Alternator Low Voltage - this worked (apparently) - PeachParts Mercedes-Benz Forum

Junkman · #1 03-14-2010, 07:39 PM

My SD alternator had low voltage ie 12v at idle. I installed a diode per these instructions (http://www.detomaso.nu/~thomast/alternator/) & now have slightly over 14V. An engineer friend of mine said that I likely have a diode out in the rectifier & that this addition compensates for that. Pack of 2 diodes were in stock at radio shack for $2.00 & change. Frequently, copying some "web mod" only covers a problem & isn't a long term fix. Any comments?

barry123400 · #2 03-14-2010, 07:52 PM

Could just be acting as compensation for an aging component or off value one in the regulator itself. Dropping one phase out of the actual alternator would not be reflected by lower output voltage. Instead in reduction of ability to meet designed output currents.

I am not saying your friend is wrong for sure. I just suspect the above senario is more likely.

ah-kay · #3 03-14-2010, 08:49 PM

I do not fully understand the mod. What I suspect is that the regulator is faulty and is giving half wave rectifier circuit instead of full wave. If you have an oscilliscope then you can see the DC waveform as what you measure is the RMS value.

I would think the the circuit would be OK as no harm is done. Other can chime in.

Diesel911 · #4 03-15-2010, 12:09 AM

The Author of the article gave his explination.
"The fix
The regulator senses the voltage between D+ and ground and adjusts the current to the rotor to obtain the nominal voltage. If one could raise the regulator ground slightly above the alternator ground, this would be seen as a higher output voltage."

What I understand from the article and other things I have read is:
The Bosch Alternator uses the Ground to alter the Voltage output to the Fields. The Diode increased the resistance to the Ground so the Voltage Regulator has to up the Voltage to overcome the Resistance.
The Author was using a Diode as if it was a Resistor not to rectify any wave form.

compress ignite · #5 03-15-2010, 01:11 AM

Produces A.C. (Which is "Rectified" to D.C. by the Diodes)
[The Alternator is a true three phase producer,so you should have at
least three different Diodes (think of them as "One Way" Electrical Valves)
to RECTIFY the ALTERNATING current TO DIRECT current]

"IF" you have a Dead (or misbehaving ?) Diode
[the question mark is because Diode (s) either work OR DON'T]
WHIPPING THE OTHER TWO DIODE POSITIONS (Like and old or dead horse)
[Which is what your friend is suggesting the VR "Alteration" accomplishes]
WILL NOT PRODUCE THE CORRECT D.C. current scenario.

Your Voltage Regulator is bad and all you've done (Rather Neatly) is postpone
it's eventual total demise.

A DMM set to A.C. voltage (With the engine running) that shows anything more than
.5 Volts AC (At the Battery terminals) denotes a Bad Diode.

Voltage regulator
From Wikipedia, the free encyclopedia
Jump to: navigation, search
Electronic symbol for Voltage regulator

A voltage regulator is an electrical regulator designed to automatically maintain a constant voltage level.

It may use an electromechanical mechanism, or passive or active electronic components. Depending on the design, it may be used to regulate one or more AC or DC voltages.

With the exception of passive shunt regulators, all modern electronic voltage regulators operate by comparing the actual output voltage to some internal fixed reference voltage. Any difference is amplified and used to control the regulation element in such a way as to reduce the voltage error. This forms a negative feedback control loop; increasing the open-loop gain tends to increase regulation accuracy but reduce stability (avoidance of oscillation, or ringing during step changes). There will also be a trade-off between stability and the speed of the response to changes. If the output voltage is too low (perhaps due to input voltage reducing or load current increasing), the regulation element is commanded, up to a point, to produce a higher output voltage - by dropping less of the input voltage (for linear series regulators and buck switching regulators), or to draw input current for longer periods (boost-type switching regulators); if the output voltage is too high, the regulation element will normally be commanded to produce a lower voltage. However, many regulators have over-current protection, so that they will entirely stop sourcing current (or limit the current in some way) if the output current is too high, and some regulators may also shut down if the input voltage is outside a given range (see also: crowbar circuits).

