Boost output voltage on a 7805 regulator

[Mad_Professor]

Re: Boost output voltage on a 7805 regulator

DCR of 0.4 Ohms is normal for that electromagnet coil.
The needle assembly is not binded up?

Well, hold on, those alligator jumpers are kind of known to have some resistance, let me take a second read.

what i was thinking.

it's pretty shit btw, the ones i used to see actually had the scale marked in ampsx10 (10,20,30 etc)

It's basic, most people don't know what it's for anyways.

BRB

[Mad_Professor]

Re: Boost output voltage on a 7805 regulator

Here are three readings.

[budm]

Re: Boost output voltage on a 7805 regulator

The resistance has to be very very low to reduce the Voltage drops and power dissipation on the meter.
If it is really 0.1 Ohm (did you zero out the meter probes resistance) and you have 30A flowing, the Vdrops will be (E = I x R) = to 30A x 0.1 Ohm = 3V! so with 12V system, the load will only see about 9~10Vdc, and the power dissipation (P = I x E) on the meter will be = to 30A x 3V = 90 Watts! 20A will be 40W.
http://www.sengpielaudio.com/calculator-ohm.htm

Did you verify that the needle is not stuck?

[Mad_Professor]

Re: Boost output voltage on a 7805 regulator

The resistance has to be very very low to reduce the Voltage drops and power dissipation on the meter.
If it is really 0.1 Ohm (did you zero out the meter probes resistance) and you have 30A flowing, the Vdrops will be (E = I x R) = to 30A x 0.1 Ohm = 3V! so with 12V system, the load will only see about 9~10Vdc, and the power dissipation (P = I x E) on the meter will be = to 30A x 3V = 90 Watts! 20A will be 40W.
http://www.sengpielaudio.com/calculator-ohm.htm

Did you verify that the needle is not stuck?

Yes, the needle is not stuck and moves freely.

Here's a wiring diagram from cluster to engine harness

Circuit A1-20RD and B1-20BK are for the ammeter.

[eccerr0r]

Re: Boost output voltage on a 7805 regulator

Looks like you should be fine to use a DMM (should still used a fused one) to measure the current. That meter is not taking all of the current (looks like it's only 22? 24? gauge wire winding?), there's another shunt in the schematic that will take the brunt of the current and reduce the total voltage drop. Looks like R6 is the shunt, and it's a piece of 10 or 6 gauge wire depending on what alternator you have. The shunt is fine if you're not getting anything at the meter, likely one of the wires to the meter is bad. You could test your meter with a current limited PSU...

[budm]

Re: Boost output voltage on a 7805 regulator

Looks like you should be fine to use a DMM (should still used a fused one) to measure the current. That meter is not taking all of the current (looks like it's only 22? 24? gauge wire winding?), there's another shunt in the schematic that will take the brunt of the current and reduce the total voltage drop. Looks like R6 is the shunt, and it's a piece of 10 or 6 gauge wire depending on what alternator you have. The shunt is fine if you're not getting anything at the meter, likely one of the wires to the meter is bad. You could test your meter with a current limited PSU...

Yes, it does look like it has shunt resistor (R6-10BK) in parallel withe AMP meter.

[fzabkar]

Re: Boost output voltage on a 7805 regulator

How many mA or uA would be required to drive the ammeter to full scale deflection?

A typical DMM outputs 1mA on the diode test range.

[stj]

Re: Boost output voltage on a 7805 regulator

stick a 9v battery across it - NOT a lithium one!

btw, with the car battery disconnected, you should meter a very low resistance between the 2 wires in the dash.
if it's not then you have a break in the loom.

[fzabkar]

Re: Boost output voltage on a 7805 regulator

Here is an interesting discussion with some actual measurements:

http://webcache.googleusercontent.co...ic.php?t=24551

So I set up a bench power supply for 12V, connected a headlight to it, then cranked the power supply current limit down to 0. I then added the car ammeter in series, and put a clamp-on ammeter on the circuit. I slowly increased the current - and the car ammeter reads full-scale at 0.2 amps. Due to the limited precision of my clamp-on ammeter, this could be anywhere from 0.1A to 0.3A, but at least it gets me in the ballpark.

