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View Full Version : How do I test Millivolt valve system - Pilot is on, but main burner not lighting



mnalep
10-24-2010, 10:40 AM
I am trying to diagnose a problem with an old furnace. I have some pictures to help clarify what I have, a ThermoPILE system.

The problem is that the PILOT flame is on, but the main burner does not always light.

I cleaned all electrical contacts today with sandpaper, and made sure they were all tight.

I also made sure the PILOT line and BLEED line were not blocked. (Took them off and blew through them).

I also took out the THERMOPILE sensor, and cleaned it off with steel wool.

http://img246.imageshack.us/img246/8505/img1519marked.jpg

http://img819.imageshack.us/img819/4227/img1524marked.jpg

http://img251.imageshack.us/img251/7003/img1521marked.jpg


And this is a repeat , but I am trying to show that there is a steel connection(steel plate) between RED(Thermopile,pos) to TH1(thermostat,pos).

Also a steel connection(steel plate) between TH2(thermostat,neg) to L2(LIMIT switch, pos):
http://img203.imageshack.us/img203/2227/img1521marked2.jpg

I was trying to use the "MILLIVOLT TROUBLESHOOTING CHART", on page 2 of this diagram:
http://www.robertshawtstats.com/spaw2/SiteContent/Files/faq/ControlTips-MillivoltSystems.pdf

But I'm not sure what connections to put my meter across.

For example, the chart says in TEST-A, to test across TP and TH for more than 100mV, with the thermostat contacts CLOSED. (On my gas valve, would that be RED to TH1, or RED to TH2, or something else?)

For example, the chart also says in TEST-B to test from TH/TP to TP for greater than 325mV, with the thermostat contacts OPEN. (On my gas valve, would that be TH1 to WHITE, or TH1 to RED, or something else?)

Also, when it says thermostat contacts CLOSED - does that mean with the thermostat temperature set to higher than the room temperature?

I attempted to do TEST-A, with thermostat set to turn furnace on(I assume that what what they meant by thermostat connections CLOSED?) and measured:
RED to TH2 = 123mV
WHITE to TH1 = 387mV
TH1 to TH2 = 20mV
RED to WHITE = 400mV
L1 to L2 = 26mV

For some reason I did not measure RED to TH1, or WHITE to TH2. Should I have?

Also, those readings were what actually displayed on my meter, when the dial was on the 2000mV range, and the 200mV range.

So I think I'm reporting the voltages correctly? That is, if the meter displayed 123 on either of the ranges(2000mV, or 200mV) - that means I measured 123 Millivolts?

If the gas valve is sticking, can this be disassembled and cleaned? Can it just be replaced?

mnalep
10-24-2010, 10:41 AM
2 more pertinent pictures:

http://img401.imageshack.us/img401/4889/img1526marked.jpg

http://img804.imageshack.us/img804/3917/img1545marked.jpg

Thatguy
10-24-2010, 10:54 AM
There must be two terminals in this connector block that powers the gas valve coil. I suspect they are the "white" terminal and TH1.

This wiring should reduce to a simple series circuit: a gas valve coil in series with a thermopile in series with a t'stat with normally open contacts in series with a limit switch with normally closed contacts.
The next time the valve opens, the current measured anywhere in this series loop gives you the current through the valve coil, limit switch and t'stat. Current specs are hard to get but it might be approx. 100 mA.

The next time the valve fails to open you need to measure the voltage across this valve coil, or, better, the current through it.

When they replaced my gas valve it was $400, and I did the troubleshooting ahead of time.

My interpretation

TH1 to TH2 = 20mV - voltage drop across closed switch contacts plus voltage drop across several yards of wire of some gauge number.
RED to WHITE = 400mV-voltage output of t'couple while under load assuming you measured it while the valve was open and working. It should be close to 750 mV when the valve is closed and off.
L1 to L2 = 26mV-voltage drop across closed switch contacts plus voltage drop across a yard or so of wire of some gauge number.
WHITE to TH1 = 387mV-?-I'd think the coil voltage should read 400 - 20 - 26 = 354 mV. [1- (387/354)] x 100 = a +9% error, which is kind of high for a digital meter.
RED to TH2 = 123mV-?