Rectifier
From Wikipedia, the free encyclopedia
Jump to: navigation, search
For other uses, see Rectifier (disambiguation).

A rectifier is an electrical device that converts alternating current (AC) to direct current (DC), a process known as rectification. Rectifiers have many uses including as components of power supplies and as detectors of radio signals. Rectifiers may be made of solid state diodes, vacuum tube diodes, mercury arc valves, and other components.

A device which performs the opposite function (converting DC to AC) is known as an inverter.

When only one diode is used to rectify AC (by blocking the negative or positive portion of the waveform), the difference between the term diode and the term rectifier is merely one of usage, i.e., the term rectifier describes a diode that is being used to convert AC to DC. Almost all rectifiers comprise a number of diodes in a specific arrangement for more efficiently converting AC to DC than is possible with only one diode. Before the development of silicon semiconductor rectifiers, vacuum tube diodes and copper(I) oxide or selenium rectifier stacks were used.

Early radio receivers, called crystal radios, used a "cat's whisker" of fine wire pressing on a crystal of galena (lead sulfide) to serve as a point-contact rectifier or "crystal detector". Rectification may occasionally serve in roles other than to generate D.C. current per se. For example, in gas heating systems flame rectification is used to detect presence of flame. Two metal electrodes in the outer layer of the flame provide a current path, and rectification of an applied alternating voltage will happen in the plasma, but only while the flame is present to generate it.

Junkman · #6 03-15-2010, 11:08 AM

Quote:

Originally Posted by compress ignite

Your Voltage Regulator is bad and all you've done (Rather Neatly) is postpone it's eventual total demise.

This is the 3rd voltage regulator. I replaced the original with a Bosch from the parts store. Still had low voltage. Took the alternator to the local rebuilder who has recently retired. I'm not sure what parts were changed but charging was better (measured 13.5+ at posts, not 14). Charging dash light began to flicker. Voltage output decreased and I still had times where battery went dead. Tested battery and checked for parasitic drains - battery tests good (at parts store & hydrometer, voltage consistent between cells), no drains measured. Put in diode & increased voltage output. I suppose that new alternator will be next step when this one fails. Am I missing something?

ah-kay · #7 03-15-2010, 12:03 PM

Quote:

Originally Posted by compress ignite

Produces A.C. (Which is "Rectified" to D.C. by the Diodes)
[The Alternator is a true three phase producer,so you should have at
least three different Diodes (think of them as "One Way" Electrical Valves)
to RECTIFY the ALTERNATING current TO DIRECT current]

"IF" you have a Dead (or misbehaving ?) Diode
[the question mark is because Diode (s) either work OR DON'T]
WHIPPING THE OTHER TWO DIODE POSITIONS (Like and old or dead horse)
[Which is what your friend is suggesting the VR "Alteration" accomplishes]
WILL NOT PRODUCE THE CORRECT D.C. current scenario.

Your Voltage Regulator is bad and all you've done (Rather Neatly) is postpone
it's eventual total demise.

A DMM set to A.C. voltage (With the engine running) that shows anything more than
.5 Volts AC (At the Battery terminals) denotes a Bad Diode.

Voltage regulator
From Wikipedia, the free encyclopedia
Jump to: navigation, search
Electronic symbol for Voltage regulator

A voltage regulator is an electrical regulator designed to automatically maintain a constant voltage level.

It may use an electromechanical mechanism, or passive or active electronic components. Depending on the design, it may be used to regulate one or more AC or DC voltages.

