SMALL - espritmodel.com SMALL - Telemetry SMALL - Radio
Reply
Thread Tools
Old Jan 27, 2004, 10:29 AM
Registered User
QLD, Australia
Joined Sep 2003
1,552 Posts
Josh,
Both static and heat can damage fets , if you cant find any other reason then maybe it is the fet .
What are you powering the Tx with ? if the battery isnt capable of delivering the required current the signal will suffer .
I have 6 off 800ma NiMh pack in mine .
Stewart
clipclop is offline Find More Posts by clipclop
Reply With Quote
Sign up now
to remove ads between posts
Old Jan 27, 2004, 10:47 AM
Registered User
Canada, NB, Fredericton
Joined May 2002
941 Posts
Josh,

I can't view your images. I get redirected to the server "phvrf.yahoo.com" which doesn't appear to exist.

D.
darkith is offline Find More Posts by darkith
Reply With Quote
Old Jan 27, 2004, 02:27 PM
Thats NOT indestructible
j8m8l's Avatar
London UK
Joined Apr 2002
1,448 Posts
Sorry guys.

The pics should work fine now, I was having trouble hosting them with Yahoo.

Stewart.

It is running from a 700mah NICAD, this should be able to handle the current of the 5 LED's.

Could one damage the mosfet in such a way that it still worked, just not fully/properly?

Josh.
j8m8l is offline Find More Posts by j8m8l
Reply With Quote
Old Jan 27, 2004, 03:48 PM
Registered User
Canada, NB, Fredericton
Joined May 2002
941 Posts
You're not using a 12F6XX for the TX are you?

Some nice work btw.

D.
darkith is offline Find More Posts by darkith
Reply With Quote
Old Jan 27, 2004, 04:12 PM
Thats NOT indestructible
j8m8l's Avatar
London UK
Joined Apr 2002
1,448 Posts
Thanks.

Thats the IR Transmitter v.2 (attached to a Sanwa VG600, I have just bought a RD8000 for when I get it all working ).

I was hoping that the I could avoid the excess resistors and transistors by using a mosfet (was a wrong !).

Josh
j8m8l is offline Find More Posts by j8m8l
Reply With Quote
Old Jan 27, 2004, 06:16 PM
Registered User
jberg's Avatar
MSC Rödinghausen, Germany
Joined Feb 2001
954 Posts
Josh,

you can use one mosfet for all leds, but you still have to use one current limiting resistor in each line of leds!

Regards, Jochen
jberg is offline Find More Posts by jberg
Reply With Quote
Old Jan 27, 2004, 08:57 PM
Registered User
Canada, NB, Fredericton
Joined May 2002
941 Posts
Odd, I didn't. I have 8 LEDs in a 1x8 parallel array attached to an IRFD123 mosfet with no limiting resistors (other than 1 between the PIC and gate pin). Average (measured) current draw is only 30 mA @ 9.6v (tho that may be a bit higher at this high a frequency). I believe Koichi mentioned that the LEDs are off 99+% of the time, which would somewhat explain the low averages I'm measuring. At the current and duration, the peak current should be well below the LEDs peak capabilities. No burnouts according to my digital camera...

Has anybody else measured the current draw on their LED array? The IRFD123 has an Rds of 0.4ohm, so I don't think that's limiting current flow. Vg is 4v, but the PIC should be outputing square pulses, so it shouldn't matter.

Odd.

D.
darkith is offline Find More Posts by darkith
Reply With Quote
Old Jan 27, 2004, 10:04 PM
Registered User
KevinMc's Avatar
Hamilton
Joined Oct 2003
29 Posts
Hi all,

