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.
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.
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.
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.
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.
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:
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.
Thanks,
Sunny.
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
Vout = 1.25*(1+R2/R1)
~Tom
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.
equirement is 1amp.
Sunny.
tomchr has read the datasheet for you and given you the formula. Are your calculator batteries flat?
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
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).
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...
R1=220 and R2=2.2k+220 gives you 15 V.
But forget about it if you really need more than about 100 mA...
Tom,
I found there is +18,0,-18volts preamplifier supply existing on my power amplifier board.What resistor i should use to decrease 18 volts to around 14 to 15 volts?I need 15 volts for my pre-amplifier.My current requirement is within 100mA.
Please suggest suitable resistor with wattage.
Thanks,
Sunny.
Depending on what opamps you are using, they may well be OK with +/-18V supply although this is normally pushing it a bit.
I talked through the method for calculating dropper resistors in my first reply and it might be a good idea for you to try too as it improves knowledge of formulas and understanding of the relationship between voltage, current and resistance. Anyway here you go:
Voltage dropped by resistors=3V (working off an 18V supply)...
...and the current required is around 100ma (plenty for any preamplifier circuit), then:
R=V/I so... R (ohms)=3/0.1=30 ohms dropping resistor.
The power dissipated (in watts) by the resistor is determined by: V^2/R...
...so: 3^2/30=0.3 watt-I would pick a resistor of around 1 or 2 watts for a safety margin.
I talked through the method for calculating dropper resistors in my first reply and it might be a good idea for you to try too as it improves knowledge of formulas and understanding of the relationship between voltage, current and resistance. Anyway here you go:
Voltage dropped by resistors=3V (working off an 18V supply)...
...and the current required is around 100ma (plenty for any preamplifier circuit), then:
R=V/I so... R (ohms)=3/0.1=30 ohms dropping resistor.
The power dissipated (in watts) by the resistor is determined by: V^2/R...
...so: 3^2/30=0.3 watt-I would pick a resistor of around 1 or 2 watts for a safety margin.
Hi Sunny,
I am not Tom. (But I quoted him and possibly the text is not showing correctly on your browser).
Using just a resistor is not a good idea.
Ohm's law will get you far in this hobby.
V=I x R
So as I changes, the V will change.
One mark of a good regulator/power supply is low output impedance. With most regulators R < 0.1 ohm. So the output voltage does not change much as the load current changes.
To get 15V from 18V, you should use an LDO type regulator. Linear has a bunch. Example:
LT1965 - 1.1A, Low Noise, Low Dropout Linear Regulator - Linear Technology
They are relatively expensive though and need more care with design.
The drop-out voltage for the 317/337 is around 1.7 V at 200mA. So they will also still work, if a little borderline. R1, R2 same as before. If the 18V supplies are quiet and stable you should not have any problems. Make sure the 18V supplies are up to the extra demand.
With only 3V drop and <100mA load you will not need heat sinks.
I am not Tom. (But I quoted him and possibly the text is not showing correctly on your browser).
Using just a resistor is not a good idea.
Ohm's law will get you far in this hobby.
V=I x R
So as I changes, the V will change.
One mark of a good regulator/power supply is low output impedance. With most regulators R < 0.1 ohm. So the output voltage does not change much as the load current changes.
To get 15V from 18V, you should use an LDO type regulator. Linear has a bunch. Example:
LT1965 - 1.1A, Low Noise, Low Dropout Linear Regulator - Linear Technology
They are relatively expensive though and need more care with design.
The drop-out voltage for the 317/337 is around 1.7 V at 200mA. So they will also still work, if a little borderline. R1, R2 same as before. If the 18V supplies are quiet and stable you should not have any problems. Make sure the 18V supplies are up to the extra demand.
With only 3V drop and <100mA load you will not need heat sinks.
Can TLO72 IC withstand upto 18 volts DC? What is the maximum working voltage of TLO72?since,i'm using TL072.
Sunny.
Sunny.
Last edited:
The datasheets for almost all IC's are online and you can find them by just typing the part number into Google. By asking questions like this, it makes people think you are lazy and deter them from replying-it doesn't take much to do a quick Google and look it up in the datasheet.
18V operation is right on the edge for a TL072 so it is definitely best to use either a LDO regulator as discrete suggested or a dropper resistor as you risk blowing up your opamps if the rails go slightly out of tolerance.
- Status
- Not open for further replies.