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Hey Bob M, A Thought about Voltage Regulators

Red Dave

Red Dave

Well-known Member
I got to thinking about the post below concerning volatge regulators suffering early failure. Specifically, contact failure due to higher than designed field current.

I've done contact protection on equipment years ago using resistor/capacitor networks or varistors. 12 volt varistors are available and will suppress the arc drawn by contact opening prolonging the life of the contacts. The proper resistor & capacitor in series will do the same thing. Somewhere in my pile of "stuff" I have a nomagraph that, given the voltage and amperage of the circuit, will specify the proper values to use. We've used it at work with a lot of success over the years.

Do you know any reason that approach wouldn't work on a voltage regulator? It would have to be wired directly across the contacts. I'll root around and see if I can find that nomagraph.
From Here
 
I been traveling n missed the VR discussion below but until Bob arrives I have some thoughts. I"ve come across what I believe were suppression diodes on some units which is one of the reasons they would be polarity senstive.... One thing to consider when trying to dampen and supress arcing across VR relay contacts is that its not quite as simple as that relay just opens n closes similar to the old manual Low High charge light switches, they can "chatter" which acts to me like pulse width modualtion for more precise regulation and that amounts to changing the frequency (duty cycle) so Im not quite sure selecting the capacitance n resistance values will be as easy as if it were straight DC On and Off switching. Of course, each is still a single discrete DC switching occurence so maybe it IS EASY AFTER ALL????

The coatings on those relay contacts is expensive and Id tend to contribute that fact to why they may be scripming to save a few bucks and why the new ones fail so soon?????

Any agreement or disagreement or thoughts ?? NO WARRANTY ON THIS LOL just brainstorming here

John T
 
Interesting thought Dave...

If one can figure out the inductance value for the generator field winding, sizing an RC network to place across the VR relay contacts to surpress arcing should be a relatively straightforward.

However it's been MANY years since I last sat took an EE course - darned if I recall the equations anymore(!)

----

Let us know if you turn up that nomograph!

Bob
 
Hey John - A clamping diode or varistor across the contacts makes sense too.

FWIW I recall the VR's on a couple 60's era, Mack and Reo farm dumps where I wrenched sometimes as a teenager. The trucks had BIG 80 - 100 amp generators (this is pre-alternator days...). The VR's had both capacitors and what looked like diodes or resistors across both the voltage and current regulator contacts inside the VR box.

At the time I dismissed them as RFI surpression components to reduce static on the AM cab radio. Could it be these extra parts etc had and a more fundamental purpose??

It might also would explain the polarity sensitive nature of those VR's...

Bob M
 
I have seen the diodes (Autolite called them rectifiers) in Autolite VR from late 50's.

John mentioned the 'vibrating/pulse width modulation' and I will add that I have observed this on oscilloscope in the hundreds of cycles per second, as best memory serves.

Also, some regulators have two field resistors. The purpose being arc suppression. The equivalent resistance seem by the arc voltage is one-half the normal, so that helps arc suppression and the pair are chosen so as to provide same field current as with a single resistor.
 
I will chime in a tad with the idea that if there is a RC, or diode element in place to reduce radio interference, it could, in addition reduce the very arcing giving rise to the RF. At any rate it is sad to see deteriorating quality on VRs. I think the chopping effect of the field circuit might be compromised if the dampening was serious, Just a thought. Jim
 
In this particular application all you need is a diode in parallel with the generator field with its cathode toward the positive terminal. The diode should be rated for the full field current. The diode will be reverse-biased when the VR is energizing the field. When the VR contact opens the field current will flow through the diode, decaying at the field's LR time constant. There will be little arcing because the voltage across the contact won't exceed battery voltage.
 
Interesting thought. It would definitly discharge the field's inductive kick when the contacts open, but I think that you would still want some protection on the contacts because they still have to break the current going to the field.

Good thinking, and my intent was to stir up some "brainstorming". Lots of talent on here.
 
