Hello

I have designed and built a basic robot circuit for use as to demonstrate basic fire detection. The ronobt circuit uses a Genie 14 microcontroller with two LEDS which track light. This then decides which motor should be controlled from one of two different slave boards. The slave boards use 4 Mosfet transistors each to control the direction of rotation of each motor.

I used circuit wizard to design and compile the circuit and it all works great in circuit wizard. Problem is I have built the circuit in real life acouple of times now and the motors do not run and the transistors on the slave boards keep over heating.

I have included diagrams/pictures below and also the circuit wizard files for any body to have a look at.

Any help would be much appreciated?

Andy

https://drive.google.com/file/d/1xFUhAMbWH1CS_UjuSPneHyIM4humihjt/view?u...

 

Welcome to AAC!
There are at least the following problems with your circuit :
1) The schematic shows BJTs, not FETs, for switching the motor.
2) The Genie's switch inputs have no pull-down resistors (unless these are inside the IC and have been activated) to prevent the inputs floating.
3) The buzzer has no voltage-spike suppression diode across it. Depending on the buzzer characteristics it may not be needed.
4) Have you checked that the Genie output pin is rated to carry the buzzer current?
5) LED D2 has no current-limiting resistor.
6) The Genie should have a decoupling capacitor (~100nF) across its supply pins.
7) The Genie power supply should be less than 5.5V. If you use 9V as shown in the schematic (typo?), the Genie will die.

 

Welcome to AAC!
There are at least the following problems with your circuit :
1) The schematic shows BJTs, not FETs, for switching the motor.
2) The Genie's switch inputs have no pull-down resistors (unless these are inside the IC and have been activated) to prevent the inputs floating.
3) The buzzer has no voltage-spike suppression diode across it. Depending on the buzzer characteristics it may not be needed.
4) Have you checked that the Genie output pin is rated to carry the buzzer current?
5) LED D2 has no current-limiting resistor.
6) The Genie should have a decoupling capacitor (~100nF) across its supply pins.
7) The Genie power supply should be less than 5.5V. If you use 9V as shown in the schematic (typo?), the Genie will die.

Hey

Thanks for the quick reply.

1) The schematic shows BJTs, not FETs, for switching the motor.
The FETS I have used in the actual circuit are ZVN43D6A. The problem is that they overheat and the motors don't move. This is the main problem which I cannot work out

2) The Genie's switch inputs have no pull-down resistors (unless these are inside the IC and have been activated) to prevent the inputs floating.
Yeah I need to add some pull downs in because the signal is a bit all over the place on the real circuit.

3) The buzzer has no voltage-spike suppression diode across it. Depending on the buzzer characteristics it may not be needed.
It doesn't seem to be needed as the buzzer works quite well.

4) Have you checked that the Genie output pin is rated to carry the buzzer current?
See point 3 above.

5) LED D2 has no current-limiting resistor.
This isn't an issue as the output current on those pins is not very big

6) The Genie should have a decoupling capacitor (~100nF) across its supply pins.
I'm not entirely sure how to put this in or why it is needed but the microcontroller itself doesn't seem to be the problem. The input pins are all working and so are the output pins, its just that on the slave boards the motors aren't moving and the ZVN43D6A transistors are overheating.

7) The Genie power supply should be less than 5.5V. If you use 9V as shown in the schematic (typo?), the Genie will die.
Yeah I don't know why I put 9V on the drawing, it is connected in real life using 5.5v. Problem with circuit wizard is that it just seems to let any Voltage supply and doesn't show accurate real world consequences.

Thanks for helping me out, I really appreciate it. If there is any thing else you can think of for why the motors aren't working and the FETS would be overheating please let me know.

Oh and I have tried different sized power sources for each of the slave boards, and it has always done the same overheating thing.

Thanks

Andy

 

1) The schematic shows BJTs, not FETs, for switching the motor.
The FETS I have used in the actual circuit are ZVN43D6A.

If you are using FETS, why didn't you simulate with FETs?
FETs and BJTs are not normally directly interchangeable.
You can't expect the simulator to show what the circuit does if you don't use the same parts.

I could find no info on a ZVN43D6A FET.
Is that the correct part number?
If it's not a logic-level type MOSFET, than that could explain the overheating of the bottom FETs at 5V.

