Diode to drop voltage on a AXE021 proto board

jensmith25

Senior Member
Hi all,

I have a new project for a model plane to create some flashing wing lights. There is an existing 6v battery supply that my customer wishes to use onboard the plane which is already controlling various nav lights rather than adding extra weight with additional batteries.

To drop the voltage to 5v for the 08M Picaxe chip I was thinking of using the 1n4001 diode used on a servo boards (AXE024) (or an LED to drop the voltage) but using it with the AXE021 proto board. Looking at the schematic for the AXE021 I am assuming that the diode would go where the additional capacitor can go, directly before the power supply between + and GND, but wanted to check this was correct before soldering.

Thanks very much,

Jennifer,
 

eggdweather

Senior Member
At the expected current levels a diode forward voltage will be 0.6 to 0.7 volts, so you should use 2 in series.

Don't use those spare holes, they are connected across the input supply and are for an input smoothing capacitor - putting a diode in there will either blow it up if placed in the forward direction, or do nothing if the converse.
 

techElder

Well-known member
jensmith, the diodes should be wired IN SERIES with the battery supply and the VOLTAGE INPUT of the board you are using.
 
D

Deleted member 67719

Guest
If you were to use a 7805 voltage regulator there is a minimum input voltage in order to get a stable 5v output. (Minimal input voltage for the 7805 is: 7.3v)
 

eggdweather

Senior Member
Even a low drop out voltage regulator would be at its limits as most require 1volt difference between imput and output. Your LED option is still viable and lowers the part count, but would need to be red or infrared for the lowest forward voltage of ~1.5volts.
 

techElder

Well-known member
Have you considered using a processor other than the 08M? From the PICAXE web site, "PICAXE Power Supply - All the latest (M2 and X2) PICAXE parts can be run at 3V, 4.5V or 5V."

Put a 3V regulator on the 6V line for the processor. Use a FET switch to make the LEDs do what you want them to do.
 

eggdweather

Senior Member
As this is an aero model application, I would be surprised if the models receiver and battery system did not include a BEC battery eliminator which always provide 4.8 or 5 volts, so there may aleady be a correct supply in the aircraft.
 

jensmith25

Senior Member
Have you considered using a processor other than the 08M? From the PICAXE web site, "PICAXE Power Supply - All the latest (M2 and X2) PICAXE parts can be run at 3V, 4.5V or 5V."

Put a 3V regulator on the 6V line for the processor. Use a FET switch to make the LEDs do what you want them to do.
It's not just a simple flashing LED. He wants the LEDs to alternate every 3 flashes so I think this requires a programme rather than an FET switch.
 

jensmith25

Senior Member
Even a low drop out voltage regulator would be at its limits as most require 1volt difference between imput and output. Your LED option is still viable and lowers the part count, but would need to be red or infrared for the lowest forward voltage of ~1.5volts.
If you use an LED to drop the voltage do you not also need a resistor to stop the LED being fried?
 

inglewoodpete

Senior Member
If you use an LED to drop the voltage do you not also need a resistor to stop the LED being fried?
Typical small LEDs have a maximum forward current of 20 or 30 mA (check the data sheets). If your PICAXE is to drive the nav lights directly, then you may have overcurrent problems in the voltage-dropping LED. Using a LED as a voltage dropper or low-voltage zener alternative has some merits but perhaps not in your case if you don't use MOSFETs.

He wants the LEDs to alternate every 3 flashes so I think this requires a programme rather than an FET switch.
I think eggdweather meant driving the FETs as switches using outputs from the PICAXE.

As eggy suggests, you could run the PICAXE at 4.2 or 3.3volts and use a surface-mount MOSFET to drive each nav light off the 6v supply. The current required for the PICAXE would be much less if using MOSFETs as switches. The 3.3v supply would come from a SMD or T092-sized regulator. Or use a red LED to get ~ 4.2v from the 6v supply.
 

jensmith25

Senior Member
Typical small LEDs have a maximum forward current of 20 or 30 mA (check the data sheets). If your PICAXE is to drive the nav lights directly, then you may have overcurrent problems in the voltage-dropping LED. Using a LED as a voltage dropper or low-voltage zener alternative has some merits but perhaps not in your case if you don't use MOSFETs.



I think eggdweather meant driving the FETs as switches using outputs from the PICAXE.