Bob, I did find that nomagraph in my desk drawer at work. Drop me an email and I'll scan it in and send it to you.

My email is davef55 at hotmail dot com.
 
I don't think that the rapid opening and closing of the contacts would be a problem. They certainly wouldn't "chatter" any more than the vibrator on the old vibrator type power supplies on tube type radios, and they used R/C contact suppression.

I agree that the hard surface coating on the contacts may be getting scrimped on to "save" a few pennies per unit. It's those thrifty chinese ;)
 
> Interesting thought.

It has been standard practice for inductive loads for decades.

> It would definitly discharge the field's
> inductive kick when the contacts open, but I
> think that you would still want some protection
> on the contacts because they still have to
> break the current going to the field.

No it doesn't: that's the point. The current simply transfers to the diode. Since the current through the inductance is not interrupted there is no inductive kick.

In some applications one adds an RC snubber network to bring the current down more rapidly (at the price of some inductive kick) than the rate determined by the RL time constant of the winding but that should not be necessary here.
 
> A clamping diode or varistor across the contacts
> makes sense too.

A varistor in parallel with the contacts would help. A diode would not (besides, at the time those machines were designed cheap high-current semiconductor diodes were not available).

> FWIW I recall the VR's on a couple 60's era,
> Mack and Reo farm dumps where I wrenched
> sometimes as a teenager. The trucks had BIG 80
> - 100 amp generators (this is pre-alternator
> days...). The VR's had both capacitors and what
> looked like diodes or resistors across both the
> voltage and current regulator contacts inside
> the VR box.

They most likely would have had a resistor and capacitor in series across the contacts. The values would have been chosen for a satisfactory compromise between limiting the voltage across the contacts and critically damping the RLC network.
 
"No it doesn't: that's the point. The current simply transfers to the diode. Since the current through the inductance is not interrupted there is no inductive kick.

In some applications one adds an RC snubber network to bring the current down more rapidly (at the price of some inductive kick) than the rate determined by the RL time constant"

We must be thinking of two different things here. I don't see how opening the contacts wouldn't interupt current through the field coils.
 
> We must be thinking of two different things
> here. I don't see how opening the contacts
> wouldn't interupt current through the field
> coils.

The diode is in parallel with the field coil and reverse-biased when the contacts are closed. It is the nature of inductors to resist sudden changes in current (v=Ldi/dt). When the contacts open in the absence of a diode or snubber the current continues to flow (though decaying rapidly). It quickly charges the stray capacitance across the contacts to a high enough voltage to arc over. The diode shorts out this voltage by allowing the current to flow through it. The current then decays at a rate determined by the RL time constant of the inductance and resistance of the coil. A resistor in series with the diode will make it decay faster at the price of higher voltage across the contacts.
 
(quoted from post at 11:39:27 06/23/09) "No it doesn't: that's the point. The current simply transfers to the diode. Since the current through the inductance is not interrupted there is no inductive kick.

In some applications one adds an RC snubber network to bring the current down more rapidly (at the price of some inductive kick) than the rate determined by the RL time constant"

We must be thinking of two different things here. I don't see how opening the contacts wouldn't interupt current through the field coils.
Click to expand...

Oh, opening the contacts interrupts it alright, but the inductor doesn't like that (picky critters that they are), so it immediately reverses polarity & begins to dump its stored energy in order to maintain that current, either thru the plasma of the arc across the contacts or the now forward biased diode, RC snubber, etc.
 
(quoted from post at 09:15:50 06/23/09) A resistor in series with the diode will make it decay faster at the price of higher voltage across the contacts.
Click to expand...

I thought I knew how TC circuits work. How will a series resistor decrease the discharge time?

On a side note, and I haven't really thought this through, but one wonders why "we" can't just go to a solid state device for the VR. It might even be possible, depending on field wiring and current, to use an Alternator regulator AS-IS at least for the VR section.

