Info on Voltage Limiter Operation

[nm9stheham]

Anohehr FWIW post here. I wanted to come up with a simple test procedure for folks to use to accurately test their original type instrument voltage limiters with a digital voltmeter, and came up with something and went out to test. It read a voltage waaay off from what I expected and modeled based the explaantion of how these limiters work from Allpar.com and other sites.

So the oscillosope was used to measure the limiter output voltage, expecting to see a 40% or so duty cycle; nope! It was in the range of 15-20%. The first conlusion was that this limiter was bad, but then it became clear that it was not; the gauges read right, sensor resistances are right, and so forth.

So WTH? The reason finally dawned: The fuel and temp systems use THERMAL gauges which respond to the POWER flowing through them, not the current. Power in a resistive curcuit is based upon the RMS voltage in the circuit, not average voltage. (RMS = root mean sqaured and equals the DC voltage that will produce the same power as a given time-varrying voltage.)

Since somebody figured out that a steady 5V DC will put the right power into these guages to make them work the same as the voltage from the old limiter, then that is obviously the RMS voltage of the old limiter. Working backwards from that to figure up the duty cycle of an on-off switched 12v voltage that gives 5v RMS, the duty cycle will be around 17%.

So the old saw of the limiter voltage being almost a 50% duty cycle is not correct. (And maybe now I can make up that simple test circuit to work right.)

 

[67Dart273]

Frankly, I never paid much attention to the "dope" on the old units. "Back then" they usually work or they didn't. After all, you can't adjust them, LOL

But about the figuring. We don't have a sine wave here, I'm not sure RMS applies, but "my math" has been awhile. Why would it not be a simple percentage off 12-14V, that is 5V/ 14 or 40 something percent?

 

[KitCarlson]

My experience with switching power supplies suggests, the duty cycle adjusts based on load. It is a crude closed loop buck regulating system. At light load, the the duty cycle will be low, increasing more load. Test with senders at minimum resistances, and see what happens.

Nominal values for a gauge is estimated at 20 Ohms, with 10 Ohms as sender full. With two gauges try a 15 Ohm load.

 

[67Dart273]

You are referring to the old pulse unit I guess? I would think so. More current means the "flasher heater" heats up faster, just like an old turn signal flasher "blinked" way too fast with more bulbs installed.

That's an interesting point because it suggests that these do NOT respond as per original with a modern stiff regulated voltage supply. It might suggest that with one or more gauge at the high end, drawing more current, the limiter actually might "back off" some.

I don't remember anymore, "who all" used these. I know Fraud did, and maybe AMC?

Had one fail back in 67 when I had the mighty 144 CID 85 hp 1960 Ferd Falcoon. My Dad worked at NAPA back then, so he knew immediately what the deal was. I did NOT of course, have a shop manual back then, just "Motor's Manual" which belonged to.........Dad. I guess I still have a couple that were his.

 

[nm9stheham]

Yeah, I know, this is all pretty arcane LOL. I am just 'into' the old fashioned stuff as my Dodge is original and I want to keep it that way.

What you have noted above is how it is written up in many places. But that computation gives you average voltage, not RMS voltage. This is like a sine wave in that way: as I am sure you know, for a sine wave, the RMS voltage is .707 times the peak voltage and the average voltage is .637 time the peak voltage. This ratio of RMS-to-average only applies to sine waves. (Hence why we need true RMS voltmeters for non-sinusoidal waveforms where the RMS-to-average ratio diverges from this ratio.)

RMS voltage is needed when you want to know power in a resistive circuit and is why RMS is even used at all; that is what is used in the old Ohms law power formula of P=V^2/R.

To figure up RMS for a pulsed on-off signal with constant voltage at the peak, it is:
SQRT(V^2 x duty cycle) (where SQRT is the square root function).
In the case of a constant 5 volt DC voltage, this becomes just SQRT(5^2 x 1.0) = SQRT(25) = 5.

To get a pulsed 12v signal with an RMS voltage of 5 volts, then the quantity (12^2 x duty cycle) in the argument in ( ) has to equal 25. The algebra becomes pretty easy then:
25 = 12^2 x duty cycle
25= 144 x duty cycle
25/144 = duty cycle
0.174 = duty cycle = 17.4%

That is right on what I saw on the o-scope.

 

[nm9stheham]

My experience with switching power supplies suggests, the duty cycle adjusts based on load. It is a crude closed loop buck regulating system. At light load, the the duty cycle will be low, increasing more load. Test with senders at minimum resistances, and see what happens.

Nominal values for a gauge is estimated at 20 Ohms, with 10 Ohms as sender full. With two gauges try a 15 Ohm load.

That assumes that the limiter's heating element is in series with the load and that load current is what heats the element in the limiter. But that is not the case; if you look carefully at the depictions and simple schematic of the limiter, the heating element is a shunt element across the load, not in series. It doesnot see the load current.

