 
Bandalo said:
No, that'd give you 17V. You need to connect the +12 to the +5 (+12+5=17), but connect the +5 backwards! (+12-5=7) It's simple math.
Click to expand...

No. You can't add two positive voltages.

Voltage is also known as potential difference, essentially the difference between two charges. If you have +12 and +5 the most you can ever get between them is 7v

In order to get 17v, you would need a -5.

Which side you connect to which would only determine the flow of current, but the voltage would still be 7v.

The flow of current - however - would determine the direction the pump runs, and you'd probably want that to stay the same as it originally was, so keep the 12 where it is, and move the ground to the 5v
 
Zarathustra[H] said:
No. You can't add two positive voltages.

Voltage is also known as potential difference, essentially the difference between two charges. If you have +12 and +5 the most you can ever get between them is 7v

In order to get 17v, you would need a -5.

Which side you connect to which would only determine the flow of current, but the voltage would still be 7v.

The flow of current - however - would determine the direction the pump runs, and you'd probably want that to stay the same as it originally was, so keep the 12 where it is, and move the ground to the 5v
Click to expand...

I was trying to be humorous, but I feel I've failed.

You can connect a load between the +12V and +5V supply, and the load will see +7V. But given that this is a 18W pump, that much current may do unpleasant things to the power supply. The +5 side is designed to supply a nice filtered +5V, not sink current from another stage of the same power supply. Depending on how the upstream stages are designed and the load, it might work, or it might cause permanent damage to your PSU.

The simple way to do this is a potentiometer and another load resister and build an adjustable voltage divider. Problem with that is you're going to be dumping a lot of current/heat with an 18W load. A PWM controller is much more complex, but won't put as much extra heat in the case.

Source: I got a Master's in EE in a frame around here somewhere...
 
Zarathustra[H] said:
Would probably take a beefy pot for 1.8a of current. Will likely get got. If you go the pot route, make sure you checked it has a high enough max current rating.
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specs from link. you no clicky? :) I made sure to cover everything but this is pwm and his pump is 3pin so not so sure now...

Qunqi 1.8V 3V 5V 6V 12V 2A PWM DC Motor Speed Control /w Potentiometer Switch 1803BKW
Input voltage: 1.8V-15VDC
  • Maximum output power: 30W
  • Maximum continuous output current :2A
  • Potentiometer with switch function.
  • Potentiometer cable length 20cm.
 
Bandalo said:
I was trying to be humorous, but I feel I've failed.

You can connect a load between the +12V and +5V supply, and the load will see +7V. But given that this is a 18W pump, that much current may do unpleasant things to the power supply. The +5 side is designed to supply a nice filtered +5V, not sink current from another stage of the same power supply. Depending on how the upstream stages are designed and the load, it might work, or it might cause permanent damage to your PSU.

The simple way to do this is a potentiometer and another load resister and build an adjustable voltage divider. Problem with that is you're going to be dumping a lot of current/heat with an 18W load. A PWM controller is much more complex, but won't put as much extra heat in the case.

Source: I got a Master's in EE in a frame around here somewhere...
Click to expand...

My bad, but you know how humor and sarcasm are very difficult to read on the internet in written form :p
 
Bandalo said:
It's always a challenge! I probably could have worded the joke better, but EE humor is challenging.
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How would the electronics inside the pump react to a pwm current instead of dc? The wiring is different for pwm and non pwm fans (the pwm pin controls a transistor that does not disconnect the whole fan when voltage is at the low point), would probably be the same here as well. An adjustable DC-DC converter might be a better idea here. You can get one for a couple of bucks that will do 3A at any voltage.
I'm a hobbyist and don't intend to argue a real EE, just curious.
 
Koween said:
How would the electronics inside the pump react to a pwm current instead of dc? The wiring is different for pwm and non pwm fans (the pwm pin controls a transistor that does not disconnect the whole fan when voltage is at the low point), would probably be the same here as well. An adjustable DC-DC converter might be a better idea here. You can get one for a couple of bucks that will do 3A at any voltage.
I'm a hobbyist and don't intend to argue a real EE, just curious.
Click to expand...

A good, separate PWM controller would supply a filtered DC voltage to the pump, so it should operate just fine. An unfiltered PWM output would probably be OK for most DC motors as well, since the PWM switching is generally done at such a high frequency that the natural characteristics of a small DC motor will filter out the ripples.

PWM fans have the PWM controller built into the fan. A typical fan has 4 wires. So they get a steady +12V from the PSU, they get a PWM signal that sets the duty cycle from the controller, and they supply an RPM signal for feedback or display.

