How do you calculate how big of power supply?

[cjruger]

Is there a formula for calculating how many amp 12v power supply is needed for how many watts of output?

 

[Beetle]

Add up the current requirements for each piece of equipment you plan to run from the supply (with the transmitter transmitting). Then multiply that figure by at least 1.3.

 

[Captain Kilowatt]

That works if you know those numbers Beetle but I suspect he wants to know how big a supply he needs to run X-amount of watts. Generally if he doubled the watts and divided that by about 13.2 he would be right on the money. Add at least another 20-25% or so to have a little headroom and he should be good. I like your idea better however.

 

[cjruger]

I read in another thread here , one member said the old rule of thumb was 10 amps per every 100 watts?

 

[Robb]

I read in another thread here , one member said the old rule of thumb was 10 amps per every 100 watts?

Yeah; but that is an estimate. In real world usage, if you don't use a power supply that is rated at/more than 25% greater than you actually use, you run the risk of over-taxing and over-heating it. The end result is a fried power supply. If you calculate that you need 30 amps; then get a 40 amp supply and it will run cooler and last longer. If 60 amps; then 75 or 80 amps.

Just because a radio puts out ~20 watts doesn't mean that all you need is a 2 amp supply. A radio will require more than that. It may use about 3.5 amps when transmitting; but a 5 amp supply would be the right choice. There is so much of the power being used in circuits end up as heat/inefficiency before it gets the finished result. Perhaps 50% IIRC.

 

[Captain Kilowatt]

I read in another thread here , one member said the old rule of thumb was 10 amps per every 100 watts?

I knew someone would mention that.That "old rule of thumb" is an old CB wives tale based on a broken thumb.Actually it is based more on running a cheap and dirty class C amp which is a bit more efficient but dirty as far as signal quality goes. If that was really the case then the typical 100 watt ham transmitter would only require 10 amps. My 100 watt transmitter has 25 amp OEM fuses in the power leads. A 10 amp supply would fall flat on it's face trying to run it.

 

[dxhound]

Is there a formula for calculating how many amp 12v power supply is needed for how many watts of output?

First you need to know what your powering. Is it class AB1, C etc....... Then determine what its peak output is. Lets say its 1000 watts for an easy number. Most TRUE AB1 amplifiers are around 60% efficient. Most class C's are 70-75%. What this means is, that a class AB1 amplifier will produce a wattage number that is 60% of the total power consumed by the amplifier. the other 40% is lost in heat. Sooooo If your 1000 watt amplifier is class ab1, then it is truly consuming 1667 watts of power during the peaks. we know that watts / volts = amps. So we get 139 amps PEAK current draw.

Now dont let that scare you too much. That figure above is assuming a very tight class AB1 amplifier. Even the "AB1" texas stars are around 70% I put AB1 in "" because they arent truly AB1 due to their biasing....thats for another discussion. Anyway, I have found most comp amplifiers to be in the 80% realm. This was observed on a service monitor and not some cheap power meter.

So if your 1kw amp is on 12v then the total amp draw would be 104 amps. This brings some merit to the 10 amps per 100 watts.

I laugh every time I hear someone say they are getting 1kw out of their amp thats on a 52 amp 12v power supply. That means their amplifier is............161% efficient. Now I do understand that the 52 amp supply might peak 60 amps briefly allowing for more power, but you can see the math is still way off! This will help give you an idea of 1) how much a meter can lie and 2) the unrealstic claims of some of the amplifiers on the market. Keep in mind, the comp amps will produce more output at 18-20v, so you need to redo the math for that change. Hope this helps.