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Newbie DC heater help

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vconsumer

Member
Joined 2004
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:confused: Any help appreciated.
 
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vconsumer

Member
Joined 2004
DF96 said:
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?
Click to expand...

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.
 
V

v4lve lover

Member
Joined 2010
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 :D
 
V

v4lve lover

Member
Joined 2010
vconsumer said:
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.
Click to expand...

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 :eek:

Edit better use a 317 its more temp stable constant current works a charm
 
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12E1

12E1

Member
Joined 2012
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).
 
Loudthud

Loudthud

Member
Joined 2007
An old National data sheet for the 78XX series shows the "Peak Output Current" to be about 0.6A with a 25V input to output differential and 125 degree junction temperature.

Also, expect the tubes to draw about 2x rated current when cold. It's a good idea to arrange some sort of soft start circuit so the voltage ramps up slowly at turn-on.
 
R

regal

Member
Joined 2004
vconsumer said:
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).
Click to expand...

Because you rectified it and probably lightly loading it. Just add a resistor in series to get it below below 30V loaded (tubes hooked up.) But start with a new regulator, yours is probably damaged. If the sink is sized appropriatley then the sink should be 60c or under, if not try going down on input voltage 5V at a time by adding resistors in series (be careful of power rating IR=V and P=IV.)
 
M Gregg

M Gregg

Disabled Account
Joined 2010
The voltage difference,
There is no magic unfortunately..

When you measure AC with your meter you are measuring RMS..when you measure the DC value you are measuring is Peak..so the AC reading is actually lower than the value that is on the AC<<ie not Peak only RMS.

Have you tried running the heaters in series with a low value resistor to drop the excess voltage, lift the heater chain in the middle. Or regulate with a soft start to overcome the low cold resistance of the heaters..keep the voltage drop across any regulator to a minimum to reduce wattage dissipation<<this will stop the regulator dropping out with over heat.

Regards
M. Gregg
 
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