# New to this want to build adjustable voltage regulator.

Discussion in 'General Electronics Chat' started by Shockmyselfsmart, Dec 18, 2010.

1. ### Shockmyselfsmart Thread Starter New Member

Dec 17, 2010
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Ok Im using a 5 volt 3 pin ic regulator. So the circuit starts out with 5 volts. Its rated at 5 volts 400ma max. What I am trying to do is use this plus some resistor and a 3 leg adjustable resistor to get 100 ma max and around 4.5 volts all the way down to 0 volts 0 amps. This is what I have come up with this far. All i could think to use is paint. I have the 4.4 volts ruing threw the adjustable resistor and the arm that slides over the resistor is the out put. Im just wondering if I did the math correct or if this will even work?

Mar 24, 2008
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3. ### tom66 Senior Member

May 9, 2009
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This won't work. As you adjust the potentiometer the output will go from a very constant 5V to a value about 2-3V below the input.

4. ### Kermit2 AAC Fanatic!

Feb 5, 2010
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you need some voltage for the regulator to 'drop' while it performs its function. At least 6.5 to 7 volts input for a 5 volt regulator.

Get the data sheet on a 7805 regulator and read it.

5. ### spinnaker AAC Fanatic!

Oct 29, 2009
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You are right about the voltage drop but as As far as I know, the 7805 is not adjustable. The OP will need the LM317.

6. ### SgtWookie Expert

Jul 17, 2007
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An LM317 regulator has a minimum dropout (difference between the input and output voltage) of 1.7v, even under light load. Therefore, 5v-1.7v = 3.3v; which is the maximum you could expect to get out of it.

Also, it won't go down to 0v out - ~1.25v is the minimum you'll get without a negative supply. This should not be a large issue though, as there are not a lot of circuits that need less than 1.25v to work.

If you want a range of ~1.25v to ~4.5v, you'll need a "low dropout" regulator, or you will need to go to a switching regulator.

If you really want to go down to 0v, you will need to either have a dual polarity supply, or you will need to create a negative supply using an IC like an ICL7660, or perhaps a 555 timer with an inductor.

7. ### Shockmyselfsmart Thread Starter New Member

Dec 17, 2010
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Ok so did i fudge the math. Lets just stick with my paint set up so I under stand (still very new to this). Would the specks on the output line after the potentiometer be 3.8885 volts to 0 volts .100 amps to .0777 amps?

If I can get the math I can solve my problem.

Let me explain what I am doing with this. I am a mechanic what I am trying to do is make a test tool to simulate a air fuel sensor. If I can get this I can cut my diag time in half. What I want to do is plug the output of this into the air fuel ratio sensor input to the ecu. Spin my knob and watch my scan tool data list to see if I get a change. The ecu speck for max limits on the air fuel ratio sensor line is 4.8 volts and .100 amps. This will be used for testing only I dont what to use this on a running motor just as a fast check to see if I have a open/short or unwanted resistance on the input line of the air fuel ratio sensor.

This way I can just watch data instead of ohm testing out the wires and inspecting every connector along the way.

8. ### thatoneguy AAC Fanatic!

Feb 19, 2009
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There are pre-made diagnostic tools that do what you are looking for. Some sensors are moving toward digital or PWM instead of analog levels, so the type for the vehicle would be the best solution.

Apr 7, 2010
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10. ### Audioguru AAC Fanatic!

Dec 20, 2007
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This circuit causes a car with a missing emissions control system to produce awful air pollution that its computer cannot detect.

Like those "kids" cars that are lowered so they bounce all the time and with blackened glass so they cannot see where they are going at night. Their pollution STINKS and burns my eyes. Cops catch most of them but not all of them.

11. ### Jaguarjoe Active Member

Apr 7, 2010
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OP states his intentions in that paragraph. He does not intend to circumvent pollution controls, just check circuitry on non running engines.

12. ### Audioguru AAC Fanatic!

