Circuit Assistance...

Community Projects
1

First post here…

I’m hoping to find some help with a circuit design, and apologize in advance if this is the wrong place to ask for help…

I have a design in Fritzing (breadboard view) that I believe is close to what I need.

But putting into use results in smoke and damaged parts.

I’m on my third MOSFET (just ordered more from SF) and second BT module.

I tried a Transistor also, in place of the MOSFET - those generate alot more smoke.

The .fzz file is located here -

http://www.bnite.com/GPMotor.fzz

Any help would be greatly appreciated.

GP

2

A lot of people aren’t going to be able to view that file, can you post a jpeg ?

3

Tx MnM

JPG here:

http://www.bnite.com/GPMotor_bb.jpg

Parts:

N-Channel MOSFET 60V 30A COM-10213

Voltage Regulator - 3.3V COM-00526

Wireless Serial 4 Pin Bluetooth RF Transceiver Module HC-06 RS232 With backplane

Schottky Diode COM-10926

10k Resistor.

Red LED

9vBattery or 5v Walwart

Small DC Motor (think door lock on Ford)

Eagle Schematic PNG image here:

http://bnite.com/GPMotorEagle.png

Update:

I added more diodes to the image and uploaded a new copy.

4

first:

What is the Peak Current draw of the motor? Or what is the DC resistance of the motor windings?

How fast does the MOSFET smoke?

The MOSFET may need a heat sink if the motor current is high.

Are you sure the MOSFET is wired correctly?

Are you really using a 9V battery in this circuit??? or another power supply.

Why are there diodes in series with the motor and MOSFET? These are not needed. You should have a diode across the motor to snub the back EMF when the motor is turned off.

Instead of using the RF board to drive the MOSFET gate start with just using 5V to turn on the MOSFET.

If you’re unsure of the motor then use a power resistor to get the MOSFET part of the circuit working. In other words, get each sub-circuit working.

Does the motor run if you only connect the battery to the motor?

To use an NPN you do need a series resistor to the base of the transistor.

5

1’st thing I see is that the voltage regulator is in backwards. See the diagram below. You should also have the recommended 10uF cap on the output. If this is in a car, use the 100 nF cap on the input. If nothing else get the regulator wired w/nothing else (except the caps) and use a DVM to verify the voltage.

2’nd - lose the diodes I’ve marked up. They really do nothing. You may keep the reverse voltage protection diode but make sure it can handle the peak current.

3’rd - What’s the LED for ? If it’s supposed to light up when the motor runs then you need to replace the diode w/a 400 - 500 ohm resistor and turn the LED around. You may want to add a reversed biased diode across the motor terminals

Lastly I can’t say much about the BT module. Does it work stand-alone ? What voltage is supposed to come out to drive the FET ? The FET seems to be on the UART Rx input, not an output.

(click on to open)

6

To fully turn that FET on, you need at least 8volts Vgs which you don’t have with this circuit. You’re applying too low a voltage and the FET is acting like a resistor.

Gaver:
First post here…

I’m hoping to find some help with a circuit design, and apologize in advance if this is the wrong place to ask for help…

I have a design in Fritzing (breadboard view) that I believe is close to what I need.

But putting into use results in smoke and damaged parts.

I’m on my third MOSFET (just ordered more from SF) and second BT module.

I tried a Transistor also, in place of the MOSFET - those generate alot more smoke.

The .fzz file is located here -

http://www.bnite.com/GPMotor.fzz

Any help would be greatly appreciated.

GP

7

lyndon:
To fully turn that FET on, you need at least 8volts Vgs which you don’t have with this circuit. You’re applying too low a voltage and the FET is acting like a resistor.

Hmmmm a lot depends on just how much current that motor is drawing. A 3.3v gate voltage will put a limit on what can be passed, as will a lack of a heatsink. I'm still betting the regulator wiring is his 1st problem.
8

Wow…

First off- let me say thank you to all that have replied with suggestions.

Secondly - I made a mistake on the fritzing image relating to wiring of the regulator - you were right Mee-n-Mac - I showed it incorrectly wired on the dwg - but on the breadboard - I wired it correctly and verified 3.3v output.

@waltr - I don’t know how to determine the Peak Current Draw of the motor. But I just measured the resistance at 32.6 ohms.

I can answer most of the questions I think by explaining more about the motor…

I purchased a 12v Door lock from ebay (cheap import).

It has a small electric motor in it that cycles an actuator in one direction, then if you reverse the polarity it reverses the direction. Pushing and pulling the actuator.

I discovered at the expense of my first mosfet, when I put a spring (rubber band) on the actuator - that when it cycled back to the extended position - the motor was no longer a motor and became a generator - producing voltage (around 1.6v) - which I believe ran back thru the neg wire and fried the NMOSFET ( https://www.sparkfun.com/products/10213 ). According to the specs @ SF the NMOSFET is supposed to cycle full on with as little as 3v, which is why I was using it on the Rx signal leg of the BlueTooth transceiver.

I was originally doing all of this development using a 12v wallwart (from SF) - It’s likely I ruined more than one device by trying to check voltage across terminals and legs of the circuit with my low end DVM.

I was hoping that I could cycle the actuator (motor) using a 3v feed from the BlueTooth device by turning the NMOSFET on and (originally) sending 12v to the motor / actuator. I’ve since discovered the actuator works fine on 5v.

