Help Identifying mistakes in control circuit for 2 24V DC pumps via 4 switch inputs

Thread Starter

ForgedAscendant

Joined Aug 26, 2017
6
ood Day,

Currently I am trying to put together a control circuit that controls 2 separate 24V pumps based off the input of 4 switches using FETs and resistors. What I would like is any feedback on mistakes I made and better ways of accomplishing my task. I am open to other solutions than the one I am currently pursuing. If you like to propose one, I would greatly appreciate a list of components needed so I can price it out and basic instructions on how to do it.

A short summary of where I am:
I bought the FETs and resistors and started putting the circuit together on solderless breadboards. Last night I tried testing the partial circuit (Inverters and AND gates) to verify that if the right conditions were given, I would indeed get a 24V signal out. Well, things went badly; very badly. I managed to get smoke, fire, scorched our wooden floor, melted parts of the breadboard, and even caused one of the FETS to blow its plastic cover right off the component revealing the scorched circuity beneath. Had I not damaged the floor I might have enjoyed looking at the carnage.

Here is a summary to the components I was using:
FET
Fairchild FQPF13N06L
MOSFET 60V N-Channel QFET Logic Level


Resistor
Carbon Film Resistors - Through Hole 10M Ohm 1/2W 5% 350 Volt

After building a truth table and doing some reduction logic I came to the following (where the opposite of signal A is A):
P1= ACD+ABC
P2=ABD+ACD

I was utilizing the following circuits for each gate:
upload_2017-8-26_16-4-23.png upload_2017-8-26_16-5-30.png upload_2017-8-26_16-6-24.png

I believe one mistake I made was with selecting hardware. While the FETS say they can handle up to 60V between the Source and Drain and flow up to 10A; they can only handle +/-20V from Gate to Source. If you see any more mistakes I made, please let me know.

I would also appreciate a little help selecting FETS that will work and something to mount the circuit on. Please let me know if you have any ideas.

Thank you for your time,
ForgedAscendant
 

WBahn

Joined Mar 31, 2012
26,398
What are the power requirements of these pumps? Are you trying to power these pumps directly from the output of the logic?

What kind of switches are these?

Where are these signals A through D coming from? What voltage are they when HI and when LO? What amount of current can they deliver?

What kind of FETs are these?
 

Thread Starter

ForgedAscendant

Joined Aug 26, 2017
6
What are the power requirements of these pumps? Are you trying to power these pumps directly from the output of the logic?

Pump
Lightobject EWP-P4580D24J High Performance DC 24V Brushless Water Pump, 12 L/minutes, 190 GPH

Power: DC24V
Output: 12.0L/m (190GPH) @24V
Vertical cutoff: 9m (30ft)
Shaft: Ceramic
Fin/blade: ABS
Intake/Outlet: PPE+20%GE
Body: Nylon

Current: 1.3A@24V 0.8A@12V
Intake: 18.0mm (0.7") Dia.
Outlet: 10.8mm (0.42") Dia.
Life span: > 20,000hrs @ 6,800rpm~7,400rpm
Noise: < 40dB
Fluid temp: 50'C/122'F (max)
Envir Temp: <40'C/104'F
Size(L*W*D): 2.95"x1.84" (75mm X 46.8mm)
Weight (net): 7.0oz

While there are 2 pumps, only one at a time will ever be on. The idea was to power them off the logic circuit however if you have a better idea I am Listening.

What kind of switches are these?

Switch
Aiskaer 6 Pieces Side mounted Aquarium Tank Side mounted Horizontal Liquid Float Switch Water Level Sensor-Black (Black-1)
Max Contact Rating :10W;Max Switching Voltage : 100V DC

  • Max Switching Current : 0.5A;Max Breakdown Voltage : 220V DC;Max Carry Current : 1.0A;Max Contact Resistance : 100m Ohm
  • Temperature Rating : -10 ~ +85 C;Float Ball Material : P.P;Float Body Material : P.P;Float Ball Size(Approx) : 2.35 x 1.7cm (L*D)
  • Thread Dia(Approx) : 1.13cm;Hex Nut Diameter(Approx) : 1.87cm ;Body Total Length : 8.5cm ;Cable Length :39cm
  • Color : Black;Net Weight : 20g;Package Content : 6 x Water Level Sensor

Where are these signals A through D coming from? What voltage are they when HI and when LO? What amount of current can they deliver?

Signals A-D are the float switches either opening or closing the circuit from the 24V power supply to the FET gate. The amount of current they carry should be minimal this way. Hi would be 24V and Low would be 0V. The switch can deliver up to 0.5A switching and 1A steady.

