Heater switch transistor

ianl · 22nd March 2006, 05:59 PM

Has anyone used the following, or similar, circuit to switch a tube heater? I'm looking for a transistor to use, but I think I might need to use a darlington pair.

Current is 600ma, hfe min 120-200. A MJE340 would almost do it with a small driver transistor with high hfe (BC547?).

Brian Donaldson · 22nd March 2006, 07:00 PM

Or maybe a power mosfet. Then there's no drive current needed.

EC8010 · 22nd March 2006, 08:35 PM

A FET would probably be better because you'll always have 200mV or so of Vsat across a bipolar transistor, whereas with a FET the crucial thing is the on resistance. Either way, it should work just fine.

Giaime · 22nd March 2006, 08:39 PM

What's the purpose?

ianl · 23rd March 2006, 01:11 PM

A FET was the answer. I tried to reply earlier, but my post was being still being moderated.

I remembered I had a few IRLZ44 power FETs in my junk box. This tested great, so I etched the board and used the FET.

The purpose is to give a PIC control of the heater. This amp (gainclone with tube preamp) is remote controlled. The PIC has a startup sequence, activated by the remote, that does the following:

1. Turn on heater.
2. 20 second delay
3. Start B+ SMPS (240 volts)
4. Start gainclone SMPS (+35 volts)
5. Increase volume to previous setting (before last shut down sequence)

There is no interface on the PCB - everything is controlled from a remote. I really like this design. I'm still waiting on the digital pot IC, but everything else works great.

I'm very happy with the SMPS for the tube supply. I substituted a soft recovery diode for my standard MUR160, and used some big caps with a choke in a C-L-C configuration. No switching noise, even at maximum volume.

22nd March 2006, 08:35 PM	#3
EC8010 diyAudio Moderator Join Date: Jan 2003 Location: Near London. UK	A FET would probably be better because you'll always have 200mV or so of Vsat across a bipolar transistor, whereas with a FET the crucial thing is the on resistance. Either way, it should work just fine. __________________ The loudspeaker: The only commercial Hi-Fi item where a disproportionate part of the budget isn't spent on the box. And the one where it would make a difference...

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22nd March 2006, 07:00 PM	#2
Brian Donaldson diyAudio Member Join Date: Jan 2003 Location: Texas, Love it or leave it	Or maybe a power mosfet. Then there's no drive current needed.

22nd March 2006, 08:39 PM	#4
Giaime diyAudio Member Join Date: Oct 2004	What's the purpose?

23rd March 2006, 01:11 PM	#5
ianl diyAudio Member Join Date: Feb 2006 Location: Netherlands	A FET was the answer. I tried to reply earlier, but my post was being still being moderated. I remembered I had a few IRLZ44 power FETs in my junk box. This tested great, so I etched the board and used the FET. The purpose is to give a PIC control of the heater. This amp (gainclone with tube preamp) is remote controlled. The PIC has a startup sequence, activated by the remote, that does the following: 1. Turn on heater. 2. 20 second delay 3. Start B+ SMPS (240 volts) 4. Start gainclone SMPS (+35 volts) 5. Increase volume to previous setting (before last shut down sequence) There is no interface on the PCB - everything is controlled from a remote. I really like this design. I'm still waiting on the digital pot IC, but everything else works great. I'm very happy with the SMPS for the tube supply. I substituted a soft recovery diode for my standard MUR160, and used some big caps with a choke in a C-L-C configuration. No switching noise, even at maximum volume.

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