Help me design lab PSU with the LT3080

Hi, I want to build a simple lab PSU with the LT3080. I have two of these regulators plus a multitap transformer, taken out from a commercial LM317 PSU. I need the whole PSU to be very small in size, smallest possible, but not SMPSU.

My proposed schematic is attached. Note that the Vcontrol is connected to the input, for simplicity. To limit the heat (and thus the size of the heatsink), I use the multitap transformer in combination with a selector switch, to select the right range.

There are two PSUs conencted in parallel to the transformer. Of course the transformer max current is shared between them.

1. Can I connect the GND of the first PSU with the positive out of the second to build a symmetrical PSU (I am not sure if this IC will have any problems connected like this)?

2. On page 18 of the datasheet a LAB supply is shown which has also current limiting. But it can go only up to 10V. Since my transformer can output max 15V, can I somehow alter this circuit so it can at least go to 12V current limited? MAybe by changing the value of set resistors or connect the Vcontrol pin of the second IC somewhere else?

Instead of two full diode bridges, you might get by with one diode for a positive supply, and one diode for a negative supply.



The loads can be unequal. Or, a load can be absent.

neazoi said:

Hi, I want to build a simple lab PSU with the LT3080. I have two of these regulators plus a multitap transformer, taken out from a commercial LM317 PSU. I need the whole PSU to be very small in size, smallest possible, but not SMPSU.

My proposed schematic is attached. Note that the Vcontrol is connected to the input, for simplicity. To limit the heat (and thus the size of the heatsink), I use the multitap transformer in combination with a selector switch, to select the right range.

There are two PSUs conencted in parallel to the transformer. Of course the transformer max current is shared between them.

1. Can I connect the GND of the first PSU with the positive out of the second to build a symmetrical PSU (I am not sure if this IC will have any problems connected like this)?

Click to expand...

No.
That will create a short.
You can only do that if the transformer has two isolated windings.

neazoi said:

2. On page 18 of the datasheet a LAB supply is shown which has also current limiting. But it can go only up to 10V. Since my transformer can output max 15V, can I somehow alter this circuit so it can at least go to 12V current limited? MAybe by changing the value of set resistors or connect the Vcontrol pin of the second IC somewhere else?

Click to expand...

The Ltspice simulation of the circuit below shows the output going to well over 12V with a 1A load (1A limit) and a 15V input.
I increased the maximum output by increasing the U4 pot resistance, reducing the current limit R1 resistor value, and connecting U2's Vcntrl input to the 15V supply (which reduces its dropout voltage).

But note that if the transformer output is 15Vrms then the bridge filtered output will be about 19Vdc.

crutschow said:

No.
That will create a short.
You can only do that if the transformer has two isolated windings.

The Ltspice simulation of the circuit below shows the output going to well over 12V with a 1A load (1A limit) and a 15V input.
I increased the maximum output by increasing the U4 pot resistance, reducing the current limit R1 resistor value, and connecting U2's Vcntrl input to the 15V supply (which reduces its dropout voltage).

But note that if the transformer output is 15Vrms then the bridge filtered output will be about 19Vdc.

View attachment 133879

Click to expand...

Thank you very much for the circuit I will try it!
Do you think that I could also use the transformer taps as shown in my schematic, in conjunction with your circuit to limit the heat produced by the regulator? But then how a lower input voltage will affect the first current regulator, will it limit the current as well due to the lower input voltage by the transformer?

- - - Updated - - -

Also do the 0.5R need to be a high wattage wirewound? How many watts?
And what is the value of U4?

neazoi said:

..............
Do you think that I could also use the transformer taps as shown in my schematic, in conjunction with your circuit to limit the heat produced by the regulator? But then how a lower input voltage will affect the first current regulator, will it limit the current as well due to the lower input voltage by the transformer?

Click to expand...

Yes, reducing the input voltage is a good (and only) way to reduce the dissipation in the regulators

The current will not be affected as long as the minimum DC input voltage is a least about 2.5V above the output voltage.

neazoi said:

Also do the 0.5R need to be a high wattage wirewound? How many watts?
And what is the value of U4?

Click to expand...

The maximum dissipation in the 0.5R will be 0.5W at 1A (I²R) so a 1W resistor would be good.
It can be any type.

As shown on my schematic, the value of U4 pot (Rtot) is 2 Meg ohms and the value of U3 pot is 50k ohms.

crutschow said:

Yes, reducing the input voltage is a good (and only) way to reduce the dissipation in the regulators

The current will not be affected as long as the minimum DC input voltage is a least about 2.5V above the output voltage.
The maximum dissipation in the 0.5R will be 0.5W at 1A (I²R) so a 1W resistor would be good.
It can be any type.

As shown on my schematic, the value of U4 pot (Rtot) is 2 Meg ohms and the value of U3 pot is 50k ohms.

Click to expand...

Thank you!
I guess I should also include the capacitors shown in the datasheet for this PSU (page 18) right?

Would I need a huge capacitance after the bridge? How much?
Thanks

neazoi said:

..............
I guess I should also include the capacitors shown in the datasheet for this PSU (page 18) right?

Would I need a huge capacitance after the bridge? How much?
Thanks

Click to expand...

Yes, I didn't show them but those capacitors are needed for filtering and stability. The input capacitor on the left can be part of the bridge filter capacitor

You will need a large filter capacitor at the bridge output, typically in the range of several thousand microfarad, depending upon the load current and the ripple you can tolerate (the negative peak of the ripple must be at least 2.5V above the maximum regulator output voltage.
Here's a calculator to help with that.