Tube Question #4: Voltage Divider for running 6 volt heater from 12 volt supply

[frequencycentral]

If you're reading this you'll already know that I'm obsessed with 6111 submini dual triodes!

I'm looking at ways of powering a single 6111 heater from a 12 volt supply. The 6111 requires 6.3 volts (300ma) ideally, but 6 volts should be fine.

The only example I have to go on is Dano12's way of powering the Subcaster, which uses an LM317 and associated parts.

I'm thinking a simple resistor voltage divider would do the trick a little more elegantly.

Just looking at the math:

6 volts divided by 300mA = 20 Ohm

6 volts multiplied by 300mA = 1.8 Watts.

So - I'm looking at a voltage divider using two 20 ohm/1.8 watt resistors. Is this correct?

Next: I only have 10 ohm/3 watt in my parts bin. Am I right in assuming two of these are more than enough?

Finally: Most voltage dividers I've seen for 'vref' purposes include an electro cap (say 10uf) between the vref and earth, should I include one in my voltage divider?

Thanks!

[R.G.]

The tubes' heaters allow 300ma to pass when there is 6.3V across them, and they are resistive. That means, as you properly compute, that the resistance of a heater is 6.3V/0.3A = 21 ohms. If you have 12Vdc and want to drive 300ma into you heaters, you can do that easily enough without using a voltage divider by putting a resistor in series with the heater. The resistor is R = V/I = (12-6.3)/0.3 = 5.7/0.3 = 19 ohms. It would dissipate 5.7*0.3 = 1.7W and you'd need to use a 3W resistor to keep it from runnig at about 200C surface temp.

A 20/20 divider will not give you what you think. A divider composed of 20 ohms / 20 Ohms from 12V will gie you 6V only when there is no load on the 'Y"s. This produces 6C only when it's not loaded.

[frequencycentral]

Thank you again R.G., that's just great! Amazingly simple solution. 

EDIT: I tried it - it works - obviously. I had two 10 ohm 3 watt resistors in my parts bin which I scavenged from an old T.V. set. With 5% tolerance, they add up to 19 ohm in series. They get HOT! But then so does the tube, or an LM317 for that matter! They actually get too hot to touch for more than a few seconds - much hotter than the tube.

[frequencycentral]

R.G. - is there a downside - apart from the heat?

Could you give me an idea of the difference in milliamps consumption between the LM317 method and the 19 ohm resistor method please?

What I mean is - those little guys are really hot - are they turning additional milliamps into heat, am I now consuming way more that 300ma?

Upside - the British summer is non-existant this year - it's nice to get some heat!

[darron]

i'd rather a 6V regulator as they probably have about 80% efficiency, but i've never used tubes so i'm no authority.

why not make a design that uses two of those tubes, then run the heaters in series off the 12 volts... no need for extra components? no wasted heat/juice.

[frequencycentral]

i'd rather a 6V regulator as they probably have about 80% efficiency, but i've never used tubes so i'm no authority.

Is there such a thing? I've come across 5 volt (7805), but am not aware of a 6 volt.

why not make a design that uses two of those tubes, then run the heaters in series off the 12 volts... no need for extra components? no wasted heat/juice.

Been there, done that: 

http://www.diystompboxes.com/smfforum/index.php?topic=69514.0

http://www.diystompboxes.com/smfforum/index.php?topic=70132.0

......................but I still have some ideas for using just a single tube in pedals too.

[darron]

yeah? the 7806

like this: http://www.jaycar.com.au/productView.asp?ID=ZV1506&CATID=&keywords=regulator&SPECIAL=&form=KEYWORD&ProdCodeOnly=&Keyword1=&Keyword2=&pageNumber=&priceMin=&priceMax=&SUBCATID=


i suppose rs in the uk would have a billion of them

[frequencycentral]

yeah? the 7806

like this: http://www.jaycar.com.au/productView.asp?ID=ZV1506&CATID=&keywords=regulator&SPECIAL=&form=KEYWORD&ProdCodeOnly=&Keyword1=&Keyword2=&pageNumber=&priceMin=&priceMax=&SUBCATID=


i suppose rs in the uk would have a billion of them

Doh! I'll have to try them! thanks Darron!

[R.G.]

R.G. - is there a downside - apart from the heat?
Could you give me an idea of the difference in milliamps consumption between the LM317 method and the 19 ohm resistor method please?
What I mean is - those little guys are really hot - are they turning additional milliamps into heat, am I now consuming way more that 300ma?

Actually, this is the best you can do without going into a step-down switching regulator.

Series resistors are conducting exactly the same current as the heaters - they have to, there is no other place for it to go except down the single path. A separate resistor divider that is not series would eat much more current.

As for a 317; the 317 has been designed to make the additional current it eats be very, very small, down in the microamps. The resistors that set the voltage are the only real additional current, and that is a few milliamperes. So in total, a 317 regulator would use only microscopically more than the series resistors. A 7806 eats about 1ma of excess current to do its job, so that's OK too.

Here's the right way to look at this. You have a 12Vdc supply. If you pull 300ma from it, that 300ma will show up SOMEWHERE as heat. You can choose to put it all into one very hot light bulb filament (or tube heater element) or to spread it out in 2000 43-ohm resistors hooked up in series-parallel to give an equivalent to a 43 ohm resistor. The resistor array will dissipate the same amount of heat, but each resistor will only dissipate 1.8mW, so it will not warm up to any noticeable degree at all. Heat is where energy goes. Temperature is what you get from concentrating heat.