shiggsy
Expert Class
Posts: 128
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posted July 23, 2007 03:54 AM
Whats the best way to get a clean power signal
I have just fitted an Digital Fuel Pressure gauge to the bike and the readout is jumping around quite a lot. When I power it from a 9 volt battery the readout is nice and steady so the bikes electronics seem to be interfering with it. I currently have it powered from the ignition cable although I have tried a couple of other cables and different ground points it still jumps about. Most things I have read on this indicate 'grounding' problems. To try and test this (and I don't know if this was wise or not) but I powered it from the 9 Volt battery running the gauges positive wire straight to the 9v battery but the negative wire first ran to a ground point on the bike and then on to the negative terminal of the 9v battery. The gauge was nice and steady so if this was a valid test it's the positive feed which is giving the problem not the ground point.
Anyway, what's the best way to go about getting wiring this gauge up to the bikes electronics to get a good signal so the readout isn't jumping around all over the place.
  
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ridgeracer
Pro
Posts: 1309
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posted July 23, 2007 10:21 AM
Well this is the same problem car audio equipment has and all the standard remedies should work for your equipment.
Besides good grounding and power feed as close to the battery as possible (be sure its fused) you could use an inline power filter. You can get these at any car audio store. These usually consist of coils or 'chokes' and capacitors and are essentially AC signal filters.
If you can't find anything let me know and I can show you how to make them out of radio shack parts.
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SHIGGSY
Expert Class
Posts: 128
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posted July 23, 2007 10:50 PM
Thanks for the answer (I know your busy with other threads at the moment  . I'll have to google for 'standard remedies' as I don't know what they are. 
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ridgeracer
Pro
Posts: 1309
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posted July 24, 2007 09:12 AM
 Edited By: ridgeracer on 24 Jul 2007 10:26
Never too busy to lecture on electronics (too many years as an high school vocational electronics instructor) Please excuse my lengthy reply. I've always found teaching a man to fish is better than handing out fish...
Electricity / Electronics is basically the study and manipulation of the flow of electrons. Because of that it lends itself to be explained using a hydrolic (flow of fluids) model. In this model:
Wires are pipes
Amps are gallons per hour.
Volts are Pressure (psi)
Resistance is pipe diameter.
The first rule of flow is the more you flow the lower the pressure. If you put a PSI gauge on your house plumping you'll read the highest pressure with all the taps closed. Start drawing a bath, run the washer, and water the garden and the pressure falls. Who hasn't been in the shower when someone flushed the toilet. To make it worse your bike is like a house with eight garden hoses, 4 injector, 4 coil, and some kid on your lawn constantly turning them full on then off rapidly. To make it worse your house doesn't get its water from the city water main but from a 12' water tower (battery) in the back yard that is kept filled by a pulsating pump (alternator)
Here is a scope shot of my bikes power measured at the rear parking light. The Key is on but the bike is not running. The line is a graph of voltage (up/down) over time (left to right). Its nice and straight, no fluctuations.
Here it is with the bike running at idle..
There is a lot of voltage fluctuations from the injectors and coils. Each of those squares on the grid represents 2 volts. The red line shows some 8 Volt spikes that didn't show up well in the photo. That is some heavy duty electrical noise. The standard filter for such a problem is shown below:
The choke is a coil of wire. It allows DC to flow but resists sudden changes in current direction. It gets rid of those big fast spikes. (sorry my water analogy's got nothing to explain how a coil works )The other component is a capacitor. In our water analogy it acts like a pressure tank or like those oil tanks you put on a bike to prevent oil pressure loss if the pump suddenly runs out of oil. Like them it can make up a pressure loss over the short term as long as on average you put more into it than you take out.
I didn't have a choke so I made one using 6ft of insulated 20 AWG solid copper wire wrapped tightly around a 16d nail. The cap you can get from Radio Shack. Its a 4700uF electrolytic polarized rated at 35VDC. On the cap you can always use larger values. The voltage should be at least twice the highest you expect. On the choke if you need current flows more than half an amp your going to need a larger gauge wire.
I hooked it up to the bike at the tail light running light wire. I used a resistor to simulate a 100mA load. If your pressure gauge draws a lot more than that you will need a bigger cap.
I put the scope on it with the bike running at idle and....
All the noise is gone. There is still a little bit of high frequency noise you cant see but its negligible. Used to be radio shack sold these in package called a noise suppressor and as I said just look up the local car stereo store. They will have them. You want the 3 wire ones. The two wire ones are just a choke. The 3 wire ones are Power in, power out, and ground for the cap. Or you can make your own like I did. Just wrap it up with lots of tape.
Do your gauge specs say how much current it uses?
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SHIGGSY
Expert Class
Posts: 128
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posted July 24, 2007 11:29 AM
Edited By: SHIGGSY on 24 Jul 2007 12:30
Wow, now that's an answer, i'll have to read this several more times.
Someone advised me to fit a relay whch I have done, triggered by the ignition wire so any acessories are now running off a (fused) line straight from the battery. Havent been out for a test run yet but previously tickover and especially some moderate revving would make the pressure guage dance and now it's holding steady so it seems to have worked.
What settings would the scope be set to, to take those measurements ?
Thanks again for the lesson
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