All About Circuits Forum Calculating Current from DC to AC
 User Name Remember Me? Password
 Register Blogs FAQ Members List Today's Posts Search Today's Posts Mark Forums Read

 General Electronics Chat Discussion forum for general chat about anything electronics related, including asking questions about material in the All About Circuits E-book, Worksheets, and Videos.

#1
10-31-2011, 03:06 PM
 AutoNub Junior Member Join Date: Oct 2011 Posts: 44
Calculating Current from DC to AC

Hello,

We have a 115VAC supply going to two 24DC supplies. One 24DC supply is 1A and the other is 2.1A. How do I figure out the total alternating current going from the 115VAC supply to the two parallel connected 24DC supplies? It can't be as simple as just adding the currents of the DC supplies (1+2.1=3.1A), or can it? Is there a square root of 3 calculation in there somewhere?

I understand Kirchhoff's current law states that current in has to equal current out, but I've never actually done a hand calculation from DC to AC before for calculating current. Thanks in advance for an clarifications!

Last edited by AutoNub; 10-31-2011 at 03:18 PM.
#2
10-31-2011, 03:24 PM
 ErnieM Senior Member Join Date: Apr 2011 Location: Lon Guyland, Noo Yawk Posts: 5,341

Perhaps the best way to approach is to do a power calculation, where the power in is the power out plus losses.

Let's guess supply 1 runs 24V @ 1A out with an 80% efficiency, that is:

P = (24*1) / 0.8 = 30W

And supply 2 runs 24V @ 2.1A out with an 75% efficiency, that is:

P = (24*2.1) / 0.75 = 67.2W

So the total power in is 30W + 67.2W = 97.2 W

The current is then just power over voltage:

97.2W / 115V = .85 A

That's a rough order of magnitude calculation for an average current. Power supplies, especially linear output stages, are notorious non-linear, requiring high current spikes for short periods of time.

Kirchhoff's current law does apply, but it yields nothing useful to finding the current. A power supply has 4 leads, 2 for AC, 2 for DC. Each pair of leads has the same current going in as going out, so they all sum to zero.
__________________
“Freedom is never more than one generation away from extinction. We didn’t pass it to our children in the bloodstream. It must be fought for, protected, and handed on for them to do the same, or one day we will spend our sunset years telling our children and our children’s children what it was once like in the United States where men were free.”
 The Following User Says Thank You to ErnieM For This Useful Post: AutoNub (10-31-2011)
#3
10-31-2011, 04:01 PM
 AutoNub Junior Member Join Date: Oct 2011 Posts: 44

Quote:
 Originally Posted by ErnieM Power supplies, especially linear output stages, are notorious non-linear, requiring high current spikes for short periods of time.
So if I'm rating the required minimum source current for the system in question, is there a general percentage used as a rule of thumb for adding to the calculated currents to account for these required spikes? I understand the inrush current only lasts for milliseconds, but if the source is literally incapable of delivering that high current for any duration of time, would this be a problem?

Forgive me for the questions. I'm only a recent graduate with very little real world experience. I deeply appreciate you sharing your knowledge. Thank you.
#4
10-31-2011, 06:33 PM
 bountyhunter Senior Member Join Date: Sep 2009 Posts: 2,498

Quote:
 Originally Posted by ErnieM Perhaps the best way to approach is to do a power calculation, where the power in is the power out plus losses. Let's guess supply 1 runs 24V @ 1A out with an 80% efficiency, that is: P = (24*1) / 0.8 = 30W And supply 2 runs 24V @ 2.1A out with an 75% efficiency, that is: P = (24*2.1) / 0.75 = 67.2W So the total power in is 30W + 67.2W = 97.2 W The current is then just power over voltage: 97.2W / 115V = .85 A That's a rough order of magnitude calculation for an average current. Power supplies, especially linear output stages, are notorious non-linear, requiring high current spikes for short periods of time. Kirchhoff's current law does apply, but it yields nothing useful to finding the current. A power supply has 4 leads, 2 for AC, 2 for DC. Each pair of leads has the same current going in as going out, so they all sum to zero.
Actually, you forgot power factor. For a single phase 115VAC design, it's usually about 0.5 - 0.6. That means the RMS current flowing into the AC lines will be close to double what you calculate based on power calculations alone.
#5
10-31-2011, 06:40 PM
 #12 Senior Member Join Date: Nov 2010 Location: 15 miles west of Tampa, Florida Posts: 9,088 Blog Entries: 9

