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46  Using Arduino / Motors, Mechanics, and Power / Re: MOSFET for DC motors on: January 22, 2013, 04:39:28 am
PWM has switching of many times per microsecond.

What if one were to use a transistor and switch at say 100ms ?

Currently I plan to put it together with relays with a 20 second timebase using a percentage of time as the driving time for relays limited to 1 second on / off.

But MOSFETs would be better for driving them at a lower current.
So, if I drove them in software with a transistor such as:
digitalWrite(fan1, HIGH);
delay(100);
digitalWrite(fan1, LOW);
delay(900);

(The fan takes longer than 1 second to slow down smiley-wink)

This would be much slower than the analogWrite function yet still give reasonable results in driving the fans.
Is that workable, or is the current consideration in using the gate driver a problem?
Transistors are like 800mA, and gate drivers around 1A (I can't find a store in Au selling 6A gate drivers), in fact - this is the only gate driver I can find:
http://www.jaycar.com.au/productView.asp?ID=ZK8878
But it's 1A, I have 800mA transistors already!

I've already got the MOSFETs I plan to use:
http://www.jaycar.com.au/productView.asp?ID=ZT2468
47  Using Arduino / General Electronics / Re: Interfacing to circuit which grounds the input on: January 22, 2013, 12:51:53 am
What size pull up resistor ? I'm thinking 1k.. (just enough to hold it high so the ECU gnds it)
48  Using Arduino / General Electronics / Re: Interfacing to circuit which grounds the input on: January 21, 2013, 03:42:14 am
I set it up the first way, that is to continuously count the number of times the pin is high out of a fixed number (3000). This appeared to work really good and smooth out any problems!

It works better than pulseIn, as the time interval is dependant on knowing the timebase, whereas merely counting the number of times in a fixed number gives you a good percentage.
Thanks heaps for that tip - worked really well.
49  Using Arduino / General Electronics / Re: Interfacing to circuit which grounds the input on: January 19, 2013, 06:56:54 am
What I'm concerned about is if the frequency is too fast, then I won't catch the transition I need to.
i.e. if I loop 20 times, reading the pin and for those 20 times I read that the input is low, but what happens is between the read for read 2, and read 3, it transitions and back again i'll potentially miss it?

How do I know what PWM period is to read 'larger' than it?
I think PWM period is a important bit of info - i.e. if the PWM signal was 10%, and the period was 10 seconds, the first 1 second would be low. I don't know what that PWM period is.
50  Using Arduino / General Electronics / Re: Interfacing to circuit which grounds the input on: January 19, 2013, 01:42:40 am
I monitored the output and tested some knowns. When I run the A/C compressor this runs the fans at 50% from previous tests.

I tried to monitor the PWM signal using this line:
  value = pulseIn(pulsepin, HIGH, 1000000);

I got values such as 229, or 99.
What happens tho is when I turn the A/C compressor on, it reads 0!
Not believing that - I metered it and saw 4.9V.

I need a way of doing it such that I can differentiate between line idling high, and line remaining low and allowed to go high.
I know that when the fans are not needed, the wire reads 0V (so I think the ECU continually drives it to 0V when it wants no fan - which makes sense, wiring harness gets damaged then it should remain '5V'...

I like '1' better - any thoughts on a circuit for that design (i.e. smooth the pwm to a voltage and feed to analog in).. This would give me great proportional control.
51  Using Arduino / Motors, Mechanics, and Power / Re: MOSFET for DC motors on: January 16, 2013, 07:19:49 am
I had using a simple transistor in mind.

In my travels the reason for a driver IC is to sink current fast - why not chuck a transistor in ? Not fast enough ? Not enough current (and if current - how do I calculate current required for gate)?

I can drive the MOSFET using batt voltage, and if I use 2 per fan (4 total), do I need two driver circuits total, one for all 4 or one each? (I guess current dictates that if current is the limiting factor)..
52  Using Arduino / Motors, Mechanics, and Power / Re: MOSFET for DC motors on: January 16, 2013, 06:30:40 am
Am I right with the following:
The datasheet shows Maximum permissible Power Dissipation of 330W.
The power dissipation for a MOSFET with RDS on specified as 0.0055 (5.5milliohms)  at 30A is (30 x 30) * 0.0055 = 3.43W
As 3.43W is below 330W this does not require a heat sink?

Seems too simple and given the expected current - I would think it needs one - but the above says not so much.

I'm working on the theory that I'll oversize the MOSFET and simply use a 169A MOSFET. As I am switching it,  and the current will always be below 169A - there won't be an issue in sizing.
The datasheet is here if it helps:
http://www.jaycar.com.au/productView.asp?ID=ZT2468
I'm looking for confirmation that using a 30A rated MOSFET such as IRF540N would not be better (I don't think it is because of greater RDSon) - but is there any other reason you'd not want to use a higher current rated MOSFET?

