have only transistors on hand, must build 3v3 logic level driver for this chip, would like to do it properly, and as efficiently as possible.

unfortunately am very amateur, and do not really know what i am doing.

so far, working in circuitjs, i have come up with the attached - these images show the circuit in the on and off state, respectively.

have also attached the circuit source, paste into https://www.falstad.com/circuit/circuitjs.html to run.

my resistors are chosen to reduce the power consumption as much as possible, and i worry if this will work, in practise.

the transistors are s8050 and s8550, and are all i have right now, and the motors i need to drive are about 2A brushed.

my primary application is IOT bilge control, and it bothers me that my circuit is consuming power just keeping the motor off, however i cannot see how to correct this, other than by floating the output pin, about a half a second after it has turned the motor off.

am i doing this right, at all?

or is there a better way?

any advice, suggestions, would be very much appreciated.

 

it bothers me that my circuit is consuming power just keeping the motor off,

Don't understand that comment.
It is only drawing 3.6nA (leakage current) when off so that is negligible power.

For just turning the MOSFET OFF and ON for a bilge pump, you only need one NPN and one PNP (Ltspice sim below):
The circuit draws only leakage current when off, and about 120µA when on.

Note: Do not connect a capacitor (C1) across the motor as that will cause a large current spike through the MOSFET when it turns ON.
Connect it from the positive to ground, as shown.

 

perfect, thank you very much.

not sure how i got the idea i needed a class B (hope that's the right term) to drive the mosfet, thank you for setting me right.

i get your point about leakage current, i tend to misjudge such small orders of magnitude.

will probably leave out the capacitor, supposed to counteract RF interference, hopefully not needed.

your help is very much appreciated.

 

The complementary transistors are not Class-B, instead they are a voltage level converter.
A +3.3V input produces +12V at the Mosfet gate to turn it on.

 

The complementary transistors are not Class-B, instead they are a voltage level converter.
A +3.3V input produces +12V at the Mosfet gate to turn it on.

yes, i think i understand - this is an non inverting circuit, and more efficient than a single transistor pulling the gate to ground against a resistor, just to turn the gate off.

actually i was referring to my own circuit at the top of the question - is that not a class B output stage, driving the mosfet?

i know it's wrong, and i wish i could remove those images now, they might cause confusion.

on the other hand, sorry to digress, however am also wondering if there are ways i can get a PWM logic signal to control the IRFZ44N as a proper amplifier, somehow. am advised it will operate as such, given a gate/source voltage of between 2 and 4 volts or so.

it seems to me i could use an ADC to monitor the output voltage of a buck circuit on the 12v supply, or perhaps a buck/boost coming from the logic supply, with PWM to control the actual voltage on the mosfet gate.

or am i overcomplicating things, are there better ways to do this?

 

if there are ways i can get a PWM logic signal to control the IRFZ44N as a proper amplifier, somehow. am advised it will operate as such, given a gate/source voltage of between 2 and 4 volts or so.

Standard MOSFETs typically require Vgs of 10V to fully turn on, and logic-level MOSFETs typically require 3-5V.
The 2 to 4V Vgs is probably the MOSFET threshold voltage where it is just starting to turn on at a very low drain current.

 

Q2 base-emitter turn-off resistor - ?

ak

 

Q2 base-emitter turn-off resistor - ?

- ?
It's for a bilge pump. where fast turn-off is not needed.

 

Do You know about dedicated Gate-Driver-Chips ?

Are You specifically trying to figure out how to accomplish this task using only Bipolar-Transistors ?,
and if so ....... Why.

Is it necessary to use Bipolar-Transistors for your project ?

12-Volts or 24-Volts Supply Voltage ?,
How many Locked-Rotor-Amps ?,
How many Full-Speed-Amps ?,
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It's for a bilge pump. where fast turn-off is not needed.

OK - how about reliable turn-off? When Q1 is off, the Q2 base is completely floating. In that kind of electrical environment, that seems unwise. And, nanoamps of leakage current through Q1 become microamps of current through R2.

ak

 

In that kind of electrical environment, that seems unwise. And, nanoamps of leakage current through Q1 become microamps of current through R2.

I'm not particularly worried about the electrical environment, since I would expect only a short connection to the base to pick up any EMI, but it likely is a good idea to add a few kΩ resistor from base to emitter of Q2 to bypass any leakage current, especially at higher temperatures.

 

The datasheet for a standard IRFZ44 Mosfet shows the threshold voltage range for the gate voltage which is that some of them barely conduct 0.25mA with 2V and others at 4V, use at least 8V.
The datasheet for a logic level IRLZ44 shows 1V to 2V, use at least 4V.

Your high value resistors turn off the transistors slowly allowing the Mosfet time to be linear and conduct current which wastes power heating the Mosfet.

 

Your high value resistors turn off the transistors slowly allowing the Mosfet time to be linear and conduct current which wastes power heating the Mosfet.

Not a problem if he is only turning a bilge pump on and off.
Would be a problem if he's going to do PWM.

 

Not a problem if he is only turning a bilge pump on and off.
Would be a problem if he's going to do PWM.

I saw the input on the schematic says, "Pulse".

 

should there be another resistor pulling Q2 base to +12v?

as has been noted, this is an on/off switch, duty cycle is minutes or hours.

this is a marine environment, in winter, and currents get into these circuits even through the air, which is loaded with salty vapours, i believe, and there can be a lot of noise in the 12v power supply also.

what would it take to trigger Q2 while in the off condition, without a pull up resistor?

or would it be safer to include the pull up? am guessing 100k would be enough?

it is important the circuit does not leak, in the off state, or it could drain the battery, and any leakage through the mosfet would also corrode the motor's commutator, over time.

