I have just purchased some standard and high power project boards and because I need them to operate on 24V I followed the instructions and fitted a 78L05 regulator IC. I have used these regulators on many other projects and have always been advised to include decoupling capacitors (I usually use 0.1uF ceramic). Because there are no capacitors fitted to the PCB I checked on the Texas Instruments web site to find out whether I was wasting my time and money - on page 3 of the data sheet (www.ti.com/lit/ds/symlink/lm78l05.pdf) I found "Recommended minimum load capacitance of 0.01μF to limit high frequency noise" and considering the PIC can have problems with high frequency noise I suggest that these capacitors should be fitted. I fitted 0.1uF 1206 size ceramic capacitors alongside the legs of the IC and because I am also going to be pulse driving an inductive load I also took the precaution of fitting a 470uF, 35V electrolytic capacitor between + & G in the bottom right-hand corner of the PCB. Would technical like to comment on the lack of decoupling on the 78L05 ?
Using 78L05 regulator on project PCBs
- Thread starter Videostar
- Start date
You are quite correct , C1 & C2 are decoupling the +5V section of the supply rail but for adequate HF decoupling, the 100nF cap should be close to the output pin of the regulator and (preferably) ceramic and not almost at the other side of the board. If you look at the data sheet from Texas instruments who make the regulator, you will see they always include decoupling on the input too but removing the link (R9) means there is absolutely no decoupling of any kind on the input. Working on the theory of "better safe than sorry", I have fitted two small SMD caps across the pins of the regulator which is really easy to achieve. The 470uF will add a bit of LF decoupling on the input supply. Because I haven't finalised the output circuitry yet (twin motors, each 24V @ 10-15A) I have obtained both the standard and the high power 18pin boards to see which is going to be more suitable - because I have to be able to reverse the motors (they are going in a "ride-on" train) I am using "H" bridge drive with high power FETs as an output stage.
I think you will find that the 100uf satisfies the needs of the reg and any demand from input circuitry, and the 0.1uf provides the decoupling needed by the picaxe, which should be, and is, as close to the picaxe as it can be. Adding more won't do any harm but might be solving a problem you don't have.
You might need to change the mosfets on the high power board, and beef up the copper tracks, or, better, make a separate power board with higher-current fets and chunky tracks, if you plan on carrying 15A.
You might need to change the mosfets on the high power board, and beef up the copper tracks, or, better, make a separate power board with higher-current fets and chunky tracks, if you plan on carrying 15A.
If this were my project...
There is a possibility that when the motor is switched on and off that some heavy inductive currents are created, which could induce some high voltage (and high frequency) spikes onto the supply lines. As I'm assuming this is going to be a battery driven unit and you can't power the PicAxe from a separate supply from the heavy current supply I might be tempted to use an LM7805 (the 1A style of regulator) not the 78L05. I think the 7805 might better handle any voltage spikes. I would probably add a series resistor before the regulator, with some hefty capacitance to try to level out some of the possible spikes.
Of course, this is speculation on my half and it may be that in practice none of this matters...
There is a possibility that when the motor is switched on and off that some heavy inductive currents are created, which could induce some high voltage (and high frequency) spikes onto the supply lines. As I'm assuming this is going to be a battery driven unit and you can't power the PicAxe from a separate supply from the heavy current supply I might be tempted to use an LM7805 (the 1A style of regulator) not the 78L05. I think the 7805 might better handle any voltage spikes. I would probably add a series resistor before the regulator, with some hefty capacitance to try to level out some of the possible spikes.
Of course, this is speculation on my half and it may be that in practice none of this matters...
...but wise counsel never the less.