I've been lurking in here for a few months now and just recently got off my duff and started building. Having just finished off my transmitter, I guess I'm just about at the same point as Josh (j8ml8). I also noticed a drop in voltage on the rails of the LED board (which I separated from the encoder section). I eventually clued into the fact that this was probably due to chokes in the lines of the trainer port of the Tx that were reacting to the immediate demand for current when all the LED's were switched on. (Good design for EMI, bad for us.) My recourse was to put a BFC across the power rails on the LED board and it made a world of a difference. (Attached is an after shot of my rails.)
KevinMc is offline Find More Posts by KevinMc
Last edited by KevinMc; Jan 27, 2004 at 10:21 PM.
Reply With Quote
Old Jan 27, 2004, 10:06 PM
Registered User
KevinMc's Avatar
Hamilton
Joined Oct 2003
29 Posts
Here's the Tx setup, not the BFC hanging off the LED board.
KevinMc is offline Find More Posts by KevinMc
Reply With Quote
Old Jan 27, 2004, 10:19 PM
Registered User
KevinMc's Avatar
Hamilton
Joined Oct 2003
29 Posts
And another scope grab of the voltage across one (of five) LED current limiting resistor, which on my board are only 0.5 ohms. Although almost all of what I've built has come straight from Koichi's web pages, I found that the current limiting resistors specified were consuming too much power, and as a result I dropped the value to get a reasonable peak current flow. Calculations based on the attached plot indicates that I'm seeing a peak of 700ma through each LED branch. (My network is a 5p x 5s array, with a current limiting resistor for each series branch.)

The one thing that I'm noticing is that my array now gets warm. (No doubt from the increased current flow.) Has anyone else experienced this?

BTW, very large "Kudos" to Koichi, Andy, and all the rest of you that have contributed so much to this thread.
KevinMc is offline Find More Posts by KevinMc
Reply With Quote
Old Jan 28, 2004, 05:37 AM
Thats NOT indestructible
j8m8l's Avatar
London UK
Joined Apr 2002
1,448 Posts
Jochen.

I do have one resistor for each series "line" of LED's !

KevinMc.

That looks like the problem I have.
I will try a 220uf across the power lines for the array (BFC?)
I will measure the current with the 10ohm, and adjust from that.

Thanks
Josh.
j8m8l is offline Find More Posts by j8m8l
Last edited by j8m8l; Jan 28, 2004 at 05:57 AM.
Reply With Quote
Old Jan 28, 2004, 07:55 AM
Registered User
KevinMc's Avatar
Hamilton
Joined Oct 2003
29 Posts
Josh- BFC: you got it!
KevinMc is offline Find More Posts by KevinMc
Reply With Quote
Old Jan 28, 2004, 08:09 AM
Thats NOT indestructible
j8m8l's Avatar
London UK
Joined Apr 2002
1,448 Posts
Thanks Kevin.

I will solder this up when I get home and hope for the best !
BFC - Big Fat Cap ?!?

Josh
j8m8l is offline Find More Posts by j8m8l
Reply With Quote
Old Jan 28, 2004, 08:46 AM
Registered User
KevinMc's Avatar
Hamilton
Joined Oct 2003
29 Posts
Josh,

The "F" has so many possibilibities, but you're on the rignt track...
(Freakin' was always my preference.)

KM
KevinMc is offline Find More Posts by KevinMc
Last edited by KevinMc; Jan 28, 2004 at 11:22 AM.
Reply With Quote
Old Jan 28, 2004, 02:11 PM
Thats NOT indestructible
j8m8l's Avatar
London UK
Joined Apr 2002
1,448 Posts
I have added the cap and tested again (with the proto rx).
Reception still seems erattic.
I will scope it tomorow (If I get to school, its snowing and the last time that happened London shut down !).
Would'nt the simplist solution be to tap power straight from the battery (Y lead or similar ?).

Josh
j8m8l is offline Find More Posts by j8m8l
Reply With Quote
Reply


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
airwars "infra-red combat system for planes" Tree_Magnet Power Systems 15 Jan 08, 2008 10:41 PM
Discussion DIY conversion to infra red Tom Harper Aerial Photography 46 Jun 25, 2006 06:43 PM
Infra-Red Thermometer for Helis rscamp Electric Heli Talk 7 Jan 08, 2002 07:11 AM
airwars "infra-red combat system for planes" Tree_Magnet Foamies (Kits) 4 Aug 02, 2001 01:15 AM
Infra red transmitter and receiver for micro size aircraft Ron MIller Power Systems 2 May 28, 2001 10:51 AM