DHT Heater Group Buy notes.

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Andrew
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#1 DHT Heater Group Buy notes.

Post by Andrew »

The PCB should be self-documenting as all the positions and values are marked on the screen print. Rs is the sense resistor that looks like a wire loop. The slots marked C1k and C470K use the carbon comp resistors. Polarity on the diodes, LED and caps should be observed All other resistors are metal film.


The regs should ideally be close to the load as possible, if they are not it's not the end of the world but you may need to tweak the output voltage up slightly to deal with cable loss between the reg and the load, also use nice chunky wires.

When mounting the bigger boards you will need the heatsink (obviously), perhaps some heatsink compound and an isolation tab (mica etc) for the pre-reg to isolate its metal tab. No heatsink is supplied with the bigger board as it is envisioned that these can be chassis mounted or on your own heatsink. You will need a 3.5 deg C/W heatsink or better depending on voltage drop and current. Use the metal tabbed version of the pre reg LD1085V in the higher current version - the limit of the LD1085V is 3A, if you're going to try to squeeze more current out of it get a LD1084V 5A part.

Andrew
The smaller PCB comes with plastic tabbed parts and includes a heatsink on the PCB, it is good for up to a 300B, obviously the bigger PCB can also do all of the things the smaller one does.

The group buy was designed for a 9v TX maximum and should be good for a 6B4G 6.3v. If you are operating above 6.3v output then use higher value caps and a higher voltage TX would be required. I used low ESR ones from the Panasonic FR Series in the group buy. Driving a 2A3 at 2.5 volts should use a 6.3v input TX, do not use a 9v TX, you might be OK with a 6v TX.



The PCB can support the small wirewound trimmer for adjustable output or use the following values, neither are supplied in the kit.

Ry
7.5v = 3k6 (12v AC in)
6.3v = 3k3 (9v AC in)
5.0v = 3k3 (9v AC in)
4.2v = 3k3 (9v AC in)
2.5v = 10R (6.3v AC in)

Rx
7.5v = 1k8
6.3v = 2k2
5.0v = 3k3
4.2v = 4k2
2.5v = 5k2


Or use a Bourns Trimmer, between 5k and 10K, 25TURN, 10% and adjust on test.

Once set it should be stable and no need to adjust if replacing valves.

[Edit, ILPS (Mark) found that Rx = 4k7 and Ry = 3k3 works best for a 4.2v 41PL DHT, thanks for the update, Mark.]

Andrew
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#2 Re: DHT Heater Group Buy notes.

Post by Andrew »

On parts substitution.

The 250mW 1K, Rx/Ry and 47k metal film resistors can use any quality and similarly rated metal film, just get reasonably decent ones.

The parts that that are best not substituted include the 470nF surface mount caps and the carbon comp resistors, please get the exact ones listed in the BOM.

Additionally, I would stick to the same the IXYS diodes and but I don't think brand on the D44H11 nor LD1085P is crucial, just as long as part numbers are the same.

Feel free to substitute the brands on the DIL socket, (if you feel you need it), the 1N4001 and the led (just keep it green) and similarly any brand of TL431 will do the job.

I also use

CAMDENBOSS - CTB5202/2 - TERMINALBLOCK, PCB, 2WAY, 5MM

CAMDENBOSS - CTB5202/3 - TERMINALBLOCK, PCB, 3WAY, 5MM

to connect the PCB to the transformer and to the wiring for the socket. Any suitable similar 5mm terminal block is good.
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#3 Re: DHT Heater Group Buy notes.

Post by Thermionic Idler »

Hi Andrew,

Can you tell me if these boards are still available? I'm back to pondering circuits with push-pull 2A3 driving 300B, a requirement of which is decent DC heating on the 2A3 to prevent unacceptable hum levels. So this application would need 4 of them at 2.5V each.

Cheers!
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#4 Re: DHT Heater Group Buy notes.

Post by Andrew »

PM Sent.
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#5 Re: DHT Heater Group Buy notes.

Post by Andrew »

I have had requests to show a completed board so folk can see how they are put together.

cheers,

Andrew
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#6 Re: DHT Heater Group Buy notes.

Post by Andrew »

Small boards in the photograph are 85 * 50 * height of a SK129 heatsink.

http://uk.farnell.com/search?st=SK129, 63.5mm, 50.8mm, 38.1mm or 25.4mm, the thermal capacity reduces with size.

You also need to add mounting pillars to the height, you can see the nylon pillars I use in the underneath photo.

Large boards are in fact the same size, but need an external heatsink, the extra room made available by removing the heatsink is used up with more smoothing capacitors.

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#7 Re: DHT Heater Group Buy notes.

Post by RhythMick »

Andrew is there a circuit diagram for the board ? I don't recall seeing one in the original thread (though I could well have missed it somewhere in the 57 pages :shock: )

I ask because my experience with SS components is EXTREMELY limited and I'd like to get my head around exactly how the circuit works.
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#8 Re: DHT Heater Group Buy notes.

Post by RhythMick »

Andrew could I ask for 2 favours please.

Could you (or someone) post a pic of a large board fully populated. I just want to make sure I correctly orient the 3-legged jobbies.

And could you add the formula for calculating resistors? I need 1.5v for the 26.
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#9 Re: DHT Heater Group Buy notes.

Post by Nick »

Andrew can confirm, but I think you will need to use the TLV431 instead of the TL431 for voltages down to 1.25v and below 2.5v.

http://uk.farnell.com/on-semiconductor/ ... ?st=tlv431

Using that, I would suggest Ry=1k2 and Rx=5k7 If I have my x and y correct.
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#10 Re: DHT Heater Group Buy notes.

Post by RhythMick »

Thanks Nick. I'm wondering if there are upstream changes required, to change the Voltage into that component?
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#11 Re: DHT Heater Group Buy notes.

Post by Andrew »

Well found, Nick.

The TLV431 should just drop in, the voltage pre-regulator, the LD1085, is referenced from the TL431, so you should be good to go. Just set Rx and Ry according for the TLV431 rather than the TL431.

cheers,

Andrew
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#12 Re: DHT Heater Group Buy notes.

Post by RhythMick »

Much appreciated Nick and Andrew. Order placed
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#13 Re: DHT Heater Group Buy notes.

Post by RhythMick »

Nick wrote: Sat May 19, 2018 6:13 pm Andrew can confirm, but I think you will need to use the TLV431 instead of the TL431 for voltages down to 1.25v and below 2.5v.

http://uk.farnell.com/on-semiconductor/ ... ?st=tlv431

Using that, I would suggest Ry=1k2 and Rx=5k7 If I have my x and y correct.
The dataset for the TLV431, page 10 figure 20, shows the formula as...

Vout = Vref * (1 + Ry / Rx)

That's the same formula as for the TL431. I'm confused, though I know it doesn't take much. If I drop in the TLV431 for the TL431, and make no upstream changes to reduce Vref, how does that formula produce a Vout < Vref ??
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#14 Re: DHT Heater Group Buy notes.

Post by Nick »

The two devices have different Vref. The TL431 Vref is 2.5v, the TLV431 is 1.24v
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#15 Re: DHT Heater Group Buy notes.

Post by RhythMick »

Thanks Nick. I guess what I'm asking is how is Vref set upstream of the TLV431?
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