charles_tan

Excuse me if this is not the right place to post this.

Found this 1.5v-3.9v dc-dc ckt in a Dorcy penlight that uses only 1 AAA cell.

Want to use this to generate 200mA 5v for my wireless video cam that needs
35mA for cmos cam and 120mA for the wireless Tx.

Don't know much about these dc-dc ckts. This ckt draws about 300mA just to generate a 8.4v 35mA output. Is this normal? Any idea? Any suggestions? Any pointers? Thanks.

redfox435cat

sounds about right.
you simply can't create power. we've been trying this for centurys. If it were posable our energy cost wouldn't be what they are. To the point when you step up voltage you use alot of energy to create it. You also loss alot of energy as heat.
8.4 v @ 25ma is .21 watts
1.5 v@ 300ma is about .45 watts

In normal cercomstances you generally loss 50% of your power in heat and of course power to operate the devises. So you power consumption sounds like its spot on

To me it would be more practical to find a power sourse above your needs and step down your voltage. Much more effeciant. I've been using 3 cell lipos @ about 12v and 2 voltage regulators that step down. One @ 5v for my radio and the other @ 9v for the camera.

charles_tan

Redfox,

Can you help me on this DC-DC circuit? If I want 5v instead of the 8.4v, what component do I tweak?

charles_tan

redfox435cat,

Let me revise the number more accurately.

"8.4 v @ 25ma is .21 watts
1.5 v@ 300ma is about .45 watts "
is really this.

"8.4 v @ 35ma is .29 watts
3.5 v@ 300ma is about 1.05 watts "

So I guess it is still 2:1 loss.
I don't really need 8.4v but that is what the ckt generate as is.
I only need 5v so I had to waste more energy thru a 5v regulator.
If only I can make this dc-dc ckt output 5v simply by change a resistor
or rewire the inductor, I will try. Please suggest.

charles_tan

redfox435cat,

Here is another penlight that uses 2x1.5v AAA cells.
How or what do I do to make it generate above 3.9v?
I need 5v. Fixed at 5v if possible so I don't need to use
a 5v regulator. Any suggestions?

redfox435cat

The only thing I could see put a 25 turn pot @ SK14 and under a comparqable dummy load ajust the pot to whatever output you want. But with a SMT board that might prove difficult.

I make my own voltage regulators
[link=http://www.southwest.com.au/~jfuller/electronics/regulators.htm]voltage regulators[/link]

I say get a 9v batterys and get a 5v regulator[8D]

charles_tan

Redfox,

I am aware of the step-down solutions.
I am trying to loose as much weight as possible.
1 Lipo is my current priority.

Back to your circuit suggestions.
On the first circuit, if I understand you correctly you think puting a 25 turn res pot across the SK14
and adjuct the output voltage while the circuit is under a comparable loading will drop the output voltage
down close to 5v. I will try that.

What is Sk14? Can you guess?

Any idea on the second circuit? It only boost from 3.0v to 3.9v after all that circuitries. I am guessing the
U1 is an osc ckt and the pin that drives the FET is fixed duty cycle for this circuit to generate 3.9v.
The FB feedback node voltage at the res R is always 3mV. I am also guessing the unused pin on U1 is a serial programming pin to program the U1's output duty cycle. I may be wrong.

charles_tan

How embarassing. I left out the inductor L from the schematic.


Sorry.

The LTC3200-5 DC-DC without inductor is great.
I might need quite a few to reduce the output ripples.

I will look at the MAX's chip like MAX682 MAX756. Most of them are 8 pins.

charles_tan

I figured the SK14 must be a Schottky diode. May be an IN4818 or equivalent.
Looking at other dc-dc ckts such as LT1111CN8-5, L may be around 47uH. C may be around 47uF.
AM50 must be a fixed duty cycle oscillator with an output stage that has a power FET to ground.
Then, the R is only a fuse? Any suggestions how to tone down the step-up from 2.3 to 1.4.
The 8.4v output is generating too much heat and waste. I need only 5v.

Is this tek-gear's 5v dc-dc part ($14) a good light weight part for wireless cam use?
Has anyone used it yet?

redfox435cat

ok this makes more sense. So putting a varistat on SK14 won't work.
The last DC-dc converto looks like it will meet your needs. I'd test in extensivley on the ground first before it goes in the air. If your worrid about ripple it would be more practical to use a 400 uF - 1000uf cap across the output to reduce ripple instead of an inductor in serise with the input. Also a .1 uf across the output for high frequency noise. But in all-in all I still don't thing this will be very reliable setup since even this small of a wireless Tx is going to drain the battery rather quickly. These Tx are unstable to begin with and a bad power sourse will only add to the problem

what kinda weight saving are you looking for?

my 3 cell lipo weights about 3 oz. another oz for a switch and 1 regulator and cables. 1.5 oz for the black widow Tx and camera.
I run it in a micro jet and I have no weight problems whatso ever

Of couse this kinda weight is nothing in my gas ships

My thermal glider I use 2 regulators. one @ 5.5v I think I said 5 before. whoops and the other @ 9 again with 1 oz 3 cell lipo. These are for the electric Heli's. very small and light. Even on the thermal glider were weight savings is a must I don't notice anything with or without the system in place

What I'm trying to get at is get a slighly bigger battery for your radio and power the camera with it as well. ANd if your running an electric model definetely use that to power your camera.

these 3 cell micro heli batterys weight about as much as a standard 4 cell radio battery pack
these battery are expensive. about 44 bucks. I did see them on ebay for 15 once. anyway have a good one

charles_tan

My 68gm multi rotor Electric only Heli Bladerunner can lift a 2.4Ghz wireless cam
and Tx for a few minutes. Heli and flight batt 53gm + 5v cam with Lens 6gm +
Tx 2gm + 140mAH Lipo 4gm + dc-dc ckt 3gm = 68gm.
The payload wireless cam/Tx at 15gm with cam battery is about 1/4 a quarter
of the total Heli flight wt. So every gm I can shave releive the tention off the motor.
Hence increase flight time and reduce wear on the motor brushes. (I have not found
a brushless solutions for these tiny Heli yet.) The lack of knowledge to design a sound
dc-dc ckt is the weakest link on this project. Hope I can get a really small, light and
powerful dc-dc ckt to power most of what I need on board.

Here is my stripped Heli with the on-board wireless cam Tx. The video that follows
is one of my many video taking from the " Bladerunner point of view".




Here is video with sound sent back from my heavily shaved Bladerunner.

?[link]http://www.smallrc.com/forum/vb_jukebox.php?[/link]

and select the " Bladerunner point of view"

Enjoy.

redfox435cat

oh a heli. ? My honey bee 2 I got that I got with a mini camera worked great on a 3 cell 1100 mAh lipo. I could only get off the ground in 3d mode. but I got about 7-9 min out of it. sold it though. It had it's own converter for 3-12 v

charles_tan

Redfox,

Thanks for all the help. Nice Chat.

Jas