What kind of FETs are these?
MOSFET 60V N-Channel QFET Logic Level

Product Category: MOSFET
Manufacturer: ON Semiconductor
RoHS:
QFET
Brand: ON Semiconductor / Fairchild
Configuration: Single
Fall Time: 40 ns
Forward Transconductance - Min: 5.5 S
Height: 16.07 mm
Length: 10.36 mm
Pd - Power Dissipation: 24 W
Rise Time: 90 ns
Series: FQPF13N06L
Factory Pack Quantity: 50
Transistor Type: 1 N-Channel
Type: MOSFET
Typical Turn-Off Delay Time: 20 ns
Typical Turn-On Delay Time: 8 ns
Width: 4.9 mm
Part # Aliases: FQPF13N06L_NL
Unit Weight: 0.080072 oz
 

ebeowulf17

Joined Aug 12, 2014
3,275
When the switch closes, it connects 24V to the FET gate, but what about when it opens? Do you have pull down resistors on all the gates?

Or are these SPDT switches which never leave the gate floating, always connecting to either 24V supply or ground?
 

Thread Starter

ForgedAscendant

Joined Aug 26, 2017
6
Those are 10a fets, how are you driving them and what are the details on the motors?
Is this just on/off control?
Max.
The plan is to drive the motors off the logic circuit using the float switches to manipulate the FET gates. The idea was on/off logic to power the motors where only a single motor will be on at any given time. They did post this in the motor product details:

Power: DC24V
Output: 12.0L/m (190GPH) @24V
Vertical cutoff: 9m (30ft)
Shaft: Ceramic
Fin/blade: ABS
Intake/Outlet: PPE+20%GE
Body: Nylon
Current: 1.3A@24V 0.8A@12V
Intake: 18.0mm (0.7") Dia.
Outlet: 10.8mm (0.42") Dia.
Life span: > 20,000hrs @ 6,800rpm~7,400rpm
Noise: < 40dB
Fluid temp: 50'C/122'F (max)
Envir Temp: <40'C/104'F
Size(L*W*D): 2.95"x1.84" (75mm X 46.8mm)
Weight (net): 7.0oz

So I would imagine you could adjust flow rate via a variable resistor if you wanted to.
 

Thread Starter

ForgedAscendant

Joined Aug 26, 2017
6
When the switch closes, it connects 24V to the FET gate, but what about when it opens? Do you have pull down resistors on all the gates?

Or are these SPDT switches which never leave the gate floating, always connecting to either 24V supply or ground?
I honestly never considered pull down resistors. For the off state I was just going to let the gate float. If this is a bad design or prone to issues please let me know and give me an extremely simple example of a pull down resistor on a switch and I can add it in.
 

Reloadron

Joined Jan 15, 2015
5,913
Fairchild told me they were 10amps? FQPF versions. FQPF13N06L
Max.
When I correctly read the part number they certainly are a 10 Amp MOSFET. I was looking at FQP30N06 and what scares me is I have done that a few times as of late. Thank God I am retired where I can't do any real damage.

Thanks Max
Ron
 

ebeowulf17

Joined Aug 12, 2014
3,275
I honestly never considered pull down resistors. For the off state I was just going to let the gate float. If this is a bad design or prone to issues please let me know and give me an extremely simple example of a pull down resistor on a switch and I can add it in.
I'm still a relative novice, so I may get since details wrong here. Hopefully the experts can clean up my mess, but here goes:

When an input is left floating, the tiniest variances in anything around (EMI, capacitive coupling to nearby circuits, static electricity from nearby people or other objects, etc.) can change the voltage on that floating input rapidly and persistently. This causes numerous different problems depending on the scenario.

In your case, any voltage between a solid on or off signal will leave the FET in its linear region where it is partially conducting, acting as the main voltage drop in the circuit, and therefore dissipating much, much more heat than it would when fully on or off. The rapid switching itself is also a cause of more heat generation.

You need to connect a resistor (10k is a common choice) from the gate to ground. When you close the switch the 24V supply will still drive the FET gate, and when you open the switch, the gate will be held at ground potential. (separate resistor for each gate.)
 

ebeowulf17

Joined Aug 12, 2014
3,275
Also, I don't claim this is the only problem or that I'm offering a complete solution.

You also need to get your gate voltages down to acceptable levels (a simple voltage divider might do the trick, and the bottom resistor in that scenario would also act as your pull down resistor.)

There may be other issues that I'm not aware of, but these are starting points for improvement.
 

ArakelTheDragon

Joined Nov 18, 2016
1,353
Do you have a cooler on the fets, transformer (if you have one) and other components?

You want to power 2 DC motors, only 1 will be on at the same time, but can the pumps supply the current for the DC motor at "24V"? Otherwise the voltage will drop! Will the motors rotate only in 1 direction?
 
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