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codingplanet
Mad Scientist
United Kingdom
195 Posts |
 Posted - May 24 2011 : 6:01:15 PM
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I started making a USB contraption that would report temperatures and voltages to a PC and also alter fan speeds with PWM. It's my first time using a PIC 18F and also my first time working with USB but it's gone well so far.
Temperatures will be read with thermocouples but I've yet to arrange the hardware to interface them to the PIC. I've done it before with a 16F + LCD but I used an all-in-one chip that costs about £20 each. This time I think I will implement coldpoint compensation and such myself along with an instrumentation amplifier for the main amplification. Something I've yet to think about properly.
The firmware was written with Microchip's C18 and is based off one of Microchip's HID examples. The software is written from scratch by me using VB.net.
The reason behind this is that I have 2 very high speed fans (rated at 3A, 190CFM) on the radiator in my watercooling setup and I would like to run them at a lower speed most of the time but be able to ramp them up when needed.
Also when I'm benching with LN2, sometimes 2 temp meters and 4 voltmeters are needed and the desk gets a little cramped :P This would also give me the ability to log the data if I wanted to.
Thread can be found on my site here: http://www.benchtec.co.uk/forums/threads/8170-Owl-s-Contraptions |
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wasssup1990
Nobel Prize Winner
A Land Down Under
2261 Posts |
Posted - May 24 2011 : 10:47:03 PM
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| Cool little contraption you've got there. |
When one person suffers from a delusion it is called insanity.
When many people suffer from a delusion it is called religion. |
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codingplanet
Mad Scientist
United Kingdom
195 Posts |
Posted - May 25 2011 : 04:46:40 AM
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Thanks Johnny. I wonder if anyone here can recommend me a fast switching diode to clamp the back emf? I was using a 1N4001 when I had a fan connected and it was getting warm (not hot), but I guess it wasn't fully switching on during the low period of the PWM pulse.
And can someone tell me how I would go about defining the specs of such a flyback diode? Such as how do I know how fast I need it to switch? Should I just choose the switching time to be a lot smaller than the minimum PWM low period that I could have? And what current rating do I need to use.
The PWM switching is done using a MOSFET from the PIC pin. Load is a 3A rated fan, continuous current is measured to be under 2.5A, I don't know what the surge current could be. PWM frequency is 30KHz. |
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wasssup1990
Nobel Prize Winner
A Land Down Under
2261 Posts |
Posted - May 25 2011 : 08:54:17 AM
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The 1N4001 is just a low spec general purpose diode. It isn't suitable for high speed applications.
http://www.aaroncake.net/forum/topic.asp?TOPIC_ID=8771
A Schottky diode practically doesn't have any reverse recovery time. Their forward forward voltage drop is usually higher than their "Ultra Fast" and standard Si diode counterparts though. |
When one person suffers from a delusion it is called insanity.
When many people suffer from a delusion it is called religion. |
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codingplanet
Mad Scientist
United Kingdom
195 Posts |
Posted - May 25 2011 : 12:29:22 PM
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I realise the nature and unsuitability of the 1N4001, I just used it because I have lots of them. The only other diodes I have are 1N4148 wish I guess would be worse given their much lower current rating (albeit faster switching).
Forward voltage drop should not concern me though right? As I am using it only as a flyback diode.
Do you suggest that I use a Schottky diode then? Should I take care of it's reverse recovery time? Or can it be ignored as you said it "practically doesn't have any reverse recovery time"?
Thanks |
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wasssup1990
Nobel Prize Winner
A Land Down Under
2261 Posts |
Posted - May 25 2011 : 8:12:10 PM
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| Can you quickely draw up a schematic? |
When one person suffers from a delusion it is called insanity.
When many people suffer from a delusion it is called religion. |
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codingplanet
Mad Scientist
United Kingdom
195 Posts |
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wasssup1990
Nobel Prize Winner
A Land Down Under
2261 Posts |
Posted - May 27 2011 : 03:34:02 AM
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Just looking at datasheets I can deduce:
1) At least put a small heatsink on your MOSFET.
