Newbie DC heater help

vconsumer · 15th June 2012, 07:02 PM

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.

disco · 15th June 2012, 07:40 PM

The regulator takes about three volts to operate stable. Can your power supply deliver 15 volts at 0.6A?

vconsumer · 15th June 2012, 08:40 PM

Quote:

Originally Posted by disco

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.

DF96 · 15th June 2012, 08:57 PM

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?

vconsumer · 15th June 2012, 09:06 PM

Quote:

Originally Posted by DF96

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.

v4lve lover · 15th June 2012, 09:07 PM

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

v4lve lover · 09:11 PM

Quote:

Originally Posted by vconsumer

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

DF96 · 15th June 2012, 09:16 PM

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.

12E1 · 15th June 2012, 09:45 PM

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).

vconsumer · 15th June 2012, 09:52 PM

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).

15th June 2012, 07:02 PM	#1
vconsumer diyAudio Member Join Date: Nov 2004 Location: Colorado USA	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.

15th June 2012, 07:40 PM	#2
disco diyAudio Member Join Date: Apr 2006 Location: Holland	The regulator takes about three volts to operate stable. Can your power supply deliver 15 volts at 0.6A? __________________ jaap

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
DC Heater Caps	Bengali	Tubes / Valves	3	24th September 2011 11:29 AM
dc heater supply help	milen007	Tubes / Valves	17	21st February 2009 07:42 AM
DC Heater Voltage	Bengali	Tubes / Valves	9	2nd February 2009 04:19 AM
referencing DC heater to B+	jarthel	Tubes / Valves	4	28th March 2007 02:43 AM
Diode for DC heater	dsavitsk	Tubes / Valves	16	2nd January 2007 04:13 AM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read


	Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving

15th June 2012, 08:57 PM	#4
DF96 diyAudio Member Join Date: May 2007	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?

15th June 2012, 09:07 PM	#6
v4lve lover diyAudio Member Join Date: Aug 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

15th June 2012, 09:16 PM	#8
DF96 diyAudio Member Join Date: May 2007	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.

15th June 2012, 09:45 PM	#9
12E1 diyAudio Member Join Date: May 2012 Location: Near Huntingdon, Cambridgeshire, UK	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).

15th June 2012, 09:52 PM	#10
vconsumer diyAudio Member Join Date: Nov 2004 Location: Colorado USA	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).