What gauge number wire and what length do you use to your t'stat?
http://en.wikipedia.org/wiki/American_wire_gauge
Usually #18 is used for physical strength and not so much to limit the voltage drop.

BTW, "test A" implies that the valve will open on as little as 100 mV, and "test C" gives 80 mV max. system voltage drop for whatever current is flowing. For 100 mA this max. resistance value would be 0.8 ohm.

mnalep
10-24-2010, 11:40 AM
There must be two terminals in this connector block that powers the gas valve coil.

Thatguy, ther are 6 terminals, 2 from pile generator, 2 from limit switch, and 2 from t'stat.



This wiring should reduce to a simple series circuit: a gas valve coil in series with a thermopile in series with a t'stat with normally open contacts in series with a limit switch with normally closed contacts.
The next time the valve opens, the current measured anywhere in this series loop gives you the current through the valve coil, limit switch and t'stat. Current specs are hard to get but it might be approx. 100 mA.

- So I should test for milliamps also?



The next time the valve fails to open you need to measure the voltage across this valve coil, or, better, the current through it..
- You again mean in milliamps?



My interpretation

RED to TH2 = 123mV-?
WHITE to TH1 = 387mV-?-I'd think the coil voltage should read 400 - 20 - 26 = 354 mV.
WHY ARE YOU SUBTRACTING 20mv and 26mv)
TH1 to TH2 = 20mV - voltage drop across closed switch contacts plus voltage drop across several yards of wire of some gauge number.
WHAT IS VOLATAGE DROP?
RED to WHITE = 400mV-voltage output of t'couple while under load assuming you measured it while the valve was open and working.
I THINK THE MAIN GAS VALVE WAS NOT ON WHEN I MEASURED RED to WHITE. I can DO THIS AGAIN
L1 to L2 = 26mV-voltage drop across closed switch contacts plus voltage drop across a yard or so of wire of some gauge number.
THE LIMIT wire WAS A 14Gauge wire, about 12 inches long



What gauge number wire and what length do you use to your t'stat?

Not sure of the T'stat wire. I'd guess it is 14gauge? The wire off the limit switches are 14, as I just had to strip one of them. The t'stat wire runs from a 2nd story flat into the basement. Could be 30 feet maybe a little more.

Thatguy
10-24-2010, 11:51 AM
Thatguy, ther are 6 terminals, 2 from pile generator, 2 from limit switch, and 2 from t'stat.

Yes, and two of those, as yet unidentified, connect to the gas valve coil.

- So I should test for milliamps also?

Amps or milliamps, whatever is flowing.

- You again mean in milliamps?

Amps or milliamps, whatever is flowing.


Not sure of the T'stat wire. I'd guess it is 14gauge? The wire off the limit switches are 14, as I just had to strip one of them. The t'stat wire runs from a 2nd story flat into the basement. Could be 30 feet maybe a little more.

the robertshaw link gives max lengths vs. gauge

The voltage "drops" in a series circuit must add and be equal to the source voltage. I guess they call them "drops" because the source voltage represents a "rise" in voltage.
Lengths of wire and closed switch contacts have small voltage drops across them when passing current.
The t'pile has an internal resistance so the voltage it supplies when open circuited will be more than when it is under load.

mnalep
10-24-2010, 12:37 PM
I forgot to mention that I got the main burner to light on 2 occassions by tapping on either the gas pipe before the main shutoff valve, or on the honeywell gas millivolt valve itself.

