DHT Heater using VCCS - PCB or Kit (Possible Group buy)

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RhythMick
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#976 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by RhythMick »

Nick wrote: Wed May 02, 2018 11:00 am Yep, the little drawing shows the currents flowing. I1 is current through R1, I2 is current through R2. We know that there is 8ma flowing out of the node where the two resistors join, so we apply Kirchhoff's current law:
The algebraic sum of currents in a network of conductors meeting at a point is zero.
So I1 is the current into that node, the current out is I2 + 0.008, so for the total to be 0

I1 = I2 + 0.008 or I1 - I2 = 0.008.

Note the direction of the current flow in R2
OK your way is simpler.

Thanks Nick
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Nick
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#977 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by Nick »

Yep, though you have silently assumed that Kirchhoff's current law is true otherwise your therefor's have no basis. You have also assumed that currents can be added in a linear fashion which is not a given without some further assumptions (probably ohm in that case).
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RhythMick
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#978 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by RhythMick »

Nick wrote: Wed May 02, 2018 11:31 am Yep, though you have silently assumed that Kirchhoff's current law is true otherwise your therefor's have no basis. You have also assumed that currents can be added in a linear fashion which is not a given without some further assumptions (probably ohm in that case).
Thanks Nick. That was well worth doing for me, glad my rusty old brain hasn't completely lost it.
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#979 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by RhythMick »

One thing confuses me Nick. Couldnt we just as easily have started with

i2 - Ia = I1

as

I1 - Ia = I2

??

Both I1 and I2 flow into the junction, while Ia flows out. I guess the difference is the direction of the heater current at the junction.
Last edited by RhythMick on Wed May 02, 2018 1:12 pm, edited 1 time in total.
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Nick
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#980 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by Nick »

Yep but then the direction of current would have been reversed for I2 and I1
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RhythMick
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#981 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by RhythMick »

I drew a little diagram showing the directions of the currents. I get it now. So obvious - I knew there was something missing that I needed to grok.

Thanks again Nick.

This shows the minimum resistors needed for the virtual cathodes in order to stay under 25% of rating...
Virtual Cathodes - Minimum Resistors.JPG
And this shows the use of 200R ...
Virtual Cathodes - 200R 0.75W.JPG
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Nick
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#982 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by Nick »

Yep, I should have put the little drawing further up the page.

One thing worth considering is that the maths show that if a pair of equal resistors are used, it will guarantee that equal cathode current will pass out of both ends of the filament. That in itself to me is a good reason for using the pair of resistors.
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#983 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by Andrew »

The parts from the US have arrived, UK based parts to arrive this week.

Hopefully, I should be able to start sending packs out towards the end of the week.

best regards,

Andrew
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RhythMick
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#984 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by RhythMick »

Brilliant thanks Andrew
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#985 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by Andrew »

Just to be clear, I don't intend to ship the kits with the two current set resistors, please see here http://www.audio-talk.co.uk/phpBB3/view ... =19&t=5500 as I don't know your intended voltage, although I assume your not using an input transformer larger than 9 volts AC; two off 250mWatt generic metal films are required.

For similar reasons, mounting hardware is not included, e.g. screws, terminals, posts, and isolating tabs....

Andrew
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#986 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by Andrew »

Folks in the group buy,

All the parts are here but despite my best efforts to avoid partial deliveries I have ended up with a back order for some of the critical resisters, these are in the area that Mark and myself spent all the time perfecting. When I changed the recipe last time, Mark tore strips off me and I never even told him there was a different part in there. I don't see the point in going through the work, nor the listening tests, to reprove this part of the design for the sake of another week or so. The resistors are on a short lead time and the suppliers are often conservative in their estimates; I don't anticipate a long delay.

I'll let everyone know when the resistors turn up.

Andrew
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#987 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by IslandPink »

Good to see you standing up for my principles there Andrew ! :D
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RhythMick
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#988 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by RhythMick »

Absolutely agree Andrew. This is a "do it once, so it right" thing.
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#989 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by Andrew »

IslandPink wrote: Sat May 12, 2018 11:48 am Good to see you standing up for my principles there Andrew ! :D
My pleasure, Mark ;)
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RhythMick
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#990 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by RhythMick »

I realise I've put my updates in the Group Buy Notes thread, sorry about that. Update here in case it's useful for anyone else.

I've got (most of) my boards and components, thanks Andrew. I've no experience building PCBs so needed to clarify a couple of things. First board was very slow while I picked my way through checking orientations. 2nd and 3rd board took 45 minutes each. I'm mounting the LD1085 and the D44H11 remotely on a shared heatsink, running wires to 3-pin terminal blocks, which connect to the vias on the PCB.

71A board tested, working perfectly, rock-steady voltage and virtually no heat increase, but it only draws 0.25A.

I realised a little late the 26 requires 1.5v which is below what the board as originally designed can provide. Nick and Andrew helped with a component subsitution (swap the TL431 for the TLV431, which goes down to 1.24v). Working perfectly. The 26 runs 1.05A and over about 5 minutes after switch on I see a 34 deg C temperature increase measured 1cm above the shared heatsink, but it stabilises there. Voltage and temperature steady. The final build will mount those heatsinks on copper sheet which will aid dissipation.

Next step is to get 4 x 26 boards and 4 x 71A boards done and mount them on the prototype amps, see if that helps with the hum.
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