Power Regulator with LM317

Thread Starter

carlosgoac

Joined Nov 16, 2010
11
AC voltage is usually labeled as its RMS value. The peak value is RMS times the square root of 2, or 1.414. 30 radical 2 = 42.426 volts peak. From that you subtract the diode loss and you will have the voltage that will accumulate on your first capacitor if there is no load.
You're Absolutely right I totally forgot that with a multimeter you measure RMS instead of Peak!!!
 

Audioguru

Joined Dec 20, 2007
11,248
AC is a rising voltage to the peak of the sine-wave then a falling voltage that passes through 0V then another rising voltage to the other polarity peak voltage then another falling voltage that passes througe 0V. Its RMS voltage (the rating of the transformer's output) is the same as a DC voltage that heats a heater the same (almost like an average voltage).

But a rectifier charges the filter capacitor to the peak voltage of the sine-wave which is 1.414 times higher than the RMS voltage. The rectifier bridge always has two diodes in series so it reduces the rectified peak voltage about 2V. So 30VAC produces a rectified and filtered +40VDC. But the ripple drops it to maybe 38VDC.

The ripple is caused by the filter capacitor discharging a little between charging pulses from the bridge rectifier. A higher value filter capacitor reduces the amount of ripple.

When you have an input voltage of 38VDC and an output voltage of 23V or less then the LM317 reduces its max output current.

The fixed resistor for an LM317 is supposed to be 120 ohms. The fixed resistor for the more expensive LM117 is 240 ohms. The datasheet says that if the resistors have a higher value then the output voltage might rise with no load.
A voltage regulator that allows the output voltage to rise with no load is not a voltage regulator.
 

#12

Joined Nov 30, 2010
18,224
The thing that causes the output voltage rise when there is no load is the current that the 317 chip allows at its adjustment pin. The intent of the 120 ohm resistor is to set up a current flow in the adjustment circuit of 10 milliamps so the current from the 317 is insignificant compared to the 10 milliamp flow.
 

Thread Starter

carlosgoac

Joined Nov 16, 2010
11
Wow I'm learning so much from you, thanks a lot!

So in various data sheets of voltage regulators they have the 1XX and the 3XX series, but in the data sheet it doesn't say what's the difference the tables show exactly the same values just when you go buy it the price of the 1XX is like the double of the 3XX, is there a real difference like in the tolerances or what's the difference?
 

Thread Starter

carlosgoac

Joined Nov 16, 2010
11
OK, so another question, I've been using 1/4W resistors but if I want to use the full output current (1.5A) do I need to use at least for the 120 Ohms resistor a 5 or 10W resistor? Because if you say from the output to the adjustment terminals a 10mA current flows then the 1/4W resistors will be more than enough
 

Audioguru

Joined Dec 20, 2007
11,248
The 120 ohm resistor will have only 1.25V across it. The power calculation is 1.25V squared divided by the 120 ohms= 0.013W so a tiny 0.05W resistor or bigger will be fine.
Use a standard 1/4W resistor.
 

Thread Starter

carlosgoac

Joined Nov 16, 2010
11
Great thanks!

Here's another question, the Vout is dependant to the Vin or can I use the chip to make a reference? because I'm trying to do that and also, is there a way to calculate the value of R2 or should I just keep doing it with a potentiometer and then getting that value to a near commercial value resistor?
 

SgtWookie

Joined Jul 17, 2007
22,230
With current being drawn from the output, there is a "dropout voltage" between the input and the output. This is typcally 1.7v, but it can be as low as 1.3v under very light load, and up to 2.5v under heavy load. Temperature also plays a big factor, there are plots in the datasheet showing various dropouts at various current and temperatures. So, if you want to plan for worst case, Vout <= (Vin-2.5v)

Vref (the voltage at OUT referenced to the ADJ terminal) is nominally 1.25v; it can be as low as 1.2v and as high as 1.3v and still be within manufacturer's specifications; the specs are tighter for the LM117 regulator - however, THAT regulator still requires a minimum 10mA load in order to provide guaranteed regulation. Audioguru is confusing the LM117 with the LM317Lx variants, which are limited to 100mA output but only require 5mA for guaranteed regulation.