Does that indicate more a gas valve problem or a thermopile generator/pilot flame problem?

mnalep
10-24-2010, 12:41 PM
the robertshaw link gives max lengths vs. gauge

I THINK I"M OK ON THE LENGTH THEN, I"m SURE ITS UNDER 40 Feet, and at least 18 guage, more likey 16 or 14

The voltage "drops" in a series circuit must add and be equal to the source voltage. I guess they call them "drops" because the source voltage represents a "rise" in voltage.

YOU LOST ME HERE.

DOES VOLTAGE DROP HAVE TO DO WITH, FOR EXAMPLE, THE PILE GENERATOR LEADS TO THE GAS VALVE BEING MEASURED BOTH ATTACHED(WITH MAIN BURNERS ON) - AND NOT ATTACHED TO THE GAS VALVE - BUT PILOT FLAME ON IN BOTH TESTS?

Lengths of wire and closed switch contacts have small voltage drops across them when passing current.
The t'pile has an internal resistance so the voltage it supplies when open circuited will be more than when it is under

.

I was looking over my notes, and I did disconnect the pilot leads with the pilot light on the pile generator. And I think that was around 700 on the 2000mV scale. So if I had 400 with the leads attached, what does that mean?

jimbo
10-24-2010, 01:24 PM
Draw yourself the circuit. It looks like this: the thermopile is a generator...puts out up to 400 MV. The gas valve is the load....need to have current flow from one leg of the thermopile...through the limit switch ( zero ohms ) througn the thermostat switch ( approx zero ohms on call for heat) through the coil of the valve...and back to the other terminal of the generator. If either of the switches is bad, no go. If the switches are good, that sort of defaults to a bad gas valve. 400 millivolts across the generator loaded...current flowing through the switches and valves...is about right.

Thatguy
10-24-2010, 01:59 PM
And we already know that the t'stat + terminal is jumpered to the t'couple + terminal, and that the t'stat - terminal is jumpered to the limit switch + terminal. Except for the 123 mV this circuit is pretty well obvious.

700 mV open circuit and 400 mV closed circuit with an internal t'couple resistance of 3 ohms means the current draw is (0.7-0.4) volts /3 ohms = 0.1 A. Maximum power is transferred when the closed circuit voltage is half that of the open circuit voltage so these numbers are entirely plausible.
For a 20 mV drop, a 60' loop distance, #14 AWG and 100 mA you should have 15 mV drop on the wires, leaving 5 mV across the closed t'stat contacts.

Almost certainly, with these readings and the tapping, your valve is sticking closed.
If it sticks open, there could be a big BOOM.

What would a person without a death wish do next? :D

jimbo
10-24-2010, 04:07 PM
Your system is quite old and somewhat unsafe, in that the pilot comes off ahead of the safety valve. I would not work on it, without completely replaceing the gas control system, and the furnace may not be worth that expense.

mnalep
10-24-2010, 04:09 PM
Draw yourself the circuit. It looks like this: the thermopile is a generator...puts out up to 400 MV. The gas valve is the load....need to have current flow from one leg of the thermopile...through the limit switch ( zero ohms ) througn the thermostat switch ( approx zero ohms on call for heat) through the coil of the valve...and back to the other terminal of the generator. If either of the switches is bad, no go. If the switches are good, that sort of defaults to a bad gas valve. 400 millivolts across the generator loaded...current flowing through the switches and valves...is about right.


I had the 400mV reading while the main burner was off. Would that count as being 'under load'

So if I put a test wire across the LIMIT switch contacts, does that remove the LIMIT switch from the circuit? (ie: eliminate that as a problem).

Could I also do that for the stat, or do I need power there? How would I do that?

mnalep
10-24-2010, 04:12 PM
I searched for that gas valve, and found this one as the replacement:

http://americanhvacparts.com/Merchan...tegory_Code=GV

How would I convert the 6 wires I have now, to that (looks like 3 terminals) gas valve?

Thatguy
10-24-2010, 05:30 PM
I would seriously consider what Jimbo said, but, yes, if you bypass the t'stat and the limit switch with jumper wires and the pilot is on, the valve should open if the schematic is what I think it is.