As eggy suggests, you could run the PICAXE at 4.2 or 3.3volts and use a surface-mount MOSFET to drive each nav light off the 6v supply. The current required for the PICAXE would be much less if using MOSFETs as switches. The 3.3v supply would come from a SMD or T092-sized regulator. Or use a red LED to get ~ 4.2v from the 6v supply.
Ok, thanks very much. I have used FETs to drive LEDs before so that I can do.

So I could use one of these: http://www.rapidonline.com/electronic-components/taiwan-semiconductor-ts2950ct33-a3-voltage-regulator-ldo-3-3v-150ma-to-92-51-7310 to regulate the voltage to 3.3v? Then use 2n7000 FETs to power the LEDs?

How do I connect the 3.3v voltage regulator into the AXE021? I see it has output/GND/Input pins. GND is obvious but where is it best to connect the input and which 08M2 pin does the output connect to? Is it pin 1 or 2?
 

techElder

Well-known member
Ok, thanks very much. I have used FETs to drive LEDs before so that I can do.
If you had actually READ several of the previous posts, you would have come to this solution quicker.
 

jensmith25

Senior Member
If you had actually READ several of the previous posts, you would have come to this solution quicker.
I think that's a bit harsh! Reading a post and actually understanding what is meant are two different things. If it's not clear what is being suggested I'm no wiser as to the end solution. I don't have years of electronics knowledge and I have to work hard to get my head round a lot of it. I don't have the background knowledge to fill in the gaps.

Yes, I have used a FET but to boost power to control more than 1 LED off a pin. In this case that's not necessary, it's only 1 LED per pin so I was unclear as to why they were necessary and what purpose they were serving.

You also confused me by saying a "processor other than the 08M" suggesting to me that the suggestion was to do without the Picaxe chip at all and the FET's would drive the LEDs directly off the 6v supply.
 

jensmith25

Senior Member
Reading the data sheet it says I need a 0.22uf capacitor with the 3.3v voltage regulator. This seems like it's getting a bit complicated in terms of the number of components required and how they are going to be fitted to the AXE021.

Perhaps the red LED or ideally the 1N4001 diode is the simpler method along with the FETs?
 

AllyCat

Senior Member
Hi,

I've been "watching" this thread throughout, but refrained from commenting until a better specification became apparent. But after almost 20 posts I'm little the wiser: Is this a "real" flying model, or just a scale "tabletop" model? How bright do the LEDs need to be, how many of them, do they all flash in unison or alternately? And what type of battery: Alkaline, or a (horrid) carbon "Power Pack", or rechargeable (5 x NiMH), or Lithium-based (as would be expected if weight were an issue) ? IMHO only one of those supplies might be reasonably assumed to be "6 volts" (but still rise to 6.5 volts on occasions).

The FET LED-driver with a 3.3 volt regulator might "work", but the 2N7000 is not rated to be driven from a 3.3v control voltage. So a Red LED in series with (only) the PICaxe supply rail (Vdd) might be the correct solution. But without a proper specification, I've no idea......

Cheers, Alan.
 

jensmith25

Senior Member
Hi Alan,

Sorry, after my initial post no-one asked for any further info.

To answer your questions:
- I believe it is a real flying model. The LEDs are to be seen in daylight.
- I don't know the specifications of the battery but I can ask. I was just told it was 6v and powering other LEDs for various navigation lights.
- The LEDs need to be visible in daylight but I've tested them and standard brightness is fine. They are 5mm wide angle 13000mcd white LEDs from Bright Components, 20mA
- There are 2 LEDs
- LEDs are to flash alternately, 3 flashes per second then alternate.

I hope that helps clarify the specifications.

EDIT: Just heard back about the battery pack -
The pack is made up of 5x1.2v aa re chargeable cells which are Ni MH eneloop
The pack is also powering another light system details of which I am unsure ( not sure if its relevant )
 
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erco

Senior Member
Be careful if the LEDs aren't always on. Silicon diodes only drop their voltage (~0.7V) when a few milliamps of current flow through them. The LEDs consume enough current to ensure that drop, but if they are off and the Picaxe-only current drops to 1 mA or thereabouts, you won't get much or any voltage drop, potentially exposing the poor 5.5V (max) Picaxe to 6V or more. Check the characteristic silicon diode curve below.

 

premelec

Senior Member
@jensmith - if you are using the PICAXE to actuate driver transistors and therefore only small current to the PICAXE part of the project; I'd suggest using a shunt regulator and resistor from the battery pack for the PICAXE voltage regulation - Like a TL431 set to conduct at 4 volts. However it depends on where what currents are going in actual operation - and that needs to be explicitly mapped out...
 

jensmith25

Senior Member
Thanks Erco - didn't know that so very useful.
Thanks premelec. I'm not sure I know how to map out currents.

