# Circuit working with 9V battery but not with power supply (7805 voltage regulator is used)

I created a circuit running on 5V DC, with an ATMEGA328P chip on it, to turn a 2-channel relay module on and off and receive signals with a 433MHz receiver.

As a power source I use a 9V battery which is connected to a 7805 voltage regulator to get 5V.

To have a more reliable power source I want to use a 9V power supply, instead of the battery. So I cut the wires of a 9V power supply (I bought for my Arduino), I connected them to the voltage regulator and plugged it in.

But when I tried to send a message over my 433MHz transmitter but the relay did not turn on. Then, I unplugged the power supply and connected the battery to the voltage regulator resent the signal and the relay turned on. I used a multimeter to make sure the power supply was working and the voltage and amperage where pretty much the same (arround 9V and 2.5 amps).

Why does not the power supply work?

Power supply label

AC/DC ADAPTER
Model No: MTR-910
Input: 100-240V ~ 50/60Hz 0.2A
Output: 9V DC 1A


Remeber that it worked with the battery, it doesn't work with the power supply.

• My guess is that the current required by your relay is more than the power supply can supply. Do you have a datasheet for your relay? Jun 15 at 17:54
• No but the relay is has printed on it the text SRD-05VDC-SL-C. Jun 15 at 17:57
• After connecting your power supply to the 7805, perform a load test of the 7805's output. That is, connect a load which will draw 100, 200, 300mA from the 7805 (like resistors) and make sure the output of the 7805 remains at 5V. Jun 15 at 17:58
• Ammeter is a meter that measures current. If your multimeter has current measuring ranges then those are ammeters. Jun 15 at 18:04
• (1) Did you add the decoupling capacitors to the input and output of the 7805 as shown in the datasheets? (2) If not, why not? What might happen if you leave them out? Jun 15 at 18:07