Electronics > Electronics
Why did my capacitor in my transmitter blow
Soeren:
Hi,
Personally, I'd up the value of the base-emitter resistor (actually, I am, as I'm just making a small simple lab-supply - to be posted later, if anyone would like).
You don't wanna get the 7812 working very hard (and needing a lot of heat sinking), so make it 10..33 Ohm. You just need the regulation from the 7812 and enough current to properly control the pass transistor, not the grunt work, which should be handled by the power PNP.
And do not, I repeat do not, use a Darlington for this, it has too high a voltage drop!
The 7812 needs an input voltage of at least 14.5V to 15V (or even higher, depending on who made the 7812) and to that you need to add the base-emitter drop of the pass transistor, which will be ~0.7+ for a non-Darlington and up to 2.5V for a Darlington.
Question you need to ask yoursefl... What voltage does your two batteries span over their discharge?
Mastermime:
Ok thanks for the help.
--- Quote ---10..33 Ohm
--- End quote ---
I'm confused on what this means? Switch to 33 ohm resistor?
--- Quote ---And do not, I repeat do not, use a Darlington for this, it has too high a voltage drop!
--- End quote ---
Ok thanks for the heads up. Luckily I'm using a tip2955. This has a drop of 0.6v
--- Quote ---Question you need to ask yoursefl... What voltage does your two batteries span over their discharge?
--- End quote ---
You know that is a good question. I bought the batteries off Ebay from a Chinese dealer http://www.ebay.com/itm/1-pcs-7-2V-5300mAH-Ni-MH-Rechargeable-Battery-Pack-RC-/270614422848
They haven't arrived yet.
Soeren:
Hi,
--- Quote from: Mastermime on November 12, 2012, 07:25:04 PM ---
--- Quote ---10..33 Ohm
--- End quote ---
I'm confused on what this means? Switch to 33 ohm resistor?
--- End quote ---
It means: Change the 1 Ohm resistor to something higher (and the 10 Ohm basis resistor you can leave out).
(10..33 Ohm is "from 10 Ohm to 33 Ohm", but that was before I realized that you're gonna use a TIP2955).
--- Quote from: Mastermime on November 12, 2012, 07:25:04 PM ---Luckily I'm using a tip2955. This has a drop of 0.6v
--- End quote ---
The drop depends on the current through it - how much current are you hoping for?
(This will determine the value of the resistor discussed above).
--- Quote from: Mastermime on November 12, 2012, 07:25:04 PM ---You know that is a good question. I bought the batteries off Ebay from a Chinese dealer [...]
--- End quote ---
OK, they're 6 cells each, 12 cells total of the NiMH variety.
NiMH cells are 1.40 to 1.45 fresh out of the charger and, with a large drain, around 0.9V when flat
So, freshly charged they're around 17V down to approx. 11V when flat.
Since you need minimum 12V+2.5+0.7V=15.2V (and more likely around 1.0..1.5V more) at the input of the regulator and the fact is, that this isn't going to work for more than a tiny fraction of the available discharge, I'd recommend you to look into switching regulators, either a SEPIC or a Buck/Boost, to get a steady 12V over the full discharge.
Dimension Engineering has got a couple that may work, depending on your current demand.
Mastermime:
--- Quote ---OK, they're 6 cells each, 12 cells total of the NiMH variety.
NiMH cells are 1.40 to 1.45 fresh out of the charger and, with a large drain, around 0.9V when flat
So, freshly charged they're around 17V down to approx. 11V when flat.
Since you need minimum 12V+2.5+0.7V=15.2V (and more likely around 1.0..1.5V more) at the input of the regulator and the fact is, that this isn't going to work for more than a tiny fraction of the available discharge, I'd recommend you to look into switching regulators, either a SEPIC or a Buck/Boost, to get a steady 12V over the full discharge.
Dimension Engineering has got a couple that may work, depending on your current demand.
--- End quote ---
Ahh shoot. The current demand is approximately 2.5 amps. If I buy this http://www.dimensionengineering.com/products/anyvolt3
I can just ditch the circuitryfor the regulator booster I have, right?
Soeren:
Yes!
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