Currently, I have several ICs connected in a circuit to a 6 VDC wall supply through a 7805T voltage regulator. The outputs of the circuit are LEDs and I also have a test point in my circuit in which I can attach a speaker to hear radio signals.

I have a sub-circuit that contains ICs, an LED and an LCD. This connects to the main circuit via the serial port (not to/from computer) through a 3 feet serial cable.

It seems that the LEDs dim slightly on the main circuit when I connect the sub-circuit but the LCD backlight turns on somewhat dim.

If I have the speaker connected as well as the LCD, the LCD back light dims even more to the point where I can't see it and the LEDs dim slightly.

all LEDs are the ultra-bright RGB type.

I rely on the internal resistor of the LCD when I connect the back light. Maybe that's my mistake. For the rest of the LCD, I have a 2.2K resistor connected between pin 3 (reference) and ground. For LED's I use a 2.2K resistor for red and 10K resistor for the remaining colours. I would expect the LEDs to be dimmer than the LCD back light but the opposite is true.

As my circuitry is extremely large, I want to try to solve this with simple methods. I'm just curious. Would replacing the 7805T voltage regulator with one that has a higher current rating all I need to increase the odds of this problem being fixed?


I then proceeded to add measurements. My circuit comes with a battery charger which is just another voltage regulator.

I tried an adapter that indicates it produces a 12VDC output with 800mA. The RGB LED's continue to have no problem lighting up. The regular LED's as well as the seven segment display also have no problems lighting up (including the larger display with forward voltage of 3.7 to 4.4 at 20mA). However, the LCD back light does not want to light up. I begin to think maybe those LCD modules I purchased for $2 from china didn't come with built-in resistors. I'm not 100% sure.

After the first regulator, the voltage output at the battery connector (which is the output of the LM317 from the charger) is 5.11 VDC. I'm not sure why its so low when the resistor ratio I used is 330 ohm and 1.8K. I was hoping for 7 or 8 VDC so I can charge the batteries as well when needed.

I then measured VCC and GND at an IC socket. VCC is connected to +ve output of the 7805T through a 0 ohm resistor. The result is 1.42 VDC with a battery symbol on the voltmeter. I wonder if my voltmeter is joking.

Next, I tried a 6 VDC 900ma adapter. The LEDs lit more dimly but the larger seven-segment display units did not light at all. And as usual, nothing on the LCD appeared, not even the back light. In both tests, I have A and K pins on the LCD connected to +ve and gnd respectively. I measured the battery terminal and it reports 3.81 VDC. I measured an IC socket and it reports 0.91 VDC.

I'm going to add my charger circuit to see if that's the culprit.


This is the circuit I use as my charger.

The leads on the far right are connected to digital circuitry and that output on the far right I was expecting to be 5 VDC since I'm using 7805T.

For capacitors, P2, P8, CHVREGIN, and CHVREGOUT are 22uF, and C7 is 2.2nF. Both op-amps are from a single LM358 IC.

CHGLIMIT is 100 ohms 1 watt. CBUFOUT is 10 ohms VDR1 is 3.3K and VDR2 is 4.7K as I'm trying to make a max charge reference point for the batteries. RHVD is 330 ohms 1 watt and RHVDG is 1.8K 1 watt. HVREG is the LM317 voltage regulator. Transistor is 2N2222A Diode is 1N4007 The solar jumper is used for an option to use the solar panel to charge batteries only if the plug-in charger isn't used.

chgen was shorted in all tests to make the charging action work and to offer a secondary source of power to the remainder of the circuit should batteries fail.


  • \$\begingroup\$ What's the actual output voltage from your mains adaptor and is your regulator actually a 7805, not a similar device? The 7805 requires at least 7 V so if your adaptor really does output 6 V (unregulated ones vary a bit) then your regulator is outputting less than 5 V. \$\endgroup\$ – TonyM Apr 16 '17 at 23:27
  • \$\begingroup\$ When I measure the 6 VDC adapter directly from the wall, the meter reports roughly 9V. I also used a 12 VDC adapter as well. The only thing with the 12 VDC adapter which is expected is that the regulator was warm. My regulator is 7805T not just 7805. The datasheet does report a 2V dropout, and I'm aware of this. \$\endgroup\$ – user143136 Apr 16 '17 at 23:40
  • \$\begingroup\$ We don't really care if the load is IC's, LED's, or a pinball machine. How much current is actually being demanded? Wdhat kind of heatsink is on the 7805? What is the current rating of the wall supply? (Assuming TonyM's comment doesn't lead you to solve your problem) \$\endgroup\$ – The Photon Apr 16 '17 at 23:44
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    \$\begingroup\$ And what voltage do you read out of the 6 VDC adapter under load? \$\endgroup\$ – The Photon Apr 16 '17 at 23:44
  • \$\begingroup\$ I have no heat sink on the 7805T because the max voltage I expected to enter it is 7 - 8 VDC. I added results in my question. \$\endgroup\$ – user143136 Apr 17 '17 at 1:34

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