Print Page - Drop Voltage with MC7815CT

Title: Drop Voltage with MC7815CT
Post by: Arn C. on June 08, 2004, 10:06:05 AM

Howdy All !!!
I have a 20 VDC Wall Wart, I want to drop the voltage to 15VDC. I have an MC7815CT 15V voltage Regulator. How can I hook this up in line with the 20 VDC line to get 15VDC. DO I need any caps or anything or just in with the +20vdc and out with the +15vdc and ground middle leg to -20 vdc line?

(Looking at the regulator with pins down, left to right= Vin, Gnd, Vout)

or

Could I just put a resistor in series with power line to drop the voltage to what I need? If so, any clue what size resistor I would need?

Thanks for your help in advance!
Peace!
Arn C.

Title: Drop Voltage with MC7815CT
Post by: Arn C. on June 08, 2004, 11:30:40 AM

Anyone?

Arn C.

Title: What's the input
Post by: petemoore on June 08, 2004, 11:47:38 AM

Input rating for voltage on that regulator???
As long as you follow the input requirements, and want to go from 20v [actual] to 15 VDC...
Not being the 'regular regulator' user, I can't really say...but I think the only consideration would be heat dissipation on the regulator itself, and that shouldn't be a problem, check it anyway...quicky or you may have a 'tatoo'.

Title: Drop Voltage with MC7815CT
Post by: toneman on June 08, 2004, 11:53:39 AM

these R really easy.

here's a link to the data sheet

http://www.national.com/pf/LM/LM7815C.html

notice it recommends a LM340 for new designs.

max input V is +35VDC.
U need at least 2V higher that the output.
20V with a BridgeRecto should do fine.
Use large input cap, say 100mfd.
Only need bout a 1 to 10mfd cap on output.

really easy
staytuned
tone

Title: Drop Voltage with MC7815CT
Post by: niftydog on June 08, 2004, 09:36:22 PM

Quote20V with a BridgeRecto should do fine.
Use large input cap, say 100mfd.
Only need bout a 1 to 10mfd cap on output.

It's already DC, but none of these additions would hurt. The bridge will drop the voltage a bit and lessen the power disapation on the reg. The cap will smooth the ripple.

Title: Drop Voltage with MC7815CT
Post by: toneman on June 08, 2004, 10:04:19 PM

Right U R, Nifty.
Don't really need the diodes if it's already DC.
Why waste power(?)
Needs to have at least 1mfd cap on output of reg.
We're talking low amps here.
tone

Title: Drop Voltage with MC7815CT
Post by: Peter Snowberg on June 09, 2004, 02:19:59 AM

For even more stability.... I would use a slightly larger cap at the input like 1000uF and then add a 0.1uF in parallel to the 1uF Toneman just recommended. The tiny cap on the output can be the most important one of all in terms of keeping the noise low.

Take care,
-Peter

Title: Drop Voltage with MC7815CT
Post by: Arn C. on June 09, 2004, 08:22:44 AM

It appears that I may be good to go on this now. Thanks to all of you!
I really appreciate all your selfless help. You guys are too cool !!!

Peace!
Arn C.

QuoteAh man, This morning when I woke up I could feel Tension mounting.....................Tension's my dog ! :lol:

DIYstompboxes.com

DIY Stompboxes => Building your own stompbox => Topic started by: Arn C. on June 08, 2004, 10:06:05 AM