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ianl 22nd March 2006 06:59 PM

Heater switch transistor
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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 08:00 PM

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

EC8010 22nd March 2006 09:35 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 09:39 PM

What's the purpose?

ianl 23rd March 2006 02:11 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.

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