An "adjustable DC-DC converter" like you mentioned will be one of two general designs internally. An analog style voltage divider, which works well but wastes a lot of power via the internal resistors, or a digital adjustable PWM style DC converter with an output filter.
 
Bandalo said:
A good, separate PWM controller would supply a filtered DC voltage to the pump, so it should operate just fine. An unfiltered PWM output would probably be OK for most DC motors as well, since the PWM switching is generally done at such a high frequency that the natural characteristics of a small DC motor will filter out the ripples.

PWM fans have the PWM controller built into the fan. A typical fan has 4 wires. So they get a steady +12V from the PSU, they get a PWM signal that sets the duty cycle from the controller, and they supply an RPM signal for feedback or display.

An "adjustable DC-DC converter" like you mentioned will be one of two general designs internally. An analog style voltage divider, which works well but wastes a lot of power via the internal resistors, or a digital adjustable PWM style DC converter with an output filter.
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But fans and pumps have brushless dc motors and also the electronics to control the switching between phases (or windings, don't know how to call it correctly). Using an unfiltered pwm controller will switch the control electronics as well at high frequencies - won't that hurt? A filtered/smoothed pwm output would just look like dc, I understand that. I was looking into making my own fan controller and found a lot of info against running non-pwm fans from a pwm source.
 
Koween said:
But fans and pumps have brushless dc motors and also the electronics to control the switching between phases (or windings, don't know how to call it correctly). Using an unfiltered pwm controller will switch the control electronics as well at high frequencies - won't that hurt? A filtered/smoothed pwm output would just look like dc, I understand that. I was looking into making my own fan controller and found a lot of info against running non-pwm fans from a pwm source.
Click to expand...

There's not really a single answer to that question, because it depends on the specifics of the particular controller and the particular device.

Simple DC motors like most fans/pumps don't have "control electronics" really, they're just simple brushless DC motors with a hall sensor to detect the position of the rotor, and a pair of FETs that switch to provide commutation to the field. Depending on the characteristics of the FETs used, their response times and the frequency of your PWM, you could see it operate just fine, or it could cease to operate at all.

A PWM fan on a non-PWM source may work, but there's no telling what speed it will operate at. It depends on how the manufacturer set it up to operate when the PWM line is open. Some will default to full speed, others minimum speed.

A non-PWM fan on a PWM source SHOULD be fine, depending on how you define a "PWM source" or "PWM controller". Your motherboard, for example, has 4 pin fan headers. One is GND, one is +12V, one is a tach signal, and one is the PWM signal. Plugging a non-PWM fan into that will work just fine, since the PWM pin won't even be connected and the fan will just get it's +12V.

A PWM-based adjustable DC controller however, that actually provides a variable DC voltage, may or may not work, depending on how well it filters the DC output. If you're building your own fan controller, just keep that in mind. You can provide your own PWM signal to the PWM fans with it, or you can just provide a variable DC output to any kind of fan, as long as you filter the DC appropriately.

Remember when you build the controller that the DC fan load is NOT steady DC current either, no matter how well your voltage ripple is controlled. A brushless motor like this draws current in pulses, with ~30% swings around the RMS value.
 
Thank you fo
Bandalo said:
There's not really a single answer to that question, because it depends on the specifics of the particular controller and the particular device.

Simple DC motors like most fans/pumps don't have "control electronics" really, they're just simple brushless DC motors with a hall sensor to detect the position of the rotor, and a pair of FETs that switch to provide commutation to the field. Depending on the characteristics of the FETs used, their response times and the frequency of your PWM, you could see it operate just fine, or it could cease to operate at all.

A PWM fan on a non-PWM source may work, but there's no telling what speed it will operate at. It depends on how the manufacturer set it up to operate when the PWM line is open. Some will default to full speed, others minimum speed.

A non-PWM fan on a PWM source SHOULD be fine, depending on how you define a "PWM source" or "PWM controller". Your motherboard, for example, has 4 pin fan headers. One is GND, one is +12V, one is a tach signal, and one is the PWM signal. Plugging a non-PWM fan into that will work just fine, since the PWM pin won't even be connected and the fan will just get it's +12V.

A PWM-based adjustable DC controller however, that actually provides a variable DC voltage, may or may not work, depending on how well it filters the DC output. If you're building your own fan controller, just keep that in mind. You can provide your own PWM signal to the PWM fans with it, or you can just provide a variable DC output to any kind of fan, as long as you filter the DC appropriately.

Remember when you build the controller that the DC fan load is NOT steady DC current either, no matter how well your voltage ripple is controlled. A brushless motor like this draws current in pulses, with ~30% swings around the RMS value.
Click to expand...
Thank you for a great answer, gave me something to think about.
 
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