I recently gutted a defective VR on my "new" Cub Cadet 102 and replaced the cutout relay with a bridge from RadShack, so that part is pretty easy
 
(quoted from post at 15:15:24 06/23/09)
(quoted from post at 09:15:50 06/23/09) A resistor in series with the diode will make it decay faster at the price of higher voltage across the contacts.
Click to expand...

I thought I knew how TC circuits work. How will a series resistor decrease the discharge time?

On a side note, and I haven't really thought this through, but one wonders why "we" can't just go to a solid state device for the VR. It might even be possible, depending on field wiring and current, to use an Alternator regulator AS-IS at least for the VR section.

I recently gutted a defective VR on my "new" Cub Cadet 102 and replaced the cutout relay with a bridge from RadShack, so that part is pretty easy
Click to expand...
There are & have been SS VRs for generators, just not wide spread, probably due to volume demand.
Time constant is RC for a resistor-cap ckt and L/R for inductor-resistor ckt.
 
(quoted from post at 12:35:58 06/23/09)
Time constant is RC for a resistor-cap ckt and L/R for inductor-resistor ckt.
Click to expand...

Yeah, I UNDERSTAND that. What was said, was, that ADDING series resistance to L with a diode in circuit would DECREASE the RC time.

The quote:


"A resistor in series with the diode will make it decay faster at the price of higher voltage across the contacts."

That to me seems backwards.
 
(quoted from post at 19:26:20 06/23/09)
(quoted from post at 12:35:58 06/23/09)
Time constant is RC for a resistor-cap ckt and L/R for inductor-resistor ckt.
Click to expand...

Yeah, I UNDERSTAND that. What was said, was, that ADDING series resistance to L with a diode in circuit would DECREASE the RC time.

The quote:


"A resistor in series with the diode will make it decay faster at the price of higher voltage across the contacts."

That to me seems backwards.
Click to expand...
ell, I might learn something too, but we can keep trying. So, now we have a loop of a circuit consisting of a diode, inductor, and a resistor. Time constant proportional to L divided by R. Contacts open, voltage across reverses, diode comes into conduction. At this point if additional resistance is added (i.e., the denominator increases), then the L divided by a now larger number has a result that is smaller. Hence, the "decay faster". Higher voltage across contacts? Well if only a diode, the voltage would be only the diode drop, but with added resistance, it is the sum of diode plus the voltage across the resistor caused by the inductor supported current. OK? If not keep asking.
 
Not very technical and probably of no help, but back in the day I replaced several VR’s on small trainer airplanes. As matter of fact, in the ‘70’s we used to go to the local KMart and get a Ford VR and it fit perfectly. The only difference I was aware of between an auto VR and a aviation VR is that the springs were stronger to keep the points from fluttering at pressure atltitudes but those old trainers never went above 3,000ft. anyway so we figured what the heck. There also was an extra circuit inside the VR connecting the points which is probably what is being referred to. Another difference was the price, KMart Ford VR was about $7 and the Cessna VR was about $80 at the time.

I’ve changed a few VR’s on tractors over the years and it seems like the ones I have changed are on tractors that sat unused for a long time. I don’t know if I have ever changed one on a tractor that was used regularly.
Dell
 
(quoted from post at 15:15:24 06/23/09)
(quoted from post at 09:15:50 06/23/09) A resistor in series with the diode will make it decay faster at the price of higher voltage across the contacts.
Click to expand...

I thought I knew how TC circuits work. How will a series resistor decrease the discharge time?

On a side note, and I haven't really thought this through, but one wonders why "we" can't just go to a solid state device for the VR. It might even be possible, depending on field wiring and current, to use an Alternator regulator AS-IS at least for the VR section.

I recently gutted a defective VR on my "new" Cub Cadet 102 and replaced the cutout relay with a bridge from RadShack, so that part is pretty easy
Click to expand...

See my post here:
http://www.ytmag.com/cgi-bin/viewit.cgi?bd=farmall&th=717360
 
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