When a power supply is rated 30V / 5A, what does it mean exactly ? - Page 1
Forum section: electronics / beginners

author: tigrou
author_rank: Regular Contributor
timestamp: December 01, 2016, 06:40:35 pm
content: When a power supply is rated 30V / A5 :Does it mean it can deliver maximum 5A (regardless the voltage) . Or can it, for example, deliver 15A @ 10V ? (so 150W in total)Also (another somehow related question) : I would like to measure current that goes up to 30A @ 10V (this is for a small electric motor inside a car). Most multimeters are rated 10A only. How can I measure such high current ?

author: suicidaleggroll
author_rank: Super Contributor
timestamp: December 01, 2016, 06:43:43 pm
content: It can deliver a maximum of 30V and a maximum of 5A. Neither of those limits can be exceeded.Use a small shunt resistor in series with the load and measure the voltage drop across it with your dmm, eg:http://www.digikey.com/product-detail/en/vishay-dale/MRS-15101L00FE1404/MRS151004-1-ND/9574601mR will generate 1mV/amp.

author: electr_peter
author_rank: Unknown
timestamp: December 01, 2016, 07:03:26 pm
content: 30V/5A rated power supply will at least supply 30V open circuit voltage and at least 5A at short circuit (or close to short circuit). Power rating is not given and can be 150W (30V * 5A) or much less.Advanced lab power supplies with 30V/5A rating sometimes can deliver full 150W, but sometimes max power is lower (for example, max 75W - up to 30V at 2.5A or 15V at 5A). Check data sheet.Consumer or industrial level power supplies with 30V/5A rating typically are designed for full 30V 5A load (150W or more).

author: tigrou
author_rank: Regular Contributor
timestamp: December 01, 2016, 07:20:33 pm
content: Quote from: suicidaleggroll on December 01, 2016, 06:43:43 pmSomething like that ?So if i measure 30mv, there is 30A going trough the load ?

author: CatalinaWOW
author_rank: Super Contributor
timestamp: December 01, 2016, 07:52:16 pm
content: As in any commercial endeavor there are variations in what it means.The simplest answer is that it will deliver 30V at up to 5A. In other words you can connect any resistor across the output from infinity down to six ohms and the supply will maintain the voltage at 30V and will not overheat or otherwise behave badly.But it gets more complicated. Variable voltage supplies are often rated in the same manor. You can set the output voltage for anything between 0 and 30 volts, and it will hold that voltages for loads that draw up to 5 Amps.But there is more. Some variable voltage supplies are power limited. So after setting the voltage, they can deliver any current which does not make the product of current and voltage exceed 150 Watts. Supplies rated this way have another limit, sometimes explicitly stated, sometimes not. If power were the only limit, you could theoretically have a really low voltage (.03 volts for example) and deliver the power through a really high current - 500 Amps for the 0.03 volt example in a 150 W supply. No supply can deliver an arbitrarily high current so the manufacturer should state what the real limit is, as has occurred in this case.Finally, I have one supply that specifies its capability in two bands, with one current limit when operating in the lower portion of the voltage range, and a significantly lower limit for voltages in the remaining upper part of the range. A power limit with very low resolution.If you are heavily dependent on some aspect of this rating then you need to contact the manufacturer and discuss with them what the limits on their supply are. This may involve talking with engineers or other informed parties, not just the marketing department. If that is not possible some testing while monitoring stress on key components of the supply may be in order.

author: suicidaleggroll
author_rank: Super Contributor
timestamp: December 01, 2016, 09:11:38 pm
content: Quote from: tigrou on December 01, 2016, 07:20:33 pmYes, if you're using a 1 milliohm shunt resistor. You could use a larger resistor to increase your voltage resolution, but you need to watch the power dissipation, with 30A going through 1 milliohm you're already burning off nearly a watt in the shunt resistor. A smaller value resistor would reduce power loss, at the expense of resolution in your voltage measurement.

author: mariush
author_rank: Super Contributor
timestamp: December 01, 2016, 10:34:35 pm
content: There are also other methods of measuring the current, for example current transducers or hall effect current sensor ICsHall effect current sensors appear to the circuit as a wire with very small resistance, typically less than around 1.2 mOhm.Depending on price of such ICs, they're not as great as if you'd use a current shunt + precision opamp but should be good enough if you want to display and set the current amount in.. let's say 10..25 mV steps.See for example ICs like ACS712 :http://uk.farnell.com/allegro-microsystems/acs712elctr-05b-t/ic-sensor-current-5a-soic8/dp/1329623which can measure -5A .. +5A and outputs 185 mV per Amp of current , with 0.5x Vin output at 0A ... so you can simply convert the voltage to mA using a microcontroller or whatever, which could also using some calibration tables (matching individual sensor IC chips) to improve the precision of the measurements.