Before we were arguing that the transistors do not have enough base current for a 200mA motor. Now the motor draws more than 1A and nobody says anything?
If your transistors are weak (but passable) then the motor might not even start running.
You obviously haven't read the datasheet for the ZTX690B.
Maximum Vce = 0.5V@Ic = 1A & Ib = 5mA.
Typical Vce about 0.35V@Ic = 1A & Ib = 5mA
Typical Vce = 0.1V@Ic = 200mA & Ib = 2mA.
**broken link removed**
Just to check, are the whole range of transistors on those two links high-gain transistors? One product on the second link seemed to be specifically marked 'high gain', and none of the others on either of the pages specified that.
The ZTX690B is the NPN version of the ZTX790A. It's fairly similar except it's NPN and that the voltage rating is slighlty lower (probably because it's B rather than an A) and it has a higher gain (NPN transistors normally do).
The datsheets list the gains and saturation voltage under cetain test conditions, see my response to audioguru above.
So to be clear, with the high-gain BJTs, or 6 "standard" BJTs instead of the four, as shown in Hero's earlier schematic, there shouldn't be a problem with passing a current approaching 1A in the stalled condition, even with the microprocessor current through the bases being limited to 4mA?
In order for you to be able to use the four transistor circuit you need the NPN transistors to have a Vce≤0.2V @Ic≥200mA & Ib ≤ 4mA. Most transistors need to have a base current of at least 20mA in order for Vce to be ≤ 0.2V.
The PNP transistors still need to have a Vce≤0.2V@Ic≥200mA but the base current can easily be 20mA as they're driven by the NPN transistors, not the micro-controller.
What I'm talking about is forced beta, the ratio of the base current (Ib) to the collector current (Ic) and saturation voltage (Vce). For standard transistors a forced beta of 10 is required to achieve low Vce but you need NPN transistors that are able to give you a low Vce with a forced beta of 50, as mentioned above, the PNP transistors as less critical as long as they can handle 1A.
5p transistors... I have GOT to find a better supplier than maplin. The cheapest transistors they sell are 21p each; works out to 12.5p per unit when buying 20+ of them. I'll take a look at rapid and see how their prices compare to maplin's... come to think of it, I think that's where I got my reel of lovely violet wire
I specifically ordered it online from them despite high delivery cost because cut cable at maplin was ludicrously priced.
Yes Maplin are a rip-off but they're useful as you can just pop-in and buy stuff.
Have you looked at Farnel and RS components?
They're cheaper than Mapin but more expensive than RS and do free delivery.
Rapid also do free delivers on orders over £35.
Also beware that the prices on their sites don't include VAt.