Hi all,
I’m undertaking this project to learn a little about electronics, with the end goal to power a sculpture. Simple, I thought!

On a breadboard, which has been troublesome in itself, I have a motion sensor to control the IRF540N mosfet to run a 12v DC motor.

This runs fine when I test with a LED (removing the resistors and diode) but nothing happens when the motor is plugged in.

Any issues with the components? Circuit? Any suggestions appreciated.

Thanks!

 

Your schematic shows the motor as having 3 totally ambiguous terminals?

The gate resistors are 1 and 10 Ohms? Totally way too low.

These should be more like 100 ohms in series, with a 10K to ground.

 

And the MOSFET source and drain are swapped.

 

Your schematic shows the motor as having 3 totally ambiguous terminals?

The gate resistors are 1 and 10 Ohms? Totally way too low.

These should be more like 100 ohms in series, with a 10K to ground.

**
Thanks, just checking schematic, I actually do have 10k to ground, schematic is wrong. The other one is 1k - is that okay or too much?

 

Your schematic shows the motor as having 3 totally ambiguous terminals?

The gate resistors are 1 and 10 Ohms? Totally way too low.

These should be more like 100 ohms in series, with a 10K to ground.

****
The 3 ambiguous terminals in the schematic actually belong to the MOSFET (ground, drain and source).

 

Something *blew*. I think it was the sensor... followed by erratic behaviour. I’m thinking of buying this one now.

Thanks alberthall and sensacell - yell out if there’s anything else I should place in my cart.

 

Your schematic shows the motor as having 3 totally ambiguous terminals?

The gate resistors are 1 and 10 Ohms? Totally way too low.

These should be more like 100 ohms in series, with a 10K to ground.

***
Oh just realised what you mean. The diode is just parallel to motor. Schematic way off. It’s connected to pos and neg of motor.

 

Re-draw the schematic to show your new understanding of the circuit, post it.

 

Thanks sensacell,

To update:
- will order the Adafruit PIR (the one I had blew)
- will order 100 and 10k ohm resisters
- schematic redrawn with motor and mosfet more ordered

Hopefully this will work!

 

will order 100 and 10k ohm resistors

You may need 2 or 3 of those 10k resistors, depending on the output configuration of your PIR sensor. Some sensors are 'open-collector' configuration, which would require a 'pull-up' resistor (10k). If that is the case then the gate 'pull-down' resistor would need to be more than 10k (otherwise, because the pull-up and pull-down resistors make a potential divider, the gate would never see more than 6V, which would be insufficient to turn most FETs on fully).

 

You may need 2 or 3 of those 10k resistors, depending on the output configuration of your PIR sensor. Some sensors are 'open-collector' configuration, which would require a 'pull-up' resistor (10k). If that is the case then the gate 'pull-down' resistor would need to be more than 10k (otherwise, because the pull-up and pull-down resistors make a potential divider, the gate would never see more than 6V, which would be insufficient to turn most FETs on fully).

Just remembered I have a pack of eight of these. The PIR guide doesn’t specify whether it is open-collector, but broadly mentions that some are. Will try that if it doesn’t work in the first instance, thanks for the tip!

 

The sensor outputs 3V. It takes 10V to turn the IRF540 fully on. 3V might be enough to turn on an LED but mot enough for the motor.

Bob

 

The sensor outputs 3V. It takes 10V to turn the IRF540 fully on. 3V might be enough to turn on an LED but mot enough for the motor.

Bob

That explains it!

So should I be looking for a value of 3 or less for the Static Drain-to-Source On-Resistance?

Thanks Bob!

 

So should I be looking for a value of 3 or less for the Static Drain-to-Source On-Resistance?

No. The parameter you want is Vgs(thr), the threshold gate voltage. That needs to be well below 3V if, indeed, your PIR output is only 3V.

 

No. The parameter you want is Vgs(thr), the threshold gate voltage. That needs to be well below 3V if, indeed, your PIR output is only 3V.

These MOSFETs were mentioned elsewhere:

NX3008NBKyW
Vgs(thr): min 0.6, typ 0.9, max 1.1
and:


FDD6530A
Vgs(thr): min 0.4, typ 0.9, max 1.2
And:



Would both drive my 12v motor (45mA no load). If so, which would you recommend and why?

If not, help!

(thanks in advance!)

 

No. The parameter you want is Vgs(thr), the threshold gate voltage. That needs to be well below 3V if, indeed, your PIR output is only 3V.

No. Vgs(thr) is typically when the MOSFET will pass 250uA. The correct place to look is in the Rdson spec, as illustrated in the post above. This tells you what gate voltages are required for various currents.

Bob

 

@Termy- What you want to do, and what you can do are two different things. Your project seems simple, but isn't (for you). You need to read the datasheet on the PIR thoroughly, and understand it. Same goes for the other components, like the MOSFET. Learning how to read datasheets is another tool you have to develop in your arsenal.

Did you start with the examples in the Datasheet for the PIR, just controlling an LED? If not, do so. Use a working example to help you understand the relationship of the components and to learn how to use a couple of really powerful circle/pyramid equations:

E = IR ;Ohm's Law
P = IE ;Watt's Law

E = volts
I = amps
R = ohms
P = Watts

Transistor gates are VERY sensitive. You control them with very little current, and a specific amount of voltage in order to get a great deal more current at a given voltage through their main line (Source->Drain for FETs and Emitter-Collector for BJTs). You use the known voltage and current you need to go through your motor and work backwards from that to determine what resistor you need on your gate. What are the specs for your motor? We don't know.

Be aware that MOSFETs usually require about 10V on the gate to fully saturate. Check your data sheet and make sure you know. Your pull-down resistor, instead of 10K, should be 100K. it should be weak, not draw off excessive current- it's sole purpose is to keep the gate at ground when the gate is not being used.

Instead of powering your circuit off of 12V, you should use a regulator to power your circuit off of 5V or 3.3V, and use the transistor to switch the motor at 12V.

 

No. Vgs(thr) is typically when the MOSFET will pass 250uA. The correct place to look is in the Rdson spec, as illustrated in the post above. This tells you what gate voltages are required for various currents.

Bob

I used to try and call it out when people say that, but it's like beating your head against a brick wall. I wish they would just hide the threshold voltage in some obscure part of the data sheet, it's only really useful for audio amplifier design use.

 

Instead of powering your circuit off of 12V, you should use a regulator to power your circuit off of 5V or 3.3V, and use the transistor to switch the motor at 12V.

Now that you mention it, using 12v to power a 3v signal is incredibly wasteful!

I do have a few days off so I will do as BobaMosfet advises - dive deeper into each component and review the PIR example etc. I’ve been trying with the mosfet datasheet, including the graphs but I’m not there yet.

Will return soon, relay some learnings, and upload a new schematic.

Thanks everyone for your help so far!

 

Hi,
Just a quick post to say thanks and my circuit works!



I have a step up/down regulator, so the circuit can be powered by 2.5 to 18v with a 33 μF capacitor to prevent voltage spikes. I’m using 9 volt batteries currently but this may change.

The Adafruit sensor works well. I bought a different logic level mosfet available to me, the IRLB8721PbF, with a low RDS(on) at 4.5V.

The suggested gate resistor works fine, could be lower, but I don’t know if I’d notice much difference. Pull down resistor 100k as suggested - I found how to calculate this - a page-long explanation, but decided not to go there, obviously!

The motor will be connected to a worm gear to drive sprockets and a chains, and then I will test with the final load. So I will see how my motor handles this, then adjust to suit.

Cheers!