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Triad F7-28 transformer question

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MikeMl

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Hi, I bought this transformer Triad Magnetics - F7-28 - Allied Electronics and I was expecting to get 28v out of it and I'm only getting 16v. I'm supplying 120 VAC to pins 2 & 4 and taking readings from 8 & 10. What am I doing wrong?
This transformer puts 28Vrms between 6 and 10. 8 is the Center Tap, meaning that you would measure 14Vrms between 8 and either 6 or 10.
 

xtrmi

New Member
Thanks for the help!
Ok that is what I thought to start with but when I built this circuit

I was getting 44v DC out and that threw me off so I was trying between 8 - 10. I will add that I'm taking my data with Extech 411 if that matters. I've been in a lot of circuit theory classes but I'm a newb about actually applying it so any help is appreciated.
 

MikeMl

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If you measure the open-circuit transformer voltage between 6 and 10 you will likely see about 29V. The reason is that Triad rates the transformer to deliver 24Vrms at 2.3A. The windings will have a resistance of several Ω, so unloaded the voltage will be higher than the loaded voltage. When the regulator is lightly loaded, the peak voltage across C1 (LM317 input) will be about 1.414*29V = 41V.

Looking at your spice sim, does the phony oscilloscope have a "floating" differential input on channel B? If not, then you have an inadvertent short.
 

xtrmi

New Member
I'm not sure what you mean by differential input on channel B. Do you have any suggestions on how I can lower the amount the voltage changes under load. Since I'm new to this I'm using this project to learn more application. So I'm open to any suggestions.
 

MikeMl

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What voltage are you trying to get out of the regulator?

Here are the things you need to think about:

The minimum voltage at C1, (reg input) Vc cannot drop below VoutMax + VdropOut with heavy loading, where VoutMax is the maximum you want out of the reg, and VdropOut is on the regulator's data sheet.

Minimum Vc occurs while C1 discharges between peaks from the full-wave rectifier.
The amount C1 discharges depends on the tranny sec. voltage, the value of C1, and the regulator load current.

The max Vc occurs when the load current is minimal.

I'm guessing that you would have done better to choose a tranny with a secondary voltage of ~ 16V.

Peak Vc
 

xtrmi

New Member
hey thanks for the help. It will help me figure out what I need to read up on.
I was shooting for a bench top that would supply 1.25v - ~25v @ 1.5amps max. So far its supplying a higher voltage than I expected. I just loaded it and pulled .5 amps at 37v. So I'm guessing when I try to pull 1.5 amps it will be closer to what I expected. I'm more worried about when I hook this up to a project and the voltage is jumping around based on the load I'm pulling. Is there any way to stabilize this? Also I have noticed that when I turn the voltage up it proceeds smoothly until about 18v then jumps very fast to 37 when it was loaded.
 

MikeMl

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Most Helpful Member
Are you using the values as shown in your Spice schematic? If so, the highest voltage that your supply will put out is 7.5V (1K and pot=5K). This means that there is about 30V dropped across the 317, which means it will run extremely hot. 30V @ 1.5A is 45W, which means it has to be mounted on a finned heatsink with ~100 square inches of area.

The way to reduce the dissipation in the 317 is to reduce the unregulated voltage you are starting from. You could have a switch that selects either half or full secondary voltage depending on what output you need out of the 317.


I suspect that you are exceeding either the max allowed voltage differential across the 317 (read the data sheet), or exceeding the max allowed temperature caused by inadequate heatsinking.
 
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