How to reduce DC voltage ?

sunny · 13th May 2011, 03:41 PM

I need around 20 to 25 volts for using 7815 and 7915 regulators for powering TL02 preamplifier IC.My power amplifier voltage is between 38 to 40 volts. Since 7815 and 7915 can withstand maximum upto 35 volts only.
I want to reduce 40 volts DC to around 20 to 25 volts so that i can use 7815 and 7915 safely.So what resistor i should use to reduce the voltage?Please give me the value of the resistor and it's wattage.
Your help is highly appreciated.

Thanks,
Sunny.

jackand08 · 13th May 2011, 04:10 PM

Assuming the amplifier power supply voltages quoted are no-load voltages and we will drop the voltage to around 30V (safe), then:

Voltage dropped by resistors=10V (working off a 40V supply)...
...and the current required is around 100ma (plenty for any preamplifier circuit), then:
R=V/I so... R (ohms)=10/0.1=100 ohms dropping resistor.

The power dissipated (in watts) by the resistor is determined by: V^2/R...
...so: 10^2/100=1 watt-I would pick a resistor of around 2 or 3 watts for a safety margin.

discrete · 04:50 PM

Use LM317/337 in place of the 78/79.
They are good for 40V differential (Vin - Vout). In your case the differential is 'only' 25V.
Also better performers than the 78/79 series.
Depending on the load current, the regulators may need big heat sinks due to the 25V drop across the regulator.

Other easy option is a zener shunt regulator if the current requirement is low.

sunny · 13th May 2011, 05:09 PM

If i use LM 317/337 then what should be the value of the resistors to get 15 volts ?I don't like using variable resistors by adjusting output voltage.Please let me know the value of the resistors.

Thanks,
Sunny.

tomchr · 13th May 2011, 05:13 PM

Sunny, you can calculate the resistors from the equation in the datasheet. Set the resistor between the output pin and the adjust pin to R1 = 120 ohm and calculate the resistor to ground = R2.

Vout = 1.25*(1+R2/R1)

~Tom

sunny · 13th May 2011, 05:21 PM

Can you please suggest suitable standard value resistors for getting 15 volts.The input voltage i'm getting around 38 to 40 volts.My current r
equirement is 1amp.

Sunny.

DF96 · 13th May 2011, 07:12 PM

tomchr has read the datasheet for you and given you the formula. Are your calculator batteries flat?

Juergen Knoop · 13th May 2011, 07:23 PM

Quote:

My current requirement is 1amp.

results in a little bit to much heat for TO-220 regulators. Maybe you need to re-think your current requirements? What do you really need?
regards

jackand08 · 13th May 2011, 07:28 PM

Quote:

My current requirement is 1amp.

Then the resistor value I gave you previously will have to be re-worked. Are you really sure you need 1A? Even a preamp with a few opamps will be quite happy with 50ma supply current. The only ill-effect of drawing a higher current than calculated for with the resistor dropper method is that the resistors will produce more heat and drop more voltage (good in most circumstances).

discrete · 13th May 2011, 07:59 PM

Standard value for R1 is 240 Ohm (but 220 is closest E12 series value).
R1=220 and R2=2.2k+220 gives you 15 V.

But forget about it if you really need more than about 100 mA...

Quote:

Originally Posted by tomchr

Sunny, you can calculate the resistors from the equation in the datasheet. Set the resistor between the output pin and the adjust pin to R1 = 120 ohm and calculate the resistor to ground = R2.

Vout = 1.25*(1+R2/R1)

~Tom

13th May 2011, 03:41 PM	#1
sunny diyAudio Member Join Date: Oct 2004	How to reduce DC voltage ? I need around 20 to 25 volts for using 7815 and 7915 regulators for powering TL02 preamplifier IC.My power amplifier voltage is between 38 to 40 volts. Since 7815 and 7915 can withstand maximum upto 35 volts only. I want to reduce 40 volts DC to around 20 to 25 volts so that i can use 7815 and 7915 safely.So what resistor i should use to reduce the voltage?Please give me the value of the resistor and it's wattage. Your help is highly appreciated. Thanks, Sunny.

13th May 2011, 04:10 PM	#2
jackand08 diyAudio Member Join Date: Oct 2008 Location: Wild North West...of England	Assuming the amplifier power supply voltages quoted are no-load voltages and we will drop the voltage to around 30V (safe), then: Voltage dropped by resistors=10V (working off a 40V supply)... ...and the current required is around 100ma (plenty for any preamplifier circuit), then: R=V/I so... R (ohms)=10/0.1=100 ohms dropping resistor. The power dissipated (in watts) by the resistor is determined by: V^2/R... ...so: 10^2/100=1 watt-I would pick a resistor of around 2 or 3 watts for a safety margin. __________________ Jack "Near enough is good enough, so good enough is right" My Site

13th May 2011, 04:46 PM	#3
discrete diyAudio Member Join Date: Sep 2010	Use LM317/337 in place of the 78/79. They are good for 40V differential (Vin - Vout). In your case the differential is 'only' 25V. Also better performers than the 78/79 series. Depending on the load current, the regulators may need big heat sinks due to the 25V drop across the regulator. Other easy option is a zener shunt regulator if the current requirement is low. Last edited by discrete; 13th May 2011 at 04:50 PM.

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13th May 2011, 05:09 PM	#4
sunny diyAudio Member Join Date: Oct 2004	If i use LM 317/337 then what should be the value of the resistors to get 15 volts ?I don't like using variable resistors by adjusting output voltage.Please let me know the value of the resistors. Thanks, Sunny.

13th May 2011, 05:13 PM	#5
tomchr diyAudio Member Join Date: Feb 2009 Location: Greater Seattle Area	Sunny, you can calculate the resistors from the equation in the datasheet. Set the resistor between the output pin and the adjust pin to R1 = 120 ohm and calculate the resistor to ground = R2. Vout = 1.25*(1+R2/R1) ~Tom

13th May 2011, 05:21 PM	#6
sunny diyAudio Member Join Date: Oct 2004	Can you please suggest suitable standard value resistors for getting 15 volts.The input voltage i'm getting around 38 to 40 volts.My current r equirement is 1amp. Sunny.

13th May 2011, 07:12 PM	#7
DF96 diyAudio Member Join Date: May 2007	tomchr has read the datasheet for you and given you the formula. Are your calculator batteries flat?

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