Reversing motors

I see... Still, a lot of trouble to go to to find out!

Dunno, can't find 16 now... I don't think it was under test conditions, but I may've been wrong. Thanks! :)

Got a postcard from the library today to say The Art of Electronics arrived (I had to order it), so I'll start going through it later - looks like a big read ;-)

Now you're confusing me!

Think I'll go read this book... ;)

On this page:

Figure 3.14 shows a couple of diodes, a couple of resistors and 4xBD679 darlington transistors. On Rapid, they're 81-0106, 25p+vat.

I assume X and Y could be connected to my PIC? Outputs are 4-5V from my pic. Would the digram there work as is for me (it's around 4-5V motor), or would I need to change anything? (How does is the voltage to the motor controlled?)

I'm gonna go start reading The Art of Electronics, but I want to order the bits I'll need asap, so I can start fiddling! :-)

Ta,

That looks less fun. I want to learn as much about electronics as I can, so the more "diy" I do, the better :-) (it does mean I'll be anal about asking questions in here a lot though!)

Excellent! Just what I was hoping you'd say ;P

That sounds too easy - what's the catch?

My motors are only around 4V, but I see what you mean. Why can't the real world be software emulated - it's much easier to work with :-)

As drawn, and stated beneath, X and Y require open-collector (or open-drain) outputs. If you omitted the pull-ups, and connected 1k resistors in series i.e. between X/Y and the bases, you could use standard outputs instead.

narrow down

drawing 1A

bistable kind

you could

energy only

darlington,

your

In the circuit of Fig 3.14, there's also a diode drop across the

my

power stage

current from

around

across the

e.g.

I recommend this circuit: it'll happily run at lower voltages; you can drive it directly from PIC outputs; you could substitute different (suitably beefy) back e.m.f diodes, if you can't get the ones shown.

gate

seems

MOSFET

to

and

To answer in context:

1. Darlingtons are 2 bipolar transistors. They require less base drive current than one bipolar, but drop more voltage.
2. MOSFETs are not suitable for low-voltage H-bridges for reasons already stated.
3. JFETs are small signal (not power) devices.
4. Complementary bipolar (NPN+PNP) is the way to go.

anything

You want a "high power silicon NPN transistor" and a "high power silicon PNP transistor." They have TIP31/32. They are bipolar transistor.

Look up "darlington", "bipolar" (etc) in the AoE index.

hey Danny, there are many possibilities constructing H-bridges, but they narrow down with your low voltages. If you have a 4V supply and your motor is drawing 1A average, or 3A peak, you can best use a relay, maybe of the bistable kind like Nais TQ2-L2-5V. This would allow the full voltage swing and you could PWM the motor with an inexpensive NMOS. A bistable relay uses energy only when changing states and also stays latched on power off. I costs 4bucks single quantity. Darlingtons are not applicable as they use at least 1V each, so for your motor there is only 2V left. :-( But you can use bipolar transistors, like the FZT788 PNP for the power stage and a FCX688 for the NPN. Those can be driven by a small (20mA) current from a PIC or via another buffer transistor. These transistors have around

But then I won't understand the differences between bipolar, darlington, FETs etc. ;)

My power supply can be increased - though if it means they'd flatten my batteries quicker, I'm open to other methods!

The gate voltage is 4-5V (from my PIC) and the N-channels I've got seems to work great. What's the difference (in basic terms!) between a MOSFET (& JFET for that matter), bipolar and darlington? I'm sure they're covered in the book I'm reading (The Art of Electronics), but I want to order as many of my bits as I can asap, so I can fiddle with things and try things out! :)

I can't find anything marked as bipolar on Rapid - can you see anything you think would be good from here:

Thanks,

Excellent. Can't find a 2N2222 on rapid, would the ZTX455 here:

be a suitable replacement? Or is there anything else there I should try? (Most of them don't seem to have the same max current current as the

I've started reading it from Chapter 1, so I'll probably find them soon :-)

Other bits I need (for both the motors & ultrasonic) haven't arrived, so I've got a weekend of no fiddling, so I'll probably read a bit more!!

When you say suitably beefy, what am I looking for - high current rating? Voltage? Any particular type (rectifier? zener? schotky?)

(sorry, I'm sure I'll know when I've finished reading, but people here can probably answer the questions quicker than I'll read the book :))

Danny, first try to find out the requirement for your motor. Take your multimeter and measure the resistance of the motor at the terminals, without spinning it. If the value was 3.3 Ohms i.e. , that would mean your motor will take a maximum of 5/3.3 = 1.5A, when stalled or switched on. Second measurement driven with 5V how much would be the average current? Insert your multimeter into one line in the 20A range. It might be 0.3A. Once you have found these values you can go on choosing the right driver transistors. The max. current capability should be above 1.5A. You might want the ZTX1048A and the ZTX1147A(both available at rapid), which have a 4A max. rating. Now you should determine the base current needed for these transistors. you divide the 1.5A by the hfe value for the current in question and with hfe above 200 you will need only 7.5mA of base drive. You choose a bit higher like 15mA, to saturate the transistor well. then you can calculate the base resistor. The circuit of the link above is very good, as long as you can control those 2 lines from the pic. But you could also insert an inverter and use only one O/P driven with a PWM-signal, that allows you a continuous speed control from forward to reverse. The diodes across the power transistors should be of Schottky type with the max. current rating above your 1.5A and the average can be smaller like 1A. SB130 are good.