Yes, the load for the t-pile is the valve coil. With the 400 mV on the valve the main burner should soon turn on.

I'd think a gas valve should just have one terminal, coil + and the valve body is ground,
or two terminals, coil + and coil -.
Maybe a third terminal would be the ground terminal.

Your present valve seems to use the valve body terminals as tiepoints for the circuitry and I'd think several wirenuts could do the same job.
Without internal schematics for both valves I can't be sure, but with an ohmmeter I guess you can figure out some of the inside wiring.

This whole project seems risky. If I lived by myself in a house some distance from the neighbors I might consider repairing this furnace if it were under 25 years old.

Years ago I heard about residential gas pressure becoming excessively high somehow. Something failed and the house filled with gas, and then a girl coming home turned on a light switch to investigate. There was not much left of her.

mnalep
10-25-2010, 12:46 PM
thatguy, I hear you. But it has been reliable for a long time.

Here is a pic of the schematic. This is the lower part, which is the Millivolt gas valve:
http://img156.imageshack.us/img156/1103/img00961.jpg


This show 2 schematics, the upper is a 24 volt gas valve, which I am assuming was optional on this furnace:
http://img684.imageshack.us/img684/1316/img00971q.jpg

Thatguy
10-25-2010, 01:18 PM
Yes, this shows the pilot gen. in series with the limit switch in series with the t'stat. The internal valve coil must then be connected to the topmost and bottommost of the four terminals.
It's a wired AND function with 3 inputs: the t'stat contacts must be closed AND the limit switch must be closed AND the pilot gen. must be putting out voltage. Only then does the valve open.

I couldn't open your link; you have specs and whatnot on the 3 terminal valve? If you connect the gas pipes to the new valve make sure you use a soapy solution to check for leaks.

Let's hope your repaired furnace fails gracefully and piecemeal rather than catastrophically. It has a 1% chance of making it to 80 years.
BTW, having three high sensitivity CO detectors will give you aircraft level reliability [so called "six nines"] since the chance of all three failing is miniscule.

mnalep
10-25-2010, 03:31 PM
Let's try that link again:

http://americanhvacparts.com/Merchant2/merchant.mvc?Screen=PROD&Product_Code=GSV820A1047A&Category_Code=GV

I do not have a schematic for that replacement gas valve(GSV820A1047A), but you can see it lists my current one (VS87A9X4X2) as one it replaces.

When you state "The internal valve coil must then be connected to the topmost and bottommost of the four terminals." (SO THE VALVE COIL is CONNECTED TO WHICH 2 terminals in my picture)?

Also, you made me think that my physical valve actually has 6 screw in terminals? I have never learned to read a schematic, but it seems to indicate 4 terminals?

Also, the two steel plates on the physical valve terminals are 'jumpered 'from a pile generator(RED) to (thermostat)TH1, and another from the other thermostat terminal(TH2) to a Limit terminal(L2).

But the schematic seems to me to show one connection from one of the limit terminals (Lx) to a Thermostat terminal(T1 or T2), and another connection from the other LIMIT Ly) to a PILE GENERATOR terminal (either RED or WHITE).

Am I interpreting the schematic wrong?

Thanks for that heads up on the CO detectors!

jimbo
10-25-2010, 04:18 PM
You should NOT short out the t'stat terminals to check it. There is a heat anticipator coil in there which limits current to the valve, and shorting that out can damage the valve.

