I have been using the k7812 to run a 12 Volt tracking unit from 24 Volts. Out of 5 units installed 4 failed in the first week.

When I contacted my component supplier they said that it was a possible transient and supplied a drawing (attached) and the components.

having built and connected the unit the resistor gets really hot . VERY quickly. the unit is drawing 150mA.

Some advice will be appreciated

 

You are dissipating 3/4 of a watt in a 1 watt resistor. That's why we commonly use oversized resistors. The manufacturers of the resistors tell us what power the resistor can survive, but make them so small that they unsolder themselves.

 

hi Rodney,
Ref the 33R resistor passing a current of 0.15A Watts = I^2 * R,, thats (0.15 *0.15) * 33, giving a figure of 0.75Watt, so it will get hot.
E
See that #12, beat me too it while compiling the post.

 

Thanks very much for the help and advice. I will change the resistor and try again. The regulator as rated at 1 Amp. Ideally the resistor should be able to handle this. However a 33Watt Resistor is just not Practical.

 

You don't have to rate the resistor for the regulator spec. You can rate it for the known load.

 

So a 2W resistor should be adequate for your load (or two 1W 16 ohm resistors in series).

 

Wait a minute. That is a switcher with ~90% efficiency. If the load is 12v*0.15A, that is 1.8W. The power input is ~1.8/0.9 = 2W, so the input current is only 2/24 = 83mA. The power in the 33Ω resistor is 33* 0.083^2 =230mW.

On the Transient suppressor, the max input on the switcher is allowed to go to 32V. Just use a 30V Zener, with a series resistor. No current will flow into the Zener unless a transient happens.

 

Fooled me!
The schematic looks just like a linear regulator. Only the datasheet betrays the switching aspect.

 

The Zener and series resistor will that be across the input without A series resistor into the load

 

The Zener and series resistor will that be across the input without A series resistor into the load

Just like you showed on your hand-drawn schematic. Replace the transzorb with a 30V 1W Zener, or use the 1.5KE30. You can leave out the 1N4937 input diode because the Zener will clamp both positive and negative input transients...