I couldn’t find a reference in fritzing to a 5v source - so I threw a 9v battery on the diagram - close enough?

In order to try to protect the NMOSFET and do something with the energy produced by the motor when cycled back to extended position - I put a diode between the leads to the motor - and an LED to help dissipate the energy being generated by the return of the actuator to the extended position (rubber band / generator).

I’m not positive - but I believe that worked briefly, the signal from the BT device cycled the NMOSFET - which completed the circuit to the motor - The LED would glow brightly for a fraction of a second. But I noticed during some cycles the red SMD led on the bluetooth device went out after each cycle of the actuator - which I also believe eventually damaged the bluetooth device, as both the Tx and Rx legs stopped working (no voltage output).

Which lead me to believe there’s more stray voltage running around after an actuator cycle and I needed to add more diodes to snub the stray voltage and try to prevent it from backflowing to the other elements of the circuit.

After damaging all my spare NMOSFETS (3v would no longer enable the switch), I tried a transistor - which resulted in lots of smoke and a small blister on the end of my index finger.

So… I ordered more parts, revised the schematic by putting diodes (1 way valves) everywhere I thought there might be a path for stray voltage, and then came here for help - which I really appreciate BTW.

Regarding the Rx / Tx connection - you may be right Mee -n- Mac - I was thinking the rx leg would go high when the receiver got a message from the phone - but it could be the other way around. I noticed the Tx leg was pulsing continuously regardless of initial connection - so It could be I’ve got the circuit backwards - I’ll need to get more info from the manufacturer to understand that - It may not be possible to do what I want with this device …

Thanks again for all the help - maybe the explanation above will help with understanding what may have gone wrong and result in a workable (better) design.

I removed most of the diodes referenced and added a 10uF capacitor to the output of the regulator and revised the schematic. After reading this hopefully someone can suggest where the diodes need to be placed to protect the transceiver from stray voltage. At the suggestion of other boards - I have a 10k pulldown resistor on the gate leg of the NMOSFET.

Revised schematic:(click for larger image)

[](http://www.bnite.com/GPMotor_bbV3L.png)

9

Ok, 32.6Ohms at 12 V is 368mAmps by Ohm’s Law. This should be the max current also know as the ‘stall current’.

The need the diode directly across the motor leads NOT through the LED as your showed in your diagram. This should shunt the reverse current out of the motor.

For now, forget the RF device and work on just the motor and MOSFET. Also search the forums here since this is not an uncommon circuit.

10

Ditto on ditching the BT to get the motor working. Use a couple of resistors and a momentary switch to make a voltage divider to apply 3.3 v to the motor. You’re using the FET as a “low side” switch. While I can appreciate the simplicity, if the latch is to remain open for any length of time you’ll be driving the motor until it hits some mechanical limit and then stalls. Most motors don’t tolerate being stalled for long periods. Personally I’d lose the rubber band and be using an H bridge driver.

https://www.sparkfun.com/products/315

https://www.sparkfun.com/products/9457

You still show the LED in series w/a diode (not a resistor) and reverse biased when power is applied to the motor. As shown they do “snub” the voltage produced by the motor when it rewinds under rubber band power. That is a needed and good thing, though not what I guess you intended.

As for the BT, the Tx/Rx pins are for serial communication. I believe there are other digital output pins that you could use.

http://en.wikipedia.org/wiki/Universal_ … ransmitter

FYI - XBees have a “virtual wire” (I/O line passing) function.

http://www.science.smith.edu/~jcardell/ … okbook.pdf

2.2.7 I/O Line Passing

I/O line passing creates “virtual wires” between two Xbees, so that selected inputs on one (the source) are reflected as

output states on the other (the destination).

To enable I/O line passing, the source must from time to time transmit the required input values to the destination.

There are two possible ways to do this:

  1. Use the IR parameter and the IT parameter to set a sampling rate. Table 2.2 shows the settings for a pair of

XBees using this method.

  1. Use the IC parameter to enable change detection on the input lines.

The IA parameter controls permissions for other modules to order this module to change the states of its output lines.

The value is a 64-bit module address, and only that module is allowed to change the line states. The value may also be

set to 0xffffffffffffffff (the default), which will prevent any other module from changing the lines, or to 0xffff which will

allow any other module to change the lines. The X-CTU help box says this is for I/O line passing. It doesn’t mention

remote AT commands.

The Tn parameter (n in 0–7) sets timeout values for output lines whose values are set through I/O line passing (not

through the IO parameter). When any such line is set to its non-default value, its timer is started and the line will revert

to the default value as set by the Dn parameter when the timer expires. The values are in the range 0 to 0xff and the time unit is 100mS. The default value is 0xff (25.5 seconds). If the timer value is 0, lines retain their levels indefinitely.

The PT parameter is a common timeout value for both analog outputs. When its value is non-zero, each time a PWM

level is set through I/O line passing, the timer will be restarted. When it expires, the PWM lines will be reset to 0V. Timer values are in the range 0 to 0xff and the time unit is 100mS. The default value is 0xff (25.5 seconds).

The IU parameter controls what happens to line state packets. It defaults to 1, which means that any line state packets

received by the XBee will be sent through the UART to the XBee’s controller and will not affect local output