Design of current flow control system 1

[Gizmokev]

I have a "system" which I install into vehicles which I need to have the ability to fine control the current flowing into the system. My original thinking was to use a variable resistor inline of the positive but I dont think these will be up too the job.

Essentially the "system" will be set to allow a max of 10A flow at 13.8V, the resistance being set by the "system". I need to be able to adjust between 0A and 10A in this scenario.

My installations so far have been to take power direct from the battery (both + & -) and take them into a Ring Smart Relay RCT460 to detect when the engine is running and provide power down to the system. I have then had to adjust the current drawn by changing the "system" internally to get the required amps. This is a very fiddly job and takes time which can be better spent hence my request here.

So my request here is simple....does anyone have the ability to draw me a diagram of how to build a unit that will do 2 functions outlined underneath.

1. Replace the need for a seperate smart relay without the need to find an ignition live
2. Allow fine control of the current taken by the "system" (I call this choking it back) whilst leaving the voltage the same.

Here is hoping you can help me in my quest.

Kev

 

[waross]

Are you aware that automotive electrical systems are subject to switching transients call "load dumps" that can result in peaks in the range of 100 volts? Any automotive electronics not designed to be immune to these transients may fail the first time the headlamps are turned off.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Gizmokev]

no sorry I was not.....how do you factor these into a solution to my problem????

 

[IRstuff]

You can't do it the way you are attempting. You would need to design and build a programmable current power supply. You should hire an EE with familiarity in designing programmable, high current power supplies.

However, on a more fundamental note, what you are asking for will not physically work, based on the limited information you have provided. You cannot arbitrarily reduce the operating current of a piece of electronics. The "resistance" as you describe is integral to the proper operation of any circuit. That applies equally to motors, actuators, lights, radios, etc.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[Gizmokev]

Thanks IR Stuff

So what I need is a programmable current PS...that helps me narrow down the search on google.

As an aside given that I want to merely reduced the current flow by increasing the resistance and the voltage is constant (or virtually) at 13.8V (alternator output) this has got to be possible. It is not driving a motor or anything similar....the "system" I speak of is able to handle any amount of current from 0.2A up to 10A (resistance range of 69 - 1.38ohm) based on the limit set be the system. All I want to do is throttle back the current to be able to tune it to the vehicle and thought that this would be a simple solution. It appears I am mistaken unless others have other solutions.

 

[IRstuff]

Based on what you have described, it's still unclear to me that you have a variable resistance "system." Does it change current as a function of voltage? If so, then a programmably current supply is NOT the answer. Most things that take current are either nearly constant current, or behave like resistors. Without knowing more about what your "system" is, it's unclear whether an actual solution has been presented herein.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[Gizmokev]

I have attached a very simplistic diagram of the installation as it currently stands and as I wants it too be.

Ignore the "system" from now on and replace it with resistor at 1.38ohms.

The new unit will need to replace the Ring Smartcom Relay (part No RCT460) which is a device that detects when the alternator is operating and supplies power to the 1.38ohm resistor.

The new unit will also need to be able to control the current being supplied to the 1.38ohm resistor. Current is the key here but how it is done I dont mind.

Sorry for the basic diagram but I aint no electrical guru.

If you need any further information let me know.

 

[IRstuff]

Still no help. If your system is a "resistor" then you can only change the current by massively changing the voltage. That's just plain physics. For a resistor, then I = V/R, which is just Ohm's Law. In order to drop the current to 0.2A, then you'd be applying about 2.76V. You haven't presented anything that indicates that your "system" can change its current demand on its own.

I really suggest that you get at least a college student who has taken at least one year's worth of electronics and gotten good grades...

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[Gizmokev]

I thought you would be able to control the resistance by using a resistor or something similar?

The "system" will not change its own resistance....I can change it at time of install but wish to have a set resistance and then use the new electronic item to change the current drawn.

 

[MacGyverS2000]

A resistor in series will drop voltage. A resistor in parallel will drop current... but if you have no control of the current going in (what you need), the source will just provide more current to compensate.

I'll reiterate... find a competent EE.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[IRstuff]

"I thought you would be able to control the resistance by using a resistor or something similar?"

Unfortunately, you thought wrong, unless you've actually got a transistor.

Again, if you have something that's like a resistor, then changing the current requires changing the voltage, but in a rather huge way, and you'd need a programmable voltage power supply. Again, you should get at least a smart BSEE student or a graduated EE to help you.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[Gizmokev]

Ok can anyone recommend someone???

Live in North Wales

 

[waross]

There may be a solution to your problem but unless you are willing to share a little more information than so far, you don't have much chance.
But 0.2 Amps at 13.8 Volts will be 69 Ohms. 69 Ohms minus 1.38 Ohms is 67.62 Ohms. Put a heavy wire wound resistor greater than 67.62 Ohms in series with your system and adjust it manually.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dgallup]

There are devices that control current to loads like injectors. National Semiconductors has a single chip injector driver LM1949 that will control an output transistor to a steady current level irrespective of load (within limits). It does provide a momentary higher current but this is a very brief transient (less than 4 ms) so may be acceptable.

http://www.national.com/mpf/LM/LM1949.html#Overview

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