Dec 20, 2007
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I quoted the post with the simulator CIRCUIT that "fools" the ECU in the car so that:
"If you install the aftermarket downpipe with no cat (which as we all know is purely for off-road applications) .... shows how to build an oscillating signal generator with just the right frequency and voltage to fool the ECU."
THEN THE CAR POLLUTES LIKE MAD AND STINKS LIKE MANY KIDS CARS.

13. ### tom66 Senior Member

May 9, 2009
2,613
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So you want a variable voltage input? Just stick the potentiometer across a 9V supply and use the wiper as the input. If you're paranoid about blowing up the input, stick a 4.7V or 4.3V zener in reverse across the wiper and to ground. Remember with any battery powered device (e.g. 9V battery powered) you need to connect the vehicle ground to the battery ground or it won't work.

14. ### Shockmyselfsmart Thread Starter New Member

Dec 17, 2010
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No Im not looking for a pulse. Im not trying to replace the sensor. Just make a tool to test with. When I get a check engine lite on and the code is related to the air fuel sensor I have to deturmine whether the sensor failed, the wire harness or the ecu. If I can make this tool It will tell me that the harness is fine and the ecu is understaning its input. It will help me with diag.

Zener in reverse good idea i could use this in my set up.

All I realy want to know is if I did the math correct the second time? lol

Ok so did i fudge the math. Lets just stick with my paint set up so I under stand (still very new to this). Would the specks on the output line after the potentiometer be 3.8885 volts to 0 volts .100 amps to .0777 amps? Remember there is a load after the potentiometer probably a resistor inside the ecu.

I have seen this http://www.mkiv.com/techarticles/oxygen_sensor_simulator/ thats not what im trying to do that may work for a o2 sensor but not a air fuel I have to make sure I dont have over .100 amps or 4.8 volts.

Last edited: Dec 19, 2010
15. ### thatoneguy AAC Fanatic!

Feb 19, 2009
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Define "Air Fuel Sensor", where is it at on the vehicle? Is it up by the intake (MAS/MAF), On the exhaust, two spots (O2), or other?

16. ### Audioguru AAC Fanatic!

Dec 20, 2007
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The O2 sensor in the exhaust pipe ahead of the Catalytic Converter is incorrectly called an "Air Fuel Sensor". It gets very hot so its life is not long.

17. ### Jaguarjoe Active Member

Apr 7, 2010
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Actually, the oposite is true. The emissions system on a American vehicle must be warrantied for 50,000 miles. Most "One wire" or non heated Lambda sensors would fail before 50k because they were constantly being heated and cooled as the car would go in and out of idle. They couldn't handle the stress and the auto makers didn't want to handle the warranty claims. 3 or 4 wire heated sensors keep the sensor at one high temperature greatly prolonging their life.

18. ### thatoneguy AAC Fanatic!

Feb 19, 2009
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I guess my point is that if he's a mechanic, he should know this.

19. ### SgtWookie Expert

Jul 17, 2007
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Look, our OP just wants to use this for testing, and is a professional mechanic.

The way to approach this is to post something that will be quickly destroyed if the engine is running, which is just about anything that isn't designed with the high peak transients that will be experienced when the engine is running.

It could be powered directly by the battery, for all that matters. The low dropout really doesn't apply - as long as the circuit can output from near 0v to near 5v and at 0mA-400mA, it'll do the job.

It's not that hard.

I can't keep answering all of these questions by myself. Someone else come up with a viable circuit with parts that can be obtained from a local Radio Shack - which is what I have in mind. It can be done, simply and cheaply.

20. ### Audioguru AAC Fanatic!

Dec 20, 2007
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A modern O2 sensor is electrically heated so it warms up fast and is able to do its emissions control job sooner. I remember adding a 25W heatsinked resistor to "fool" the ECU that the heater was intact when it and the O2 sensor were removed for more power (and lots of emissions). I think an oscillator replaced the O2 sensor because the ECU expects the O2 sensor to oscillate between too rich and too lean.

I had a failed O2 sensor with a big hole burned through it.