Since Vref can be as low as 1.2v, then 1.2v/120 Ohms = 10mA, providing guaranteed regulation under all conditions within the regulators' limitations.

Nominally, you'll get 1.25v / 120 Ohms = 10.41666... mA current flow through R1.

So, if you want a range of 1.25v to 30v, you will then need:
(30v - Vref) / 10.41666mA = (30-1.25)/0.01041666 = 28.75/0.01041666 = 2760 Ohms from ADJ to GND to get 30v out. Iadj will be fairly small; in the range of around 50-80uA. With a resistor that small, it's around a 150mV to 200mV addition to the output voltage.

Of course, 2760 is not a standard value of resistance. Pots usually go 1k, 2k, 10k, 20k, 50k etc. You need to decide how precisely you want the output to be regulated; if you want it to be within the specifications of the LM117/LM317, you'll have to ensure that there is at least 10mA load on the output at all times.
 

Audioguru

Joined Dec 20, 2007
11,248
....the specs are tighter for the LM117 regulator - however, THAT regulator still requires a minimum 10mA load in order to provide guaranteed regulation. Audioguru is confusing the LM117 with the LM317Lx variants
No.
The LM117 requires half the minimum load current of the LM317 which is why it can use 240 ohms for R1 instead of 120 ohms for the LM317.
 

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Thread Starter

carlosgoac

Joined Nov 16, 2010
11
OK, thanks on that but you didn't answer me if I can also use the 317 for a reference voltage, as I previously stated I will be using batteries and at some point because they will have to provide the same amount of current at all times the voltage will drop, to ensure the protection of the batteries I need to be monitoring them constantly but I'm using an OpAmp, the actual value of the batterie on one of the terminals and the reference on the other so when the voltage of the battery goes beneath that threshold the OpAmp will saturate and trigger a relay so the load will stop asking for current.
 

Audioguru

Joined Dec 20, 2007
11,248
For a reference voltage you can use an LM317L which is in a small TO-92 transistor case. You could also use an LM10 which has a low voltage opamp plus an adjustable voltage reference in an 8-pins DIL case.
 

SgtWookie

Joined Jul 17, 2007
22,230
No.
The LM117 requires half the minimum load current of the LM317 which is why it can use 240 ohms for R1 instead of 120 ohms for the LM317.
OK, you're looking at the minimum load current, and I've been looking at the Load Regulation specification. For the latter, both the LM117 and LM317 are specified at 10mA <= Iout <= Imax.

So in order to achieve guaranteed load regulation, you still need to have at least 10mA current being sunk from the output; and using 120 Ohms for R1 satisfies that requirement under all conditions.
 

Audioguru

Joined Dec 20, 2007
11,248
I have never seen an LM117 and never intend to use one. I always use 120 ohms with all my LM317 regulators even when the input voltage is much less than the spec'd 40V.

There is a discrepancy in the datasheet where the minimum load current for an LM117 is rated at only 5mA which is with a 240 ohm resistor but all the guaranteed spec's are with a 10mA minimum load. Almost every schematic in the datasheet with an LM117 shows a 240 ohm resistor.
 

SgtWookie

Joined Jul 17, 2007
22,230
I honestly don't know why they show R1=240 Ohm resistor for a number of the examples, as it wouldn't meet the load regulation requirement per their own specifications. I consider it to be an inconsistency, rather than an error.

The only LM317 series regulator where R1=240 Ohms is valid for the entire range is the LM317Lx, which has it's own datasheet.

There is also the LM317M, which has a max Iout of 500mA, but it also requires 10mA <= Iout <= Imax for guaranteed load regulation.
 
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