BLR121 battery for AVO

[boater sam]

Sounds interesting Alan, but what is the doofer?

[AlanBeckett]

Herewith the Doofa.

         

Made by/for RS must be 30 years ago now.

Alan

[boater sam]

Fascinating, never knew they existed, wonder what is in it?  I assume it converts 1.5v to 15v but how?  Need to know!
Sam.

[Terry]

It will be an SMPS inside - some kind of switching oscillator, a step up choke or transformer and rectifier/smoothing for the output.

Here's an example (although whether the LT1615 was around 30 years ago, I don't know!)


It will work quite happily with a 1.5V input and the value of R2 sets the output voltage - 111kΩ for 15V, using the formula shown and 1.5V input.

Reducing the input to the normal voltage of a dry cell - 1.4V - will produce an output of 15.4V with R2 = 100kΩ.

[Mark Hennessy]

Reducing the input to the normal voltage of a dry cell - 1.4V - will produce an output of 15.4V with R2 = 100kΩ.

No need to change the resistor. We have a thing called negative feedback!

The fact that Vin is not part of the equation is also a strong clue.

This is a pretty standard boost converter. Usually the switching frequency is fixed and the duty cycle varied accordingly to regulate the output voltage. But this one uses a fixed off time, so the frequency varies according to input voltage. Note how the output current also isn't a factor in the equation, but this assumes "perfect" components. In practice, small variations in duty cycle (on on-time here) with load current are seen because of losses in the components.

[Mark Hennessy]

It will work quite happily with a 1.5V input and the value of R2 sets the output voltage - 111kΩ for 15V, using the formula shown and 1.5V input.

Also, this is incorrect.

1.23V is the reference voltage. That's fixed inside the chip. To arrive at 15V, you've replaced 1.23V with 1.5V in that equation.

With R1 set to 1M, the correct resistor for R2 would be 89.3k. The nearest E96 value is 88.7k, but 91k gives 14.75V, which might well be close enough. Of course, R2 doesn't have to be 1M. The bias current going into the FB pin should be 30nA, so errors caused by that should be nice and low.

[Terry]

Also, this is incorrect.

1.23V is the reference voltage. That's fixed inside the chip. To arrive at 15V, you've replaced 1.23V with 1.5V in that equation.

Thanks for pointing out my error, Mark - I don't know what I was thinking of - although I was supposed to be doing something else at the time!

[The Philpott]

Aforementioned Doofa (probably a VC1 or equivalent) has been opened up by Richard Allan (but i am not sure if a circuit diagram has been extrapolated from it.) It looks to contain, among other things, an inductor and resistors. On rare occasions they turn up they are now fetching in excess of £70.

The Avo VC1 converter block takes a trickle current when not in use, and when in use has a stable supply capacity of 80 or 90uA.

The copper track flexi tape is pretty unsatisfactory and can be replaced if required with wires and solder tags once the relevant screws have been undone.

Personally, i am not comfortable with the VC1 drawing current full time (because of wear & tear) so would probably fit the D cell with a discrete switch to effectively isolate it.

VC1 fit AVO 8 Mk4 or 5 (pictured previously) but if end screws are removed it can be shoehorned into a Mk3. But not a Mk2.... so i am told.

[AlanBeckett]

Odd. That one's been in there for Decades and I can't remember when I last changed the Duracell.

Alan

[The Philpott]

That's a (relatively) recent D you have in there at least...the previous version had shiny copper tops if i remember right, but we are going back a bit..

It's certainly a tiny current drain when not in use, a good D should be happy with it for 4 to 8 years depending on all the usual variables,
i would think. (i never risk alkalines below 1.3v, around about the pd at which they no longer operate one of those little quartz clock modules!)