Volts constant but amps will fall off and run negative
- Thread starter aeroaddict
- Start date
aeroaddict
MEMBER
Boise ID area
Was able to put in another hour of social distancing without any 'neg amp' issues.
I'm going to call it 'problem solved' and a success.
I'm going to call it 'problem solved' and a success.
wireweinie
FOUNDER
Palmer, AK
Did you keep the old alternator? Did it have bearing issues? Did you split the case and see if anything was loose inside?
Web
Web
skywagon8a
MEMBER
SE Mass MA6
Exactly! Replacing components just for the sake of replacing something which solved an issue, sometimes is the answer and sometimes just hides the real issue. Unless you can put your finger on something and say "Ahah, That's it", you will never be certain until at a most inopportune time and place the original issue shows up again.
aeroaddict
MEMBER
Boise ID area
Agree with your autopsy comment but unfortunately the alternator was done under a warranty exchange. I may get notice or may not from the mfg.
I'm a little suspect of the 'bearing issues' as this was a passing comment by the auto parts store employee.
supercub1999
Registered User
Did you find out if this is the alternator? I have one of these was going to use on a project, maybe I wont.
wireweinie
FOUNDER
Palmer, AK
Ok. Here's why it's a bad idea to use automotive alternators with built in regs, in aircraft.
The regulator picks off power from the output of the alternator. The controlling element of the regulator is a transistor. Semi conductors, such as transistors, almost always fail in the shorted state, not open. In normal ops, the reg pulls power from the output circuit of the alternator and sends the current through the field coils. If the voltage is low, the reg allows more current flow in the field. When voltage rises the reg will reduce the current flow in the field. This happens dozens of times a second.
If something causes the reg to fail, that transistor will go to a shorted state. This means it turns into a piece of wire. So, now current flows from the output circuit, straight through the transistor (shorted), into the field coils. There is NO control of current flow in this condition, which means that output voltage will rise and keep rising as long as the alternator is turning. Since there is no field breaker for this system, there is only the output breaker.
But pulling this breaker does nothing to stop the path of current flow into the field coils. Since the alternator is already putting out power, the reg will continue to pull power from the output. That output breaker only isolates the alternator from the bus. The result is an uncontrolled runaway. In my past life, I worked on several cars that had this same scenario happen. The result was melted solder and copper slung around the engine compartment. Nothing that I would ever want in an aircraft engine cowling.
This is the reason that most aircraft alternators are still externally regulated. In those systems the regulator pulls power from the bus, through the field breaker, and sends it to the field coils via a wire. In the case of a runaway, if the field breaker is pulled (or the master switch turned 'off', as they should be in series), the path of current flow to the field is PHYSICALLY opened. With no current flow in the field, the output of the alternator will go to zero.
Web
aktango58
FRIEND
18AA
You have stated that battery voltage goes to 13V when output goes to zero. Good. If there really was a negative flow, voltage would HAVE to be less than 13V. The neg flow indication is bogus. Simple test: One of the two small shunt wires is used for voltage sense, both are used for current. Break the one not used for voltage and see if the Dynon current ind goes neg. I'm guessing the mV signal to the Dynon floats around in an undefined state causing a neg indication.
When the EGT connector on my old Dynon D180 is removed, the indications float all over, they don't go to zero.
When the EGT connector on my old Dynon D180 is removed, the indications float all over, they don't go to zero.
Last edited:
wireweinie
FOUNDER
Palmer, AK
I don't agree. Current flow indication is a function of the small voltage drop measured between the terminals of the shunt. A broken wire = no voltage drop. And no voltage drop means a zero indication, NOT a negative indication.
As for the disconnected EGT readings, was this with the engine running or shut down? One of the quick checks for disconnected temp probes is to power up the gauge and see which temp readings match each other but not engine temp. I.e., if the engine has just been shut down, power up the gauge and note the temp readings. If a couple have some lower reading (usually about 65*F) and all others have readings that match the expected temp of a warm engine, the lower readings usually indicate disconnected probes at those positions. This is because the temp circuits are set to go to a default when no input is sensed.
Web
As for the disconnected EGT readings, was this with the engine running or shut down? One of the quick checks for disconnected temp probes is to power up the gauge and see which temp readings match each other but not engine temp. I.e., if the engine has just been shut down, power up the gauge and note the temp readings. If a couple have some lower reading (usually about 65*F) and all others have readings that match the expected temp of a warm engine, the lower readings usually indicate disconnected probes at those positions. This is because the temp circuits are set to go to a default when no input is sensed.
Web
aeroaddict
MEMBER
Boise ID area
Agreed, that is valuable information.
In respect to the two small shunt wires, in a conversation with Dynon, they recommended to check/re-do the small wires that sense the V and amps if the alternator did not fix the problem. The Dynon tech person did mention that a faulty small wire could cause neg amps. I agree that if the circuit is 'open' then is should show 0 amps/volts.
