Author Topic: My ready easy PWM 12V 3A transistor  (Read 2047 times)

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Offline assassa1221Topic starter

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My ready easy PWM 12V 3A transistor
« on: April 15, 2013, 04:02:54 AM »
Hello

I want to share this easy PWM-plan:
  • Power supply: Only 12V !!!(Change resistors if other supply)
  • PNP-Transistor: Select according load's current(A) needs(TIP32C=max.3A and so on...)
Comments welcome ::) My first electronic project:


http://i1321.photobucket.com/albums/u556/12356562/PWMDC6-12V3Aatleast_zpseda3c5f0.jpg

Offline newInRobotics

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Re: My ready easy PWM 12V 3A transistor
« Reply #1 on: April 15, 2013, 05:56:06 AM »
Hi

I already explained it in PM previously that 3A transistor rating is not enough, as 3A Stall Current motor can draw double that when starting or changing direction. Also, 100 Ohm base resistor will not give enough current to the base to fully switch transistor ON, if we consider worst case scenario of hFE = 10 (as in TIP32C Datasheet), meaning that transistor will heat up and possibly burn.
"Computer games don't affect kids; I mean if Pac-Man affected us as kids, we'd all be running around in darkened rooms, munching magic pills and listening to repetitive electronic music." - Kristian W

Offline assassa1221Topic starter

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Re: My ready easy PWM 12V 3A transistor
« Reply #2 on: April 15, 2013, 07:54:39 AM »
Sorry but I trust Mads Barnkob. He said it'll work so we'll see. ;) (see link)
"with a 100R base resistor and a 10K pull-up you should have a sturdy driver for your TIP32C." On below blog's comments
http://kaizerpowerelectronics.dk/calculators/transistor-base-resistor-calculator/#comment-14351
Motor 60rpm:
http://dx.com/p/high-torque-60rpm-12v-dc-geared-motor-91625

Hmmm...but I'll buy PNP TIP36C. I=25A +Heatsink. If you think it is a safer option ? (I'll ask Mads precise resistor values later)

Phiuuu... Spent 3 months and still stuck in the sandbox even if that won't work :-[
« Last Edit: April 15, 2013, 08:12:12 AM by assassa1221 »

Offline Pogertt

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Re: My ready easy PWM 12V 3A transistor
« Reply #3 on: April 15, 2013, 09:23:13 AM »
@ assassa1221

Please read note (6) on page 3 of the data sheet.

http://www.ti.com/lit/ds/symlink/lm555.pdf

Your 555 may have a limited lifespan depending on motor current.
Mmmmmmmm....Cookies

Offline newInRobotics

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Re: My ready easy PWM 12V 3A transistor
« Reply #4 on: April 16, 2013, 01:40:14 AM »
Spent 3 months and still stuck in the sandbox even if that won't work :-[
That is because You keep asking people whether something will work instead of trying to understand it Yourself. It's not that difficult at all:
  • If motor stall current is 3A, go for transistor capable of at least 6A.
  • Calculate transistor base resistor:
    • Find Min hFE (hFE is gain, it shows how many times base current is amplified and output as Ic (collector current)) value in datasheet
    • Divide desired output current (6A) by MinhFE / 2 (In case of TIP36C, Ib (base current) = 6A/(10 / 2)) = 1.2A
    • Calculate base resistor using Ohm's Law: R = V/I = 12V / 1.2A = 10 Ohm
       
  • Check if base current required by transistor matches to what 555 timer can provide:
    • Based on 555 timer datasheet Pogertt provided (refer to datasheet of 555 timer You are using), Power Dissipation = 1.18W, hence Max Current @12V can be I = W/V = 1.18W/12V = 0.098A, which is much lower than 1.2A required by transistor, meaning that something will burn (if base resistor is 10 Ohm (to satisfy base current) - 555 timer will burn, if base resistor is 123 Ohm (to satisfy 555 timer parameters) - transistor will burn (or at least get very how reducing its lifespan)).
       

