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Another newbie... working with MOSFETS

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Skydiver

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I'm building a scuba sled and I need to create a PWM to control the DC motor. I could use resistive speed control but I don't want heat or the inefficiency of this design. I've found a few resources on 555 ICs and found a neat Win OS app to test different resistors and caps to change the frequency (glad to get the URL if anyone's interested).

My question involves the two TIP35A transistors that I have. I need to handle approx 40amps of current (12V, 35Ahr battery). Seeing I'm at or above the limit of the transistors individually how do I set these in series and how are they to be triggered? Do they only require a 5V trigger to close the circuit? I could always get a higher current transistor but what fun would that be?

If possibly please explain to me what needs to be done in simple terms. No big words or scary electrical jargon :)evil:) . Thanks for any input!

Rich
 

Phasor

Member
To get obtain a higher current rating, you don't put them in series - you need to put them in parallel. That is, wire the collecters together, the emitters together, and the bases together.

I'm not certain that you will be able to drive these transistors directly from the 555, as the required base current is too high. You may need another, smaller transistor (say, BD139), and operate them as a darlington pair. (Let me know if you need this explained)

The other problem with resistive control, is that you lose a lot of torque :( . Not so much a problem with PWM.
 

Phasor

Member
Oh, and just another comment - you wrote the subject as "working with MOSFETs", but your TIP35A is not a mosfet - it is a regular, run-of-the-mill bipolar transistor.

Although, if you DID use a mosfet, instead of the transistor, you can drive it directly from logic gates. That's the beauty of them. And then there's IGBTs, which combine the best of both worlds :wink:
 

Skydiver

New Member
Phasor said:
I'm not certain that you will be able to drive these transistors directly from the 555, as the required base current is too high. You may need another, smaller transistor (say, BD139), and operate them as a darlington pair. (Let me know if you need this explained)

The other problem with resistive control, is that you lose a lot of torque :( . Not so much a problem with PWM.
:D

Okay, so would it be possible to have this explained?? Thanks!
 

Phasor

Member
To understand the Darlington pair, firstly, we must understand that one of the important properties of a transistor, is GAIN (often written as hFE). This is the ratio of Collector current to Base current.

Taking your TIP35A as an example - it has a nominal gain of 40. That is, for every milliamp flowing into the base, 40 milliamps can flow into the collector.

A Darlington Pair, is 2 transistors wired, such that their gains are effectively MULTIPLIED together. You would normally have a smallish transistor, and a larger transistor. Now, in an NPN darlington (as we are dealing with), you feed base current into the smaller transistor. The collectors are wired together. The emitter of the small transistor is connected to the base of the larger transistor. Thus, if you put them both in a black box, connected as described, you would have something that has one base, one collector, and one emitter. In other words, two transistors, ACTING like one transistor, but with much larger gain.

The whole point of doing this, is now, instead of needing to feed, say, 1A into the base of your large transistor, you now only need to feed the pair, say 10mA (these numbers are just examples), which is within the capability of your 555.
 
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