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[Boncuk]

I prefer MosFet transistors to bipolar transistors because of less voltage drop.

The BS170 has an ON-resistance of 1.2Ω at a gate voltage of 10V, resulting in a voltage drop (loss) of 24mV at 20mA, whereas a bipolar transistor normally drops 0.7V.

Boncuk

 

[colin55]

Here is a simple Kitt scanner circuit that can be modified
**broken link removed**

**broken link removed**

The Knight Rider circuit is available as a kit for less than $15.00 plus postage

You can add transistor buffers to increase the current.

 

[audioguru]

I prefer MosFet transistors to bipolar transistors because of less voltage drop.

The BS170 has an ON-resistance of 1.2Ω at a gate voltage of 10V, resulting in a voltage drop (loss) of 24mV at 20mA, whereas a bipolar transistor normally drops 0.7V.

Boncuk

No.
An ordinary little transistor like the 2N4401 has a collector saturation voltage of only 0.05V at 20mA, not 0.7V.
Maybe you were thinking of a huge 2N3055 power transistor at 9A.

 

[Boncuk]

No.
An ordinary little transistor like the 2N4401 has a collector saturation voltage of only 0.05V at 20mA, not 0.7V.
Maybe you were thinking of a huge 2N3055 power transistor at 9A.

Very true, but still more than double the drop of a BS170 under the given conditions.

 

[angie1199]

So, 'scuse my ignorance again but, on small circuits with very few 'voltage drop' components, a bipolar transistor would be ok, but when the circuit it large and therefore the total cumulative voltage drop is large, a mosfet is preferable? Is that about right?

Angie

 

[Boncuk]

Each (unwanted) voltage drop in a circuit requires power.

In the example given the bipolar transistor dissipates 0.001W (1mW) while the MosFet transistor dissipates 0.00048W (480µW).

You decide which one you use to preserve energy.

Boncuk

 

[MrDEB]

wouldn't an ULN2803 be a better choice as far as cost and parts count?
not sure of current capabilities but I recall 500ma?
seeing how the op is fairly new at this using mosfets might be a mistake as static electricity and soldering heat not to mention cost. $1.50+ per mosfet as opposed to $1 for the uln2803 with 8 outputs.