[panman]

Hi,

I'm currently in the process of building a portable META42 using 3 lithium ion cells for power (this should give me nominally 3.6volts x 3 = 10.8volts power at 1200 mAH and still fit into a serpac H-65 case!) If all of this works as planned, this power solution (batteries plus charger) should give the best sound and battery life for portable META42's in a serpac case for less than the cost of two Plainview or Tysonic 9volts and a charger. I'll let you know if all of this works out!

The problem I'm having is setting up the LED circuit. Li-ion cells are very intolerant of deep discharging and shouldn't be discharged past 2.8volts per cell (8.4volts for all three cells). That's why I need to set up this circuit. However, I'm not sure if I really understand it.

If I use a blue LED with a 3.5 volt drop and I want the cutoff to be 8.4 volts, should the zener be 5.1 volts (the closest I could find to 4.9 volts)?

But I guess my biggest question is about CRD. The part number tangent lists on his site is not found at Newark, and the only CRD's I could find were surface mount. Would this circuit work with just ZNR and RLED? Or does anyone know of a source for CRD's that would fit this application?

And just to make sure I've got this all straight, if I do find a suitable CRD and ZNR and populate those positions on the board, would I leave RLED empty?

Thanks!

 

[morsel]

The CRD determines the current through the LED. If you are trying to save power and keep the LED brightness reasonable, 1-2mA is a good choice. If you can't find a barrel type CRD you can use a 2N5484 FET with source and gate tied together instead, however Mouser sells the CRDs you want.

The zener determines the cutoff voltage. If you don't want your LED to go out when the batteries drop below a certain level you could replace it with a jumper wire. Choosing the zener voltage is partly trial and error because there is a ~1.5V "slop zone" during which the LED fades out. Vled + Vzener is approximately the voltage at which the LED fades.

You don't use RLED if you are using a CRD and zener.

I believe Tangent discusses this on his website. Here is the parts advice page:
http://tangentsoft.net/audio/meta42/advice.html

 

[panman]

Thanks Morsel, you answered every question I had. And thanks for the tip about Mouser. I found the CRD there. I have read all of tangent's site (in fact I have all of it printed out for reference), but I guess I'm just a little boneheaded at times

 

[tangent]

I said it on the META42 pages, but I want to stress that the LED voltage drop plus the zener reverse voltage drop is the point where the LED is totally dark. You might look at the voltage curves for the batteries you're using -- you might find that there's a better place to make it start dimming than at 8.6V.

Also, to hit a precise value you might combine a reverse-biased zener with a forward-biased regular diode. For example, a 4.3V zener and an 0.6V generic diode like a 1N5817.

 

[charlesb]

Quote:

Originally posted by panman Hi, I'm currently in the process of building a portable META42 using 3 lithium ion cells for power (this should give me nominally 3.6volts x 3 = 10.8volts power at 1200 mAH and still fit into a serpac H-65 case!) If all of this works as planned, this power solution (batteries plus charger) should give the best sound and battery life for portable META42's in a serpac case for less than the cost of two Plainview or Tysonic 9volts and a charger. I'll let you know if all of this works out!

Neat. In a recent discussion on 9 V lithium batteries, I turned up some links to the rechargeable 3.7 V li-ion batteries and realized that a stack of 3 could fit in the space of a 9V battery. The links to the sources were
http://www.ulbi.com/product-display.asp?ID=35
and
http://www.ulbi.com/product-display.asp?ID=47

Is this what you're using? What charger? How much did it all cost? I'd love to know how it works out, because this would be a reasonable retrofit to existing portable META amps for higher voltage and longer battery life.

 

[panman]

Thanks for the advice Tangent!

And Charlesb, at first I was thinking about using li-poly batteries, but then I realized that very few people would have a proper charger for them (or could get their hands on one for that matter!) So then I looked at my cell phone battery. It's a Nokia 8260. the batteries are 3.6v li-ion @ 1200 mAh. The battery dimentions are 53 x 33 x 8mm. The best part is that a new aftermarket battery and desk charger goes for around $8 on ebay or a bit more from discount cell phone accessory stores. I have one of these cheapie aftermarket batteries and chargers--they work fine, but I still have to check the capacity of the batteries. I doubt that they are really 1200 mAh, but I'm very sure that they are well above the 200 mAh that the plainviews and tysonics are. My big question is about them fitting into the serpac H-65-AC case. This case is like the 9v case, but without the moulded part for the 9v battery. It only has the door. Unfortunately, the blueprints/datasheet from serpac never lists the size of the door, so I don't know if the batteries will even fit. I'm ordering the case along with the last few remaining parts for my META42 tomorrow, so hopefully I'll have an answer by next week.

 

[charlesb]

Quote:

Originally posted by panman Thanks for the advice Tangent! And Charlesb, at first I was thinking about using li-poly batteries, but then I realized that very few people would have a proper charger for them (or could get their hands on one for that matter!) So then I looked at my cell phone battery. It's a Nokia 8260. the batteries are 3.6v li-ion @ 1200 mAh. The battery dimentions are 53 x 33 x 8mm. The best part is that a new aftermarket battery and desk charger goes for around $8 on ebay or a bit more from discount cell phone accessory stores. I have one of these cheapie aftermarket batteries and chargers--they work fine, but I still have to check the capacity of the batteries. I doubt that they are really 1200 mAh, but I'm very sure that they are well above the 200 mAh that the plainviews and tysonics are.

Very clever, and inexpensive too! I will have to look into this. The higher voltage and the higher mAh is very interesting. Having (theoretically) 6-7 times more mAh could give you more freedom in design, especially in buffers. I'd like to know how this works out for you. In the meantime, I'm off to look at ebay...