2) Your rise and fall times on the gate appear to be adequate, with assumptions.
3) Good to see a gate sinking resistor. Resistor value is adequate.
4) MOSFET is suitable for load.
5) You may be cutting a little fine with a VGS(th) max of 4V, assuming a signal of 0-5V. Logic-Level MOSFET recommended if that's the case.
6*) Your flyback diode is inadequate. Reason: trr parameter is too large (or isn't specified).
*Solution:
Choose a Schottky, or Silicon Carbide Schottky rectifying diode. Other specifications should be easy to decide such as VF, VR, IF and IFpeak(repetitive). Choose a diode with a VR >= 15V and a VF as small as possible. IF(max) doesn't have to be huge; you may be able to get away with 100mA. 500mA may be enough for IFpeak(repetitive).
Good luck.
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When one person suffers from a delusion it is called insanity.
When many people suffer from a delusion it is called religion. |
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codingplanet
Mad Scientist
United Kingdom
195 Posts |
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wasssup1990
Nobel Prize Winner
A Land Down Under
2261 Posts |
Posted - May 27 2011 : 10:13:00 AM
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| You're welcome. It looks like you take this over-clocking thing pretty seriously. |
When one person suffers from a delusion it is called insanity.
When many people suffer from a delusion it is called religion. |
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codingplanet
Mad Scientist
United Kingdom
195 Posts |
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wasssup1990
Nobel Prize Winner
A Land Down Under
2261 Posts |
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codingplanet
Mad Scientist
United Kingdom
195 Posts |
Posted - Jun 01 2011 : 09:32:13 AM
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| lol that sounded rather sarcastic. The back emf only generates a small current then? I was told elsewhere to use a diode rated for 6A continuous current as my fan is 3A rated. |
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codingplanet
Mad Scientist
United Kingdom
195 Posts |
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wasssup1990
Nobel Prize Winner
A Land Down Under
2261 Posts |
Posted - Jun 01 2011 : 10:42:09 AM
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quote:
Originally posted by codingplanet
lol that sounded rather sarcastic. The back emf only generates a small current then? I was told elsewhere to use a diode rated for 6A continuous current as my fan is 3A rated.
The back EMF generates a high current through the diode but only for a very short amount of time (a spike). The purpose of the diode is to clamp the spike to prevent potential damage to other circuit components (this is critical in a computer).
You are not going to get 6A of continues current, you are only going to get a small fraction of that if you take the average back EMF current through the diode over time. So yes, over time the average continues current that flows through the diode is small. The current that will be operating your fan is obviously not going through the diode so the 6A continues forward current spec is overkill. The repetitive peak forward current is what's more important to your application.
I know the repetitive peak forward current isn't specified in the datasheet but a "Non-Repetitive Peak Forward Surge Current" as the datasheet puts it, of 5.5A is a good indication that this diode will handle the back EMF without a sweat.
quote:
Originally posted by codingplanet
Would this one be better? http://uk.farnell.com/taiwan-semiconductor/b0520lw/diode-rectif-20v-0-5a-sod123/dp/1863141
Lower reverse breakdown voltage and lower forward voltage.
Yes that one would still work just fine. The reason why I said "Choose a diode with a VR >= 15V" is so you could have some headroom above your 12V supply, but 14V is still enough.
I hope this clears things up for you.
Ah! I'm so busy.
I've got an assignment due in a few hours which I am submitting now and after that I have to give a presentation in Artificial Intelligence. Then over the next few weeks I would have completed a grand total of 14.5 hours worth of exams. Fcuk me!  |
When one person suffers from a delusion it is called insanity.
When many people suffer from a delusion it is called religion. |
Edited by - wasssup1990 on Jun 01 2011 10:54:04 AM |
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codingplanet
Mad Scientist
United Kingdom
195 Posts |
Posted - Jun 01 2011 : 10:55:46 AM
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| Thanks a lot for the help Johnny. I'll go for the one with higher VR. |
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USB contraption