Linear or switching power supplies start by filling a capacitor as a reservoir. As long as the output power is less than the supply limitations, it will eventually get there. Maybe a dozen cycles or a 5th of a second. It depends on things you haven't told yet.

When the supply fails to have enough current, the tops of the sine waves get clipped off. A linear regulator will simply deliver lower (and very noisy) voltage until the supply catches up. A switching regulator might get irritated and go into self-protect because it is struggling to throw maximum amps which are not available.

That's as far as I can go with what I have to work with.
__________________
It's only my opinion, and sometimes I'm wrong.
(I learn something almost every day on this site.)
#6
10-31-2011, 07:20 PM
 Adjuster Senior Member Join Date: Dec 2010 Location: London UK (GMT) Posts: 2,147

Quote:
 Originally Posted by AutoNub So if I'm rating the required minimum source current for the system in question, is there a general percentage used as a rule of thumb for adding to the calculated currents to account for these required spikes? I understand the inrush current only lasts for milliseconds, but if the source is literally incapable of delivering that high current for any duration of time, would this be a problem? Forgive me for the questions. I'm only a recent graduate with very little real world experience. I deeply appreciate you sharing your knowledge. Thank you.
Many power sources, such as batteries or transformers may supply a moderate short-term current excess with just a slight fall in voltage, provided that it's not too excessive or too prolonged. Take things too far though, and something might overheat, or at least a fuse or breaker might blow.

In a simple situation, turning the system on in sections rather than all at once may be enough to mitigate the surge. Devices such as larger motors are routinely provided with current-limiting starting systems for their own benefit as much as that of the supply. Similarly, regulated power supplies especially switched - mode types may be made to "soft start", so that the inrush is less.

Some specialised power sources like solar batteries and (very rare) series fed supplies have inherent current limits: try to take too much current from these and their output voltage will collapse. This is an especially delicate situation where some types of regulated SMPS are used, as they can have a negative input voltage/input current slope. This can lead to a regenerative collapse of the input voltage unless things are arranged properly.

If you are in such a situation, it is necessary to manage inrush currents carefully. Starting up in a programmed sequence and the use of "soft-start" systems may be sufficient, but extra reservoir capacitors or even storage batteries may also be required, especially if previously un-powered devices may be suddenly connected to the system.
#7
10-31-2011, 08:12 PM
 ErnieM Senior Member Join Date: Apr 2011 Location: Lon Guyland, Noo Yawk Posts: 5,341

Quote:
 Originally Posted by AutoNub So if I'm rating the required minimum source current for the system in question, is there a general percentage used as a rule of thumb for adding to the calculated currents to account for these required spikes? I understand the inrush current only lasts for milliseconds, but if the source is literally incapable of delivering that high current for any duration of time, would this be a problem?
The inrush starting current is larger, but I mean during normal operation an old school transformer/bridge/cap input scheme draws a very non-linear non-sinusoidal set of current slugs. A better more expensive supply should smooth these out some.

Perhaps you should let us know how you are getting this AC voltage. Sometimes the source can be a problem, I have some friends who claim to have blown out their microwave ovens when they tried to run them off a DC battery to AC inverter. (Or was it the inverter that failed? Not 100% sure but I think it was the oven).

