NDRIVE - Nichrome payload cutdown advice

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toddhampson
Posts: 6
Joined: Mon Mar 18, 2013 10:06 am

NDRIVE - Nichrome payload cutdown advice

Post by toddhampson » Mon Mar 18, 2013 12:12 pm

Hi all, Im building a nichrome hotwire cutdown device for our balloon project and need to lean on the expertise in the forum for some assistance.

I am trying to use a Freetronics Eleven as a timer and a NDrive module as the switch to turn on the nichrome circuit. See picture pf the circuit
cutdown-schematic.jpg
Cutdown schematic
If I remove the Eleven and the NDrive from the circuit, The nichrome will will glow red and the Ammeter shows about 4Amps, which is about what Ohms Law calculates. So this part works as I expect.

On the Eleven side, here is my code, I replace Pin 2 with Pin 13 to light up the onboard LED to make sure the code works as expected. I use the serial connection to keep track of where the program is too.

Code: Select all

int fired = 0;    // 0 not fired 1 or more is fired

void setup(){
  //start serial connection
  Serial.begin(9600);
  //configure pin13 output
  pinMode(2, OUTPUT); 

}

void loop(){
  // set the pin low
  digitalWrite(2, LOW);
  //print out the value of the pin
  Serial.println("Output is low");
  Serial.println(fired);
  
  if (fired == 0) {
    Serial.println("Going into 20 second delay .. ready to fire");
    delay(15000);
    Serial.println("5 seconds to fire ...");
    delay(5000);
    digitalWrite(2, HIGH);
    Serial.println("FIRED ...");
    delay(1000);
    Serial.println("1 second ...");    
    delay(1000);
    Serial.println("2 seconds ...");    
    delay(1000);
    Serial.println("3 seconds ...");    
    digitalWrite(2, LOW);    
    Serial.println("Output is low");
    fired = fired + 1;
  }
  else {
    Serial.println("We are in sleep mode");
    Serial.println("Sleeping 10 seconds");
    delay(10000);
  }
}

When digital pin13 was used it did exactly as I expected, went into the first part of the IF loop, waited 25 seconds then lit the Led for 3 seconds then turned off, did the loop again then went into the second part of the loop and basically went into sleep mode.

OK great, changed to digital pin2, wired up the NDRIVE as per the schematic. Uploaded the new code.

Eleven started and went into the first part of the IF statement as expected, when it came time to bring Digital Pin 2 HIGH, nothing happens on the nichrome wire ... .. hmmm serial shows the FIRED statements, Digital PIn2 sent to LOW, then increments the counter, loops and goes into the second part of the IF statement ... while Im sitting there looking at the code scratching my head i feel heat on my arm and look down to see the nichrome wire glowing .. ?? ... and sadly , smoke coming from the ndrive MOSFET ... :(

Not sure if there is something obvious Im missing on the electronics side with the MOSFET, ie:
- as the nichrome coil is acting as an inductor and a reverse EMF is to blame
- I need a diode int he circuit
- Ive got something backwards as it turned the circuit on when it should have been off ..
- Not sure if the earths need to be tied together so there isnt a floating voltage ? IF so which pin S on NDrive to earth of Eleven power as well as battery providing power to nichrome ?
- Im not sure why the MOSFET smoked, it said it would take 60V and 20A, neither of which I think I exceeded :(

So would very much appreciate assistance from some more knowledgeable people as I dont really want to fry another one messing around ..
Last edited by toddhampson on Sun Apr 07, 2013 8:21 am, edited 1 time in total.

toddhampson
Posts: 6
Joined: Mon Mar 18, 2013 10:06 am

Re: NDRIVE - Nichrome payload cutdown advice

Post by toddhampson » Tue Mar 19, 2013 11:36 am

thanks for the reply tronix. I will give this a go :)

-todd-
CHNSA
http://hab.hampson.net.au

toddhampson
Posts: 6
Joined: Mon Mar 18, 2013 10:06 am

Re: NDRIVE - Nichrome payload cutdown advice

Post by toddhampson » Fri Mar 29, 2013 2:05 pm

Hi again everyone. I setup a slightly different circuit to test the NDRIVE without melting it. I used a simple circuit with a 9V battery, 450 Ohm resistor and a LED (2.1V Voltage drop). The circuit works fine by itself connect the positive and the LED lights. MY program is the same as above in the original post

So I purchased a new NDRIVE module and took tronixstuffs advise and wired it up as below
cutdown schematic v2.jpg
Cutdown schematic v2
First i put my voltmeter between G and S on the NDRIVE module and watch to see it go high when the program put pin2 high. It did as I expected in the LOW state it was reading 0.06V, when it was in the HIGH state it was reading 4.7V. SO that was what I expected.

