Hi,

We have a test board setup with 5 meter long cables between the power supplies and the test board setup.

I would like to understand the voltage drop calculations along the long wires.

The voltage required by the test board setup is 2.5 V and the current requirement is 4 A. This is needed by the test board setup.

If I use AWG 16 (Resistance 13 Ohm/Km) for power and ground cables between the power supplies and the test board setup.

Which length do I need to consider for voltage drop calculation along the long wires.

The one way voltage drop with 4 meter cable is

V_drop = Current x Resistance
V_drop = (4) x (4x13/1000)
V_drop = 0.2 V

The two way voltage drop with 2 x 4 meter cable is

V_drop = Current x Resistance
V_drop = (4) x (2x4x13/1000)
V_drop = 0.4 V

Which output voltage should I set on the power supplies, 2.7 V or 2.9 V ? For some reason we do not have sense wires.

 

Try short circuiting the points at your test table and perform a measurement, and you will see how much offset are the cables adding.

 

Hi,

We have a test board setup with 5 meter long cables between the power supplies and the test board setup.

I would like to understand the voltage drop calculations along the long wires.

The voltage required by the test board setup is 2.5 V and the current requirement is 4 A. This is needed by the test board setup.

If I use AWG 16 (Resistance 13 Ohm/Km) for power and ground cables between the power supplies and the test board setup.

Which length do I need to consider for voltage drop calculation along the long wires.

The one way voltage drop with 4 meter cable is

V_drop = Current x Resistance
V_drop = (4) x (4x13/1000)
V_drop = 0.2 V

The two way voltage drop with 2 x 4 meter cable is

V_drop = Current x Resistance
V_drop = (4) x (2x4x13/1000)
V_drop = 0.4 V

Which output voltage should I set on the power supplies, 2.7 V or 2.9 V ? For some reason we do not have sense wires.

First, you just don't do things like that in a professional testing environment. I know, this needs to get done but think about it.

You never depend on supply wiring resistance (if you have no choice, you use larger wire until the lower resistance reduces the voltage drop to a much smaller variable for the remote voltage) for voltage regulation at those low voltages because load changes at the far end test devices can easily pull voltages out of spec high or low during startups or shutdowns. You must use proper remote sensing power supplies or local regulation near or on the device under test.

You said you didn't have the proper supplies. OK budget and buy some because it's a IMO critical requirement for proper testing.

https://www.tek.com/en/blog/why-bother-with-remote-sensing-in-dc-power-supplies

 

To elaborate on what @nsaspook said, your calculations are only valid if the current draw is constant. If the current changes, the voltage drop changes, so the load side is not regulated.

 

To elaborate on what @nsaspook said, your calculations are only valid if the current draw is constant. If the current changes, the voltage drop changes, so the load side is not regulated.

I forget to mention, the current is constant.

 

If you can't use remote sensing, then I suggest you use larger wires to limit the drop to what can be tolerated by the circuits being tested.

 

First, you just don't do things like that in a professional testing environment. I know, this needs to get done but think about it.

You never depend on supply wiring resistance (if you have no choice, you use larger wire until the lower resistance reduces the voltage drop to a much smaller variable for the remote voltage) for voltage regulation at those low voltages because load changes at the far end test devices can easily pull voltages out of spec high or low during startups or shutdowns. You must use proper remote sensing power supplies or local regulation near or on the device under test.

You said you didn't have the proper supplies. OK budget and buy some because it's a IMO critical requirement for proper testing.

https://www.tek.com/en/blog/why-bother-with-remote-sensing-in-dc-power-supplies

We do have voltage regulators at the test board.
I need to find the lowest possible voltage setting at power supply that reduces thermal losses at linear regulators.

 

If you can't use remote sensing, then I suggest you use larger wires to limit the drop to what can be tolerated by the circuits being tested.

That's a goal. To use thick wires that reduces the voltage drop.

In order to calculate voltage drop along power supply wires, should I include twice the length 2 x 4 meters ? When it comes to include resistance of wires.

 

n order to calculate voltage drop along power supply wires, should I include twice the length 2 x 4 meters ?

Of course, since that's the total wire length that carries the current.
But you initially stated the wires were 5 meters in length.

 

Of course, since that's the total wire length that carries the current.
But you initially stated the wires were 5 meters in length.

Sorry, it's 4 meters.

 

We do have voltage regulators at the test board.
I need to find the lowest possible voltage setting at power supply that reduces thermal losses at linear regulators.

These sorts of facts are important. Why didn't you give this sort of information on the first post?

My post still applies about not using wire resistance as a factor for proper operation of the circuit. That is a design error, not something to be compensated for. Reduce the wire losses to something under 1% at max load instead of greater than 10% and then adjust the power supply source voltage to your hearts content knowing it will be stable under all expected design conditions.