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Old 25th August 2007, 07:50 AM   #1
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Default High Current Power Supply 12v, with zener on base

Hi,

Iím busy building a 12v regulated power supply for my UPS

http://www.aaroncake.net/circuits/supply2.asp

Click the image to open in full size.


What do u all think about this power supply, to me it looks cool.

Iíve tried a 6 volt version because I dint had the Zener for the 12 volt version, but itís on the way


I'll need to draw 3 ~ 5 amps for 2 sec and then 1 amp all the way

thx
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Old 25th August 2007, 08:28 AM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
this will work fine.
voltage will drop slightly as load current increases.

Beware,
the current rating of the transformer should be at least double the continuous output current. The original is risking overheating by saying output + 1A.

Check the temperature of the heatsink when operational. It may need to be very much bigger than you expect.
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Old 25th August 2007, 08:58 AM   #3
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I used an old heat sink from an old amp to mount the transistor and used an indicator / brake light, 12v car bulb as load
And let it on for 1 ~ 2 min for a temp test on the transistor and it kind of started to get hot, but not hand burning hot, so if I must take a guess that the bulb draw 1.5+ amps because my other variable LM317T 1.5A power supply can only make the light burn for a couple of second before it starts to dim, I get a max of about 3.6 Amps on this regulator "Power Supply 6+v, with zener on base"

My transformer is kind of small but not tot small I have a smaller one also.

Itís a 28volt transformer and I get a 7 ~ 8Amps reading then I hook up my Amp meter onto a short, ďI know itís dangerous and stupid, but this way I can see the real power in a surgeĒ I only do this with small transformers and not with my 28V which is rated 1500VA
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Old 25th August 2007, 09:32 AM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
your 12V brake light bulb is probably drawing 1.7 to 1.8A.

Used as a continuous load then the transformer must br rated for 3.5A continuous.
To get 12V regulated you need at least 12Vac or maybe 14Vac to have sufficient drop across the regulator to work properly, particularly when mains voltage is low.
The voltage of the transformer is measured while it is delivering rated current into a resistor load.

That 14Vac & 3.5A means you need a 50VA transformer rating.

If you only need 1A continuous then 12Vac & 2A only requires 24VA rating.

When mains voltage is high the regulator dissipates more heat. You should design for both conditions, not for the happy mid-voltage range.

Don't try to use short circuit currents to estimate the capability of a transformer.
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Old 25th August 2007, 10:01 AM   #5
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Iíve just located the rating on the specs,

Output: 24v ~ Max 28.8VA


Andrew T. can you please check out my other thread maybe you can also help out there, because nobody want to reply



12v UPS relays Trigger, 1 ~ 11v no trigger
12v UPS relays Trigger, 1 ~ 11v no trigger
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Old 25th August 2007, 10:06 AM   #6
AndrewT is offline AndrewT  Scotland
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that transformer is not suitable
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Old 25th August 2007, 10:10 AM   #7
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Some noob questions:

What is the purpose of R1 and how do you figure what value should be used?

How many 2N3055 can a 1w zener drive?
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Old 25th August 2007, 10:10 AM   #8
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On this regulator project?

Well the 6 volt version worked out fine, the transistor only heats up then I apply a big load


Iím ordering 1.1W 13V zeners for this project, because the small zeners seem to heat up abit
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Old 25th August 2007, 10:16 AM   #9
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Quote:
Originally posted by theAnonymous1
Some noob questions:

What is the purpose of R1 and how do you figure what value should be used?

How many 2N3055 can a 1w zener drive?

Well I might say R1 is there to power the base on the NPN transistor and to get a reference voltage from the zener diode or something, my only fear is then the zener or transistor blows then Iím getting a full load of 24+ DC into my 12V line
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Old 25th August 2007, 10:34 AM   #10
AndrewT is offline AndrewT  Scotland
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Hi,
that other thread links to a site showing a crowbar.
Use it to short out the fused 24V supply when the output goes over voltage.

A 400mW Zener can just about do the job, if the correct supply voltage is fed to the correct resistor.

If you use too high an input voltage everything gets too hot.

Remember, design for the two limiting conditions:-
1.) low mains input voltage and
2.) high mains input voltage.
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