When I connected the circuit in the above configuration, again I tested voltage on G & S and had the same result. However as soon as power was applied to the circuit the LED lit. I checked the voltage between D & S, when pin 2 (G) was LOW, the voltage between D & S was 0.9V

When pin 2 (G) went HIGH, voltage between D & S went to 0.0V and stayed there until pin 2 (G) went LOW again. Although all the time the LED was lit.

Here is a picture of the wiring.
v2 wiring.jpg
Photo of wiring
I checked it a few times, put it back to just the LED circuit and worked fine, rewired it with arduino, same thing. :(

Positive thing is I didnt cook the NDRIVE module although IM not getting much closer to getting my circuit working. Any advise or pointers from here to try next would be much appreciated ... :(


-todd-
CHNSA
http://hab.hampson.net.au

cef
Freetronics Staff
Freetronics Staff
Posts: 106
Joined: Wed Nov 09, 2011 12:53 am

Re: NDRIVE - Nichrome payload cutdown advice

Post by cef » Sat Mar 30, 2013 6:39 am

Edited. Original post quoted below:
Sounds like you're getting voltage leakthrough from the Drain (D) to the Gate (G), causing the FET to stay switched on.

Put a resistor between the Gate (G) and the Source (S), probably about 100k, and see if that helps.

Note: It's also advisable to put a 10 ohm resistor between the signal output of the Arduino (eg: Pin 2) and the Gate (G). The Gate (G) on any FET has a capacitance, so this limits in-rush current from the Arduino. It's not strictly necessary, but it's usually a good idea.
Ignore this. I just realised that the NDrive has an in-rush and Gate-Source resistor on it already, so it should not have any issue with this.

I've left it quoted however as this is standard fare for driving a FET directly, and could prove useful to someone.

toddhampson
Posts: 6
Joined: Mon Mar 18, 2013 10:06 am

Re: NDRIVE - Nichrome payload cutdown advice

Post by toddhampson » Sun Mar 31, 2013 1:46 am

Thanks cef.

I have replaced the NDRIVE with a single relay module and it works exactly as I want it to. So I will probably just continue with that solution for our cutdown. I will just have to test it at a low temperature to make sure the relay mechanicals dont bind up.

I noticed this modules is one of the few without a tutorial. Maybe someone at some stage could write up a tutorial on how to use it.

thank for your help

-todd-
CHNSA
http://hab.hampson.net.au

cef
Freetronics Staff
Freetronics Staff
Posts: 106
Joined: Wed Nov 09, 2011 12:53 am

Re: NDRIVE - Nichrome payload cutdown advice

Post by cef » Sun Mar 31, 2013 5:27 am

Welcome.

I'd have a crack at using the NDRIVE in the way you tried, to see what the issue is, but I don't have one to hand. Will see if I can arrange to try it myself and get back to you.

FWIW: If you have problems with the relay at low temp, I would suggest trying using a FET directly (eg: something like IRLB8743, FDP8870, STP55NF06L or even a BUZ71) with the resistor arrangement I mentioned. I've used all the above FET's in this arrangement with supply voltages of 24V with no issue.

toddhampson
Posts: 6
Joined: Mon Mar 18, 2013 10:06 am

Re: NDRIVE - Nichrome payload cutdown advice

Post by toddhampson » Sun Mar 31, 2013 10:35 am

Thanks cef,

I will see how the relay goes with the temperature tests and if it does have issues then I will surely have a look at those FET's you have so kindly mentioned.

If you do get a chance to test out an NDRIVE that would be great :)
Last edited by toddhampson on Sun Apr 07, 2013 8:21 am, edited 1 time in total.

toddhampson
Posts: 6
Joined: Mon Mar 18, 2013 10:06 am

Re: NDRIVE - Nichrome payload cutdown advice

Post by toddhampson » Sat Apr 06, 2013 3:29 pm

I just thought that I would post back to the group with an update.

I managed to get the N-DRIVE to work. I think it was probably just my bad soldering :oops:
I unsoldered the wires, cleaned up the connectors on the board and resoldered, then cleaned any flux away, then it worked the way it was supposed to first go.

I did put the LED before the resistor as per the first diagram and used the blink program to turn the LED on and off via the N-DRIVE mosfet. Worked fine.

I then replaced the LED and resistor with my coiled nichrome wire which has a resistance of approx 2 Ohms. Changed the blink program to 5 seconds on and 20 seconds off, then ran the program.

The N-DRIVE successfully turned the nichrome on and off glowing red each time (about 4 to 5 Amps through the nichrome circuit) .

Note that the negative from the battery running the nichrome circuit was connected to the earth on the eleven as per tronix recommendation. As in last diagram above.

Thanks for the assistance. I still think a tutorial on the freetronics site would be very helpful to others if someone gets a chance.

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