Thatguy
10-25-2010, 04:40 PM
6 terminals + 2 jumpers = 4 effective terminals = 4 nodes
http://en.wikipedia.org/wiki/Node_(circuits)

You need to find out which terminals on the replacement valve go to the coil. I couldn't make it out from the link but if you can get a photo of the terminals their labelling might tell you, or if their cust. service rep has one handy, he/she can read it to you.

mnalep
10-25-2010, 09:09 PM
Jimbo, Thanks for that tip. I tried that once already, but I won't again.

mnalep
10-26-2010, 06:38 PM
thatguy,

Here is a white-rodgers replacement millivolt gas valve installation instructions:
http://www.emersonclimate.com/en-US/products/valves/Documents/0037-6264.pdf

Page 3, FIGURE-3, has diagram of the wires and seems to show where all 6 wires would go. ;-)

now I would just need to verify if the input is 3/4" (which I think it is), or 1/2". I think the output is 3/4" also. Besides me taking it apart, is there any way to tell from the picture, or somewhere else online?

Thatguy
10-27-2010, 07:11 AM
In figure 3, TH and PG should be the coil connections and probably measures about 3 ohms. I don't know what they mean by 'power unit' or how this terminal ties in to the rest of the circuit.

The Figure 4 wiring is a series circuit and makes perfect sense.

mnalep
10-27-2010, 08:39 AM
Figure 4 is 24 volt. Fig 3 is millivolt, which I have. I went and looked at this valve at the store, and the 'power unit' is a large nut screwed into the valve body with a black wire coming out the middle of it. If you look at the 2 diagrams on the first page, the one on the right shows the wire, with a slide on clip on the end of it. I think that is the 'power unit'. I don't get it either, but what else could it be?

When you mention 3 ohms, are you saying I should be ohm-ing these?

Thatguy
10-27-2010, 02:29 PM
I would use an ohmmeter to confirm what terminals the coil is wired to, and what it is not wired to. You want to know where the current goes and how it goes because fig. 3 is puzzling.
This circuit has to work on paper first.

If there is only one black wire coming out of the power unit then the case of the valve is being used as an electrical conductor. If your current valve doesn't do this you may need to add a jumper wire.

Fig. 3 seems to be having the t'stat alone turning on the valve, so the power unit working with the hi limit can somehow disable the valve turning on, but how it does it is not obvious from this figure.

There may be something wrong with fig. 4. There doesn't seem to be any way to prevent the valve from opening if there is nothing to ignite the burner, unlike in fig. 3 which uses the pilot gen. to supply current to the valve.

For testing you can simulate the t'pile with a 1.5 v "D" cell and two 3.6 ohm 1/4 w resistors from Radio Shack.

The "power unit" could be an 18 mV thermocouple but it is not clear to me what function this serves.

mnalep
10-27-2010, 05:11 PM
thatguy

here is another diagram they have for that gas valve.

http://www.emersonclimate.com/en-US/products/valves/Documents/Cat_06_pg0250_253.pdf

Frankly, I'm overwhelmed by this diagram. Can you make sense of it?

Thatguy
10-27-2010, 07:44 PM
thatguy

here is another diagram they have for that gas valve.

http://www.emersonclimate.com/en-US/products/valves/Documents/Cat_06_pg0250_253.pdf

Frankly, I'm overwhelmed by this diagram. Can you make sense of it?
I know figs. 4 through 14 and fig. 1 don't apply. If fig. 3 doesn't physically look like your valve it doesn't apply either.

It looks like fig. 2 applies for the 36C03U and you should measure the coil resistance between TH and PG. The terminal TH-PG is just a tiepoint and does not connect internally inside the valve housing.

Ask them what the power unit is and how it works with the limit switch and how come it only has one wire.

mnalep
10-28-2010, 07:04 AM
Fig 2 looks like the 3 screw terminal on the valve. Fig 3 cold have been the 'power unit' lead, but this picture shows 2 wires, and the one I'm looking at only has one.

Thatguy
10-28-2010, 07:08 AM
but this picture shows 2 wires, and the one I'm looking at only has one.
And so this figure doesn't apply.

mnalep
11-10-2010, 07:50 AM
Sorry it took me so long to get back here. I've had some other problems that needed attention.

So I took apart the pilot burner and orifice, and cleaned it out.

I took a couple of still pictures of the flame before and after. And a video clip of the flame is posted below also.

