Comments for page: uTracer-HR You Can Build

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Posted by Steve L. February 17, 2022 - 05:12 pm
Hi Rico, Yes, with some additional details you could drive a relay from the uTracer board heater output but I wouldn't recommend doing that unless you're pretty knowledgeable about the electronics issues involved. As was discussed in the section, "The Issue with the Heater Supply," what appears at the board's heater output is actually a 20kHz pulsed signal which swings between 19V and 0V. Unless measures are taken, it could cause damage. Note that the filament output is between the +19V supply and the drain of driver, T3. I will assume the coil of the relay would be in place of the filament. When T3 turns on, 19V is applied to the relay coil. Due to the coil inductance, current will ramp up linearly with time until T3 turns off. It is essential that the coil have a diode across it to absorb the inductive spike which will occur when T3 turns off. The spike will be opposite polarity to the applied voltage. Without the diode, the spike can be 1000V or more. With the diode, it will only be the forward voltage of the diode. If the polarity of the diode is wrong, there will be damage.

The problem is, we don't know what the inductance of the relay is. It's not usually specified so one would need to measure it before trying it in the circuit. The key question is whether the inductance is high enough to keep the peak current acceptable during the on-time of T3. One wouldn't want it to be more than, say, two times the coil rating. Bear in mind, this is with 19V applied. A better solution would be to use an 18V relay such as an Omron G2RL-24 DC18 (available at Mouser) and set the GUI heater voltage to 18V or more. The duty cycle would then be close to 100% and there wouldn't be a risk of excessive pulse current. It still needs the protective diode though. So you could use the relay contacts to control power to the HRB.

By the way, if you did try to do it with a 6V relay, the GUI setting would likely have to be much higher than 6V. The reason is that the relay inductance will act as a low pass filter on the pulsed signal. As discussed in Appendix A of the article, filtered PWM requires a very different duty cycle than unfiltered, to produce the same effective DC voltage.

Posted by Rico February 17, 2022 - 11:23 am
Thank you for your answer. Would it perhaps be possible to connect a 6V relay (when you choose a heater voltage of 6.3V in the GUI) to the connections of the heater of the uTracer. And when you press Heater On in the GUI, the contact of this relay turns on the 19V of the HRB. And when you're done with the measurement, it also switches off the power of the HRB. Would it be possible?

Posted by Steve L. February 17, 2022 - 09:51 am
Hi Rico, Thank you for your kind comment. You are correct that the new heater regulator board (HRB) is on whenever the main power switch is on in this design. Of course, you could add a separate switch for the HRB if you wish. And you're also correct that the Heater On function in the GUI doesn't affect the HRB heater voltage. However, you do need to click the Heater On button in the GUI so it will change to the "Measure Curve" button needed to start the test.

Posted by Rico February 17, 2022 - 07:53 am
Hi Steve, I read your article about the uTracer with great enthusiasm. And mainly the heater regulator design.
Now I would like to apply this design in my uTracer as well. But now I was wondering, the current heater voltage of the uTracer is activated via the GUI. But in your design I don't see a release from the uTracer anywhere to start the heater voltage. Does this mean, for example, that the 6.3V is always present when the unit is connected to the 19V power supply? And does the whole "Heater on" function on the GUI do nothing when you get the heater voltage from the heater regulator circuit? Or am I missing something?

Posted by Steve L. May 01, 2021 - 02:04 pm
Very happy that the HRB is working favorably. Congratulations on the accomplishments! Your reports that the version with the XL4015 module works fine and that the alternative reference ICs work okay are especially appreciated. It's a relief to have confirmation that the later module is viable. You know, I really wish I had addressed the use of cheaper references in the article but since I hadn't tested any, I didn't feel I could just throw that in without verification, even though I knew that the pinout was fairly common. So your testimonial is particularly valuable.

My attitude was that the $12 reference was in the "why-not" category, since it only added 15% to the cost of the HRB board or about 2% to the cost of the uTracer project. Heck, the way I see it, the cost of the whole project pales in comparison to the value of one's time spent doing it. But I know that some people are put off by what they see as fancy or excessive designs so it's nice to have an alternative just to make them feel better, if for no other reason.

Posted by Gary K. May 01, 2021 - 02:00 pm
[Ed. note: This is extracted from a recent email thread--posted with permission---shl.]

I'm happy to report that the board works amazingly well on the bench, and hopefully soon in uTracer 3+

I also built up a second board with the XL4015 and it seems to work fine. Indeed the module still uses the 10K/270 ohm combination. There are several XL4015 modules on Aliexpress and I was careful to select the one with the same basic layout and position of the connectors.

Also I had a small pile of REF43FZ voltage reference IC's and they work just fine in place of the MAX6325 in the second regulator I built. A brief test with the REF03 also seemed ok - a significantly less expensive part for less obsessive folks - I share your views on accuracy :)

Thank you so very much both for your support and for a very nice project.

Posted by Steve L. January 27, 2021 - 03:49 pm
Hi Pete, Thank you so much for your kind message. I truly appreciate your encouragement. Sincerely, Steve

Posted by Pete Barth January 27, 2021 - 01:56 pm
Your article is wonderful. I have been saving for a kit, and I do need to measure and compare transmitting tubes.
Keep up the good work, and fantastic writing.
Pete W6LAW

Posted by Markus January 20, 2021 - 11:25 am
Hi Steve, thanks for your advice! I will try to find a heatsink in case I have the impression that it might become critical! Regards,

Posted by Steve L. January 19, 2021 - 04:34 pm
Hi Markus, Without the heatsink you would certainly be okay with 12AX7 and other low-level tubes. I guess 6L6 and EL34 would be okay. Perhaps the main concern would be rectifier tubes like 5U4 and GZ34. By the way, if the issue is mainly sourcing the heatsink, a similar-sized chunk of aluminum could suffice. In fact, I considered doing that because it would allow me to use a bigger piece with more thermal mass. However, since the heater may be left on for an extended period, I realized that the extra thermal mass wouldn't necessarily be a key factor. Since the heatsink was endorsed by the module vendor, I decided that was the safest course.

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