With the exception of passive shunt regulators, all modern electronic voltage regulators operate by comparing the actual output voltage to some internal fixed reference voltage. Any difference is amplified and used to control the regulation element in such a way as to reduce the voltage error. This forms a negative feedback control loop; increasing the open-loop gain tends to increase regulation accuracy but reduce stability (avoidance of oscillation, or ringing during step changes). There will also be a trade-off between stability and the speed of the response to changes. If the output voltage is too low (perhaps due to input voltage reducing or load current increasing), the regulation element is commanded, up to a point, to produce a higher output voltage - by dropping less of the input voltage (for linear series regulators and buck switching regulators), or to draw input current for longer periods (boost-type switching regulators); if the output voltage is too high, the regulation element will normally be commanded to produce a lower voltage. However, many regulators have over-current protection, so that they will entirely stop sourcing current (or limit the current in some way) if the output current is too high, and some regulators may also shut down if the input voltage is outside a given range (see also: crowbar circuits).

Rectifier
From Wikipedia, the free encyclopedia
Jump to: navigation, search
For other uses, see Rectifier (disambiguation).

A rectifier is an electrical device that converts alternating current (AC) to direct current (DC), a process known as rectification. Rectifiers have many uses including as components of power supplies and as detectors of radio signals. Rectifiers may be made of solid state diodes, vacuum tube diodes, mercury arc valves, and other components.

A device which performs the opposite function (converting DC to AC) is known as an inverter.

When only one diode is used to rectify AC (by blocking the negative or positive portion of the waveform), the difference between the term diode and the term rectifier is merely one of usage, i.e., the term rectifier describes a diode that is being used to convert AC to DC. Almost all rectifiers comprise a number of diodes in a specific arrangement for more efficiently converting AC to DC than is possible with only one diode. Before the development of silicon semiconductor rectifiers, vacuum tube diodes and copper(I) oxide or selenium rectifier stacks were used.

Early radio receivers, called crystal radios, used a "cat's whisker" of fine wire pressing on a crystal of galena (lead sulfide) to serve as a point-contact rectifier or "crystal detector". Rectification may occasionally serve in roles other than to generate D.C. current per se. For example, in gas heating systems flame rectification is used to detect presence of flame. Two metal electrodes in the outer layer of the flame provide a current path, and rectification of an applied alternating voltage will happen in the plasma, but only while the flame is present to generate it.

WOW. A rectifier/diode/voltage regulator 101 in the forum.

Unless you have a scope, you really do not know the quality of the DC. It could be half wave or full wave or a lot of ripple. You cannot tell with a DMM. If you can fix it-ish by putting a diode in there then I can only suspect one of more of the diodes in the bridge circuit is faulty. I expect your fix will work well.

hey_allen · #8 03-15-2010, 12:41 PM

Having just gone a few rounds with the regulator in my 500SEC, including digging up the diagram for the wiring in our alternators, and watching it with a scope, I found out a few interesting (at least to me) things about how MB did it.

If the diode bridge is functioning properly, it's a full wave, three phase rectifier.

When one of my diodes failed, it showed up initially as a charging system light glowing brighter with increase in engine rpm, as well as the brake wear light and empty fuel tank light.

Once I hooked my oscilloscope to the car to check the output waveform, I found that instead of the specified even ripple, mine had a sharp drop out on every sixth wave.

That being said, the 14 volt charging system check was at ~2300 rpm on my car, as the alternator just isn't spinning fast enough to be putting out full voltage until it's up to it's target rpm. The ETM had a list of different vehicles and their respective minimum RPM's for testing.

Rafi · #9 03-15-2010, 01:43 PM

Quote:

Originally Posted by Diesel911

The Author of the article gave his explination.
"The Diode increased the resistance to the Ground so the Voltage Regulator has to up the Voltage to overcome the Resistance.
The Author was using a Diode as if it was a Resistor not to rectify any wave form.

There is 0.6 v drop across a diode (semi-conductor) if it is silicon which most diodes are, germanium diode has 0.3 v drop across. So if you do not need to bump up the charging voltage as much you use germanium diode.
If you want to increase the charging voltage even more then put 2 diodes in series it will raise the ground to 1.2 v as far as sensing is concerned.