I do have my visual and audio alarms set in case of 0 amps or volts. I can disconnect the alternator from the system via a 5amp breaker for the relay on the main 50amp breaker. Then I'm on just the battery.
In respect to the two small shunt wires, in a conversation with Dynon, they recommended to check/re-do the small wires that sense the V and amps if the alternator did not fix the problem. The Dynon tech person did mention that a faulty small wire could cause neg amps. I agree that if the circuit is 'open' then is should show 0 amps/volts.
I do have my visual and audio alarms set in case of 0 amps or volts. I can disconnect the alternator from the system via a 5amp breaker for the relay on the main 50amp breaker. Then I'm on just the battery.
Last edited:
It depends which wire has the bad connection on which way the amp(indication) will go.
Sounds like Dynon has installed some pull up or pull down resistors on the inputs since mine was built. This keeps the easily influenced high impedance input from wandering around and gives you a warning. If you want to make the input experience 0 volts, a sensitive input needs to be shorted, you can't assume its 0.
wireweinie
FOUNDER
Palmer, AK
Look at the shunt in #56 and imagine it intact, without the burnt spot. The gauge senses current through the two small wires that have the red ring terminals. The reason for the shunt to exist is that it has a small, fixed, resistance when measured from one block to the other. Lets assume it to be .1 ohms, for easy math. If 15 amps passed through the shunt it will produce a voltage drop. Volts = Amps X ohms. this works out to 15 amps X .1 ohms = 1.5 volts. That means that those two wires with the red ring terminals are reading the 1.5 amp drop. If either of those two wires are broken/disconnected, they would sense a voltage drop of zero. This would result in a reading of zero amps. The only way a broken wire could result in a reading other than zero is if the broken wire was grounding out to the airframe. Then, depending on which of these wires was grounded, the indicated amp reading will spike up OR down. And only momentarily as the grounded wire would melt almost immediately. Since the OP did not find or smell burnt wires, this was not the case.
Web
Web
wireweinie
FOUNDER
Palmer, AK
Another way to look at it. Take the shunt as in #56. Put your voltmeter leads on the screw terminals with the red rings. You will read a very small voltage while current is flowing through the shunt. Remove one of the voltmeter leads to simulate a broken wire, and note the reading. It will be zero.
Web
Web
wireweinie
FOUNDER
Palmer, AK
If an intermittent connection, it would vary from the correct reading to zero, not into the negative range.
Web
wireweinie
FOUNDER
Palmer, AK
Because an ammeter can indicate current in either direction which would reverse the polarity of the voltage differential.
wireweinie
FOUNDER
Palmer, AK
An ammeter is a passive instrument. It cannot reverse a signal. Yes it can indicate a negative (reverse) current flow but only if the polarity on the sense wires was reversed.
Web
I meant to say a "shunt."
wireweinie
FOUNDER
Palmer, AK
Doesn't matter. For a shunt to cause a negative reading of current flow, the current flow would have to reverse. This is the only way the sense wires will read a negative flow.
Web
Web
You don't need reverse current, just a negative voltage differential. It could be as simple as a cold solder joint somewhere in the Dynon or in the fusible links.
wireweinie
FOUNDER
Palmer, AK
A high resistance connection will only produce a low current indication. It has to be a reverse polarity condition in order to have a negative current flow indication.
That big ol chunk of shunt is an effective short to the Dynon input. (assuming no breaks and no current thru the shunt.) The resistance of the small wires and shunt is so small it is for all practical purposes a short and the reading is 0.
Come to think of it, the cracked shunt idea is backwards. A crack would lessen the cross section, up the resistance, so the voltage drop would go more positive.
I'm with Casey, cold solder on the homemade fuses. After all the poking around it may not reappear for a while. I like Peters' inline ATC fuses.
Come to think of it, the cracked shunt idea is backwards. A crack would lessen the cross section, up the resistance, so the voltage drop would go more positive.
I'm with Casey, cold solder on the homemade fuses. After all the poking around it may not reappear for a while. I like Peters' inline ATC fuses.
Last edited:
wireweinie
FOUNDER
Palmer, AK
The OP conditions were normal voltage indication and negative current flow.
Web
Web
wireweinie
FOUNDER
Palmer, AK
Once again, an open circuit on one of the small wires will not result in a negative current reading only a zero current flow. Both of the small sense wires need to be connected in order to have any current flow reading at all.
Web
Web
Yes both wires must connected to impose shunt voltage drop on the input. If you don't tell the input what the voltage drop across the shunt is, the input doesn't go to zero, it wanders around, or according to the OPs Dynon contact, gets pulled down so you notice and take action. This is an improvement from my older dynon where it drifts slowly and you may not notice for awhile.