[fzabkar]

Re: Boost output voltage on a 7805 regulator

How about a test?

With the engine off and meter disconnected, turn on the headlamps.

Measure the battery voltage (Vbat) and the voltage between the meter terminals (Vshunt).

Calculate the headlamp current using Plamp x 2 = Vbat x Ilamps, where Plamp is the rated power of one headlamp.

Now Vshunt = Rshunt x Ilamps, so this should allow us to calculate Rshunt.

If the meter goes to fsd at 30A, then Vfsd = Rshunt x 30A.

If fsd occurs at a meter current of 0.2A, then the resistance of the meter should be ...

Rmeter = Rshunt x (30A / 0.2A) = 150 x Rshunt

[stj]

Re: Boost output voltage on a 7805 regulator

meter FSD must be a minimum of 60A, probably closer to 90A

[Mad_Professor]

Re: Boost output voltage on a 7805 regulator

So we took the wrap on the wiring harness off on the engine side of firewall, and found the welded splices and I think a diode of some sort. Not sure what it's for yet, but we hooked up the ammeter to two places where the welded splices were and we were able to see changes on ammeter, with the A/C blower and head lamps on. Our guess is there is a fault between the firewall.

My dad and I are thinking of modding a volt meter inplace of the ammeter.

[stj]

Re: Boost output voltage on a 7805 regulator

but the ammeter is bi-directional.

anyway, there are nice led ammeters on ebay if they work bi-directional.

[kaboom]

Re: Boost output voltage on a 7805 regulator

My dad and I are thinking of modding a volt meter inplace of the ammeter.

Which will break the ckt- ammeters go in series and are low impedance. Put a voltmeter there and its high impedance opens the ckt.


It's not beyond either of you to figure out- someone put this in, someone can fix it.

Ammeters are more useful too. "At a glance," you get a "picture" of what's going on. A voltmeter either tells you you're not charging, or the regulator ran away...

Ideally, you'd use both.

But no "fireworks," OK? They'd be the wrong kind for the 4th!

[stj]

Re: Boost output voltage on a 7805 regulator

*if* the ebay ameters are bidirectional and support the "-" sign, you could use one.
they use an inductive loop.

so you bypass the original shunt resistor, and put the inductive loop over the battery cable.

[Mad_Professor]

Re: Boost output voltage on a 7805 regulator

Which will break the ckt- ammeters go in series and are low impedance. Put a voltmeter there and its high impedance opens the ckt.


It's not beyond either of you to figure out- someone put this in, someone can fix it.

Ammeters are more useful too. "At a glance," you get a "picture" of what's going on. A voltmeter either tells you you're not charging, or the regulator ran away...

Ideally, you'd use both.

But no "fireworks," OK? They'd be the wrong kind for the 4th!

We didn't plan on using the existing wiring, we would just supply a new run.

As far as I can see everything is working without the ammeter.

No fireworks, awww you're no fun. What's 4th of july without an electrical fire?

*if* the ebay ameters are bidirectional and support the "-" sign, you could use one.
they use an inductive loop.

so you bypass the original shunt resistor, and put the inductive loop over the battery cable.

Hmmm interesting, this is really up to my dad, whatever he decides, I'll do it.

[fzabkar]

Re: Boost output voltage on a 7805 regulator

So we took the wrap on the wiring harness off on the engine side of firewall, and found the welded splices and I think a diode of some sort. Not sure what it's for yet, ...

IIRC, Chrysler added a diode for back-EMF protection. Does the truck have a mechanical regulator for the alternator output, and does it have an electronic ignition (with magnetic reluctor pickup)? If so, then that's probably the reason for the diode.

Which will break the ckt- ammeters go in series and are low impedance. Put a voltmeter there and its high impedance opens the ckt.

There is a shunt (R6) in the cable between the alternator and battery, so there would be no problem in removing the ammeter.

Just FYI, here is a kit for a "Compact 0-80A Automotive Ammeter":
http://archive.siliconchip.com.au/cm...1/article.html
http://archive.siliconchip.com.au/st...30551_12mg.jpg

The eBay device would probably be much easier and cheaper to implement.