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Why does my motor is so slow after using two transistors as switches? Answered

Hi guys! As i'm waiting to my H-Bridge to arrive i built this just to play around, but theres a problem. The motor works, goes foward and backwards as i programmed on the arduino, but its incredibly slow, to a point that it can't make the rc car body move. If i connect the power suply directly to it, it goes like a bullet. What  could it be? The circuit is attached.
*I'm using a BC546 transistor
**All the resistors are 1k ohm
***My power supply is 9v/600mah.Thanks in advance!
http://i.imgur.com/lIgNf.jpg

12 Replies

user
-max- (author)2017-08-19

Your transistors are a fairly poor choice. They are only rated for 100mA continuous. From my experience, if you try to pass currents close to the maximum the transisor can handle, you will have significant Vdrop across the transistor even when saturated. (base-emitter voltage actually higher then the collector-emitter voltage, the transistor literally cannot be turned on any "harder.")

For a motor you should consider using something like a pn2222 or better yet, some low on-state resistance MOSFETs. N channel devices or NPN devices are typically better, so it's not uncommon to see H bridges use all N type devices. This does however require special care because when you have N channel devices on the high side, you need to drive the gate/base with a higher voltage than your vcc.

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user
Sowee (author)2012-10-14

I'm using NPN transistors. My idea was to build a simple system that could make the motor go forward and backward. So, i had to place two transistors for each state (back and forward) so no energy would flow backwards and cause a short circuit. I seriously have no ideia why this isnt working.

When pins 3 and 4 are on HIGH, transistors 1 and 2 would allow the current to flow in a direction and when pins 5 and 6 are on HIGH the current would flow backwards.

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user
rickharris (author)Sowee2012-10-14

Look at the diagram - to make an H bridge you NEED NPN AND PNP transistors because one pair had to be OFF and the other pair ON.

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user
rickharris (author)rickharris2012-10-14

Diagram shows current flow depending on which half is on.

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user
rickharris (author)rickharris2012-10-14

tHE BLUE ARROW HEAD IS ON THE WRONG END SORRY

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user
Sowee (author)rickharris2012-10-14

Welp, i think ill get some diodes and pnp transistors tomorrow, and make one H bridge that way. Thanks for the help :)

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user
rickharris (author)2012-10-14

Not connected right depending on hat type of transistors your using

And H bridge looks like this from a circuit point of vies although this one uses relays.

A bipolar transistor H bridge looks like this diagram.

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user
rickharris (author)rickharris2012-10-14

The relay diagram is easier to follow:

IF Rly A is on AND Relay B is off then current flows + 12 to - 12

If Rly B is on AND Rly A is off current flows -12 to +12

IF Both are on OR Off then no current flow.

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user
Sowee (author)2012-10-14

Oh my fault, i finished building it, but didnt finish the diagram. Heres the correct one. I'll upload a photo as soon as my phone charges up a little bit.

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lemonie (author)2012-10-14

It's not wired up right, but I don't understand your picture as a circuit diagram. E.g. it looks like the Arduino is only connected to two transistors.

L

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user
rickharris (author)2012-10-14

Show circuit and picture if possible.

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