Hmmm...but I'll buy PNP TIP36C. I=25A +Heatsink. If you think it is a safer option ? (I'll ask Mads precise resistor values later)
This transistor is better suited for 3A stall current motor (although motor that You linked to does not state its stall current), however it still does not solve problem that 555 timer is unable to provide enough current for transistor to switch properly.
« Last Edit: April 16, 2013, 01:25:12 PM by newInRobotics »
"Computer games don't affect kids; I mean if Pac-Man affected us as kids, we'd all be running around in darkened rooms, munching magic pills and listening to repetitive electronic music." - Kristian W

Offline Jon_Thompson

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Re: My ready easy PWM 12V 3A transistor
« Reply #5 on: April 16, 2013, 02:21:11 AM »
As it stands, this circuit will positively rip current through the 555 and burn it out. It needs a pre-driver stage before the TIP32A to provide enough juice to fully switch it on.

Don't believe me? Try it!
You can still do a lot with a small brain.

Offline jwatte

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Re: My ready easy PWM 12V 3A transistor
« Reply #6 on: April 16, 2013, 11:38:47 AM »
Yes, try it! You will likely burn some components. Once you see smoke, turn off power, and figure out what parts are smoking, and why. Then replace those parts, make appropriate changes to the circuit, and try again. That is the only way to learn! And, yes, it costs some money in components. That's the (very low) cost of learning. I always buy three of anything; I'll burn the first one; I'll use the second one, and the third one is a spare for unexpected accidents.

Btw: For switching power, you want to use a MOSFET, not a BJT. (BJT is the "normal" kind of transistor you're currently using.) BJTs suck current all the time while they are on. MOSFETs only suck current for a short while while they switch state (to on, or to off.) A 555 might be able to drive the gate of a MOSFET fast enough to make the MOSFET and 555 survive, as long as the duty cycle is OK (say, 200-400 Hz PWM.) A 555 will not be able to drive the base of a power BJT with a hFE of 10 to get to 3A. The hFE means that you need to put 300 mA through the base to get 3A through the collector/emitter.

Offline assassa1221Topic starter

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Re: My ready easy PWM 12V 3A transistor
« Reply #7 on: April 17, 2013, 12:34:08 AM »
OK. Thanks I guess... I give up.
I understand that nobody hasn't invented a working "easy to do it self for cheap pwm" for my specific purpose or that the diy-plan is highly kept as a secret(Meaning that I won't be ever possible to solve this because it is impossible to create it myself.)

That is why I must buy a ready made controller:(and simply copy the product)
I should have done it in the first place :-[
Bye

Offline newInRobotics

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Re: My ready easy PWM 12V 3A transistor
« Reply #8 on: April 17, 2013, 12:56:49 AM »
I give up.
I understand that nobody hasn't invented a working "easy to do it self for cheap pwm" for my specific purpose or that the diy-plan is highly kept as a secret(Meaning that I won't be ever possible to solve this because it is impossible to create it myself.)
What are You on about? This whole thread is explaining You how to achieve it and for some reason You refuse to take that knowledge on-board. Circuit You want to build has been built many times since creation of 555 Timer, which was 41 years ago.
"Computer games don't affect kids; I mean if Pac-Man affected us as kids, we'd all be running around in darkened rooms, munching magic pills and listening to repetitive electronic music." - Kristian W

Offline jwatte

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Re: My ready easy PWM 12V 3A transistor
« Reply #9 on: April 17, 2013, 12:24:01 PM »
I think what he's looking for is a step-by-step instruction for how to put one together without having to know any electronics.

If you have trouble doing Ohm's law in your head, and don't understand what the hFE of a transistor is, and haven't heard of the difference between a MOSFET and a BJT transistor, then designing the appropriate circuit is likely to be impossible.
Meanwhile, if you know enough electronics to understand those things (which is not particularly hard for a dedicated hobbyist) then the circuit designs itself, and you don't really need to document it, because it's so obvious.

Thus, that's likely a reason why there's no PWM circuit for high current loads available with step-by-step no-understanding-needed instructions.

Although the second hit on google is this image:

It comes from this project description: http://www.apdigitallight.com/2010/08/diy-laser-spirograph-part-2.html
Which I think is a pretty good description of a DIY PWM controller circuit. So saying "there's no DIY project available" really isn't true -- it just shows that you didn't think of putting "pwm controller diy" into Google and following a few of the links.
Other links include this: http://3lectronics.com/wp-content/uploads/2011/11/schematic-pwm-controller-eagle.png  and this: http://3lectronics.com/pwm-controller-regulator-diy-555/

« Last Edit: April 17, 2013, 12:25:22 PM by jwatte »

 


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