View Full Version : Reducing 12v DC to 0.5V DC no more than 1A draw

10-07-2013, 12:08 AM
Right lads what do i need as far as components and what order or even better if someone can do me some schematic diagram to reduce 12v DC to 0.5V DC no more than 1A draw what i have got to do is replicate the steady output from a post cat O2 sensor voltage and i'm told that around 0.5v is good.

How do i go about making a regulator if i tap of a 12v feed i.e the heater wires to the O2 sensor?

Sure Jonathan or Irvin might be able to shine some light on this on..

10-07-2013, 12:28 AM
Seeing as you already have a smooth DC supply, then all you need to reduce the voltage is use 2 resistors to create a "potential divider". Values can be calculated using Ohms Law (https://en.wikipedia.org/wiki/Ohm's_law). Or you can cheat and use this online calculator: Potential Divider Calculator (http://www.random-science-tools.com/electronics/divider.htm)

Not sure where you're getting your 12V supply from, but if it's from the vehicle battery, then beware that it is greater than 12V, particularly if the engine is running as you will then see around 14V+ from the alternator/charging circuit.

10-07-2013, 12:49 AM
Cheers birchy, yes the car voltage will be 14.4v so i have used your link to calculate that i need a 620k and 20k Resistor if the voltage is 14.4v and the output was say 0.45v

As for getting my supply voltage there is normally 4 wires to an O2 sensor and two are the heater wires that are the voltage of the car i.e 14.4v when running, i will tap into this supply. looking at the schematic on that Potential Drive Calc will the resistors get hot or blow if i wire them like it suggests and take my feed from between the two?

10-07-2013, 01:01 AM
To be honest, I think you'd be better off using a small AA battery or similar for the 0.5V signal. It will certainly be a lot safer as you don't want to be shorting out a battery with 300A+ peak current!

10-07-2013, 01:15 AM
On further thinking...I don't quite understand what you are trying to do here? If you have a suspect O2 sensor (i.e. the output signal voltage is not within tolerance), then it's faulty. If it's not faulty, then you can use the (0.5V?) signal from it to test the ECU or whatever.

Might be worth making yourself a little variable power supply. Basically all you'd need is a 9V PP3 battery and a potentiometer.

10-07-2013, 01:28 AM
The problem with just using a resistor divider is that the output volts will vary with input voltage, and the O2 sensor heater voltage may vary from 10v under cranking to 14.4v when charging from the alternator.

I'm guessing this is to simulate a post-CAT sensor when using a straight thru exhaust (obviously for track use only as it wouldn't be road legal :) )

Where does the 1A requirement come from? And how accurate does the 0.5v need to be? And are you sure you want a DC voltage as many O2 sensors actually have a pulse output, the frequency of the pulse representing the O2 level.

Assuming you do want a DC voltage, then the correct way to do this is a resistor, a zener diode and a trim pot (variable resistor). A 560ohm 1/2w resistor from the nominal 12v supply connected to a 1.5v zener diode which is then connected to the ground side. A 500ohm trim pot is then connected across the zener diode and the output voltage taken from the centre of the trim pot and ground. The pot can then be adjusted to give a suitable output between 0 and 1.5v

10-07-2013, 09:39 AM
Hi Irving, yes the o2 sensor should normally output a oscillating voltage but the post cat should see a fairly stable voltage as it thinks the CAT is working and giving a stable O2 output. Is just to clear things up could you do a diagram so I don't balls things up please?

10-07-2013, 10:37 AM
Irving, automotive petrol lambda sensors take a direct output from the sensing element so they output a varying voltage.

Post cat sensors still change occasionally, although I'm not sure how much the ECU monitors their performance, so even by applying a steady voltage faults may still get set.

Another snag you may have to allow for is some ECUs control and monitor the Heater section, so you may need to add a pretty beefy resistor to apply a dummy load to avoid any faults relating to the heater circuit.

10-07-2013, 10:47 AM
M_c I was going to leave the sensor in still and just intercept the signal from the o2 that's all so the ecu will still see the right load on the heater side of things. The car in question will be fine seeing a steady voltage as this has been done before on this particular car.

10-07-2013, 11:14 AM
The setup suggested by Irving should do nicely then -

10-07-2013, 11:28 AM
Is it critical for the value of the zener diode as I can only get my hands on a 2.7v jobbie today

10-07-2013, 11:51 AM
No, but a higher voltage zener will mean the potentiometer will be working a bit harder.

10-07-2013, 11:54 AM
Is it critical for the value of the zener diode as I can only get my hands on a 2.7v jobbie today

Not really, your output voltage will be 0 - 2.7v rather than 0 - 1.5 so trimming it will be slightly more fiddly.

I'd set it to zero and connect it up and then bring it up to the voltage you want - this will negate any effects due to the load the ECU puts on the potential divider.

10-07-2013, 11:55 AM
Ahh ok. Well thanks for your help guys I'll try and slap something together today. Cheers :)

10-07-2013, 12:10 PM
Can I ask what car this is for?

10-07-2013, 12:22 PM
It's for a Toyota Corolla Compressor

10-07-2013, 03:01 PM
Well works like a charm on the meter so now to plumb it in.