I also bought a new limit switch and installed it, as the old one was looking kind of rusty. You can see the two limit controls side by side below.

The furnace has still been working well, although we have had temps in the 60's this last week. So it's been a light duty test.

The pilot generator was putting out about 750mV on my last meter reading. I was worried at first because my initial reading was around 600mV. I took it all apart and made sure it was real clean, and after I put it back in the 2nd tome the reading was at the normal 750mV. Not sure why it was low the first time?

So I am hoping this furnace is set.

Thank you guys for helping me with this!!!



http://img121.imageshack.us/img121/3128/img0253vh.jpg
http://img121.imageshack.us/img121/3128/img0253vh.jpg

http://img607.imageshack.us/img607/7226/img0266b.jpg
http://img607.imageshack.us/img607/7226/img0266b.jpg

http://img202.imageshack.us/img202/5348/img0267zf.jpg
http://img202.imageshack.us/img202/5348/img0267zf.jpg

http://img829.imageshack.us/img829/2461/img0283ed.jpg
http://img829.imageshack.us/img829/2461/img0283ed.jpg

mnalep
11-10-2010, 07:51 AM
ans some more pics and a video of the pilot flame:

http://img560.imageshack.us/img560/9652/img0290q.jpg
http://img560.imageshack.us/img560/9652/img0290q.jpg

http://img201.imageshack.us/img201/5387/img0292i.jpg
http://img201.imageshack.us/img201/5387/img0292i.jpg

http://img577.imageshack.us/img577/3131/img0295h.jpg
http://img577.imageshack.us/img577/3131/img0295h.jpg

http://img152.imageshack.us/img152/2363/mvi0254.mp4
http://img152.imageshack.us/img152/2363/mvi0254.mp4

mnalep
11-10-2010, 08:15 AM
I would like to get a vacuum in there and start cleaning the burners and heat exchange area. you can see in the picture there is a lot of dust that needs cleaned.

what is the best way for me to clean? vacuum? brush, steel wool, or ? it seems really dusty right by the pilot flame area, and on the cabinet just under the main burners. any advice?

Thatguy
11-10-2010, 10:10 AM
and after I put it back in the 2nd tome the reading was at the normal 750mV. Not sure why it was low the first time?

I assume 750 mV is open circuit voltage. The voltage for a t'pile is related to flame temperature. There's probably a graph somewhere online for the relationship between temp. and voltage.
A blue flame indicates more nearly complete combustion than a yellow flame.

Two things define a voltage source: open circuit voltage and output voltage when current is being drawn.
Voltages sources have a Thevenin equivalent circuit and it's pretty useful to figure out the two values in this equivalent circuit.
Open circuit voltage is only half the story.

You might want to buy a 3 ohm resistor and measure the t'couple voltage output when loaded down with this many ohms. Independent tests of each system component comes in handy.
If the internal resistance of the t'pile is 3 ohms you will get 750/2 = 380 mV across the resistor.

jimbo
11-10-2010, 10:59 AM
AH....my old buddy Thevinin, and his right hand man Kirchoff! I believe you are about right on the loaded source measuring about 380,

Thatguy
11-10-2010, 11:09 AM
AH....my old buddy Thevinin, and his right hand man Kirchoff!

I used to go drinking with these guys. . .:D

mnalep
11-10-2010, 03:52 PM
thatguy,

I'm not sure what the meaning of 'open circuit' is? I measured with the pilot flame on, and the leads unhooked from the gas valve. Is that what you meant?

how would I physically hook up " a 3 ohm resistor and measure the t'couple voltage output when loaded down"? I've never heard of this, would you educate me please?

where would i get a 3 ohm resistor?

when you said "open circuit voltage and output voltage when current is being drawn. " do you mean with vs without the gas valve being hooked up to the leads of the pilot generator?

