1.5 amp fast blow for the secondary, no more than 500ma for the primary if the original was 35va. full rated load would be about .3 amps, so a 250ma would be boarderline. (but the transformer should NOT be running full rated load, either)
Take the va rating and devide by the voltage to get the current rating in amps. 40va at 24 volts= 1.66 amps. 40va at 115 volts = .347 amps.
40 va at 127 volts = 0.315, etc.On 4/9/2011 2:21 PM Steve Turner spake thus:
Guess I missed that.
35 VA/24 V = 1.45A, so you need a 1.5A fuse. Fast-blow.Get the kind that'll fit in an in-line fuse holder (the little glass jobbies with metal contacts on the ends). Should be available even at Radio Shack.
Not to worry; I read the directions. The red and orange wires were indeed taped up and not used; only the white and black wires on the input side were hooked up in the normal fashion. The blue and yellow wires on the 24V output side of the transformer were hooked to the blue and red wires (respectively) leading to the control board. I measured input and output voltages after installing the transformer, and I ensured that I was getting 24V on the output side before I hooked those leads to the control board. The control board then came up with it the flashing red LED to indicate normal ("no call for heat") operation.
Well, that blew that theory! ^_^
TDD
Interesting; thank you. My meter is an Extech MN26T:
Great information David. Thanks!
On 4/9/2011 3:11 PM Steve Turner spake thus:
Yep. You've got a better meter than I, and it's up to this task. Just connect it as I described (be sure to use the correct connectors on the meter, the ones marked "10A" and "COM"), make sure it's on the highest current range (10A), and you're good to go.
It's not that hard, or that critical.
Put this on the primary:
This on the secondary:
You can put them in this:
It is OK for the transformer to run a little warm, it shouldn't be hot.
There are two kinds of failures, one is a long term thermal failure (which seemed likely when you said years had gone by), the other is a transient short, or maybe not so transient. If that is the case then something else will be bad. It may look burnt, or if it is a diode or transistor it may just be shorted. You can Google how to test those.
Don't be afraid to put a quick finger on a transistor to see if it is hot, or on the transformer.
As far as calculating what size fuse, you should know Ohms law. Current
Note that there is a small surge when you turn on a device. So it may peak higher at the instant of turning on. A fuse usually can absorb that. The white ceramics are faster blowing and the slow blow ones usually have a little coil inside the fuse to show it down and are marked slow blow. When/if you fix this, leave in the fuses and we can give you some guidance on more properly sizing them. But some fuse in the ballpark is definitely better than nothing.
Jeff
Just a thought,
When you put the new transformer in does the "Diagnostic Light" LED=20 turn on ? Does the LED blink or flash in one of the patterns indicated in the=20 "diagnostic codes" section that you posted a link to ?
The "diagnostic codes" imply that the control board can detect=20 several of the problems mentioned by others. Maybe a good starting point as there is not enough time to test with=20 test equipment.
robb=20
"robb" wrote in news:WfKdnQOvu_d2bz3QnZ2dnUVZ snipped-for-privacy@earthlink.com:
a bad start cap would not affect the 24v control transformer,that just powers the controller board and relays. either something is loading the transformer or a problem with input voltage.
a line voltage monitor may be needed to see if there's some short-term overvoltage applied to the transformer,and a oscilloscope may determine if the input waveform is sinusoidal or otherwise,because 60hz iron core tranformers don't like extreme distortion on their input,it gets converted to heat,not output voltage. a DMM will not show line distortion or short term overvoltages. are the wires burned close to the transformer,or over their entire length?
Pardon? every one of my digital meters has an ac current range.. The one on my desk right now has a 200ma and a 20 amp scale, while for DC it has 2ma,20ma, 200ma, and 20 amp.
My "pocket" meter has 2 amp and 10 amp AC and dc
My "bench" meter has 300ma and 10 amp, both AC and DC.
The first digital meter I ever owned (and I still have it) has 2ma,
200ma and 10 amp ranges (and it is 30 years old) And my"amp clamp" reads 200 or 1000 amps
The rating is "output" which means the secondary. Primary power will always be slightly higher due to efficiency issues.
We don't need to guess here. The transformer has a rating. NEVER fuse above the transformer rating. If the transformer is a 40va 24 volt transformer the MAXIMUM fuse rating is 40/24=1.66 amps - so a 1.5 amp fuse will protect the transformer - a 2 amp fuse will NOT.
As noted previously, this is NOT the case. An AUTOMOTIVE DMM might not have an AC current scale, but virtually all general purpose meters do. They generally have a "third terminal" used with the common for the high range, which will be what he wants to use since the low range that uses the standard (vom) connections generally tops out at 300ma, more or less.
He needs to fuse the secondary and meter the primary, and have someone switch all the loads while he watches the meter. This will tell if he has a neutral issue, because a load on the "other phase" will cause the voltage to go up while a load on the "same phase" will cause the voltage to go down.
Even simpler, when the blower motor comes on, if he has a neutral issue the voltage on the primary WILL DROP. Absolutely no question. (and more than 1 volt - virtually guaranteed)
On 4/9/2011 6:11 PM snipped-for-privacy@snyder.on.ca spake thus:
Well, all I can say is that *none* of my 3 general-purpose multimeters (2 analog and 1 DMM) have AC current settings. But apparently this is a non-issue in this case, as the OP's meter definitely has AC current capability.
leads in the common and 10a jacks.
It's AC so there is no polarity. It's single output, non grounded, so it doesnt matter which direction either the primary or secondary are connected. Good to have the white on neutral and the bloack on live, but it will have NO EFFECT on the operation. - or the life of the transformer.
Or just another crappy transformer. You do not just ASSume it is the transformer
Transient overvoltages of less than 20% or there-abouts are unlikely to blow the transformer primary of the furnace without manifesting themselves elsewhere in the house - and over 20% would definitely manifest themselves elsewhere.
I peeled the insulation back from the hot and neutral wires and the burning only occurred right at the terminals, no more than 1/4" into the wiring.
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