The adjustable voltage regulators have a max voltage difference from input to output. that's why you can use them with a pre-regulator to regulate hundreds of volts.
So yes, you can use 36 Volts in on a 317 if your output voltage is more than a couple of volts.
What you have to be careful of is the power dissipation within the device. Every amp produced at the output is an amp going through the output transistors in the device. So the total power dissipated in the device is output current times the voltage drop across the device. That can be substantial if you are sourcing a lot of current.
Sheldon
So yes, you can use 36 Volts in on a 317 if your output voltage is more than a couple of volts.
What you have to be careful of is the power dissipation within the device. Every amp produced at the output is an amp going through the output transistors in the device. So the total power dissipated in the device is output current times the voltage drop across the device. That can be substantial if you are sourcing a lot of current.
Sheldon
imperfectcircle said:
Ohms Law; R = V/I.
If you know the maximum load current you can calculate a resistor to bring the supply down to around 3V more than the required output. However, at a very light minimum load it may then not drop enough to keep the differential safe, so take possible load range into account.
And for the resistor wattage, W = I2R
Going down the resistor line...
the best way I can see is with a voltage divider....
I.e. at least two resistors in series...
Imagine you want 2.5 amps max and 2.5V output (as and example) your input is 10V.
current flowing through two resistors in series will be the same regardless of their values using ohms law.
R= E/I, i.e. the total resistance for the two resistors will be 10V/2.5A = 4R
so the divider will have to split 4R into whatever the ratio of the output to the input is... i.e 1/4 +3/4
so one of the resistors is 1/4 of 4 and the other is 3/4 of 4.
Power over the 2 resistors will be 2.5V @ 2.5A and 7.5 @ 2.5A respectively.
the best way I can see is with a voltage divider....
I.e. at least two resistors in series...
Imagine you want 2.5 amps max and 2.5V output (as and example) your input is 10V.
current flowing through two resistors in series will be the same regardless of their values using ohms law.
R= E/I, i.e. the total resistance for the two resistors will be 10V/2.5A = 4R
so the divider will have to split 4R into whatever the ratio of the output to the input is... i.e 1/4 +3/4
so one of the resistors is 1/4 of 4 and the other is 3/4 of 4.
Power over the 2 resistors will be 2.5V @ 2.5A and 7.5 @ 2.5A respectively.
You could drop some of the voltage using a transistor as a pre-regulator. No matter whether you use a transistor or a resistor, you are going to be dissipating the same power. The transistor will actually be easier to get rid of the power since it's designed to be mounted to a heat sink.
Sheldon
Sheldon
cpemma said:
Right, you use the output of the 317 as the reference. You run a zener from the output to the raw supply through a resistor. The base (gate) goes to the zener resistor junction. And the emitter (source) goes to the input of the 317. The collector (Drain) goes to the raw supply. Pick a zener value with a bit of headroom for the 317, like 5 volts. Then you heat sink the transistor and most likely don't need to heat sink the 317. It works well, and is right out of the data sheet if memory serves me right.
Sheldon
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