The limiter IS sensitive to input voltage and will cut back its duty cycle with increased voltage; and keep the RMS output somewhat regulated.

 

[KitCarlson]

Yes the heating is in shunt with load, that is how a regulator works. Same for series pass, or switching buck regulator. The control elements are referenced to the output via comparator. Very basic control theory. The IVR is thermal, but the control is still hysteretic. The on - off contact is driven, hot opens, cold closes. The system seeks balance. The gauge is thermal, and slow so, filtering is less important.

A modern low cost pwm switching regulator with current limit and high input range is available for about $5. I posted information on that months ago, but can't remember now.

I will not comment on your rms theory.....

 

[RedFish]

In the beginning everything operated on 6 volt systems. When 12 volt systems came about, the simplest solution to the instruments was to reduce the current to approx' 6 volts. Every mfgr. except GM added a mechanical gauge voltage limiter. The mechanical limiters will put out a lot more current during the warm up period. That's why the gauges are able to climb so fast.
The ONLY REASON a 5 volt solid state regulator was substituted is because there wasn't a suitable 6 volt regulator in the catalog. 5 volts continuous does work and does not cause the gauges to read lower. It does cause them to warm up/climb slower. They'll reach the same temperature and readings eventually.

 

[KitCarlson]

Great info.

 

[nm9stheham]

In the beginning everything operated on 6 volt systems. When 12 volt systems came about, the simplest solution to the instruments was to reduce the current to approx' 6 volts. Every mfgr. except GM added a mechanical gauge voltage limiter. The mechanical limiters will put out a lot more current during the warm up period. That's why the gauges are able to climb so fast.
The ONLY REASON a 5 volt solid state regulator was substituted is because there wasn't a suitable 6 volt regulator in the catalog. 5 volts continuous does work and does not cause the gauges to read lower. It does cause them to warm up/climb slower. They'll reach the same temperature and readings eventually.

I can see your idea of the 'warm up' characteristic of the limiter; the 'on' period of the limiter is set by the temperature and if cold, it could take longer for the heating element to heat the bimetallic arm to where it will open. After 'heat soaking', it will likely open more quickly, thus a lower 'on' duty cycle. (But is should also slow the time to close the arm; hmmm, I may need do some more tests!)

But the mechanical limiters don't regulate current per se; they regulate voltage; there is nothing the way they are built to sense or react to current going through them. The heating coil is in shunt across the limiter input. A higher 'on' duty cycle would give a higher RMS voltage when cold and that is what would push more current through the guages.

Now you have me wondering if there is any resistive changes in the gauges themselves as they read higher; they are thermal movements after all, with a resistive heating element.

 

[nm9stheham]

Yes the heating is in shunt with load, that is how a regulator works. Same for series pass, or switching buck regulator. The control elements are referenced to the output via comparator. Very basic control theory. The IVR is thermal, but the control is still hysteretic. The on - off contact is driven, hot opens, cold closes. The system seeks balance.

I will not comment on your rms theory.....

I am not sure we are looking at this the same way, Kit. The heating element in the mechnical limiter is in shunt on the input side of the limiter. It never sees the load current and has no reference to what is going on at the output; it isn't a feedback-control type of device. You can disconnect the load completely, and the limiter will contiune to click on and off the same way; it lives in its own little thermo-electric world, and the gauges go along for the ride with whatever voltage it puts out. Thinking that it reacts to the load is like thinking that turning up the radio will make the headlights change brightness.

RMS is not theory; it is offered as the explanation of the observed behavior. If there are others ideas, I am wide open to that. I'm much more interested in understanding and explaining this arcane little animal than anything else.

 

[RedFish]

I can see your idea of the 'warm up' characteristic of the limiter; the 'on' period of the limiter is set by the temperature and if cold, it could take longer for the heating element to heat the bimetallic arm to where it will open. After 'heat soaking', it will likely open more quickly, thus a lower 'on' duty cycle. (But is should also slow the time to close the arm; hmmm, I may need do some more tests!)

But the mechanical limiters don't regulate current per se; they regulate voltage; there is nothing the way they are built to sense or react to current going through them. The heating coil is in shunt across the limiter input. A higher 'on' duty cycle would give a higher RMS voltage when cold and that is what would push more current through the guages.

Now you have me wondering if there is any resistive changes in the gauges themselves as they read higher; they are thermal movements after all, with a resistive heating element.

