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Old 6th July 2006, 06:51 AM   #1
jarthel is offline jarthel  Australia
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Default help with transformer secondary

I am thinking of getting a 9VAC 4.17A transformer (no CT) to supply a basic bridged rectified LM1085 circuit (similar to LM317 but lower voltage drop-out). The intent was to have an output voltage of 8.2V.

The hammond website (http://www.hammondmfg.com/pdf/5c007.pdf) says:

average voltage after the 1st cap: .9 * Vsecondary
peak voltage after the 1st cap: 1.41 * Vsecondary.

if the average is .9 * Vsecondary, a 9VAC transformer won't be enough. plus you have the voltage drop-out associated with the LM1085.

I intend to draw 2.6A from the transformer. just wondering what are people practical experience with the topic.

Thanks for the help.
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Old 6th July 2006, 09:16 PM   #2
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I don't know about your specifics, but, in general, you'd like to have some head room so that you regulator can do its job correctly. Often the datasheets for the regulator will suggest a minimum input % over your expected output. Just shooting from the hip here, I'd say you might want a Vsec rms that is 25% higher than your expected Vp(rect).
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Old 6th July 2006, 09:25 PM   #3
poobah is offline poobah  United States
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You would be better off to use a 12 VAC secondary...

Go to Duncan's Amp Pages and download PSUDII. Nifty program... FREE!

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Old 6th July 2006, 11:20 PM   #4
jarthel is offline jarthel  Australia
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Quote:
Originally posted by poobah
You would be better off to use a 12 VAC secondary...

Go to Duncan's Amp Pages and download PSUDII. Nifty program... FREE!

I have used it and am using it.

the problem is, that is software. and sometimes reality is different
=========================
I just want to avoid a situation where I need to drop alot of voltage on the LM1085 when I do not need to
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Old 6th July 2006, 11:27 PM   #5
poobah is offline poobah  United States
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Understood... what does your model suggest?

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Old 6th July 2006, 11:47 PM   #6
jarthel is offline jarthel  Australia
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Quote:
Originally posted by poobah
Understood... what does your model suggest?

since I cannot simulate an LM1085 in PSUD2, I have placed the resistance load right after the 1st cap.

the voltages on the first cap according to PSUD2 are:

Vmin: 3.14
Vmax: 9.8
Vmean: 6.2
Vrms: 6.7
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Old 6th July 2006, 11:59 PM   #7
poobah is offline poobah  United States
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You can change the resistive load to a "current sink"... this will help your sim. Right click on the resistor and you can change it.

Seems like your cap is too small because the Vmin & Vmax are so different.

OR,

Be sure to set the simulation to 5 seconds or so to evaluate... you could be looking at the first start up voltages rather than the steady state conditions after the supply has charged up.

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Old 7th July 2006, 12:03 AM   #8
jarthel is offline jarthel  Australia
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Quote:
Originally posted by poobah
You can change the resistive load to a "current sink"... this will help your sim. Right click on the resistor and you can change it.

Seems like your cap is too small because the Vmin & Vmax are so different.

OR,

Be sure to set the simulation to 5 seconds or so to evaluate... you could be looking at the first start up voltages rather than the steady state conditions after the supply has charged up.

why is a current load be different from a resistive load? I have the voltage that it requires and I have the current requirements. it's easy enough to derive the resistance using those 2.
ps. the psu will be supplying tube heaters.

4700u is too small?

I've set it to simulate for 30secs. (or 30000 ms in PSUD2)
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Old 7th July 2006, 12:05 AM   #9
jarthel is offline jarthel  Australia
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Default Re: Re: help with transformer secondary

Quote:
Originally posted by paulb

Just from curiosity, what do you have that requires 8.2V and 2.6A?
tube heaters
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Old 7th July 2006, 12:08 AM   #10
paulb is offline paulb  Canada
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Just saw your reply and deleted my question.
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