# Variable DC Input Volatge, Constant DC Output Voltage

Discussion in 'The Projects Forum' started by mdweiss, Jan 28, 2010.

1. ### mdweiss Thread Starter New Member

Jan 28, 2010
14
0
Im looking to make a circuit that will be take anywhere from 10-16 volts DC in my car and output a constant voltage in the 14-16 volt range. Im having an issue with the consistency of the voltage to my fuel pump and its causing the fuel pressure to change. The pump can draw up to 20 amps.

They sell a device called a boost-a-pump that can be programed to ramp in more voltage as the engine needs it, but i dont require that just the voltage stabilization.

http://www.kennebell.net/accessories/boostapump/boostapump.htm

thanks for any help

2. ### SgtWookie Expert

Jul 17, 2007
22,182
1,728
... and something that costs less than 7 grand, right?

Seems to me it would be a whole lot cheaper to simply upgrade your alternator, and use a somewhat smaller alternator pulley to generate more power. If your electrical system is dropping below around 13.8v, your existing alternator/wiring just ain't makin' it.

You could slap on a DC-DC converter, but that's just a temporary band-aid; that power has to come from somewhere, and that "somewhere" will be the battery, until it's dead - and your power usage is so excessive, the alternator can't keep up.

Let's say you made or bought a DC-DC converter that was really pretty efficient - somewhere around 90%. If you had it set to output 15v and your pump still drew 20A, and your alternator was putting out 14v, then the input current would be:
I(in) = 20A * 15V/(14*90%) = 20A * 1.1905 = 23.81 Amperes.

But then let's say your alternator wasn't putting out enough current, and you were running off a partially discharged battery - let's say it was down to 11.5v.
I(in) = 20A * 15v/(11.5*90%) = 20A * 1.4493 = 28.99 Amperes.

Your load on the battery has gone up considerably to keep up the voltage on the fuel pump, and the lower it gets, the more power the DC-DC converter will try to pull out of it.

Basically, what you have right now is a power management problem. You can't run all of your stuff with your existing alternator. Adding on yet more stuff will just make the problem worse.

You have to figure out what to do about increasing your electrical power generation, or decreasing your use of electrical power, or a combination of the two.

3. ### mdweiss Thread Starter New Member

Jan 28, 2010
14
0
ok, I do have a brand new GM 10si alternator that has a built in voltage regulator, my problem is not the need to up the voltage, i would be able to get away with a constant 12 volt feed to the pump.

Whats happening now is that im seeing anywhere from 13.5 to almost 16 volts on my volt gauge and Ive connected it to a change in fuel pump psi and in turn the engine air fuel ratios are not consistent. You are correct in my alternator not regulating correctly, this is the reason why i would want to implement a circuit like this, so that if i did have a voltage drop i don't blow an engine.

The 20 amp draw on the pump is its max, im guessing around 8-10 amps is its average draw.

on a side note im currently using a boost-a-pump in a different car and its being set to constantly output 18 volts, i would like to make a circuit to replace this as well. it should be very similar im guessing.

Thanks for the help!!!

Jan 28, 2010
14
0
5. ### MikeML AAC Fanatic!

Oct 2, 2009
5,450
1,066
There is something seriously screwed up in your charging system. With the engine running at fast idle or above, the voltage at the battery should be between 14.2 and 14.7V. It is that way way in every car, truck, boat, airplane I have ever owned (and it needs to be that way based on the chemistry of the battery).

If it is not that way in yours; then fix the underlying problem instead of using a bandaid on the fuel pump.

6. ### SgtWookie Expert

Jul 17, 2007
22,182
1,728
Yep, I agree with MikeML.

Besides, I don't think you realize how difficult it would be to design and construct such a DC-DC converter for use in an automotive environment. It's minimum rating would need to be around 320 Watts. It's not like you could just buy a regulator IC, a couple of resistors, slap it in a box and expect it to work. It would take some careful engineering and a good bit of time to come up with a reliable yet cost-effective solution - and it would require someone with at least intermediate electronic skills to assemble and test it.

I suggest what would be more appropriate for your situation is a low voltage circuit interrupt; if the voltage to the fuel pump fell below a threshold level (say, 12v), the power to the pump is disconnected, shutting off the fuel supply. Better yet, monitor the fuel rail pressure sensor(s), and cut the ignition if fuel pressure dropped below a threshold value; that should eliminate the danger of fuel starvation/detonation damage. However, such a mod may not be legal for on-road use in many states due to emissions regulations.

7. ### mdweiss Thread Starter New Member

Jan 28, 2010
14
0
In my new Mercedes and GM Truck i see voltage fluctuation similar to what im experiencing in my race car, this is a common issue in the automotive world and short of running an external regulator between the alternator and the battery there is almost nothing you can do about it. Thats why companys sell these products, but i will take your advice that it would be hard to build and ill look to another solution.

Thanks for the help

8. ### SgtWookie Expert

Jul 17, 2007
22,182
1,728
I certainly don't want you to be completely disappointed - it's just that if you can find a commercial off-the-shelf product that will perform the desired task, you will be far better off than trying to build your own.

I've attached a couple of .pdf documents; the first one describes a number of different types of switching power supplies; the 2nd is a presentation of a design for a 100W DC-DC converter that is >90% efficient.

The latter is not suitable for your requirements as-is, because you need more than 3x the power output, the input & output voltage ranges are different, and whether or not the regulators used are certified for operation in automotive environments is an unknown. However, these two documents will help to give you some background knowledge on DC-DC conversion; and perhaps help you to appreciate the fact that I am not simply "blowing off" your request.

File size:
286.8 KB
Views:
69
File size:
715.3 KB
Views:
62
9. ### mdweiss Thread Starter New Member

Jan 28, 2010
14
0
Thanks SgtWookie ill take a look at the .PDFs, Im a Mechanical Engineer and I have moderate electrical experience, its just been a while.