How to set up 7805 regulator

Discussion in 'General Electronics Chat' started by riverrunner06, Jan 17, 2011.

  1. riverrunner06

    Thread Starter New Member

    Jan 17, 2011
    I have purchased a board kit called JectorRate. I have assembled it and tested it with no problems. The board requires 12v, 5v and ground to function. I am using it to test automotive fuel injectors and coils. While accessing 12v is not an issue, finding a 5v reference can be bothersome. I have investigated the 7805 as an addition to the board to regulate 12v to 5v. I have purchased a 7805 from Radio Shack and a heat sink (not sure if the heat sink is necessary, but would rather be safe than sorry).

    My initial thought was to simply add this chip into the aprpriate points on the board. then after reading more about regulator circuits I see that caps are used (I would imagine to filter the voltage). My question is are these caps necessary and benificial or would my idea of simply adding the chip be sufficient?

    I am providing the schematic for the board for reference. Thanks for any help that can be given. Please keep in mind that I am likely not as experienced nor educated as most here in electronics. I am a lowly auto tech trying to make my life a little easier at work :rolleyes:
  2. Wendy


    Mar 24, 2008
    Be sure to use a 0.1µ cap on the output and input. Larger caps won't hurt, but those are critical. It is in the datasheet of the parts.
  3. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    First off, the FI Tester[1] includes a 5V source. Why do you need another?

    Second, how much current do the injectors draw at 5V?

  4. PatM

    Active Member

    Dec 31, 2010
    Use the caps.
    Here is a example of a circuit.
  5. nigelwright7557

    Distinguished Member

    May 10, 2008
    I was tempted to use a 7805 without capacitors and found the 7805 output oscillated when current was drawn from it.
    Quickly fixed with some 0.1uf caps.
  6. Macgiver007

    New Member

    Jan 17, 2011
    Electrical spikes are very common in electronic circuits and using capacitors (o.1uF caps) are a common practice to use in voltage regulators and power supplies so that the signal is clear of these damaging "voltage spikes" Also, depending on the amount of current and how long you'll have the (7805) operated will decide if you will need the "heat sink". Even in low current setup; it is a good idea to use a "heat sink" because of the possibility of environmental heat, length of time at the "on" state, and it can increase its lifespan with the "heat sink" installed.
  7. riverrunner06

    Thread Starter New Member

    Jan 17, 2011
    Thank you everyone for the input! I will use the caps. I was concerned of varying and spikes, so my thoughts were valid. So now to fab a board to hold the components and tie it in. :)
  8. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    If you are concerned about spikes, add a 10Ω 5 watt resistor in series with the input of the 7805. That combined with the input cap will reduce spikes to something the 7805 can handle, at the cost of 500mA max output (most you can get without a heatsink anyway).

    The input cap should be 10x the size of the output cap on the 7805 to prevent oscillations as well. So 47uF on input side, and a 4.7uF aluminum electrolytic + a 100nF (0.1uF) film (poly/mylar/ceramic) on the output. The smaller caps are MUCH better at suppressing high frequency noise.

    You could reduce the input resistor to 5 ohms or so if the current draw is higher, this will also result in the 7805 dissipating less heat than with no resistor, as a chunk of the voltage will be dropped on the resistor. The regulator will dissipate Vin-Vout * current. Though Vin needs to be at least 2.5V higher than Vout for regulation with the 7805, so you can't carry it too far. I'm doing napkin calculations assuming 12VDC input.
  9. riverrunner06

    Thread Starter New Member

    Jan 17, 2011
    12vdc is correct, and spikes in that are not a concern as it will be from an automotive battery. Spikes on the output is where the concern would be. I do not believe the 5v demand would be high as the component being driven is using the 12v current, the board supplies pulse width to the ground for activation.
  10. Jaguarjoe

    Active Member

    Apr 7, 2010
    What are you using for rest of set up to test injectors- like a 40+psi source of non flammable fluid with characteristics similar to gasoline? I made a test stand for P & H injectors with a couple of 555's but never could find a good fluid to mimic gasoline. I couldn't get accurate flow rates but the repeatability was more than good enough to match up sets of injectors.

    What other coils can this device test? How does it test them?

    According to the write up, you'll need to supply 60ma at 5 volts. This puts 7 volts across the 7805 which will then dissipate 420mw. Its a good thing you bought the heatsink.
  11. marshallf3

    Well-Known Member

    Jul 26, 2010
    Unless these are tiny fuel injectors you need one heck of a lot of surge current to activate them.
  12. Adjuster

    Well-Known Member

    Dec 26, 2010
    An isolated auto battery may not give you any spikes, but the electrical environment in a vehicle is quite another matter. Surges due to such things as engine starting currents are notorious for damaging electronic devices. You would do well to take all reasonable precautions in the latter case.