Added details about the battery pack to previous specifications post.
I'd prefer the simpler red LED option if it will work.
 

inglewoodpete

Senior Member
With 5 x NiMH as the battery pack, the voltage would be as high as 5 x 1.3v = 6.5v when freshly charged.

Rather than hunt for special low-voltage gate MOSFETs, consider some common garden NPN bipolars like BC548 or 2N2222 etc.

You asked earlier: where would you mount the extra components (like the voltage regulator, capacitors, or zener diode etc)? I was not familiar with the AXE021 board. It is ideal for your task! Unsolder one or both of the power-supply wires and replace the power pads with link wires into the prototyping area. With some careful design work, it should be easy to fit your voltage-dropping components (whatever circuit you settle on) as well as the driver transistors and current limiting resistors. If you have a bread-board, I suggest you design, test and prove your circuit before committing to your final solution on the AXE021.

-Peter
 

jensmith25

Senior Member
Thanks Peter. I've not yet connected the power supply leads anyway as I know it's harder to desolder than it is to get it right the first time.
I do have a breadboard so will try out options but just want to make sure I don't blow up the chip before doing so.

I have used the 2n7000 FETs before with 4.5v supply with no problems to power additional LEDs and as I have those available I assumed I could use those again.

I had another Google of the problem and discovered that the advice from the Picaxe website and manuals is to use the 1n4001 x2 diode solution on the positive input.
A 6V Battery Pack should not be connected directly to a PICAXE which is rated only for 5V operation. Doing so may damage the PICAXE. The 6V supply should be taken through two diodes to reduce the voltage by around 1.2V to a safe limit. - See more at: http://www.picaxe.com/Circuit-Creator/Power-Supply-Circuits/6V-Battery-Pack/#sthash.iLjSlJDu.dpuf
It's also in the Picaxe manual 1.

If the issue is maintaining current draw through the Picaxe chip to keep the diodes operating at correct levels as mentioned by erco, is the easiest solution to have an LED on the board that is always on, but dim, so it's pulling a few mA but not bright enough to be a nuisance?
 
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premelec

Senior Member
It's empirical when you do this sort of thing - if it works it works - however how well regulated is the 6volts? Does it go up to 8 volts when charged etc... Instead of the diodes you can use an LED in series with PICAXE V+ . I prefer distinct solutions that don't depend on various possible aberrations of other parts of the circuit to stay safe... If you are going to use a shunt regulator - like the LED to draw current - why not use a TL431 instead which will have a distinct operating voltage?
 

jensmith25

Senior Member
It's empirical when you do this sort of thing - if it works it works - however how well regulated is the 6volts? Does it go up to 8 volts when charged etc... Instead of the diodes you can use an LED in series with PICAXE V+ . I prefer distinct solutions that don't depend on various possible aberrations of other parts of the circuit to stay safe... If you are going to use a shunt regulator - like the LED to draw current - why not use a TL431 instead which will have a distinct operating voltage?
5 x 1.2v AA cells are not going to charge to beyond 6.5v at the most so dropping 4 volts is surely too much when I need to power white LEDs that need at least 3v? Assuming the TL431 can drop say 2v, I have no idea how to achieve that. Until you mentioned it I'd never heard of one and on Googling it I'm still not sure how you get a specific voltage drop from it. Is it by using resistors and how is it calculated?

I just wanted the simplest solution that will work so I can implement it successfully. The more complicated it gets the more out of my comfort zone I am and the harder it is for me to understand what I'm doing and get it to work successfully.
 

erco

Senior Member
Either an LED with an appropriate series resistor or simply a resistor will keep enough current flowing. For instance, a 1K resistor from the negative (banded) side of the diode to ground will keep 6 mA flowing on 6V. So basically the resistor is in parallel with everything else. It's downstream of the power switch, obviously. No need to draw 6mA when everything else is off.
 

premelec

Senior Member
@jensmith Actually I was suggesting using an LED to drop voltage _instead of_ 2 diodes - however a lot depends on just what your circuitry is and my preference is still for crisp voltage regulation rather than diode drops and such... whatever works!
 

premelec

Senior Member
erco knows best! - note that there are TO-92 packaged low dropout regulators if you don't need high currents....
 

jensmith25

Senior Member
Thanks all for your help. In the end I went with the 2 diodes in series because it works and I had the parts available. I have checked the voltages and it all seems to be running smoothly.
 
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