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MOSFET IRF540A circuit.

JPU

May 19, 2012
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Hi All

I have tried to create a simple circuit using a IRF540A mosfet. I have a PICAXE 08M2 in the circuit. The picaxe has a simple program that monitors the ADCVAL of pin 3. The pin is connected to a positive source via a home made touch switch i.e two screws that when you touch the screws the ADCVAL increases, Picaxe reads this increase and turns on or sets high pin 6. This is then attatched to the gate of a IRF540A mosfet. The source is connected to negative of the battery and the drain is connected to a buck/boost micro puck 2009sho which is connected to an LED which is connected to Positive.

All works well. The Picaxe works perfectly and so does the home made contact switch. The LED works and so does the MOSFET. However for some reason the LED will not burn as bright as it should it seams as though the mosfet is acting like a resisitor??? Its getting a little warm and if I remove the MOSFET by simply attaching the drain to negative the LED burns as bright as is expected.

I understand that the MOSFET is the problem here but please can someone explain why? and what should I look at in the TECH SPEC to overcome this,,,or do I need an alternative. My understanding of technical wording is very limited so please explain technical phrases. If someone can suggest an alternative Id be gratfull.

The system runs on 5V supply.

Thanks for you help.

Justin
 
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duke37

Jan 9, 2011
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The IRF540 has a threshold voltage of about 3V and needs considerably more than this to turn on fully. They work well on 10 or 12V.

You will need to find a FET which will turn on with 5V, they are available. Or use a resistor feeding a junction transistor.
 

JPU

May 19, 2012
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Hi

Thanks for your reply.

I ve had a look on the farnel site and I cant find exactly what I need. I know it sounds kinda lazy, but any chance you could give me a model number as I don't really know what Im looking for ie where to look on the SPEC sheet.

Thanks

Justin
 

CDRIVE

Hauling 10' pipe on a Trek Shift3
May 8, 2012
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Justin, the NDS7002A is a logic level FET but the data sheet lists max sustained current (Id) @ 280mA, which won't be suitable for you. So far, I haven't found a high current logic level FET. Perhaps another member knows of one? Your other option, as previously stated, is using a BJT.

Btw, regarding using the ADC input to sense finger resistance: I guess the concept is OK but I think Capacitive Touch Sense might be a better approach.

Chris
 

CDRIVE

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Ah, I just found the IRLD014PbF in my files. Check it out. It's a logic level FET that can handle 1.7A @ 25degC.

Chris
 

duke37

Jan 9, 2011
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I looked on the Farnell site fot the first time and selected 'logic level mosfet'
The STP40NF03L (30V 40A) with threshold of 1.7V may do.
 

JPU

May 19, 2012
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Hi Guys

Thanks for your replies I will give those a try...appreciated!

Justin
 

JPU

May 19, 2012
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Btw, regarding using the ADC input to sense finger resistance: I guess the concept is OK but I think Capacitive Touch Sense might be a better approach.

Chris

Hi Chris

As you probably read, Cocacola pointed out a flaw in my plan. "What if you are wearing gloves" he asked:p. An important aspect I had failed to take note. However I have had a read of the wiki on Capacitive Touch Sense and am I right in thinking, "rubber gloves" will not make a difference and Capacitive Touch Sense is the way I should go?:confused:

Ps I started a new thread for this topic so that the other thread would not become cluttered even though somehow, I knew you would have the answer I needed.;)

Thanks

Justin
 

CocaCola

Apr 7, 2012
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am I right in thinking, "rubber gloves" will not make a difference and Capacitive Touch Sense is the way I should go?:confused:

Gloves will make a difference with capacitive switches (especially most if not all DIY designs), some new commercial designs are much more tolerant but just the same the glove does cause potential issues, even when they have conductive material embedded in them...

As novel as they are, IMO they are not all that practical for you planed implementation vs tactile switches...
 

CocaCola

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Look over Ebay there are some generic ones, and honestly designing your own isn't overly complicated, the membrane can be printed very reasonable with your own graphics and then it's just a matter of making a matching PC board and attaching the 'metal dome' that goes under the membrane, many dome com with the adhesive sheet attached already making it pretty painless...

Remember these will be momentary switches so you need something like the micro to latch them...
 

CDRIVE

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Justin, are we talking about thin surgical latex type gloves?

Chris
 

JPU

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Justin, are we talking about thin surgical latex type gloves?

Chris, yes, any other type would prevent the operator from feeling the condition of the horses teeth.

Justin
 

CDRIVE

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I think CocaCola's 'membrane switch' suggestion is a good one. They've been used in marine electronics for many years now.

Chris
 

CDRIVE

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Justin, what's the battery supply voltage for this head lamp?

Chris
 

JPU

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Chris, Its a 3.7v Lipo with 5000mAh. I run it through a bucktoot which ups the supply voltage to around 7V and limits the Current to 500mAh.

Its a bit of a fiddle to do the electrics as the 2 * LEDs that I use are in two separate enclosures (ie 1 LED per Module) The light can be 1 on or 2 on or all off so I use a double pole double throw switch with on - off - on. Its a pain to wire them up (a real fiddle). I was hoping to simplify and improve the circuit by using a PICAXE chip. I thought the user could then step through the settings, ie off -> 1 On -> both On ->off

However, If I use a picaxe I will have to find a way to isolate the Bucktoot when not in use as it must have a load when on. The DPDT switch allows me to turn off the source as well as choose 1 on or both on. The reason for the ability to choose 1 or 2 lights on was to save power, however the light has been such a success that most people who have bought one say they use both lights and still get days of light!! The next best on the market last only upto 3Hrs?? I sold one of mine and the girl said she charged it only 6 times in 6 months and used it every day to do about 10 horses!!!

Regards

Justin
 
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CDRIVE

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Sometimes..., actually, most times, KISS is preferable. I would imagine a toggle switch is easy to find on your head. ;)

Chris
 

JPU

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CocaCola

I think we have come up trumps here with your idea of a membrane switch. The goods turned up today and look perfect. I will be using this in future builds and will also be using it on the step up/down push button circuit that Chris devised for me. (The brushless motor controller)

Great call, Sir.;)

Here is a pic.

Thanks

Justin
 

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CocaCola

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CocaCola

I think we have come up trumps here with your idea of a membrane switch. The goods turned up today and look perfect.

Good to hear, just remember it's a momentary switch so you need to use the micro to latch it and control functions... Learn to put the micro into deep sleep with wake on port change interrupt when the button it pressed, the battery consumption of the micro in deep sleep waiting for that button press (if properly put in deep sleep) will be in the same area as the batteries self discharge just sitting there by itself so in the end it has very little effect on the battery life...
 

CDRIVE

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If you have any trouble putting the AXE to sleep (I doubt you will) you could consider using the ubiquitous CD4017. Very versatile and draws very little current. It can easily step through OFF - LED1 ON - LED2 ON - LED1 & 2 ON - OFF.

Chris
 
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