I would also stick a diode on the supply side of the hefty capacitance so that starting surges from the motors cannot pull the amps from the capacitors and starve the voltage regulator.
erco
Senior Member
I think perhaps this might be a good idea too - anything which limits potential problems (sorry !) has to be a good idea. I think I will go with resistor, diode, capacitor, just to be totally sure. The only problem will be incorporating them all on the project PCBs and I'm still undecided whether to use the standard board which inverts the output (and I don't know how to change the software - see below)
#Picaxe 18m2
#No_Data '// Do not write to or erase the EEPROM memory
'// Pin Constants
symbol ADC_PIN = B.1 '// Center of pot connects Pin B.1
Symbol PWM_PIN = B.6 '// PWMOUT on Pin B.6
'// Variables
symbol ADC_value = B0 '// 0 - 255
symbol duty_cycle = W1 '// 0 - 1000
'// Initialize PWM
pwmout pwm_pin, 127, duty_cycle '// Start pwm at 1000 hz with a duty cycle of zero
MAIN:
DO
readadc ADC_Pin, ADC_value '// Read the Pot (0 - 255)
duty_cycle = ADC_value * 200 / 51 '// Do the math to map the value for 0 - 100 percent duty
pwmduty pwm_pin,duty_cycle '// Change the duty cycle according to pot position
pause 100 '// Short Delay
Loop
(The above was provided by "Goeytex" - with many thanks and I hope he will be better soon)
Or should I use the high power board which doesn't invert the output but costs more and contains the 4 FETs which I won't be using ?
The inversion presumably due to the Darlington Array ? You can take a non-inverted signal direct from the PICAXE output pins.
If you are extensively modifying a PICAXE project board to achieve what you want; it is worth considering designing your own circuit on veroboard / stripboard or even designing your own PCB.
If you don't need the functionality a particular project board provides it will also be worth looking at other project and starter boards and other PICAXE chips as they may be more appropriate or better lend themselves to what you want to achieve.
I'm afraid I'm very much a "newby" with regards to PICs & PICAXE projects so I was sticking to what was available, rather than get too involved and finishing up with a "non-starter".
Basically I need to control FOUR 24V motors and the total load with all four running could be as high as 40Amps and the only way to keep everything as cool as possible is with PWM - a feature available on the PICAXE chips. I already have a few 18M2 chips and a couple of each of the standard & high power project boards + the board that comes with the starter pack so that is why I am going down that route. If this project is successful I may have a repeating demand for it and the project boards will provide a cheaper route than designing and manufacturing my own boards (I doubt I will ever need 100 off or more, much rather perhaps 10).
Basically I need to control FOUR 24V motors and the total load with all four running could be as high as 40Amps and the only way to keep everything as cool as possible is with PWM - a feature available on the PICAXE chips. I already have a few 18M2 chips and a couple of each of the standard & high power project boards + the board that comes with the starter pack so that is why I am going down that route. If this project is successful I may have a repeating demand for it and the project boards will provide a cheaper route than designing and manufacturing my own boards (I doubt I will ever need 100 off or more, much rather perhaps 10).
erco
Senior Member
Ebay? Interested. Got a link? Thanks.
I've used ITEAD Studio, prices start at USD 10 (plus reasonable shipping) for 10 off 5 x 5 cm 2-layer boards. Quality seemed fine to me.
PWM does not necessarily keep things cool. A poorly designed / implemented PWM drive can easily over heat and / or require huge heat sinks. How cool things run will depend upon the MOSFETS selected and how the MOSFETS are driven. Whether or not your Project is a "non starter" depends upon the choices you make.
I suggest you take the time to draw up a complete schematic of how you propose to drive the MOSFETs. Include the MOSFET part number and all related parts. Then post the schematic here for review. This could save you a lot of unnecessary grief.
The vendor is grand_sky in Macau.
Use this link, then click 'see other items' for other board types.
http://www.ebay.com/itm/PCB-Manufacture-Prototype-Etching-Fabrication-2-Layer-/110819255410?pt=LH_DefaultDomain_0&hash=item19cd57a472
Or search for
PCB-Manufacture-Prototype-Etching-Fabrication
and see which vendor you like best ;-)