Thatguy
11-10-2010, 04:17 PM
thatguy,

I'm not sure what the meaning of 'open circuit' is? I measured with the pilot flame on, and the leads unhooked from the gas valve. Is that what you meant?
Yes, it means no current is flowing out of the source. The current drawn by the voltmeter is in the microamp or nanoamp range and can be considered to be zero in this case.
A wall outlet with nothing plugged in is open circuited.
A short circuit means the maximum current available from a source is flowing.
A closed circuit means some current is flowing.

how would I physically hook up " a 3 ohm resistor and measure the t'couple voltage output when loaded down"? I've never heard of this, would you educate me please?
Instead of hooking up the gas valve to the t'pile, hook each lead of the resistor to each lead of the t'pile. Then you'll have a simple series circuit with only two components, a source and a load.

where would i get a 3 ohm resistor?
I hate to send you to Radio Shack but there aren't many other places left where you can get components.
http://www.google.com/imgres?imgurl=http://electronics-for-beginners.com/pictures/resistor.PNG&imgrefurl=http://electronics-for-beginners.com/pages/page/2060/&usg=__a9rI42q-bizlHtMiu_tqqdOF_nc=&h=408&w=1082&sz=736&hl=en&start=0&zoom=1&tbnid=qt_v6xqbgOjeRM:&tbnh=66&tbnw=174&prev=/images%3Fq%3Dresistor%26um%3D1%26hl%3Den%26safe%3D off%26client%3Dsafari%26sa%3DN%26rls%3Den%26biw%3D 1488%26bih%3D772%26tbs%3Disch:1&um=1&itbs=1&iact=hc&vpx=1083&vpy=145&dur=1&hovh=138&hovw=366&tx=287&ty=75&ei=hzbbTPy4JYP6lwe99uC1CQ&oei=hzbbTPy4JYP6lwe99uC1CQ&esq=1&page=1&ndsp=32&ved=1t:429,r:7,s:0

when you said "open circuit voltage and output voltage when current is being drawn. " do you mean with vs without the gas valve being hooked up to the leads of the pilot generator?
Yes, the t'pile can be loaded down with the valve or a resistor.

This is assuming somewhere in this thread that the valve coil resistance of a working gas valve has been determined to be 3 ohms.

mnalep
11-13-2010, 05:56 PM
thatguy, i've started reading that site, electronics-for-beginners.com, but McAfee Site Advisor (my antivirus) is saying there are potential security risks with that site. have you used it safely?

mnalep
11-13-2010, 06:12 PM
ok, so an open circuit has no load device on it?

I thought a short circuit was where a device, say, a toaster, was not getting any current? That sounds like the opposite of "maximum current available from a source is flowing" - wouldn't it?

Isn't a short circuit when, say, a wire between the fuse panel and, for example, a toaster - has a bare spot in the rubber, and the copper wire is touching something metal - short circuiting the electricity away from the toaster?

So a closed circuit has a load device? and 'some current' would still be enough to run the device properly?


This is assuming somewhere in this thread that the valve coil resistance of a working gas valve has been determined to be 3 ohms. - I never tested the resistance(ohms) of the valve...it sounds like i'd need to?


If the internal resistance of the t'pile is 3 ohms you will get 750/2 = 380 mV across the resistor. - so if I test for DC mV with a 3ohm resistor attached to the t'pile generator (and assuming the t'pile also had 3ohms resistance) - then I'd measure 380mV DC ?

Thatguy
11-14-2010, 09:20 AM
ok, so an open circuit has no load device on it?
Right; no current flows.

I thought a short circuit was where a device, say, a toaster, was not getting any current? That sounds like the opposite of "maximum current available from a source is flowing" - wouldn't it?
?

Isn't a short circuit when, say, a wire between the fuse panel and, for example, a toaster - has a bare spot in the rubber, and the copper wire is touching something metal - short circuiting the electricity away from the toaster?
Yes, it's a path that causes current to flow in the wrong places and usually at values much higher than the design value.