Diesel911 · #10 03-15-2010, 01:56 PM

Quote:

Originally Posted by compress ignite

Produces A.C. (Which is "Rectified" to D.C. by the Diodes)
[The Alternator is a true three phase producer,so you should have at
least three different Diodes (think of them as "One Way" Electrical Valves)
to RECTIFY the ALTERNATING current TO DIRECT current]

"IF" you have a Dead (or misbehaving ?) Diode
[the question mark is because Diode (s) either work OR DON'T]
WHIPPING THE OTHER TWO DIODE POSITIONS (Like and old or dead horse)
[Which is what your friend is suggesting the VR "Alteration" accomplishes]
WILL NOT PRODUCE THE CORRECT D.C. current scenario.

Your Voltage Regulator is bad and all you've done (Rather Neatly) is postpone
it's eventual total demise.

A DMM set to A.C. voltage (With the engine running) that shows anything more than
.5 Volts AC (At the Battery terminals) denotes a Bad Diode.

Voltage regulator
From Wikipedia, the free encyclopedia
Jump to: navigation, search
Electronic symbol for Voltage regulator

A voltage regulator is an electrical regulator designed to automatically maintain a constant voltage level.

It may use an electromechanical mechanism, or passive or active electronic components. Depending on the design, it may be used to regulate one or more AC or DC voltages.

With the exception of passive shunt regulators, all modern electronic voltage regulators operate by comparing the actual output voltage to some internal fixed reference voltage. Any difference is amplified and used to control the regulation element in such a way as to reduce the voltage error. This forms a negative feedback control loop; increasing the open-loop gain tends to increase regulation accuracy but reduce stability (avoidance of oscillation, or ringing during step changes). There will also be a trade-off between stability and the speed of the response to changes. If the output voltage is too low (perhaps due to input voltage reducing or load current increasing), the regulation element is commanded, up to a point, to produce a higher output voltage - by dropping less of the input voltage (for linear series regulators and buck switching regulators), or to draw input current for longer periods (boost-type switching regulators); if the output voltage is too high, the regulation element will normally be commanded to produce a lower voltage. However, many regulators have over-current protection, so that they will entirely stop sourcing current (or limit the current in some way) if the output current is too high, and some regulators may also shut down if the input voltage is outside a given range (see also: crowbar circuits).

Rectifier
From Wikipedia, the free encyclopedia
Jump to: navigation, search
For other uses, see Rectifier (disambiguation).

A rectifier is an electrical device that converts alternating current (AC) to direct current (DC), a process known as rectification. Rectifiers have many uses including as components of power supplies and as detectors of radio signals. Rectifiers may be made of solid state diodes, vacuum tube diodes, mercury arc valves, and other components.

A device which performs the opposite function (converting DC to AC) is known as an inverter.

When only one diode is used to rectify AC (by blocking the negative or positive portion of the waveform), the difference between the term diode and the term rectifier is merely one of usage, i.e., the term rectifier describes a diode that is being used to convert AC to DC. Almost all rectifiers comprise a number of diodes in a specific arrangement for more efficiently converting AC to DC than is possible with only one diode. Before the development of silicon semiconductor rectifiers, vacuum tube diodes and copper(I) oxide or selenium rectifier stacks were used.

Early radio receivers, called crystal radios, used a "cat's whisker" of fine wire pressing on a crystal of galena (lead sulfide) to serve as a point-contact rectifier or "crystal detector". Rectification may occasionally serve in roles other than to generate D.C. current per se. For example, in gas heating systems flame rectification is used to detect presence of flame. Two metal electrodes in the outer layer of the flame provide a current path, and rectification of an applied alternating voltage will happen in the plasma, but only while the flame is present to generate it.