 

Why is a standard "Float-Switch", designed specifically for a Bilge-Pump, not being considered ????
They are pretty-much bullet-proof when installed correctly.
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Why is a standard "Float-Switch", designed specifically for a Bilge-Pump, not being considered ????
They are pretty-much bullet-proof when installed correctly.

not in my experience.

without naming brands, i have tested the most common ones, in multiple situations, and so far only two switches have survived longer than a couple of years. this is in excess of 80% failure rate.

i have not known a reliable float switch since they banned mercury.

and that is without all the mechanical failures, and what happens when crud gets trapped under the float, or on top of it, in a busy bilge.

how does one incorrectly install a float switch? they have no polarity that i know of, but they corrode awfully rapidly, perhaps one current direction is better than the other? are they better on the postive rail? i have been wiring mine in ground in case of the inevitable leakage currents.

and i just got sick of 80 dollars a unit, two and three times a year, when a raspberry pico board, a transistor H-bridge driving a AC into a couple of stainless steel probes, plus a resistor to measure the current, and a power supply, and i've got a wifi connected IOT switch that reports every minute, for a little more than ten dollars.

after learning that the simple moisture detectors do not work, have developed a somewhat foolproof probe sensor based on TDS measurement techniques, and everything was working fine until the mechanical relay burned out..

 

The datasheet for a standard IRFZ44 Mosfet shows the threshold voltage range for the gate voltage which is that some of them barely conduct 0.25mA with 2V and others at 4V, use at least 8V.
The datasheet for a logic level IRLZ44 shows 1V to 2V, use at least 4V.

ok, so the 2 to 4 gate volts in the spec sheet refers to a variance in manufacturing?

i had assumed it meant that drain/source begin conducting at 2v, and are on at 4v, but since then i have studied the graphs more closely, and i see their reference plots show the test device beginning to conduct at 4v, on a curve going all the way up to 8v.

this, to me, suggests that i can use these devices as linear amplifiers, somewhere in this range..

and to function properly as a switch, i must pull at least 8v or more.

Your high value resistors turn off the transistors slowly allowing the Mosfet time to be linear and conduct current which wastes power heating the Mosfet.

should i fear any surge current through Q2, if i turn it on too quickly?

 

Do You know about dedicated Gate-Driver-Chips ?

yes, thank you, and if i had known what i was doing when i ordered the mosfets, i would have paid the price for IRLZ44N, and saved all this bother.

Are You specifically trying to figure out how to accomplish this task using only Bipolar-Transistors ?,
and if so ....... Why.

unfortunately all i have available, i'm even out of the usual 2N3906, and am having to substitute s8050.

Is it necessary to use Bipolar-Transistors for your project ?

no, but i don't think drivers are needed either, plan to substitute IRLZ44N in future, if this works.

12-Volts or 24-Volts Supply Voltage ?,
How many Locked-Rotor-Amps ?,
How many Full-Speed-Amps ?,

12 or 24 needed, currently 12.

if locked rotor means what i think it means, then it gets close to short circuit, i would assume, when stuff gets stuck in the pump.

intend to add a small resistance to measure the pump current, or am hoping i can read the voltage drop across the 12v supply fuse (5, 7 or 10 Amps, depending on the pump) and am wondering if there are better ways to measure current draw through a mosfet circuit like this.

also plan to stick a dallas 1-wire temperature sensor on the mosfet - much cheaper than replacing burned out pump motors.

full speed amps are below 5 in the smaller pumps, and below 10 at maximum - at 12 volts.

 

The IRL44Z will probably not withstand very much abuse,
I would recommend a much tougher FET like a IRFP4368PBF,
with a FET Gate-Driver number IXDN630MCI.
It comes in a big fat TO-247 Package and is Current-Limited only by the Package-Limitations,
just about guaranteeing that it will start a fire before it fails.
The Supply must be Fused for the Wire-Gauge used of course.
The FET-Driver will require a 1uF Ceramic-Capacitor attached close to the Power-Input-Pins
to insure operational-stability, and fast switching-speeds.
This FET can easily drive 2-Pumps simultaneously.

This is all over-kill because if it fails your Boat may sink or be damaged.

The FET Gate-Driver only draws 10-micro-Amps in stand-by mode,
an insignificant load for a Lead-Acid-Starter-Battery.
The Battery should be connected to a "Battery-Maintainer" at all times when not in use in any case.

Then use a Standard-Float-Switch to activate the Input of the FET-Driver,
this will remove all Motor-Loads from the Float-Switch,
leaving only a ~1K Load-Resistor to switch.
The FET-Driver has a CMOS-Input which is very High-Impedance, this means it
could easily pickup odd Electrical-Noises without some moderate Resistive-Load attached to it.
A ~10K Input-Resistor will protect the Input from any possible over-Voltage anomalies.

The FET will need to be protected from Brushed-Motor-RFI-Hash by
a Common-Mode-Choke, DigiKey number 399-SCF47C-400-1R8C040JH-ND,
and a Freewheeling-Diode, DigiKey number 497-19457-ND.

All of these Components need to be screwed-down
inside a Cast-Aluminum-Box which will also act as a Heat-Sink,
and then Potted to seal-out the Salt-Air-Environment.

The Control-Box requires a permanent ~5-Amp-rated Battery-Connection
which is separate from the Power-Supply(s) to the Pump(s).

If You still want to pursue this very-simple, but moderately-expensive project,
I'll provide You with a Schematic.

I would not ever recommend the use of any type of bare-wire probe in a Salt-Environment.
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