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

What people are working on at the moment
Andrew
Eternally single
Posts: 4206
Joined: Thu May 24, 2007 2:18 pm

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

Post by Andrew »

Thanks, chaps, looks like everyone who contacted me has confirmed.

Please consider the group buy closed.

Andrew
Analogue, the lost world that lies between 0 and 1.
Andrew
Eternally single
Posts: 4206
Joined: Thu May 24, 2007 2:18 pm

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

Post by Andrew »

Quick progress update. I am waiting for delivery of an order from the US for parts not available in sufficient quantity in the UK.

best regards,

Andrew
Analogue, the lost world that lies between 0 and 1.
RhythMick
Old Hand
Posts: 747
Joined: Fri Jun 23, 2017 12:24 pm

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

Post by RhythMick »

Thanks Andrew, appreciate the update
RhythMick
Old Hand
Posts: 747
Joined: Fri Jun 23, 2017 12:24 pm

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

Post by RhythMick »

RhythMick wrote: Wed Mar 28, 2018 7:48 am
Nick wrote: Sat Mar 24, 2018 4:34 pm
a couple of 10R or 22R would be my choice in most cases.
Sorry I'm having a bit of a mental block thinking about the virtual cathode thing. I might have untangled it in the process of typing the question and editing it as I've worked through it - grateful if someone could check I haven't gone off the rails. It's been known !

...

Or I'm missing something, not for the first time? Do I need to stand in the corner with the pointy hat again sir?
Having worked my way through my previous mental block, can I just get a check on the voltage drop across the virtual cathode resistors please ? My mental blocks still seem to mostly revolve around where multiple currents flow through the same component. Let me give a worked example.

Take the 71A valve, which will have Ia = 16mA at the chosen operating point. Vfil = 5v, Ifil = 0.25A. Let's say I pick 22R resistors for the Virtual Cathode (0.75W Amtrans Carbon Film I'm looking at).

Assuming the current flows evenly through each resistor (ignoring any claims about greater current flow from one end of the cathode, for now), then each will see 8mA through it.

The 2 x 22R resistors will also draw current from the heater boards though : 5.0v / 44R = 114mA.

So the total current through each 22R will be 8 + 114 = 122mA. Since V = IR, the voltage drop across each resistor will be 2.7V. Is this correct ?

Actually I wouldn't choose the 22R for this valve, because the dissipation will be 44% of it's rating (I try not to exceed 25%), but also because the heater current is excessive and would increase the heat dissipation requirement on the board. I'd more likely go for 100R, which will draw 25mA heater current for a total of 33mA, the dissipation will be 15% of rating but the voltage drop would be higher at 3.3V.

The voltage drop isn't a problem, I'm just trying to make sure I haven't got the wires wrong in my head (which happens).

Appreciate a quick sanity check pls.
User avatar
Nick
Site Admin
Posts: 15708
Joined: Sun May 06, 2007 10:20 am
Location: West Yorkshire

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

Post by Nick »

Yes, close enough to know what you need to know. You know that there will be 5v across the two ends of the resistors and you know that 8ma will flow out of the centre point of the two resistors.

Based on this and using 22R I calculate that one resistor will pass 109.63ma and the other 117.63ma

If I can get the scanner to work I will show my working to prove I didnt cheat and use spice :-)
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
RhythMick
Old Hand
Posts: 747
Joined: Fri Jun 23, 2017 12:24 pm

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

Post by RhythMick »

Haha. Thanks Nick and yes please upload today's lesson when ready.