Yes the resistance within each gauge changes with the temperature of the resistor ( in this case a length of wire. This wire measures 1.5 ohms per inch at 68 degrees F in some gauges and 1.1 ohms per inch at 68 degrees F in others.
The gauges operate on a range of 80 ohms to 10 ohms. Your calculator will suggest the center of that range is 35 ohms. Wrong. The approx. center of the range is actually 23 ohms. That doesn't mean 23 ohms is the exact center of the scale printed on the screens of every gauge. Anyway...
Since a nichrome resistor wire is used in the limiter, similar variance exists there also.
Believe it or not a somewhat similar variance exists in the typical T7805 regulator too. Hook one up outside a proper heat sinked application and watch its output voltage climb.
If I could have mounted that regulator inside the rally fuel gauge, I would have.

 

[nm9stheham]

^^ Thanks for the info! I assume you mean that the sensors work in the 80 to 10 ohm range? I think I have seen reference to the gauge resistance being around 50-52 ohms; that would be 3' to 4' of wire....does that make sense?

Edit to add: BTW, I just ran calculations to figure up the fuel sender resistance that puts the gauge power in the middle-of-the-range guage dissipation power, in the fuel guage circuit with the sender resistance ranging from 10 ohms full to 73 ohms empty. Turns out that middle-of-the-range power occurs when the sender resistance is in the mid 20 ohms range (just like you have listed), and does not vary much with guage resistance from 20 to 50 ohms. Results:
Sensor R for mid-range power/ Gauge R
27.6 / 60
26.3 / 50
24.6 / 40
22.7 / 30
20.4 / 20

This seems to make sense if the gauges are thermal and respond to power, not current.

 

[cudajim]

In the beginning everything operated on 6 volt systems. When 12 volt systems came about, the simplest solution to the instruments was to reduce the current to approx' 6 volts. Every mfgr. except GM added a mechanical gauge voltage limiter. The mechanical limiters will put out a lot more current during the warm up period. That's why the gauges are able to climb so fast.
The ONLY REASON a 5 volt solid state regulator was substituted is because there wasn't a suitable 6 volt regulator in the catalog. 5 volts continuous does work and does not cause the gauges to read lower. It does cause them to warm up/climb slower. They'll reach the same temperature and readings eventually.

I tried a variable regulator (cause I had nothing better to do) and set it to 6.0 volts output. It did NOT read correctly on either gauge. I then went back to using a 5v regulator and all was well. Just to prove it wasn't a fluke I installed one on all 3 of the Barracudas I have with the same results.

 

[nm9stheham]

6 VDC instead of 5 VDC would make the power to the guage and sensor circuit 44% higher, so it would maek sense that it would change the readings a lot. Thanks for the info on what you did.

 

[RedFish]

There is a difference in heat generated by constant voltage and a pulse voltage.
A constant 6 volts would make some gauges read a tad higher. Constant 5 volts is very close in some applications and a tad low in others. I already mentioned that all gauges are not the same. That dictates that all mechanical limiters are not the same either. Some supply only 2 gauges, others supply 3 gauges. Those can be 3 at 20 ohms, or 2 at 13 ohms, or 1 of each, 2 at 20 and 1 at 13. It's a thick book. In the end they are simple range indicators so a tad one way or the other isn't enough to loose sleep over.

 

[nm9stheham]

OK, thanks. With the way the limiters work, they don't care how much load is on them. They work via the internal heating element, and none of the load current flows through that; it only flows through the contact, not the heating element. (Sorry for the blurred pix...)

I'm still tryng to work up a good way to test this pulsed voltage on a standard voltmeter for folks asking about their gauges. The original idea was to make a RC small filter and use that to test for some sort of average voltage. But the duty cycle is critical to that, and getting and hooking up a couple of resistors and a capacitor is not what most gearheads want to do, I suspect. So any good ideas on that would be helpful to the whole Mopar community.

 

[BillGrissom]

I would say the best simple tester would be a resistance heating element w/ a temperature sensor - thermistor, thermocouple, or maybe re-purpose a digital thermometer (cheap @ the Dollar Store). The resistor averages the electrical output, making the reading easy. This is the way your dash gages work (heater w/ bi-metal arm). For consistency, always test at same 70 F ambient temperature, and your data is the rise in temperature from starting and with voltage limiter on. Of course, you could use an old dash gage for this, if just interested in comparing Vreg's.

If you want a more quantitative method, you need an A/D converter and post-processing. You can get software that turns a PC sound card into a voltage digitizer. You would then need to process the voltage output time trace into a power value. You would time-average V^2 (i.e. time-integrate [V(t)]^2 and divide by t2-t1). If you don't know what that means (or refuse to learn), paragraph 1 is better for you.

 

[nm9stheham]

LOL I think I will pass on reccommending the A/D and PC card method for a field test....

Right now, I'm leaning towards using a 6v lamp at the end of the temp or fuel gauge wire. It'll flash brightly but the duty cycle of a properly operating limiter and the gauge resistance ought to keep it from burning out; an old #47 lamp worked well yesterday. If the limiter is stuck on; it'll take out the 6v lamp. Maybe a video of how the lamp flashing ought to look would be helpful.