I am trying to replace a transformer for a Kitchen Aid oven. The part is no longer available from the vendor. The needed output is 1.95 VAC with only 200 mA and the input is 120 VAC. It feeds part of the control panel for the device. I have found a standard model that uses 120 as an input, but outputs 2.5 VAC, with a 6 Amp output rating. I would like to get the output as close to 2 Volts as possible. If I could drop the input voltage by 20%, would that drop the output by the same percentage? Or would it be possible to lower the output voltage?

 

Almost any small transformer could be adapted by just winding on a couple of turns of enameled wire, wind on a couple of turns and check the voltage to see what is required, probably no more than 2 or 3 .
Or use the transformer you have and take turns off until the requirement is met. You would soon be able to tell the turns/volt.
H.D. has a selection, but bell transformers may be a little large.

 

Almost any small transformer could be adapted by just winding on a couple of turns of enameled wire, wind on a couple of turns and check the voltage to see what is required, probably no more than 2 or 3 .
Or use the transformer you have and take turns off until the requirement is met. You would soon be able to tell the turns/volt.
H.D. has a selection, but bell transformers may be a little large.

You mention both removing and adding turns. If I remove turns, ( seems easier ) should they be removed from the input winding? Or the output winding?

 

The secondary. (output winding)

 

Plus the secondary is usually wound on last, so much easier.
One way to find out the turns/volt is to wind on, say 10 turns of small gauge wire and measure voltage result across output pair.

 

The only application of 1.95 volt AC power on a consumer product is the heater section of a vacuum florescent display. That is a very stable load not subject to much grief or problems.

So my very first question is why do you think that transformer has failed? It would be more likely that the heater in the VF display has failed, and in that case you may simply be out of luck.

Often that transformer is combined with the logic power supply, which provides AC to the rectifier and filters to power the logic section of the control board.

The six amp transformer is physically far to big to use. If you still have that 1.95 volt transformer available you should neasure the resistance of both windings to verify that it has actually failed. A cheap source of a similar sized transformer could be a failed TV set of the early all transistor flat screen variety, or even a failed flat screen computer monitor should have a small transformer. OR another microwave oven from a similar era.

OR, for a greater expense, check the Digikey electronics catalog. That is an honest company that is knowledgeable about the products that they sell.

A final suggestion is to attack a low voltage wall wartpower module of the older transformer type. Max has mentioned that a small transformer can be rewound, or in the case of a six volt supply module, secondary turns can be unwound until the voltage is what you require. That can be done with the transformer removed from the housing, the main requirement being adequate lighting and patience. Maxcan provide more detailed advice.

 

The original is only a .4Va transformer, if that is all it supplies. which is quite small!
@TexasRedbud Can you post a pic of the Txfr?
Do you know what it operates/feeds?

 

The original is only a .4Va transformer, if that is all it supplies. which is quite small!
@TexasRedbud Can you post a pic of the Txfr?
Do you know what it operates/feeds?

In the microwave ovens that I have seen with the VF display, that winding only feeds the heater of the VF display. And I still question that it actually has failed. That is a very low-stress part of the circuit.
Really, transformers with that same low voltage winding should be in every old VCR that has a VF display. Lots of other windings also, but always one for the VF display. So there could be another cheap source.

 

Could the VF heater supply be DC?
That would be much easier to arrange.

 

In the microwave ovens that I have seen with the VF display, that winding only feeds the heater of the VF display.

Not sure it is a microwave type,?
The OP states KitchenAid which make a lot of convection ovens, if this is so, I don't quite see what the task of the LV would be?

 

he needed output is 1.95 VAC with only 200 mA a........................I have found a standard model that uses 120 as an input, but outputs 2.5 VAC,

To make the math simple. Have 2.5Vac and want 2Vac at a load of 200mA. (often the load is now what you think)
2V 200mA so the load must be about 10 ohms.
Add a 5 ohm resistor in series with the load. (5.1 ohm is standard value)

 

With this in mind:

The needed output is 1.95 VAC with only 200 mA and the input is 120 VAC.

