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Pararelling PIC pins and get more current?

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hantto

Member
Hello, the topic says it all, but I have in mind driving a 4,5V 100mA lightbulb directly from a pic without any buffer transistor (to save weight because it's comming in my RC model). So is this possible if the pic is put to output 4 pins high at the same time (25mA per pin, isn't it so?)?
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Yes, you can parallel pins in this way, but obviously you have to be very careful that the pins all switch the same way at the same time. Assuming the spec per pin is 25mA, only using 4 for 100mA is pushing things a bit, I would prefer to see more used. The chips also have a maximum total current capability, I should check what that is as well.
 

ivancho

New Member
I would be careful because depending on the PIC each port has a current limiting capabilities. For example the 16F84 MaX current out in PortB is 150mA and each pin is MAX 25mA. The PIC16F877 the PortB has 200mA and each pin 25mA... bare in mind that this are abosolute maximums.....!

How much more heavy can a transistor be.... :lol:

Ivancho
 

hantto

Member
About 1gram? But why would I use a transistor when I can do without? much simpler, I do not even need a pcb, just a pic somewhere with some wires comming and going(and maybe a decouling cap).

And i'm also learning pic so I want to learn the sofware side etc..
 

ivancho

New Member
Well I try to weight the transistor just for kicks! :lol: and I could not weight it. It is not even 0.1oz which is the minimum weight of the scale.

What are you trying to do? Are you going to blink a light?

Ivancho
 

crust

Member
You might also check the datasheet. Often the spec is such that the maximum current can be drawn given the condition that the output is still a valid logic high. For instance, you might be able to source 40mA out of a pin without damaging the part. But the output voltage might not be considered a valid high if you drove another part with it. On the other hand, if you are driving an LED ... check the datasheet and give it a try.
 

hantto

Member
ivancho said:
What are you trying to do? Are you going to blink a light?

Yup, to be totally honest i'm blinking 2 superbright red leds, one one each wing. So i just want to draw about 60mA @ 2V. 8)

Just wrote a light because i have them hardwired them in pararell inside the wing, so I can't anymore disconnect them from eachother. And I was to lazy to write the whole thing in my original post. :p
 

ivancho

New Member
You are going to need a limiting resistor for your LED in that case because they will try to take as much current as possible until they burn.

Why not use a Astable Multivabrator IC.. for US $0.50 can't beat that and you will only need a resistor and a capacitor, no programming and a battery as low as 2.5V ;)

Check this one out from Digikey: Part#296-13642-1-ND

Ivancho
 

hantto

Member
You know what? :wink: I did that at first. I used a 555, resistor, cap and it worked fine. But then my sample request from microchip arrived and fortunately i ordered a 12f675 smd version. It's too easy to write a blink program (thx to nigel) so that even i'm capable of it. With the pic I don't need anything else. And I just tested, it works down to 2V. If i'm good enough i might write a program that will act like a pwm and thus maybe not need a resistor, but I doubt that, so i'll just use a resistor. And the best of all: it was FREE

Take that :D

P.S. I don't like to order stuff from abroad (ofcourse i could have gotten the chip from my local dealers, but I already have the pic..). But thx anyway. :)
 

ivancho

New Member
just one question... how are you going to program the PIC without being in a board since it is surface mount?

The PWM is a really good idea, and even better if you run your PIC from your 4.8V battery. The bad thing about PWM leds is that they will be dimmer. So ig you run only 2V and PWM then they will be dimmed...... but if you were to run a higher voltage that might compensate for the PWM.

Let me know how it turns out.... Have you seen this by the way? Realy cool for night flying :) https://www.glowire.com/rcairplanes.htm

Ivancho
 

hantto

Member
ivancho said:
just one question... how are you going to program the PIC without being in a board since it is surface mount?
Just solder some wires to its leads and then stuff them in the programmer.

And about the battery, it is a 4.8V batterypack. The plane is a glider.

I just discovered that the 675 has a A2D, maybe I will also write a "Battery low" indicator program, that's when I learn to use a2d's. So it will take a looong time before i'm completed with this project.
 

ivancho

New Member
that sounds like it will be an onging project.... next thing you know you are building an autopilot :lol:
 

MarkK

New Member
hantto said:
ivancho said:
just one question... how are you going to program the PIC without being in a board since it is surface mount?
Just solder some wires to its leads and then stuff them in the programmer.

And about the battery, it is a 4.8V batterypack. The plane is a glider.

I just discovered that the 675 has a A2D, maybe I will also write a "Battery low" indicator program, that's when I learn to use a2d's. So it will take a looong time before i'm completed with this project.

I fly model gliders and electrics too and have made a couple of speed controllers using pics. You might like to know that there is a new pic coming out in spring that is IDEAL for a speed controller, Its basically a 675 but with pwm, so there isn't the problem of the pic being busy decoding input pulses while it should be putting out a smooth pulse train to the mosfets.
Its the 12F683 :
https://www.microchip.com/1010/pline/picmicro/category/embctrl/8kbytes/devices/12f683/index.htm
 

pike

Member
ivancho said:
Well I try to weight the transistor just for kicks! :lol: and I could not weight it. It is not even 0.1oz which is the minimum weight of the scales

have you tried putting say, 20 transistors on the scales and dividing the result by 20???
 

hantto

Member
Re: SOT23

Rescyou said:
If you are going to be using SMD circuitry with the PIC anyway, a SOT23 transistor or MOSFET would work slick and they are are extremely lightweight:

It's not only the weight. They cost, and they're not so easy to find. And I say it again, why would I use a transitor if i don't need to?
 

ivancho

New Member
For kicks I just wnet and weight 30 of them and nice enough they weighted 0.3oz so each one is under 0.01oz ( they were in a package) jajajaja :lol: Better start doing something more productive with my time 8)
 

Exo

Active Member
Re: SOT23

hantto said:
Rescyou said:
If you are going to be using SMD circuitry with the PIC anyway, a SOT23 transistor or MOSFET would work slick and they are are extremely lightweight:

It's not only the weight. They cost, and they're not so easy to find. And I say it again, why would I use a transitor if i don't need to?

Using pic pins in parallel would require a bigger pic (more pins) then when using only one pin. The cost & weight of 1 transistor versus a bigger pic...
I think the transistor will do better.
 

Rescyou

New Member
Cheap

A fairchild MMBT3904 costs $0.17 US for a quantity of 1.

I think you may find that SMD transistors are pretty mainstream and are probably almost more readily available than the bigger/standard TO92 varieties.

As for why you may need to, it's cheap insurance based on the value of the PIC as what you are doing is somewhat of a gray area with respect to pin output on the PIC.

R.
 
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