From what I read in the article the guy put the Diode where I have the Red Arrow pointing in the pic. At that point the the currnt has already been rectified.
Comment from the article:
"The regulator senses the voltage between D+ and ground and adjusts the current to the rotor to obtain the nominal voltage. If one could raise the regulator ground slightly above the alternator ground, this would be seen as a higher output voltage."
In the 2nd pic I put the D+ circuit to the Voltage Regulator in Red.

compress ignite · #11 03-16-2010, 06:01 AM

My point (and the article's)
Forcing/Fooling the Regulator to have the Alternator produce MORE voltage
does not FIX the "less than full waveform" caused by a bad Rectifier (Diode).

from the Pinpernell's post earlier:
"I would recheck the alternator, by disconnecting all the wires attached to it, including the heavy battery cable, put your multi tester in continuity mode, and place one lead of the tester on the alternator battery connection post, and the other lead to the engine block or any ground point. If you get a continuity reading, it means that one or more of the alternator diodes have failed, and is allowing current to flow from your battery to the engine block. I had that happen to my car, and if the battery were left connected, it would totally discharge in a matter of a few hours with the key in the off position. "

JHZR2 · #12 03-16-2010, 07:39 AM

That is fine, but what if the rectifier is still fine, but the voltage output is still too low. AFAIK, the voltage regulators on these cars arent even 14.1V. This is a problem for many modern battery chemistries, which like a good charge up to 14.1-14.5V.

A battery is full when it takes less than 0.5A at somewhere in that 14.1-14.5VDC output range. It is a lot more difficult to fully recharge a battery at 13.4V, which is roughlhy what the DC bus in a w123 seems to be.

If on a working alternator, the output stage can be fooled into slightly higher voltage, there is working benefit across the board. Im not proficient enough to say if this is a good/bad/right/wrong way to do it, I just too would like to see slightly higher operating voltage.

compress ignite · #13 03-16-2010, 12:19 PM

I'm just saying it won't fix a blown Rectifier DIODE.

For the trick to work
(and the Alternator to produced a fully rectified waveform)
the Diode Pack (Rectifier) must be good.

http://www.autoshop101.com/forms/alt_bwoh.pdf

http://www.bcae1.com/charging.htm

http://images.google.com/images?client=firefox-a&rls=org.mozilla:en-US

fficial&channel=s&hl=en&source=hp&q=Alternator+Rectifier&um=1&ie=UTF-8&ei=ErCfS-HIOeWutget5cj7DQ&sa=X&oi=image_result_group&ct=title&resnum=6&ved=0CCsQsAQwBQ

Parts Source:
http://www.nationsautoelectric.com/wai-global-link.html

JHZR2 · #14 03-16-2010, 03:03 PM

Absolutely agree with you. That said, the other part of the question was the long-term implications of this. A band aid fix on bad equipment is no good, but what about using this trick long-term on good equipment?

hey_allen · #15 03-17-2010, 10:53 AM

The first place I saw the diode trick to pull the voltage up the 0.6-0.8v was as a fix on a perfectly functional alternator, just one that wasn't pushing enough voltage for the application it was in.

The owner had installed an AGM (Optima type...) battery and it required above 14.1 to even charge.

After all my trouble with my car's failed diode, the new alternator only puts out 14.1 at peak, and the failed diode one was doing the same. It was setting numerous lights off on the dash when the diode failed, regardless.

I'm not sure what happens in a w123 that one might be running in the 13v range consistently, but I'd look into voltage drops in the main wiring runs before doing a mod on the alternator, but wouldn't stress over the alteration too much.

If someone does decide to modify their regulator, make sure to check the voltage drop on the diode you choose. The latest electronics project run at Radio Shack (due to being too impatient to wait for a Mauser or DigiKey order) was showing they're stocking .8v drop for the majority of their diode selection, and I wouldn't want to accidentally push the voltage too high and cause a battery to fail due to overcharging.