# What amp rating of 24vac transformer for LM317T?

I want to know what size transformer for LM317 at full 30 VDC. I know its a 24 VAC center tapped transformer but is 1 A or 5 A transformer best suited for a linear power supply of 1.5-30 VDC? I will be using a standard LM317 variable power supply circuit with the 2200 uF 50 V smoothing and the 240 ohm drain resistor and potentiometer. I have a homemade oversized aluminum heat-sink that is quite large.

• You are confused about voltage and current. The LM317 will draw as much current as it needs to supply current to the load. If the load doesn't pull much current, you won't need to deliver much to the LM317. – TimWescott Apr 25 '19 at 20:24
• This project is likely to be disappointing overall; if you attempt it, do it for the learning experience not the utility of the result. – Chris Stratton Apr 25 '19 at 20:52
• Care to inlighten mew as to why or just form unjust opinions – queennikki1972 Apr 25 '19 at 20:53
• @user646333 What is your goal with this project? How much current do you need to source at, say, 1.5 V? – evildemonic Apr 25 '19 at 21:41
• The circuit you show uses a non-center-tapped transformer - the secondary only has two connections. – Peter Bennett Apr 26 '19 at 1:28

You first have to figure out the maximum output DC current of your power supply. This can be determined by your load, by the maximum current handling capacity of your regulator or diodes, or (most likely the case) by the maximum power dissipation at your regulator.

Once you know the maximum DC current, you have to determine what will be the RMS current at the transformer's secondary windings. This is actually more complicated than what it seems because the current waveform at the transformer is not trivial - the current only flows through the windings of the transformer when the voltage is higher than the capacitor voltage plus the voltage drop across the diodes.

I'll give you two choices at this point. If you're really enthusiastic about it and want to know more than 99% of the linear supply designers, I encourage you to simulate the circuit and calculate the worst case RMS current (square root of average squared current) of the transformer for your case. If you just want a quick answer, a good rule of thumb to use for the rms current at the secondary is (see reference below):

for center tap secondary with 2 diodes: $$\I_{rms} = 1.2 I_{DC}\$$

for 2-wire secondary with 4 diodes: $$\I_{rms} = 1.8 I_{DC}\$$

Keep in mind that the current at the primary of the transformer will be the current at the secondary multiplied by the inverse of the voltage ratio of your transformer (well, there is also magnetization current at play, but we don't need to get there). For example, if the primary voltage is 10x the secondary voltage, then the primary current will be 1/10 of the secondary current.

P.S.: I recommend you take a look at https://www.smcelectronics.com/DOWNLOADS/1980-VOLTREG.PDF, chapter 8 starting at page 74. Don't be impressed by the age of the document: linear power supply design is an old and almost forgotten art and therefore relies on almost forgotten references. :^)

Calculate how much current you intend to draw.

$$\frac{30 V}{240 Ω} = 0.125 A$$

That is how much current you can expect the secondary to carry. Add a little bit for safety.

The primary current will be much less, so you shouldn't need to worry about it.

EDIT: I will leave the above as a simple example of using Ohm's Law to calculate current.

Now that I have seen your schematic I realize that your load is in fact not 240 Ω. Unless you are otherwise limiting your load, you should accommodate the full 2 A that the LM317 can draw.

If you are indeed limited to a choice between a 1 A and a 5 A transformer, go with the 5 A.

• The problem is the original description of 240 ohms as a "load" resistor. Reference to the circuit shows it is the drain resistor (from output to reference) for the LM317. The actual load is unspecified. Consequently, the transformer should be capable of supplying the LM317 maximum load current, or around 2A. – henros Apr 25 '19 at 21:29
• @henros Ah, good point! That schematic wasn't posted when I wrote this answer. I will adjust my answer. – evildemonic Apr 25 '19 at 21:33
• Somehow got off subject in another thread about the same thing. I will post the schematic again at the end. I have a 0-30vdc 5 amp variable workbench power supply. I am trying to mimic that in a linear power supply. 30v 5 amp max. using lm317t if possible otherwise I will make a 1.5 amp max. – queennikki1972 Apr 25 '19 at 21:45
• electrosome.com/variable-power-supply-lm317-voltage-regulator – queennikki1972 Apr 25 '19 at 21:46
• I also mentioned in the other thread that i already have a transformer i purchased by accident that is not center tapped rather only two wires green and red on secondary. 240/208/120vac primary and 24vac secondary 50/60 hz 40va – queennikki1972 Apr 25 '19 at 21:48