So a closed circuit has a load device? and 'some current' would still be enough to run the device properly?
A circuit can be closed and still have abnormally high or abnormally low current.

- I never tested the resistance(ohms) of the valve...it sounds like i'd need to?
Yes.

- so if I test for DC mV with a 3ohm resistor attached to the t'pile generator (and assuming the t'pile also had 3ohms resistance) - then I'd measure 380mV DC ?
With one assumption, yes

0.750 =open circuit t'pile voltage [Voc] in volts
2.890 =presumed t'pile internal resistance [Rint] in ohms
3.000 =load resistor [Rload] in ohms

0.127 =current [I] = Voc/(Rint + Rload) in amps
0.382 =voltage across load resistor [Vload] in volts = Rload x I

Try electro tech forum. Those guys seem to know their stuff.

My Apple doesn't much care about viruses. My other computer, which hasn't been online for more than a year, still warns me about potential viruses.
Anti-virus s/w is a money machine. What's to keep them from generating new viruses so they can sell you more upgrades?

mnalep
11-14-2010, 10:40 AM
thatguy, Thanks, I am starting to understand. So if I did all this and found .382v DC, would that mean my t'pile is good, or bad, or weak? Sounds like it might be weak - if it is supposed to generate 750mV closed circuit am I saying that correctly? Or is the 750mV stated for this part just the open circuit voltage rating with no load on it (gas valve, or, resistor)?

By "electro tech forum" - do you mean "ELECTRICAL FORUM" on this website?

I hear what you say about anti-virus s/w makers. Who knows what they are up to.

I am surprised to hear its not an issue on Apple! I'd be creamed on PC without it. And I still have not found one I really like. McAfee is seeming effective, but updates daily and stops me from working for about 15 minutes whenever it updates. It's free with my ATT DSL internet though.

mnalep
11-14-2010, 10:44 AM
btw, do you ever have a problem replying here - get a message like 'the token has expired'?

This just happened to me, and even reloading the page did not help. I'vw begun copying my replies, before clicking on POST REPLY, in case this happens - to avoid losing a message i've taken time to type. But sometiem forget to copy firts and lose a message I've typed.

Thatguy
11-14-2010, 02:12 PM
thatguy, Thanks, I am starting to understand. So if I did all this and found .382v DC, would that mean my t'pile is good, or bad, or weak?

By "electro tech forum" - do you mean "ELECTRICAL FORUM" on this website?

I hear what you say about anti-virus s/w makers. Who knows what they are up to.

I am surprised to hear its not an issue on Apple! I'd be creamed on PC without it. And I still have not found one I really like. McAfee is seeming effective, but updates daily and stops me from working for about 15 minutes whenever it updates. It's free with my ATT DSL internet though.
Pass/fail limits for these components seem pretty wide.

If the load voltage is half that of the open circuit voltage then maximum power is transferred from source to load and this is desirable for a setup like this.

Run some experiments. Your t'pile can be simulated with a D cell and two 6.2 ohm resistors and your valve with a 3 ohm resistor.
Some t'couples only put out 28 mV but I can't find a spec on their internal resistance.

http://www.electro-tech-online.com/

jadnashua
11-14-2010, 03:46 PM
For anything electrical to work, there must be a complete circuit from the source of power back to the power source. A thermocouple is a power source. When it is disconnected from it's load (the gas valve), and you measure it with a high impedance multimeter (this is almost like 'nothing', since it's internal resistance is generally quite high) you are measuring it as an open circuit. This is sort of like measuring the output of a D-cell when it isn't in a circuit. A closed circuit generally consists of a power source, and a load the power source is designed to run. A short circuit is where the power source is connected directly to the return on the power source, bypassing the load. A short-circuit is generally catastrophic - things overheat and can burn up.

To test the thermocouple with a load, out of its normal circuit, you'd take the two leads and connect each to one end of the resister, then measure the voltage across the resister.