Transformer help needed.

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Frosty_47

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Hello every 1,

I have purchased Triad 24V transformer (FD6-24). To my surprise, the trusty Fluke 45 DMM measured 29.5V when I hooked everything up. I was shocked and so I rushed back to the store and returned this transformer. Off I went to electro sonic and bought HAMMOND 24V (166L24) transformer and immediately ran into similar problem. My DMM measures 27.8V output. What in the F%#$@ world is going on with these transformers? Why are the voltages so off the ratted? Is this normal or am I a victim of two unsuccessful products from world famous manufacturers?

Thanks
 
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They are dinky little transformers. The Triad has a max current of only 1.25A and the Hammond is 2A. They have internal resistance that reduces their output voltage at full load. Then the voltage is too high without a load. It is normal for dinky little transformers.

Measure your mains voltage. Mine is 123VAC but it is supposed to be 110V, 117V or 120V.

The very old Electro-Sonic catalog listed the no-load voltage of all their transformers but not anymore. The bigger, more powerful transformers have much better voltage regulation.
 
Thank you !

Thanks again Audiogury!

I just tested the transformer with 300 Ohm load and no changes. Still reading 28V. My AC line measures 122V. The transformer input is ratted for 115V. Anyway I can reduce AC input ? I really don't won’t to add more diodes to drop the excess voltage. Do toroidal transformers have better regulation ? If they do, than I might as well exchange this one for a torrid.

Thanks a lot !
 
Look at the datasheets for the transformers to see their output voltage without a load.
One time I e-mailed Hammond about a transformer of theirs and got an immediate reply.
 
are you rectifiying the output? if you are you are going to get a peak voltage rather than the 24v rms
 
Peak Voltage, I wonder what that is ?

No **** I am going to get 24/0.707=34V

I am not that much of a noob....
 
A bridge rectifier with some current in it has a loss of about 2.0V. Then you are down to only 32VDC if there is not any ripple. You might have less than 30VDC on the lowest parts of any ripple.
 
Ayt

I apologies for being rude....

I can actually fit three more diodes by attaching them to the other already soldered 3 diodes and set them up vertically. That should drop the input to about 34.5V (39.6V input minus 1V drop for the bridge, 4.1V for 6 diodes). 34.5V is right on the boundary of the LM317.

The bridge I am using clearly states 1.1V drop at full current load.

Anyways, thank you all for your help!
 
The LM317 will get too hot and will shut-down if you set the output voltage to about 5V and the current is higher than about only 0.6A.

The bridge rectifier has very high current pulses, not continuous DC. The pulses charge the main filter capacitor with maybe 15A. Then its drop is a lot more than only 1.1V.
 
One person introduced me to LM317HV which actualy supports up to 48V on the input so that kinda solves my problem. No more need for stupid diodes.

Thank you all!
 
Omg !

OMG !!! I just found out that LM317's TO-220 package has a typical current limit of 0.8A ! Damn ! That realy ruins a lot of things for me ! I always thought that LM317 TO-220 can handle up to 1.5A of current, guess I was wrong...
 
Frosty_47 said:
Is this normal or am I a victim of two unsuccessful products from world famous manufacturers?
Click to expand...

It's entirely normal, the rated voltage is at FULL load, with lower loads the voltage will be higher - it will also, obviously, vary according to your incoming mains voltage as well. As suggested, the smaller the transformer the larger the variation will be.
 
Frosty_47 said:
OMG !!! I just found out that LM317's TO-220 package has a typical current limit of 0.8A ! Damn ! That realy ruins a lot of things for me ! I always thought that LM317 TO-220 can handle up to 1.5A of current, guess I was wrong...
Click to expand...

Not sure if you've looked at switchmode power supplies, but they are ideal for your design requirements of 5v output from 30V+ input. Natsemi has several that are *very* easy to use and will deliver an amp or so without any heatsink, and 3A or 5A (depending on the chip used) if you really need it. They have HV versions that will accept 60V...

I had a similar problem in that one part of my circuit required 20Vdc, but the logic needed 5v. LM317's just got too hot supplying just 100mA given the voltage drop. Switchmode runs coooool

P.
 
Frosty_47 said:
OMG !!! I just found out that LM317's TO-220 package has a typical current limit of 0.8A ! Damn ! That realy ruins a lot of things for me ! I always thought that LM317 TO-220 can handle up to 1.5A of current, guess I was wrong...
Click to expand...

It would help if you Told us WHAT Exactially you want at the Output.

What Voltage at What Current?
I Assume you want a DC. Output?

We could than Recommend the Best Transformer and Regulator.
 
Frosty_47 said:
OMG !!! I just found out that LM317's TO-220 package has a typical current limit of 0.8A ! Damn ! That realy ruins a lot of things for me ! I always thought that LM317 TO-220 can handle up to 1.5A of current, guess I was wrong...
Click to expand...
The LM317 is guaranteed to supply at least 1.5A if the voltage across it is 15V or less and it is kept cool enough.
When the voltage across it is more than 15V then it reduces its output current to protect itself.
With an input of 35V and an output of 5V then its output current is 0.3A minimum to 0.6A typical, if it is kept cool enough.
 
would it be possible to use more than one lm317 in series takeing the voltage down in steps ? that way each one would dissapate a portion of the heat?
 
A power resistor in series with the LM317 will dissipate a portion of the heat.
If two LM317 regulators are in series then the max output voltage will be reduced and the min output voltage will be increased.
 
Gaston said:
would it be possible to use more than one lm317 in series takeing the voltage down in steps ? that way each one would dissapate a portion of the heat?
Click to expand...

Yes its Possible, BUT its Far More Practical to just use the Correct voltage of Transformer in the first place.
 
Wide range adjustable output voltage at any real useful current is always non-trivial problem when dealing with linear voltage regulation. Adjustable bench type linear DC power supplies use various methods to solve this problem including switched transformer secondary windings, preregulators regulators, etc. Even SCR control of transformer primary power has been used by some in the past. The design challenge is to figure out how to spread out power dissipation (at the expensive of total efficiency) over several devices or to limit the unregulated DC voltage such that the regulator doesn't have to drop more then just a few volts while maintaining load regulation.

Newer switch mode type DC regulators are one method that can work well but do require additional components, but I would consider them the current state of the art in wide range adjustable, high efficiency DC power sources.

There is a reason wide range bench DC supplies cost so much and are so much more complex then just a typical IC based linear adjustable regulator.

Lefty
 
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