Also, the top transistors in the bridge are operating in a follower mode (where did you get that circuit?), so are not fully turned on, which will definitely cause overheating.
The top devices need to be p-channel devices, driven by a signal with the same phase as the bottom transistors.

Problem with circuit wizard is that it just seems to let any Voltage supply and doesn't show accurate real world consequences.

Yes, circuit simulators generally do not warn you if a part's operating voltage, current, or power limits have been exceeded.
You have to look at the simulation values for those and determine that for yourself.
(It's normally part of the design process to determine if the parts are operating well within their limits.)
Did you look at the simulation results to see what the motor voltage and currents were, for example?

 

It doesn't seem to be needed as the buzzer works quite well.

The buzzer may well be ok, but if it generates voltage spikes the Genie's output port could be damaged. Likewise if the LED overloads the output port. Just because the Genie has survived thus far could be down to luck.

If it's not a logic-level type MOSFET, than that could explain the overheating of the bottom FETs at 5V.

That, and if all the MOSFETS are n-channel, could also explain why the motors don't run at all.

 

Are the Mosfets ZVN4306A?
You have the source pins of the upper n-channel Mosfets connected to the motor. Then for the motor to be fed +9V from the source pin, the Mosfet must be turned on. Your Mosfet fully turn on when the Vgs is 10V but they turn on fairly well (but get hot) when Vgs is 5V. Then the gave drive voltage must be 9V + 10V= 19V, or be 9V + 5V= 14V.

Your Genie is powered from +4.5V that drops to +3V as the battery runs down and its output will be the same, far from the 19V or 14V that is needed by the gates of the upper Mosfets.

 

@andynalley Welcome and how exciting- you have your dream project!!! It is excellent to believe in castles in the sky, but you first must learn enough to build a stairway to them. Your first step begins here:

Title: Understanding Basic Electronics, 1st Ed.
Publisher: The American Radio Relay League
ISBN: 0-87259-398-3

 

What are the motors, do you have a datasheet for them?

As mentioned already, you're not providing enough gate drive to the upper MOSFETs which isn't helping but the main reason they're overheating is that they need to be mounted with drain, and probably source, pins soldered to at least 1 sq inch of copper as a heatsink. As wired you have virtually no way for them to dissipate heat to the air so its not surprising they get hot.

 

they're overheating is that they need to be mounted with drain, and probably source, pins soldered to at least 1 sq inch of copper as a heatsink.

That may not be true if the MOSFETs have a low enough on-resistance so that the dissipation is less than a half watt each.

 

Hello everybody,

First of all, woooowwww!! I wasn't expecting such good input, and so much of it

Second, please be patient with me as this is a learning curve for me and I am clearly a moron, haha

I have a few questions from what people have said:

If you are using FETS, why didn't you simulate with FETs?
FETs and BJTs are not normally directly interchangeable.
You can't expect the simulator to show what the circuit does if you don't use the same parts.

I don't really know much about different transistors, I just have an understanding of how the work, the pin layouts and the difference between n-channel and p-channel. So I just let circuit wizard decide the n-channel transistor and then tried to swap it out for a fet that seem rated for high current use. But clearly I was wayyy of.

I could find no info on a ZVN43D6A FET.
Is that the correct part number?
If it's not a logic-level type MOSFET, than that could explain the overheating of the bottom FETs at 5V.

I don't even remember where I got it from now but its definitely what is written on it. I have loads of different transistors stored away in the workshop but no idea which to use.

That may not be true if the MOSFETs have a low enough on-resistance so that the dissipation is less than a half watt each.

Is there a good n-channel FET that anybody could recommend me trying?? Don't particularly want to be solder big copper heat sinks on there if I can help it.

Also, the top transistors in the bridge are operating in a follower mode (where did you get that circuit?), so are not fully turned on, which will definitely cause overheating.
The top devices need to be p-channel devices, driven by a signal with the same phase as the bottom transistors.

I designed it myself which will be why its wonky! you say the top two transistors are in follower mode (not sure what that is?) but you say they should be pnp. Thing is I designed it like that so that q1 + q4 work together with the steering diodes to turn the motor motor clockwise. I then use the opposite transistors to turn the motor anti clockwise. See my childlike diagram below for more explanation of what I mean. I f I changed the top two to pnp it wouldn't have the effect as they would always be on when there is no signal? (sorry if i sound patronising, I am just trying to work it out myself )

Are the Mosfets ZVN4306A?
You have the source pins of the upper n-channel Mosfets connected to the motor. Then for the motor to be fed +9V from the source pin, the Mosfet must be turned on. Your Mosfet fully turn on when the Vgs is 10V but they turn on fairly well (but get hot) when Vgs is 5V. Then the gave drive voltage must be 9V + 10V= 19V, or be 9V + 5V= 14V.

