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Old 15th June 2012, 07:02 PM   #1
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Default Newbie DC heater help

I built my first regulated DC supply for a 12B4 preamp. It uses the LM7812 and puts out a steady 12v without a load. When powering the heaters on one tube, everything is fine. However, it shuts down after a few seconds when I hook up the second tube. The 12B4 consumes about 300 mA per tube, and the 7812 is rated up to 1 amp when heatsinked (the regulator is bolted to the chassis in my build).

I'm confused Any help appreciated.
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Old 15th June 2012, 07:40 PM   #2
disco is offline disco  Netherlands
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The regulator takes about three volts to operate stable. Can your power supply deliver 15 volts at 0.6A?
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Old 15th June 2012, 08:40 PM   #3
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Quote:
Originally Posted by disco View Post
The regulator takes about three volts to operate stable. Can your power supply deliver 15 volts at 0.6A?
Yes. The power supply can deliver up to 24v @ 1.040A.
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Old 15th June 2012, 08:57 PM   #4
DF96 is offline DF96  England
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Measure the input voltage to the 7812. Check that the PSU is actually delivering what it says it can.

How good is the heatsinking? Did you deburr the mounting hole? Is there good thermal contact between the regulator and the chassis?
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Old 15th June 2012, 09:06 PM   #5
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Originally Posted by DF96 View Post
Measure the input voltage to the 7812. Check that the PSU is actually delivering what it says it can.

How good is the heatsinking? Did you deburr the mounting hole? Is there good thermal contact between the regulator and the chassis?
Voltage on the input of the 7812 is 38v, although the transformer is wired for 24v.

I'll go back and deburr that hole. Thought I'd cleaned it up pretty well and had the regulator mounted tightly to the chassis, but I'll check again.
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Old 15th June 2012, 09:07 PM   #6
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are you sure the regulator is not going into short circuit mode ? the cold resistance of tubes is quite low .

7812 is a cheap regulator buy a second and use one per heater use the decoupling caps like in the datasheet add a diode from the ground pin to ground to lift the 7812 to 12.6 volts for correct application and ofcourse less dissipation
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Old 15th June 2012, 09:09 PM   #7
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Quote:
Originally Posted by vconsumer View Post
Voltage on the input of the 7812 is 38v.

I'll go back and deburr that hole. Thought I'd cleaned it up pretty well and had the regulator mounted tightly to the chassis, but I'll check again.
that is out of spec use a 7824 and put both tubes in series whit two diodes to lift the regulator to 25.2 volts
overvoltage or overdissipation kicking in or both

Edit better use a 317 its more temp stable constant current works a charm

Last edited by v4lve lover; 15th June 2012 at 09:11 PM.
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Old 15th June 2012, 09:16 PM   #8
DF96 is offline DF96  England
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38V might be enough to cause thermal shutdown, as well as being over spec anyway. Read the device datasheet. Do a rough estimate thermal calculation.
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Old 15th June 2012, 09:45 PM   #9
12E1 is offline 12E1  United Kingdom
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There are many versions of 7812 from different manufacturers. A quick scan of a few datasheets shows some have a maximum input voltage as low as 27 volts, others up to 35. At 38 volts you are likely to have some problems. Some versions will also limit current just because the input to output voltage drop is high (even though the regulator may not be overheating at that time).
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Old 15th June 2012, 09:52 PM   #10
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Thanks for the help, gents. My next step is to find out why I'm getting 38v out of a 24v transformer (Triad VPS24-1000).
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