One for each fan.
53  Using Arduino / General Electronics / Re: Interfacing to circuit which grounds the input on: January 13, 2013, 04:39:11 am
Ahh and that timing could indeed be critical to operation.
This would mean that to take it over correctly, I must first monitor it's current operation.

I can get data from it using the diagnostics port, but the information I get there is simply the duty cycle ("128 = 50%").

To monitor the signal (with no oscilloscope) and time the interrupts to get the true time base for the PWM - can I join two signals to the one ECU output?
i.e. Arduino pin 2, held high with a resistor (1k !) to 5V, on from there to the ECU pin.

And then both the arduino and fan controller get the PWM signal?
54  Using Arduino / General Electronics / Re: Interfacing to circuit which grounds the input on: January 13, 2013, 04:05:26 am
Bugger, I exported it and forgot!
55  Using Arduino / General Electronics / Interfacing to circuit which grounds the input on: January 13, 2013, 03:51:09 am
I'm want to interface to a circuit which grounds the input.
What it expects is the arduino to hold the line at 5V, and then it has a transistor or similar to ground that 5V signal to give a percentage of speed.

It's drawn as attached - the transistor at the top (ECU) takes the 5V held high signal and grounds it proportionally to the speed it wants to drive the two fans at.

My intent is to remove the fan controller, but obviously maintain interaction with the ECU for it's desired fan control.
To do that based on the image, common the grounds between the two controllers, hold the line high at 5V with a resistor sized to limit current (say 5mA) - 160ohm or so. Then the ECU will ground that signal and so vary the reading the arduino sees on the analog pin - right?

So by design, if the arduino ADC Is then reading 4096 - it's expecting 0% fan output. If it's reading 0, then 100% fan output - or have I got the circuit concept wrong?
When I metered V on it today, with the fan controller unplugged it read 0.03V, and as it reached higher temps and wanted cooling, the V dropped to -0.01V.
56  Using Arduino / Motors, Mechanics, and Power / Re: MOSFET for DC motors on: January 12, 2013, 09:53:21 pm
Using relay logic with two fans could I not have both fan outputs tied together to produce a medium fan setting (in series).
And then parallel for full speed?

I don't think the current controller is PWM as such as it's Medium and "High".
If I can electrically make them medium / high that'd be just as good...

Although the MOSFET idea is valid, I'm still not getting the current requirements right I think.
57  Using Arduino / General Electronics / Re: Single Supply Op Amp / Voltage Follower on: January 10, 2013, 11:31:54 pm
I'm sure I can forgive an error of 0.1V if the input is 5, and output is 4.9.
58  Using Arduino / General Electronics / Re: Single Supply Op Amp / Voltage Follower on: January 10, 2013, 03:36:35 pm
It needs to be single supply too - I thought TLV2371 was a good pick?
Why not?
59  Using Arduino / General Electronics / Single Supply Op Amp / Voltage Follower on: January 10, 2013, 06:30:36 am
I'm looking at the TLC2272 (or alternatives). What I need is a single supply Op Amp, capable of operating at 5V, and able to Follow a voltage from gnd to +5V.

Looking at the TLC2272 datasheet - it's noted the recommended operating condition is to VDD - 1.5V (so I can only expect an input of 3.5 at the output in voltage follower mode)?
I'm looking for it to have a high input impedance (no interference on circuit being monitored).

I think I can see the TLV2371 op amp as doing it - the datasheet shows that the common mode input voltage range is 0 to VDD.
I take it to mean that if I power it with a single 5V supply, and place 5V on the input, and got the DMM, I'd expect to read 5V on the output.
I think 1000Gohms means 1000 giga ohms of input resistance - so this would mean it would not load the circuit being monitored, therefore ensuring it is as if a DMM was monitoring it..

Am I right with that?
60  Using Arduino / Motors, Mechanics, and Power / Re: MOSFET for DC motors on: January 10, 2013, 02:06:14 am
That's interesting, and perhaps introduces a design issue.

So in terms of program control I don't want to say - analogWrite(MOSFET, 127) for an hour?

I'm thinking of two MOSFETs to spread the amp load between the two of them to avoid the current issue - but from what you've noted the best MOSFET is the one sized to be fully on at the fan maximum ?
Is it a good idea to run a MOSFET at 100% for 2 hours (since it acts like a switch in that state)?
i.e. Is it better for heat reasons to use a large MOSFET and switch it at 50% duty - or is that still bad for it?

I'll take a look for a high current gate driver, but this is still all new to me - any links ?
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