Your Genie is powered from +4.5V that drops to +3V as the battery runs down and its output will be the same, far from the 19V or 14V that is needed by the gates of the upper Mosfets.

Hmmm I see do you think I am over egging it a bit by using 2 9v batteries on the slave board?? I have never been super good with maths side of engineering!! haha. Anyways any advice on what exactly I should do would be massively appreciated???


Cheers guys

Andy

 

Also.......

The buzzer may well be ok, but if it generates voltage spikes the Genie's output port could be damaged. Likewise if the LED overloads the output port. Just because the Genie has survived thus far could be down to luck.

That, and if all the MOSFETS are n-channel, could also explain why the motors don't run at all.

Sorry if it sounded like I was ignoring your advice, I wasn't, I will put pull downs in and some small resistors on the buzzer and lights, I was just trying to deal with the main problem of the damnnnnnn motor not turning and me burning my finger with super hot transistors!!! hahaha

Do appreciate the input though

 

@andynalley Welcome and how exciting- you have your dream project!!! It is excellent to believe in castles in the sky, but you first must learn enough to build a stairway to them. Your first step begins here:

Title: Understanding Basic Electronics, 1st Ed.
Publisher: The American Radio Relay League
ISBN: 0-87259-398-3

Ermmmmm thanks, I guess.

 

Also I cocked up that drawing, this is what I meant:

 

As noted by many, you need to have a P-Channel (or PNP) devices for the top two bridge transistors.
Otherwise you are just spinning your wheels.

 

As noted by many, you need to have a P-Channel (or PNP) devices for the top two bridge transistors.
Otherwise you are just spinning your wheels.

Ok I will try it tomorrow morning when I am in work and might make a video to show you the results. It just seems weird that it would work exactly how I intend it to on circuit wizard but not in a real circuit

 

Doesn't the schematic wrongly show a 9V battery exploding the Genie but the wiring layout shows a 3V to 4.5V battery?
If P-channel Mosfets are used for the upper transistors powered from 9V then the Genie will not be able to turn them off.

This circuit needs additional level-shifting transistors for its two supply voltages to work.

 

Ok I will try it tomorrow morning when I am in work and might make a video to show you the results. It just seems weird that it would work exactly how I intend it to on circuit wizard but not in a real circuit

But the circuit you simulated wasn't what you built. Your simulation uses NPN bipolar transistors (BJTs) which are current driven devices but you've built the circuit using N-channel MOSFETs which are voltage driven devices and work in a completely different way.

You can't just randomly select a part and use it without reference to its data sheet and operating parameters.

 

Doesn't the schematic wrongly show a 9V battery exploding the Genie but the wiring layout shows a 3V to 4.5V battery?
If P-channel Mosfets are used for the upper transistors powered from 9V then the Genie will not be able to turn them off.

This circuit needs additional level-shifting transistors for its two supply voltages to work.

Ahhhh ok that’s interesting. If I put in p-channel mosfets on the top two and lowered the battery voltage for each of the slaves would that work??
If not then I’m not exactly sure how to set up level shifting transistors?

really appreciate the feedback on this, your all being super helpful

 

But the circuit you simulated wasn't what you built. Your simulation uses NPN bipolar transistors (BJTs) which are current driven devices but you've built the circuit using N-channel MOSFETs which are voltage driven devices and work in a completely different way.

You can't just randomly select a part and use it without reference to its data sheet and operating parameters.

haha, no I guess I can’t! are you saying then that the layout won’t work mosfets as it is? Or do you think I could run the motors using npn bipolar and pnp bipolar transistors instead of mosfets?
Any recommendations on some more suitable transducers would be greatly appreciated??

 

If you use PNP transistors or P-channel Mosfets in the H-bridge motor drivers and power the H-Bridge circuits from the same 3V to 4.5V as the Genie, then you must also re-program the Genie to invert its drive to the PNP transistors or P-channel Mosfets.
Will the motors run from being powered by only 3V to 4.5V?