Quote:
 Originally Posted by AutoNub Forgive me for the questions. I'm only a recent graduate with very little real world experience. I deeply appreciate you sharing your knowledge. Thank you.
Hey, it's what we do.
__________________
“Freedom is never more than one generation away from extinction. We didn’t pass it to our children in the bloodstream. It must be fought for, protected, and handed on for them to do the same, or one day we will spend our sunset years telling our children and our children’s children what it was once like in the United States where men were free.”
#8
11-03-2011, 02:59 PM
 AutoNub Junior Member Join Date: Oct 2011 Posts: 44

Quote:
 Originally Posted by bountyhunter Actually, you forgot power factor. For a single phase 115VAC design, it's usually about 0.5 - 0.6. That means the RMS current flowing into the AC lines will be close to double what you calculate based on power calculations alone.
Can anyone confirm this assessment? Did the efficiencies (0.8 and 0.75) already account for the power factors or do we need to add to our calculations? I believe efficiency and power factor are related (efficiency might be determined by using power factor), but are the two terms interchangeable in our calculations?

In college, we used the simple efficiency formula η=(Pout/Pin)*100%. The only time we ever got more detailed in calculating efficiency was in reference to generators and motors (electrical or copper losses, brush losses, core losses, mechanical losses, and stray load losses). In these cases we'd represent our losses by drawing a power-flow diagram. In contrast, PF is simply cos(θ)=P/S (refering back to the power triangle, [(real power)/(apparent power)]). Unfortunately, the data sheets corresponding to the power supplies in question don't appear to provide enough information.

The 115VAC single phase supply is coming from large ships. This is a marine application. EMIs are not an issue (everything is very thoroughly shielded and extensively tested). I can't be more specific due to confidentiality issues. The DC power modules described are Acopian power supplies. I have data sheets which show efficiencies, but don't see power factors. I could contact the manufacturer if necessary.

Last edited by AutoNub; 11-03-2011 at 03:48 PM. Reason: Added more information
#9
11-04-2011, 12:50 AM
 Adjuster Senior Member Join Date: Dec 2010 Location: London UK (GMT) Posts: 2,147

Power Factor is the ratio of real power (W) to apparent power (VA) in a circuit. It is a separate issue to efficiency, and only really affects it in the sense that a low power factor increases the losses in the distribution system relative to the amount of actual power delivered.

http://en.wikipedia.org/wiki/Power_factor

 Tags calculating, current

 Related Site Pages Section Title Worksheet Basic AC-DC power supplies Textbook The DIAC : Thyristors Textbook Low-pass filters : Filters Video Lecture Troubleshooting Transistor Circuits - Transistors and Transistor Circuits Textbook Series resistor-inductor circuits : Reactance And Impedance -- Inductive Textbook The common-base amplifier : Bipolar Junction Transistors Textbook Simple AC circuit calculations : Basic Ac Theory Textbook AC phase : Basic Ac Theory Textbook Measurements of AC magnitude : Basic Ac Theory Textbook Rectifier/filter circuit : Discrete Semiconductor Circuits

 Similar Threads Thread Thread Starter Forum Replies Last Post daviddeakin General Electronics Chat 49 06-26-2012 07:19 PM b2386 The Projects Forum 25 11-19-2010 09:23 PM fabieville General Electronics Chat 22 09-15-2010 12:29 AM cjdelphi General Electronics Chat 3 05-01-2009 03:58 AM Ratch Off-Topic 120 06-29-2008 08:56 AM

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is Off Forum Rules
 Forum Jump User Control Panel Private Messages Subscriptions Who's Online Search Forums Forums Home Electronics Forums     General Electronics Chat     The Projects Forum     Homework Help     Electronics Resources Software, Microcomputing, and Communications Forums     Programmer's Corner     Embedded Systems and Microcontrollers     Computing and Networks     Radio and Communications Circuits and Projects     The Completed Projects Collection Abstract Forums     Math     Physics     General Science All About Circuits Commmunity Forums     Off-Topic     The Flea Market     Feedback and Suggestions

All times are GMT. The time now is 10:59 PM.