[assail]

I have a number of circuit cards that are failing and this could possibly be attributed to a design problem.

The circuit consists of a +24VDC supply, connected to a 100 Ohm 1/4W resistor which is connected in series on the input side of a 7805 voltage regulator. Measurements indicate approximately 13-14V dropped across the 100 Ohm resistor and 10VDC input to the 7805 and 5VDC output from the 7805. The load draws approximately 170mA from the output 7805.

The problem is: the resistor that is connected in series with the 7805 appears to be showing signs of heat stress or blown altogether. Is the resistors power rating too low, or is the resistance value incorrect for the 170mA current draw.

 

[Militoy]

You have about 2W of dissipation across the 1/4 W resistor. Why do you feel you need the resistor at all?

 

[davenn]

I have a number of circuit cards that are failing and this could possibly be attributed to a design problem.

The circuit consists of a +24VDC supply, connected to a 100 Ohm 1/4W resistor which is connected in series on the input side of a 7805 voltage regulator. Measurements indicate approximately 13-14V dropped across the 100 Ohm resistor and 10VDC input to the 7805 and 5VDC output from the 7805. The load draws approximately 170mA from the output 7805.

The problem is: the resistor that is connected in series with the 7805 appears to be showing signs of heat stress or blown altogether. Is the resistors power rating too low, or is the resistance value incorrect for the 170mA current draw.

The resistor is dropping the 24V to a value that the 7805 can handle ... it wouldnt last long with 24V going into it ~ 15V is a prefered max for a 7805 (data sheet would be specific)

yes at 1/4 Watt the resistor is way under rated I would be replacing it with one rated at least 2W which will still get warm

Watts = Volts x Amps to find amps I (amps) = Volts / Resistance

14volts dropped across 100Ohms = 14/100 = 0.14Amps

14V x 0.14 Amps = 1.96Watts dissapation in the resistor

cheers
Dave

 

[assail]

Ok, i have found a circuit card that appears to work correctly and taken some measurements. The good card has 24VDC supply with about 1.4V dropped across the 100 Ohm resistor and 22.6V at the input to the 7805, 5V at the output of the 7805 with the load drawing approximately 170mA.

My question now is, what would cause the voltage drop to increase across the 100 ohm resistor in the faulty circuit.

I also forgot to mention that there is a 0.1uF cap located at the input and output of the 7805 (to ground).

 

[Resqueline]

1.4V drop with 170mA indicates a resistance of 8.2 Ohms. The 7805 will dissipate 3W and will need a moderate heatsink. Capacitors are a common good practice here.
Are you sure the 100 Ohm resistance value you are referring to is correct?

 

[Militoy]

The resistor is dropping the 24V to a value that the 7805 can handle ... it wouldnt last long with 24V going into it ~ 15V is a prefered max for a 7805 (data sheet would be specific)

yes at 1/4 Watt the resistor is way under rated I would be replacing it with one rated at least 2W which will still get warm

Watts = Volts x Amps to find amps I (amps) = Volts / Resistance

14volts dropped across 100Ohms = 14/100 = 0.14Amps

14V x 0.14 Amps = 1.96Watts dissapation in the resistor

cheers
Dave

Admittedly - I switched over to LM217 / LM317 adjustable regulators years ago - and rarely find a use for fixed 3TRs. But my Fairchild data sheet on the 7805 calls out a max input of 35V. I don't recollect a problem inputting around 24V in the past - especially at only 170 mA. Is there an issue I'm missing?

 

[davenn]

Admittedly - I switched over to LM217 / LM317 adjustable regulators years ago - and rarely find a use for fixed 3TRs. But my Fairchild data sheet on the 7805 calls out a max input of 35V. I don't recollect a problem inputting around 24V in the past - especially at only 170 mA. Is there an issue I'm missing?

Yes 35V is the ABSOLUTE max and almost garanteed to kill it

for a 5V reg the safe working max for good regulation is 20V (7.5 ≤ VIN ≤ 20) 10V typical
for a 12V reg the safe working max for good regulation is 27V (14.8 ≤ VIN ≤ 27) 19V typical
for a 15V reg the safe working max for good regulation is 30V (17.9 ≤ VIN ≤ 30) 23V typical

Taken from the National semiconductor datasheet

this is why you will often see a 5V reg taken off a lower voltage rail than what the 12V or 15V regs are (often from the 12V or 15V reg rail assuming that regulator can handle the current thats gonna be pulled through the 5V reg)

Cheers
Dave

 

[Militoy]

Yes 35V is the ABSOLUTE max and almost garanteed to kill it

for a 5V reg the safe working max for good regulation is 20V (7.5 ≤ VIN ≤ 20) 10V typical
for a 12V reg the safe working max for good regulation is 27V (14.8 ≤ VIN ≤ 27) 19V typical
for a 15V reg the safe working max for good regulation is 30V (17.9 ≤ VIN ≤ 30) 23V typical

Taken from the National semiconductor datasheet

this is why you will often see a 5V reg taken off a lower voltage rail than what the 12V or 15V regs are (often from the 12V or 15V reg rail assuming that regulator can handle the current thats gonna be pulled through the 5V reg)

Cheers
Dave

I'm sort of running between jobs right now - and too busy (or maybe too lazy) to dig into the National datasheet. Are these limits based on max input/output differential voltage - or max dissipation at full load?