Is This A Valid Approximation For LM78xx Power Dissipation?

[Rory Starkweather]

Power Dissipation(LM79xx) = Output current x (Input voltage - Output voltage)?

I know it won't be exact but will it be of any value at all?

Speaking of LM79xxs, for some reason I can't find any numbers for voltage loss in LM7912. Does it depend on how clean the input signal is?

What I'm trying to do is figure out what the smallest transformer is that I can use to give me the full 12 volts out. I'm going to use a recommended filter between the Bridge and the LM79xx.

I have to admit that I have most of the pieces and I could do this experimentally, but it has been about 5 years since the last time I got the electronics bug.

 

[davenn]

Power Dissipation(LM79xx) = Output current x (Input voltage - Output voltage)?

I know it won't be exact but will it be of any value at all?

yes ... so say you have 12V in and 5V out and drawing 1A ( pretty much the max for a 78(79)xx series reg)
you are dropping 7V x 1 amp so the reg will be dissipating 7W, its going to get quite warm (hot)

yes, its good value will help you ( if nothing else) decide what size heatsink you will need

Speaking of LM79xxs, for some reason I can't find any numbers for voltage loss in LM7912. Does it depend on how clean the input signal is?

voltage loss ? do you mean the minimum difference between the input and output voltages ?
of so, its the same as the 78xx positive series and the LM317 series
to keep good output regulation its best to keep the input around 2V higher than the output

There are low dropout regulator models (LDO's) that get down to around 1V or less

What I'm trying to do is figure out what the smallest transformer is that I can use to give me the full 12 volts out. I'm going to use a recommended filter between the Bridge and the LM79xx.

I have the maths somewhere that I did for some one else in the past .... but you should be OK with a transformer with around 12VAC out of the secondary

cheers
Dave

 

[davenn]

OK ... found my tutorial .......

As a design example consider we wanted a small unregulated bench supply for our projects. Here we will go for a voltage of about 12 - 13V at a maximum output current (IL) of 500ma (0.5A). Maximum ripple will be 2.5% and load regulation is 5%.
Now the rms secondary voltage (primary is whatever is consistent with your area) for our power transformer T1 must be our desired output Vo PLUS the voltage drops across D2 and D4 ( 2 * 0.7V), all divided by 1.414.
This means that Vsec = [13V + 1.4V] / 1.414 which equals about 10.2V. Depending on the VA rating of your transformer, the secondary voltage will vary considerably in accordance with the applied load. The secondary voltage on a transformer advertised as say 20VA will be much greater if the secondary is only lightly loaded.
If we accept the 2.5% ripple as adequate for our purposes then at 13V this becomes 13 * 0.025 = 0.325 Vrms. The peak to peak value is 2.828 times this value. Vrip = 0.325V X 2.828 = 0.92 V and this value is required to calculate the value of C1. Also required for this calculation is the time interval for charging pulses. If you are on a 60Hz system it is 1 / (2 * 60 ) = 0.008333 which is 8.33 milliseconds. For a 50Hz system it is 0.01 sec or 10 milliseconds.

The formula for C1 ( main filter capacitor) is:
C1 (uF) = [ ( IL * t ) / Vrip ] X 10 6 C1 = [ ( 0.5A X 0.00833 ) / 0.92V ] X 10 6 C1 = 0.00453 X 10 6 = 4529 or 4700 uF
Remember the tolerance of the type of capacitor used here is very loose. The important thing to be aware of is the voltage rating should be at least 13V X 1.414 or 18.33. Here you would use at least the standard 25V or higher (absolutely not 16V).

Power Supply diode rectifier rating
With our rectifier diodes or bridge they should have a PIV rating of 2.828 times the Vsec or at least 29V. Don't search for this rating because it doesn't exist. Use the next highest standard or even higher. The current rating should be at least twice the load current maximum i.e. 2 X 0.5A or 1A. A good type to use would be 1N4004, 1N4006 or 1N4008 types. These are rated 1 Amp at 400PIV, 600PIV and 1000PIV respectively. Always be on the lookout for the higher voltage ones when they are on special.

Transformer Rating for Power Supply
In our example above we were taking 0.5A out of the Vsec of 10V. The VA required is 10 X 0.5A = 5VA. For our power supply this is a small PCB mount transformer available in Australia and probably elsewhere. This would be an absolute minimum and if you anticipated drawing the maximum current all the time then go to a higher VA rating.