12V - 7.29A ac/dc converter project Need Help

Thread Starter

Butterworth

Joined May 6, 2009
135
Wait a minute - in the very first post of this thread, you gave a Vf @ 100mA - where did you get that 100mA specification from? And is that 100mA continuous, or 100mA for a very short duration?

Show us where you bought them; or better yet, supply a manufacturer's datasheet for the LEDs.
I got the info from here...

Have a look here:
http://club.dealextreme.com/forums/F...hreadid.611038

Listed the specs you mentioned, plus a response from dav850 who said he tested 200 of these LEDs.
Given the Vf is actually 1.42 not 1.2 as suspected before.
 
Last edited:

SgtWookie

Joined Jul 17, 2007
22,210
Ahh, ok. I seem to have 20 or so threads going on; I jump around so much I can't remember all of the details for all of the threads.

Somewhere about page 3, I had you measure your 12v 2a supply, and you mentioned that you measured 12.82v. Would you measure the voltage again, this time with a load on it? I want to see if the voltage changes with a load.
 

SgtWookie

Joined Jul 17, 2007
22,210
The way it's looking at the moment, if the supply comes down to 12v under load, then you can use 10 strings of 8 LEDs with ~6.3 Ohm resistors in each string to get ~100mA per LED. If the supply stays up at 12.82v, you'll need more like 15 Ohms per string.

7 strings will have one of the 1.43v and 7 of the 1.42v LEDs
2 strings will have eight of the 1.42v LEDs
1 string will have the remaining LEDs.
There will be 30mV difference between the highest and lowest Vf total.
 

Thread Starter

Butterworth

Joined May 6, 2009
135
BUMP, if anyone, can give me an idea for a PWM circuit to use in my project, please let me know, I am running out of creative juices trying to figure it out. Thanks.
 

Thread Starter

Butterworth

Joined May 6, 2009
135
Sorry, I've been preoccupied.

Have a look at the attached. Slight change of plans, but will still work with the things you have already.
Thank you for the update Sgt.Wookie.

If I read this correctly, it only an on/off switch, will it not work with the Low/Off/Hi switch? My original plan was to use a hi & low power to be able to toggle between the two, in case the light is too "bright" for some locations.

Also will any n-channel mosfet work? I do not recognize that model you noted.
 

SgtWookie

Joined Jul 17, 2007
22,210
Drat, I forgot all about the 3-way switch.

Yes, that could be done. You want off, an adjustable dim, and full brightness, right?
 

Audioguru

Joined Dec 20, 2007
11,249
Your hearing's sensitivity to loudness and your vision's sensitivity to brightness are not linear (half the current seems to be half the output) but instead they are logarithmic so the range from very high to very low is very wide.
Then half the power sounds only a little less and double the power sounds only a little more.
Half the current in an LED is a little dimmer and double the current is a little brighter.

10 times the power sounds twice as loud. 1/10th the power sounds half as loud.
You can calculate the LED brightness yourself.

33% of the current in an LED will make it appear dimmed a little. Try 10% instead.
 

SgtWookie

Joined Jul 17, 2007
22,210
Sorry - there's a lot of people needing a lot of help; things tend to fall through the crack.

Have a look at the attached. I didn't have a 3-position switch, so I used two SPDT switches. Don't let that throw you.

You'll only have one common switch terminal; represented by the left side of S1a and S1b.

The dim side is represented by S1a. When it is selected, the 555 timer runs, and the brightness is set via VR1.

The bright side is represented by S1b. When it is selected, the 555 timer is prevented from running by R15 pulling up the base of Q1, which keeps C1 from charging, which keeps the output of the 555 timer high, which means the LEDs are on all the time.
 

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Thread Starter

Butterworth

Joined May 6, 2009
135
Sorry - there's a lot of people needing a lot of help; things tend to fall through the crack.

Have a look at the attached. I didn't have a 3-position switch, so I used two SPDT switches. Don't let that throw you.

You'll only have one common switch terminal; represented by the left side of S1a and S1b.

The dim side is represented by S1a. When it is selected, the 555 timer runs, and the brightness is set via VR1.

The bright side is represented by S1b. When it is selected, the 555 timer is prevented from running by R15 pulling up the base of Q1, which keeps C1 from charging, which keeps the output of the 555 timer high, which means the LEDs are on all the time.
This looks awesome :) Thank you for the schematic!

I see what you mean about the switch.

There are zener diodes on the schematic after the switch? What V rating are they?

Thanks again for this, it is very much appreciated!
 
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