Power Supply Question

[SK63]

I know this is totally showing my ignorance but I'm really confused on this and need to ask the question....so a 100 Watt HF rig is connected to a power supply and apparently pulling 20 Amps off the power supply. What is the amperage being pulled off the 120V house circuit in which the PS is connected? How can I figure this out?

 

[krokus]

I know this is totally showing my ignorance but I'm really confused on this and need to ask the question....so a 100 Watt HF rig is connected to a power supply and apparently pulling 20 Amps off the power supply. What is the amperage being pulled off the 120V house circuit in which the PS is connected? How can I figure this out?

You would have to know the amount of overhead loss that power supply has.

The easiest way is to put an ammeter on the circuit supplying power.

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[byndhlptom]

Power supply current

rough estimate:

20A x 12v = 240watts

p/s efficiency (switchers typ 75 %, linear typ 55-60%)

switcher = 60wt loss (mostly heat)

= 320w load

320w / 120vac = 2.66A

ballpark, aprox 3A on AC for switcher, probably close to 4A with a linear supply.

$.02

 

[Rred]

20 amps at 13.8 volts (which is what a nominal 12v supply may be set at) is 276 watts. Without knowing the conversion loss and efficiency of the supply, you might guess it is pulling 350-400 watts on the AC side. 400 watts at 117 volts (because in the US, mainline power is about 117 volts plus or minus ten percent) means you will be pulling less than four amps from the wall socket.

For closer figures you read the manuals, or you buy a Kill-A-Watt for $25, which plug into the wall socket and records instant and average power draw over the circuit for a real number.

This is why learning Ohm's Law is perhaps the first thing you should do before considering any ham license or equipment, and why just learning the test answers is a really bad idea.

 

[n5ims]

I know this is totally showing my ignorance but I'm really confused on this and need to ask the question....so a 100 Watt HF rig is connected to a power supply and apparently pulling 20 Amps off the power supply. What is the amperage being pulled off the 120V house circuit in which the PS is connected? How can I figure this out?

A quick and easy way (but not all that accurate, see the later comments) is this. Since power = current times volts you can make a quick in-your-head approximation. The radio is 20 amps at 12 volts. The question is how many amps from the 120 volt power supply for that radio. It's easy to see that 120 is ten times what 12 is. You could then take that 20 amps and divide by that ten to see that about 2 amps would be needed. You would need to divide here to get the same power output since 20 times 12 would equal 2 times 120.

Now the problems with that quick calculation. First off, that assumes that the power supply is 100% efficient, a very bad assumption. For our quick estimation it would be a good idea to assume a 50% efficiency so you can quickly double that 2 amps to 4 and be fairly safe in your guess on how much current is on the AC side.

Now you can be a great deal more accurate using several methods. The most accurate would be to use a calibrated meter on the AC line. The more accurate this meter is, the more accurate your readings would be (well DUH!!!).

You could also use math like others have shown, but you would need to know several factors for those calculations to be accurate. These would include the current draws, the voltages, the efficiency of the power supply and probably several others. The current draws may vary depending on what the radio is doing (you should probably use the maximum values to be safe). The voltages are most likely approximate (that 12v is probably closer to 13.8v and 120v may be 118v or so for example).

The best thing is you really don't need all that accurate of an answer in most cases. So long as you over estimate the current draw and don't exceed the current on that circuit you should be fine with a reasonable estimate.

 

[prcguy]

You also have to consider what transmit mode is used on the HF radio. FM or continuous carrier CW would be close to 20A on the 12V side but average transmit current in SSB mode may only be 6 or 8A depending on mic gain and compression settings. I've run a 100W HF SSB transmitter from an 8A power supply in a pinch and it works.
prcguy

 

[SCPD]

An interesting aside to this--- years ago an old engineer told me of the problems electric companies had with how to bill high power'd coastal commercial SSB stations. Its seems that the-then mechanical electric meters used by the electric company could not respond fast enuff to measure the voice peak demands of SSB, and thus the bills were very low in comparison with what the stations actually were using. I am assuming the modern meters have advanced since those days, but maybe not-- operating SSB may get you a few cents off your electric bill..... but don't say I said this....

..................................CF

 

[Rred]

Look at it this way. When voltage sags under heavy summer loads, that forces you to consume more amperage to compensate, and then you're using MORE electricity than you actually should be. (Typical regulations for a utility company call for 117VAC +-10% so there's room for a lot of variation.)

In the long run it all evens out. The companies generally are required by law to maximize the profit for their shareholders, and they're pretty good at making that happen.