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400ma bipolar 12v 2 ohm drain

Circuitz

Dec 7, 2013
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hey all,

What is the easiest and most reliable source of power consumption of these specs? My first thought is a 30amp Bosch 12v automotive relay but I think they only draw 100ma or so, the polarity can be switched and it will still function, if I add a properly rated resistor will that increase the drain in heat and still say under 3ohms? I could use an electric motor or solenoid but those aren't as reliable as a relay I would not think. 4 relays would work but not a very elegant approach. Would an inductor help? Obviously I need a zener diode for protection.

Any ideas?

Thanks!
 

AnalogKid

Jun 10, 2015
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Without some kind of context, I have no idea what you are talking about. When you say "drain", are you talking about the drain pin of a FET or MOSFET? What is a "2 ohm drain"? If you are observing 400 mAdc passing through a 2 ohm resistor, the voltage across the resistor will be 0.8 V. What does a relay have to do with any of this? A zener diode to protect what? An electric motor and a relay are two very different things, so I don't see how you could compare their reliability?

What is the application?
What is the problem you are trying to solve?
What is the input to the system?
What is the output from the system?
What is the power to the system?
Where are you located?

ak
 

duke37

Jan 9, 2011
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Please describe what you want to do, preferably with a diagram.
12V with 2Ω gives 6A not 400mA.
12V at 400mA needs 30Ω.
 

Circuitz

Dec 7, 2013
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I am trying to interface with an automotive pcm without settng a diagnostic trouble code. The power source is automotive 12-14 volts (alternator) pulse width modulated. It changes polarity as well. If the computer sees less than 2 ohm resistance or absolute value 400ma draw it sets a code.
 

BobK

Jan 5, 2010
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You still have not said what you want to do with it. You would not generally use a PWM signal to drive a relay, which is what you seem to be implying.

Bob
 

Circuitz

Dec 7, 2013
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I apologize for my lack of clarity. I intend to unplug the stepper motor that the car was designed to use and as I do not want the computer to know that I have unplugged it, I intend to plug in some sort of load that has the 2 ohm 400ma draw so that the computer will not know what I have done and set a code.
 

Audioguru

Sep 24, 2016
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Why are you tampering with your car's pollution controls?
 

Alec_t

Jul 7, 2015
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A stepper motor isn't a simple 2Ω resistor, even if that is its DC coil resistance. The coil has inductance which causes coil current to build up over time. I doubt the PCM will be fooled by a simple 400mA or 2Ω load (I could be wrong).
Won't disconnecting the stepper have an adverse effect on the car's stability?
 

Harald Kapp

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Simply disconnect the stepper shaft from the load and keep the motor itself within the loop. This the controller will still "see" the motor but there will be no effect on whatever the shaft is driving.
 

Circuitz

Dec 7, 2013
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A stepper motor isn't a simple 2Ω resistor, even if that is its DC coil resistance. The coil has inductance which causes coil current to build up over time. I doubt the PCM will be fooled by a simple 400mA or 2Ω load (I could be wrong).
Won't disconnecting the stepper have an adverse effect on the car's stability?
Yes it is DC coil resistance and yes it does build over time and temperature. The computer only looks for an absolute value range of 300-800ma on startup. The motor can draw 3 amps at max.

I would disconnect the stepper motor shaft and it would have an effect on stability. Instead I want to control it with my own pwm circuit instead of the onboard pcm.

Thank you for the replies so far. I could buy an extra one but they are rather bulky and expensive and would rather not carry the excess weight. Can anyone help me to make a 12 volt D.C. 2-3 ohm coil resistance that draws a constant 400ma and maintain the draw even with a polarity change?
 

Circuitz

Dec 7, 2013
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Please describe what you want to do, preferably with a diagram.
12V with 2Ω gives 6A not 400mA.
12V at 400mA needs 30Ω.

Yes slapping a 30ohm resistor in the loop would fulfill the amp draw through heat if it would last but I need 2 ohms D.C. Coil resistance and 400ma
 

Alec_t

Jul 7, 2015
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The computer only looks for an absolute value range of 300-800ma on startup.
In that case the 2Ω is irrelevant; a 400mA current source would suffice. How long does the start-up check take?
 

Circuitz

Dec 7, 2013
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In that case the 2Ω is irrelevant; a 400mA current source would suffice. How long does the start-up check take?
The computer also checks for 1.6ohm to 3.1ohm resistance. I do not know the duration of the check yet.
 

Circuitz

Dec 7, 2013
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That's physically impossible with a 12 V power source. Look up Ohm's law. 12 V / 2 Ω = 6 A.
Basta.

Perhaps I am incorrect by doesn't that assume 12volt constant and not pulse width modulated? At 50% duty cycle is that 6 volts or 3 amps? Or both?

Also I have measured the resistance of the motor with my multimeter and it does measure 2 ohms.
 

Harald Kapp

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Also I have measured the resistance of the motor with my multimeter and it does measure 2 ohms.
That's the DC resistance of the coil(s). When operating the motor with a pwm signal, several other factors come into play:
- pwm has an AC component. Impedance to AC depends not only on the DC resistance but on the inductance, too.
- when the motor is running, there will be a counter-emf due to the induction of a voltage in the coils when the motor magnets pass by.

Both factors contribute to the average current of the running motor being 400 mA. If that's what the controller is looking for, read post #13 again.
 

Alec_t

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Do you know how the computer does its 2Ω check and its 400mA check? I think you would have to simulate those operations correctly, particulaly any voltage switching and timing constraints, to fool it into believing the stepper was attached.
Assuming your stepper has multiple coils, does the computer check each coil?
 
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Circuitz

Dec 7, 2013
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I have no idea. I think am just going to start sourcing 2 ohm coil resistance 18henry relays and motors and solenoids and plugging them in to see what happens and then go from there. I bought a couple 18 Henry inductors a while back and coupling them to an led bulb on a project breadboard I managed to slip by the computer and maintain the pwm on my meter into the circuit but then when I got back to it about a year later, I could not reproduce it. I think the led burned out as a result of flyback em from the inductors as I didn't use any zener diodes. I am going to start tinkering more than thinking as this is getting nowhere.
 
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Alec_t

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I think the led burned out as a result of flyback emf from the inductors
You will need to make sure that back emf doesn't fry the computer if you start substituting inductors.
 
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