I would do as suggested and just remove turns from the secondary of any suitable transformer. You are only looking at a secondary current of 200 mA. Next a transformer like you have is merely a radiometric device. While we say 120 VAC / 1.95 VAC is a 240 : 3.9 ratio. US power distribution is 240 VAC split phase so we end up with 120 VAC. While it looks good on paper it's all about being "about". Just as an example my residence comes in at 248 VAC and my split is 124 VAC for my residential outlets. That's today right now. An hour or two from now or tomorrow it may be a little more or less so "about" is how it works. Since we can't change the ratio which is a function of the transformer what happens if we change the line voltage by only 4.0 volts? Now we have a 248:3.9 ratio. We see where this is going. Trying to resolve a secondary voltage off a transformer two places to the right of the decimal is sort of fruitless. Pretty sure 1.9 to 2.0 volts more or less will do just fine.

Just My Take
Ron

 

Digikey has a small 2.5v CT transformer so it could be adapted easily.
Triad F-1X model.
Turns/volt on a typical E I transformer is 3 to 5 turns/volt.

 

Usually that low of an AC voltage is the heater of a vacuum-florescent display device. and an oven could certainly have a digital temperature display with a clock/timer function as well. The TS did say it was an older device, Kitchen aid products are some of those that last long enough to get old. I am not aware of any other component used in consumer electronics that requires an AC voltage that low. And I did post the question for the T asking how they knew it was the transformer.

 

The original is only a .4Va transformer, if that is all it supplies. which is quite small!
@TexasRedbud Can you post a pic of the Txfr?
Do you know what it operates/feeds?

The original transformer had 4 different output voltages. 33VAC 11VAC 3.3 VAC and 1.95. The various voltages feed into the control panel of a KitchenAid combo microwave/oven. I know the transformer is burnt because I wired a 240 V 4 wire plug direct on the leads from the oven. It was a used unit and I wanted to confirm it worked before I installed it. And of course, I managed to feed 240 VAC to one of the leads intended to have 120 VAC. The primary winding for this transformer if now open. I found another part number from KitchenAid that use the same plugs and provide three of the output voltages. Only the 1.95 output is missing.

 

Could the VF heater supply be DC?
That would be much easier to arrange.

The original transformer has a chart that shows all four of the output voltages. All four are AC Voltage.

 

The primary winding for this transformer if now open. I found another part number from KitchenAid that use the same plugs and provide three of the output voltages. Only the 1.95 output is missing.

Have you examined the transformer closely? They often embed a thermal fuse in the windings, if you are lucky, it is easily accessible.
In some case the fuse is buried in the windings, it is very unusual for a transformer to go open, the fuse is in the primary..

 

The original transformer has a chart that shows all four of the output voltages. All four are AC Voltage.

I meant to ask whether a DC supply would work for this application. If a DC supply would work for whatever the 1.95V is used, then it is much easier to get an accurate DC voltage than an AC one.

 

The original transformer had 4 different output voltages. 33VAC 11VAC 3.3 VAC and 1.95. The various voltages feed into the control panel of a KitchenAid combo microwave/oven. I know the transformer is burnt because I wired a 240 V 4 wire plug direct on the leads from the oven. It was a used unit and I wanted to confirm it worked before I installed it. And of course, I managed to feed 240 VAC to one of the leads intended to have 120 VAC. The primary winding for this transformer if now open. I found another part number from KitchenAid that use the same plugs and provide three of the output voltages. Only the 1.95 output is missing.

I meant to ask whether a DC supply would work for this application. If a DC supply would work for whatever the 1.95V is used, then it is much easier to get an accurate DC voltage than an AC one.

Unfortunatly, I do not have much information about what these various output voltages do. One responder made this comment " Usually that low of an AC voltage is the heater of a vacuum-florescent display device. and an oven could certainly have a digital temperature display with a clock/timer function as well. " That is more than I know about this appliance and its controls.

 

DC is not a good choice for heaters in displays , lights, or most vacuum tubes, because it tends to produce material migration by a process that I am not clear on, even after reading a detailed description. And probably the VF display in that combination oven would be vastly more difficult to locate a replacement for than the transformer. And now that we are aware that it is not replacing a similar transformer, there is no failed one to examine.
But now I am wondering if there is enough clearance between the winding cover and the transformer laminations to allow a single layer of #24 wire to be added to provide that voltage. That would be the graceful solution for installation with the added benefit of low cost. And even at ten turns per volt that is less than 20 turns. Certainly the task would be tedious but by no means terribly difficult. And even if there does not appear to be a gap. it should be possible to produce a gap wide enough for the thin wire. Just be careful to not scratch the insulation varnish off.

But now, knowing how the original transformer met it's end, I am wondering if any damage was done in the circuits fed by that transformer, given that doubling the input voltage will also double the output voltage in most cases.