Replacement for BC557B and 547B which can handle 1 Amp

Use a small logic level N channel mosfet.....

says 100ma here.

BC 639/640 ?

I suggest SpeedPigz take a look at the Zetex Super E-Line transistors, TO-92 parts that switch up to 10A or more. The ztx851 and ztx951 (npn, pnp) have maximum Vce(sat) under 100mV at 1A with Ib = 100mA.

The Super E-Line parts feature high beta, even at high collector currents, which means one can use them with much less than the traditional Ib = Ic/10 drive current. For example, these plots show Vce(sat) under 50mV at 1A with Ib = Ic/50 or only 20mA.

This actually outperforms many small MOSFETs.

I suggest SpeedPigz take a look at the Zetex Super E-Line transistors, TO-92 parts that switch up to 10A or more. The ztx851 and ztx951 (npn, pnp) have maximum Vce(sat) under 100mV at 1A with Ib = 100mA.

The Super E-Line parts feature high beta, even at high collector currents, which means one can use them with much less than the traditional Ib = Ic/10 drive current. For example, these plots show Vce(sat) under 50mV at 1A with Ib = Ic/50 or only 20mA.

This actually outperforms many small MOSFETs.

Most of the ZTXnnn series will switch an amp or more (just a few won't, so check the specs)

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~ Adrian Tuddenham ~
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"Winfield Hill"

** BC547s and BC557s have a maximum ( useable) collector saturation current of 100mA with 5mA of base drive ( max Vce =0.6 volts).

Now, the OP is aiming to *replace* these in an existing circuit with similar pack devices that will saturate at 1 amp.

But can only assume his circuit has circa 5 mA of drive available - definitely not 100mA.

** These numbers *might* be OK for the OP if he can select devices to avoid ones below the typical Hfe spec - PLUS if he does not mind the rather high cost of doing that.

The devices are about US$2 each at Mouser and 1.5 UKP each at Farnell.

..... Phil

2N4401 does up to 800mA if you push it (I've measured this, although I'm not particularly confident as it was measured on a wirewound resistor at 5MHz :) ), although it's rated for 600mA. 2N2270 does up to an amp, in a TO-39 case. And obviously, power transistors easily do an amp with various other specifications. Offhand I'm not sure what other generic small transistors are good for relatively high current; almost anything Zetex comes to mind, but they're all relatively expensive.

There's no shortage of cheap Japanese transistors available though. Just the other day I ordered some 2SC5001s which claim Vce(sat) < 0.3V at Ic Hi All

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They are relatively expensive.

But I'm puzzled as to the constraints. Presumably if SpeedPigz can change the transistors, he can also change the base-drive resistors. However, if this isn't the case, it's still possible the Zetex parts will be up to the task. That's because they are very high beta devices, and don't suffer as much Vce(sat) increase as other parts when the Ic/Ib base-drive ratio is increased. For example, the ztx851's saturation voltage only increases from 40mV to 60mV at 1A if the base drive is decreased from 100mA to 20mA. That means the power dissipation would increase from 40mW to 60mW.

Now imagine that the saturation voltage quadruples to 240mV for 5mA drive. That'd be 240mW power dissipation, which would likey still be acceptable. Zetex doesn't give us much direct evidence on the datasheet about ratios over 50, but they do have a plot of normalized Hfe vs current for Vce =3D 1V and 5V. This shows the beta drops only about 5%, i.e., to 200 typical, at 2A. Oops, if it was 200 at 1A, that implies a base drive of 5mA. And a power dissipation of 1W in our case, which I'd say is way too much.

It'd be best if SpeedPigz could slow down a bit and also change the base-drive resistor.