Do you mean 16mA will flow from the centre point down through the LTP, 8mA will flow through each resistor to combine at the centre point?
RhythMick
Old Hand
Posts: 747
Joined: Fri Jun 23, 2017 12:24 pm

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

Post by RhythMick »

As always I revert to spreadsheets for this stuff... The accuracy isn't required but it gives a reusable model ...
Virtual Cathodes.JPG
RhythMick
Old Hand
Posts: 747
Joined: Fri Jun 23, 2017 12:24 pm

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

Post by RhythMick »

Virtual Cathodes.JPG
User avatar
Nick
Site Admin
Posts: 15708
Joined: Sun May 06, 2007 10:20 am
Location: West Yorkshire

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

Post by Nick »

My mistake, I thought Ia was 8ma
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
RhythMick
Old Hand
Posts: 747
Joined: Fri Jun 23, 2017 12:24 pm

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

Post by RhythMick »

Nick wrote: Wed May 02, 2018 8:53 am My mistake, I thought Ia was 8ma
No problem I probably didn't make it clear. Thanks for checking me.

I would be very interested in understanding why the different currents in the two resistors and how you calculated it.
User avatar
Nick
Site Admin
Posts: 15708
Joined: Sun May 06, 2007 10:20 am
Location: West Yorkshire

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

Post by Nick »

This is my working, it was late.

http://www.lurcher.org/nick/cathode_current.pdf
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
RhythMick
Old Hand
Posts: 747
Joined: Fri Jun 23, 2017 12:24 pm

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

Post by RhythMick »

Nick wrote: Wed May 02, 2018 10:18 am This is my working, it was late.

http://www.lurcher.org/nick/cathode_current.pdf
Thanks.

Fallen at the 1st hurdle. "I1 - I2 = 0.008" ?

Let's not worry about the fact that it's 16mA, not. Let's go with 8mA and follow your working. I follow the maths and get the same result, I just don't understand that first equation.

So ignoring heater current flow, a total of 8mA is flowing up the LTP, split flow through both VC resistors, up into the electron cloud space and off to the anode. The only question is how that 8mA is split between the two resistors. Right ?

Can you explain that first equation for me ? I would have started with the assumption that I1 = I2 = Ia /2, but if I accept that the split may not be equal I would at least have said I1 + I2 = Ia ?
RhythMick
Old Hand
Posts: 747
Joined: Fri Jun 23, 2017 12:24 pm

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

Post by RhythMick »

RhythMick wrote: Wed May 02, 2018 10:40 am
Nick wrote: Wed May 02, 2018 10:18 am This is my working, it was late.

http://www.lurcher.org/nick/cathode_current.pdf
Thanks.

Fallen at the 1st hurdle. "I1 - I2 = 0.008" ?

Let's not worry about the fact that it's 16mA, not. Let's go with 8mA and follow your working. I follow the maths and get the same result, I just don't understand that first equation.

So ignoring heater current flow, a total of 8mA is flowing up the LTP, split flow through both VC resistors, up into the electron cloud space and off to the anode. The only question is how that 8mA is split between the two resistors. Right ?

Can you explain that first equation for me ? I would have started with the assumption that I1 = I2 = Ia /2, but if I accept that the split may not be equal I would at least have said I1 + I2 = Ia ?
Lets start with definitions ...

I1 = ? Total current flowing through R1 ?
I2 = ? Total current flowing through R2 ?

Why would the DIFFERENCE between those two be equal to Ia ?

I feel like I'm on the verge of the penny dropping, providing a piece of jigsaw I've been missing. To mix metaphors.
User avatar
Nick
Site Admin
Posts: 15708
Joined: Sun May 06, 2007 10:20 am
Location: West Yorkshire

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

Post by Nick »

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
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
RhythMick
Old Hand
Posts: 747
Joined: Fri Jun 23, 2017 12:24 pm

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

Post by RhythMick »

WOW - I think I just proved it from 1st principles. Going to type it here for checking.

Ia = total anode current
Let I1 = total current through R1, ditto for I2/R2
Let I1v = proportion of Ia flowing through R1, ditto for I2v/R2
Let I1f = filament current through R1, ditto for I2f/R2
Let If = total filament current through R1 in series with R2

I1 = I1v + I1f, therefore I1f = I1 - I1v
I2 = I2v + I2f, therefore I2f = I2 - I2v

Since R1 = R2, I1f = I2f, therefore...

I1 - I1v = I2 - I2v

Rearranging, and substituting Ia = I1v + I2v...

I1 - Ia = I2

QED - blimey
Post Reply