7805 & 12VDC Power Supply Problem

Thread Starter


Joined Feb 29, 2008
Dear All,

I have a single board controller prototype with a power supply problem (circuit diagram attached).

Question: Can a circuit be created to provide a 5VDC microprocessor with sufficient power, when the single 12VDC power supply is also driving other 12VDC components?

I have an externally supplied 12VDC, 2A power supply normally responsible for energizing one 5VDC item through an LM7805CT voltage regulator, but occasionally required to power four items simultaneously.

The items are:
o One 5VDC Microchip PIC16F876 microprocessor
o Two, 12VDC SPDT relays with 12VDC, 200 milliwatt coils
o One external 12VDC, 3 watt coil load, connected, long-distance to the relays via a 25 foot long, 24 gauge wire pair.

Normally the microprocessor operates perfectly; however, when the 12VDC relays are actuated (occasional 100 millisecond pulse), to send 12VDC to the distantly-located 3 watt load, the microprocessor’s 5VDC drops too low and it stops working mid-program.

I think my 12VDC, 2A external power supply is sufficient to power all components simultaneously; however, when current starts flowing into the 25foot wire pair, it momentarily starves the LM7805CT voltage regulator which stops the microprocessor.

Case 1: When the relays are actuated, with the distant load connected, the PIC microprocessor is denied sufficient power and it freezes.
Case 2: When the relays are actuated, with the distant load disconnected, the relays and the SBC PIC continue to operate okay.



Joined Feb 25, 2008
25ft of 24awg ?? what's the "R" and "Z" values of that wire pair?

place some "large" caps on both sides of the 7805.

those relays are inductive, when they energize the voltage will drop almost to zero for a brief few us or ms.

just a thought.


Joined Jul 17, 2007
You might want to put a nice big electrolytic cap out there at the remote relays, and add another bypass cap at the CPU, say 10uF tantalum. As DC Kid mentioned, it's the transients that are killing the CPU.

If that doesn't work, you could also try putting a small resistor in series with the relay's coils, which would limit the peak inrush current. 200mW @12V works out to 720 Ohms. Try a 150 Ohm or smaller resistor, 1/4W or larger.

You could also try putting a relatively large resistor from the collectors to ground, to keep some current flow going in the relays' coils. Just how much depends upon the dropout voltage of your relays. Try 2.2k; that will drop just under 3v across the relay when the transistor's off, and draw about 4.1mA current.


Joined Oct 16, 2008
i think a 7805 regulator is enough for microcontroller supply. but in your circuit you have used an half wave rectifier;a 220uF capacitor.it is the cause of your problem.you should use a full-wave rectifier or bridge rectifier with atleast 1000uF/25V capacitor as power supply ripple filter. no need of a 12V regulator(7812) for relay power supply. you can connect the relays to the +12vDC unregulated end or if you wish to use one regulator you can...
please don't connect the relay ground near to u-controller's ground;connect it directly to power supply ground.a 12V-0-12V / 2A tranformer will be enough for your usage.
please refer to my attachment...


Ron H

Joined Apr 14, 2005
JDC, your schematic shows the relay contacts shorting +12V to Vss. Is that a drawing error, or is that your problem?


Joined Dec 20, 2007
A relay coil is an inductor. An inductor doesn't have inrush current. A capacitor has inrush current. Current builds slowly in an inductor.

The 12VDC power supply is 24W. The loads in this circuit are only 3.4W. One relay feeds 12VDC to a 3W load. What does the second relay feed 12VDC to?

Ron H

Joined Apr 14, 2005
I just noticed that this thread is from 02/29/2008. No wonder we don't hear from JDC. I sure would like to know what the resolution was, though.


Joined Oct 16, 2008

Your schematic is misleading. 12V will not be produced from 12VAC, it would be more like 15Vdc.

yes....you are right..after rectification of 12VAC the DC volt will be boosted to +15V to +16V by the filter capacitor.but it won't affect the operation of a 12V relay..also it's not necessary to use a +12V regulator to supply a relay, the circuit woks fine for me all the times...
thanks for your suggestion:)


Joined Dec 27, 2007

I was referring to karthik_dm's posting, not the original OP, who is long gone.


I gotcha, just making the clarification in case someone actually expects 12V to be present there. Thanks!