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AC to DC calculations #213506 05/26/14 04:44 PM
Joined: Mar 2004
Posts: 947
twh Offline OP
This was explained to me once but I don't remember the explanation. Maybe you can help!

I built a controller for a heater - a purely resistive load. The supplier didn't have an AC ammeter in stock so I used a DC ammeter with a bridge rectifier. Only the Ammeter is DC. The load is still fed with AC. So, the rectifier and ammeter are in series with the load.

An AC ammeter reads 4.5 Amps. The DC ammeter reads 4 Amps, which is correct. 4.5 Amps AC has a peak-to-peak sine wave of 6.36 Amps (4.5 / .707) The DC equivalent is 4 amps (6.36 x .636).

Just to keep this on track, I don't care that the DC ammeter reading is low. I'm not trying to fix that. I'm just trying to remember why the DC current is lower than the AC current. I think the explanation I was given was to do with harmonics but it was so long ago that I'm not sure.

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Re: AC to DC calculations [Re: twh] #213516 05/27/14 12:14 AM
Joined: Nov 2005
Posts: 826
JoeTestingEngr Offline
So you were reading the AC current with a true RMS AC ammeter, and reading the pulsating DC current with an averaging DC ammeter. I guess that makes sense and your heating load won't mind the crossover distortion.Did you consider going with a meter and current shunt?

Re: AC to DC calculations [Re: twh] #213517 05/27/14 12:30 AM
Joined: Mar 2004
Posts: 947
twh Offline OP
Sorry, the AC isn't peak-to-peak. It's just peak. Peak-to-peak is double.

Re: AC to DC calculations [Re: twh] #213518 05/27/14 05:07 AM
Joined: Jun 2004
Posts: 1,273
Tesla Offline
The RMS calculation for AC power (sine waves) was invented precisely because it is power equivalent to straight, steady DC at that same voltage.

In this way the Tesla/Westinghouse scheme could be directly compared to Edison's DC current -- which is mathematically simpler to understand.

The FULL sine wave is used.

If your meters are accurate, then your circuit build has spurious resistance/ reactance.

You did not build the classic 'low pass band filter' -- ie you did not include a capacitor to buffer the pulses of DC. They may well have been in conflict with the DC ammeter. That is: it couldn't swing fast enough to capture the impressed signal before the signal faded. (It's pulsing at 120HZ, BTW) I'm assuming that the DC ammeter is using a magnetic coil in its pick-up.

(Alert: inductive element thrown into circuit, more than enough to explain such a lightly loaded circuit.)


Your DC reading (4A) is off.

The DC equivalent in amps is 4.5 vs 4.5 -- it's definitional.

(6.36 x .636) I have no idea where that calculation came from. The RMS of a DC steady state circuit has to be 1.000)

A fluttering DC wave form has no obvious, direct mathematical calculation. One merely tests for it. Normally, a capacitor is used to make it stop fluttering. That's what a 'low pass band filter' achieves.

Re: AC to DC calculations [Re: twh] #213519 05/27/14 05:17 PM
Joined: Mar 2004
Posts: 947
twh Offline OP
Tesla, I'm pretty sure that AC is RMS and DC is Average. RMS is .707 and Average is .636. If you put 100 VAC into a bridge rectifier your DC voltmeter will read 90 VDC on the output.

Re: AC to DC calculations [Re: twh] #214675 01/07/15 05:31 PM
Joined: Oct 2014
Posts: 6
SparksFlying Offline
New Member
What about adding up all 24VDC Devices and summing them up in order to determine what size Power Supply to use?

Re: AC to DC calculations [Re: twh] #214679 01/08/15 08:49 AM
Joined: Jun 2004
Posts: 1,273
Tesla Offline
The 90VDC was not/ is not an Edison standard.

It was selected as a DESIGN NORM for small DC loads driven by rectified AC current of as low as 110 VAC.

The difference between 110 VAC RMS and 90 VDC was 'chasmic' because the rectification technology then existing was a POWER HOG.

Indeed, MOST DC motors were power gluttons -- for every manner of reasons:

Variable speed -- and resistive controls being at the top of the list.

New solid state technologies have 'rewritten the book' -- so much so that many classic DC motor loads have been entirely switched over to 'smart' AC schemes.

BART subways (San Francisco) has entirely remotored from DC to AC. IIRC, the rails still send DC -- in switched blocks -- they only heat up when the subway approaches -- then smart inverters generate the three synthetic phases at variable speed.

ALL modern diesel electric power systems have also gone over to smart AC.

In EVERY engineering text I've read it is explicitly spelled out that RMS IS the mathematical correction to SINUSOIDAL AC power equivalence to steady state, DC, power flow.

As to economics, the Poco BILLS you at the RMS calculation for Watt-Hours consumed. That's a HINT right there.



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