Can I hook up a 9V unregulated power supply to a 9V voltage regulator to stabilize the output?

Question

I have a 9V, 200mA unregulated power supply (wall wart.) With no load, the output of the device measures about 13 volts. Can I wire that to a 9V voltage regulator IC to stabilize the output, or do I need a power supply with a higher rating to connect to a 9V regulator?

Answer 1

With no load, the output of the device measures about 13 volts. Can I wire that to a 9V voltage regulator IC, to stabilize the output?

Yes, you can - and you should get a regulated 9 V out...however, you won't be able to pull 200mA and still have a regulated 9 V out. Only with a very light load, which pulls far less current, will regulation be maintained.

The internal circuit of a typical wall-wart might look like the following. A fuse might replace the 6 ohm resistor - I've used 6 ohms to very roughly simulate the transformer internal resistance:

^{simulate this circuit – Schematic created using CircuitLab}

Typically, these wall-warts give an average rated voltage of 9V when rated current (200mA) flows into a load (R2=45 ohms).

Since it is unregulated, that average voltage will have significant 50/60 Hz ripple, sometimes above 9V, sometimes below. How much ripple depends on how much internal filter capacitance is used - a guess would be in the range of 200uf - 1000uf.
Perhaps ripple might be 2V when 200mA flows. So output varies from 8V to 10V. A linear regulator can't provide 9V out during those input dips to 8V.
You'll have to de-rate output current significantly below 200mA. For the example wall-wart shown, you might expect about 60mA before the regulator chip fails to output a steady 9V.

Answer 2

The first problem you'll run into is dropout voltage, the minimum (positive) difference between the input and output voltages required by the regulator's circuitry. For the ancient 78xx series, this is about 2V, but for more more modern linear regulators, this can be less than 500mV. The 9V/200mA rating of the wall wart implies that the output voltage will be 9V at 200mA draw; if you are only planning on drawing a few 10s of mA out of the regulator, it's possible that the output of the wall wart will stay high enough to keep a linear regulator in operation.

The second problem, which isn't as severe in your specific case, is heat. Linear regulators "burn" the difference in voltage between the input and output as waste heat which must be rejected to the environment. Assuming worst-case: that the output voltage of the regulator doesn't sag at all (stays at 13V) and you are drawing 200mA from the regulator, the amount of heat produced is (13V-9V)*0.2A = 800mW. In a typical un-heatsinked TO-220 package, this translates into a die temperature rise of ~50C. The maximum die temp is 150C so your regulator should be fine, though expect a shorter lifespan and of course the case will be a burn hazard. If the wall wart voltage sags (which it will) and you are drawing less than 200mA from it, then the power dissipated will be less and so will the temperature.

Of course, the easy way to avoid both of these problems will be to use a DC/DC converter. There are drop-in modules that require no external components, have 2:1 or 4:1 input voltage ranges (9-36V is especially common), and have efficiencies in the mid-90%s. I would recommend using one of those in your case.

Answer 3

I recently built this 5 - 30-mimute timer circuit (https://www.elprocus.com/30-minute-timer-with-circuit-diagram-using-555-ic-7555-ic/) and decided to power it with a 9V, 200mA unregulated wall wart that measures close to 12 volts, no load. I have no idea why, but I decided to add a 9V regulator IC to the circuit.

I'm using the timer to turn on/off and power a panel with 8 firefly-type flickering LEDs in it. The low-current 9V wall wart with the 9V regulator chip in the circuit powers both the timer circuit and the LED panel, no problem.

The other thing I learned building this: You can put a lot of resistance on an LED and still get a lot of light. I have a power indicator LED on the timer circuit and I put a 100K resistor on it. For the flickering LEDs, I used resistors ranging from 75K to 100K. Anything less, and the panel is blinding. Even from across the room. (I'm using it to try to get into a meditative state to help me sleep. A smaller resistor for the power indicator on the timer circuit lit up a corner of my bedroom.) I no longer have to think about what value resistor to put on an LED! 100K with 9 volts, and you can still get plenty of light! So, reach for 10K and you can't possibly go wrong either. (Unless you're building a flashlight and want all the light you can get.)