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I bought a new power supply. It is possible to set both current and voltage on the unit. I have previously been using a transformer with fixed voltage modes at 3, 4.5, 6, 7.5, 9.12 volts. The current is rated at 2000 mA, 24 VA (max). I have not cared so much about the current but now when I can set the current, I get a bit confused about proper settings.

How should I think about how to spec how many amperes I should feed?

I have been thinking that since the current is regulated by the load, i.e., that if I supply 12 V to something, the current is regulated by itself based on the load and I do not have to think so much about the current more then not exceeding what is capable of delivering. For example a charger to a phone, a charger with 2 amps charges your phone faster than one with 0.5 amps. And it is the phone that determines how much power it should draw right?

So how carefully do I need set the current? Can I think of the current settings as the maximum number of amperes it is capable of delivering or do I need to be more accurate than that. (Assume so otherwise you would have no need for specifying it except for maximum value as a reference point, in my case around 18 amps).

Another example: If I feed a driver A4988 to a stepper motor from the unit, there is a current limiter in the driver so I assume that it is not strange to set manually as in this description:

https://ardufocus.com/howto/a4988-motor-current-tuning/

Another example ... if I supply a Raspberry Pi with 5 V and 3 A (same as the official adapter). Is 3 A max what RPi can deliver and if I, based on Ohm's law, connect loads with more than 1.66 ohms (15/3), can it break? But it still can not deliver more 3 A?

Null
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acroscene
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3 Answers3

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If connecting up to a load that is supposed to be powered at constant voltage then set the current to limit at a safe value. e.g., If powering something that should only draw 400 mA then set the current limit a bit higher than that whilst allowing for switch-on surge.

If connecting up to a load that requires constant current then set the current limit to that value and adjust the voltage setting a bit higher than required to drive that current through the load.

Is 3A max what RPi can deliver ...

No, the RPi doesn't deliver power; your power supply does that.

If you overload the power supply it may overheat - or if it is properly protected it will limit the output current to a safe value by reducing the output voltage.

Transistor
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The current and voltage settings on an adjustable power supply are both maximum values.

If you set the supply for 5 volts and 1 amp, it will deliver 5 volts at up to 1 amp, with the actual current delivered determined by the load - if the load only wants 0.5 amp, that is all the supply will deliver.

If the supply is set to 1 amp but the load wants 1.5 amp, the supply will reduce the voltage in an attempt to reduce the current to 1 amp.

Peter Bennett
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    Re, "in an attempt to reduce the current..." My experience with bench power supplies tells me that if the limits are set to 5V and 1A, then "it will deliver 5 volts at up to 1 amp" \*OR\* it will deliver one amp at less than 5 volts. There is no "attempt." – Solomon Slow Oct 19 '21 at 01:40
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Edit the question to list the make and model of your power supply if you want specific answers to specific questions. Otherwise there is speculation concerning the assumption about features of your power supply.

For a DC supply this video shows the current limit forcing voltage reduction when the load is below the critical resistance:

https://youtu.be/JJqAGLxijXc?t=223

It is not clear whether your device limits at 2A and 24V (less than 12 ohms critical resistance) or whether it has a 24 watt power limit for voltage and current output.

Linear and Switching Power Supply Fundamentals:

https://www.ti.com/lit/an/snva558/snva558.pdf?ts=1634745241875&ref_url=https%253A%252F%252Fwww.google.com%252F

The voltage regulator selected must be able to provide sufficient current to the load under worst-case operating conditions, if system performance is to be reliable.

The thermal shutdown, current limiter, and voltage error amplifier make up three distinct and separate control loops that have a definite hierarchy (pecking order) which allows one to "override" the other. The order of command (and importance) of the loops is: 1) Thermal Limit (IC is regulating junction temperature/power dissipation) 2) Current Limit (IC is regulating load current) 3) Voltage Control (IC is regulating output voltage) This hierarchy means that a linear regulator will normally try to operate in "constant voltage" mode, where the voltage error amplifier is regulating the output voltage to a fixed value. However, this assumes that both the load current and junction temperature are below their limit threshold values. If the load current increases to the limiting value, the current limiting circuitry will take control and force the load current to the set limiting value (overriding the voltage error amplifier). The voltage error amplifier can resume control only if the load current is reduced sufficiently to cause the current limiting circuits to release control."

SystemTheory
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  • VA and W are not the same thing, if it's an AC power supply. – Hearth Oct 19 '21 at 00:36
  • So if I know the voltage rating but not the current, will it in theory hurt (even though it is better to know) to just put the power supply to the rated voltage and say 10 A. Even though if I suspect it to just need 1 A? – acroscene Oct 20 '21 at 14:04
  • If the load is only expected to draw 1A worst case then it would not hurt to put the current limit at 10A unless there is a system fault or wiring mistake and you don't want the power supply to flow all 10A that it can source at that voltage if it is rated to put out that many amperes at that voltage. – SystemTheory Oct 20 '21 at 15:58