Ask Me Help Desk - Repeated furnace transformer burns

Thanks a lot. The new transformer blew the lights out in the house 30 sec after I powered it on. The load side was connected at that time. Do you mean that there is a chance that it will not burn if I do not connect the load side, and I will have time to read the voltage?

Terminals look like figure 2 on this page:

http://onlinecatalog.digikey.com/Web...PageIndex=361#

A transformer of this size is incapable of blowing the lights. But the covers touching the primary side might cause some sparks if there are mechanical problems.

Inspect all wiring. Make sure the transformer will be secure.

You need to tape the ends of the wires that would go on the secondary and put your meter safely where the primary would connect and measure the voltage before permanently connecting the transformer.

Wires should pass through grommets and wires should not be pinched.

Thanks for the important info and the image, H.

Still, cannot get the furnace running. I have measured the input voltage from the line - it is 122V. I installed two inline fuses. For the primary line the fuse rating is 0.5A, 250V. For the secondary: 3A, 32V (it's a car fuse). As soon as I turn the power on, the primary 0.5A fuse burns, the secondary fuse is OK. I am hesitant to install a stronger fuse as the power given to the primary line of transformer is already more than 40W (0.5A*122V). What else may be wrong? How come the power is so high? Appreciate any suggestion. H.

I would start looking at something else than the transformer. The transformer problem is just the end product of the problem.

First make sure the line and the neutral are not reversed both in the furnace junction box and in the house panel. Do not trust color of wires (Black/white) but check with a meter to be positive it is correct.

If that is OK then disconnect all other 120 volt circuits that feed off the incomming line in the furnace except the transformer. Now activate the unit and see if the fuse still blows.

If the transformer does not pop the fuse then the problem lies elseware in the furnace.

Over the years I have seen circuit boards/motors both vent assist and blower/controls and relays cause similar problems and you will have to fish through them all till you find the culpret.

I am sure by now you have disconnected all the thermostat wires at the thermostat itself and also disconnected all low voltage wires at the furnace to eliminate a low voltage wiring/thermostat problem. If not you may try that but when you said the short took out the circuit breaker for the furnace that would somewhat eliminate low voltage wiring since a short of that magnitude would have burnt that low voltage wiring to a crisp by now.

Something is shorting to ground in that unit or circuit. These are the kind of problems I loved when I had the shop especially after 2 or 3 other companys could not figure it out. Over the years I have camped out at customers homes drinking coffee, smoking cigs and testing circuits till I finally figured it out.

You will figure it out it is just a matter of time. Just isloate one component at a time.

I used to carry mini circuit breakers used in the electronics industry so I could reset them and not keep blowing fuses.

You don't need fuses in both the primary and secondary. There is an inrush current when the transformer is connected. The primary fuse MUST be of the slo-blow or time delay fuse if you decide to use one.

Get yourself two 211-2 lamps from an auto store. These draw about 1A at 12V. Put them in series. And put the series connected lamps in SERIES with the secondary.

This means that if connected in this fashion and connected to your furnace. If something was shorted in the furnace, the WORST that can happen is the bulb will light.

BUT, remember that a bulb is worse than a transformer because it's inrush current is 10x times the normal draw.

Trust me and connect the primary directly like it supposed to be connected, no fuse and connect the secondary to the two bulbs in series. Any bulb that draws from 1-1.5 at 12V will be fine. 1*24 = 24 W, 1.5*24=36 W.

Complete the circuit. Show that this works. It will.

Now cut the secondary circuit and connect the two ends where the secondary would go in the furnace. You just proved that the transformer can light the bulb with no problems so nothing bad will happen if something in the furnace is shorted.

If something on the series connected secondary+2 bulbs+furnace shorts, the bulb lights.

Start disconnecting things UNTIL the light goes out or severely dims.

The bulb is a current dependent resistor. Since it's resistance when off is about 10X lower than when on, at low currents it's essentially a short circuit.

IT WILL WORK!

hvac1000 and KeepItSimpleStupid: Really appreciate your input. I am in business again as there are several more things to check and work on. H.

I have done some checks with two 12V bulbs in series (as suggested by an expert in one of the previous answers) and found couple of things I do not know what to do with, but think it might be important. I first connected all the elements in the secondary line in the following order: Transformer - Furnace - Lamps - 3A car fuse - Transformer. The bulbs were brightly lit. When connected in different order, Transformer - Lamps - Furnace - 3A car fuse - Transformer, the bulbs were out... because the 3A fuse burns right away. I do not know if this finding is of any use. When reconnected in the first order with a new fuse the bulbs are on again. I then disconnected the thermostats (two, connected in parallel) and the bulbs went out - the shortage eliminated? I am not sure if replacing thermostats will make it work, would it help? H.

The order thing is a fluke. Bet the wires were touching at the bulb.

Do the same thing, but disconnect the t-stats one at a time at the tstat to eliminate a wiring issue between the furnace and the stat.

Well, I guess the problem has been located. As it was suggested in many responses, it turned out that the two-wire line of one of the thermostats was cut by and in contact with a sharp edge of the furnace's metal cabinet, hence the short circuit. Simply disconnecting that thermostat's wiring got the furnace running again. Thanks for all the help. H.

Glad you got it going

Quote:

Originally Posted by harum

Well, I guess the problem has been located. As it was suggested in many responses, it turned out that the two-wire line of one of the thermostats was cut by and in contact with a sharp edge of the furnace's metal cabinet, hence the short circuit. Simply disconnecting that thermostat's wiring got the furnace running again. Thanks for all the help. H.

I do believe I told you this a week ago lol. Glad you got it up and running.