Transistors?

MichelB

Hi all, is the transistor ZTX1049A used for the little pump in Applied Sensors can be replaced by the BS170 MOSFET used for the fan in Process Control?

Leon

Have you compared the data sheets?

Leon

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Amateur radio callsign: G1HSM
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MichelB

No, just the use, one is for a little pump the other for a little fan.

Leon

Check the data sheets for the transistors!

leon

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Amateur radio callsign: G1HSM
Suzuki SV1000S motorcycle

Tracy Allen

The data sheet parameter you are looking for for the BS170 mosfet is Vgs, the gate to source voltage that will turn on the mosfet. Better yet is a graph that might be called "on-region characteristics", that relates the drain current to different Vgs voltages, and find the Vgs that is necessary to deliver 200 milliamps. When you find that parameter you will see that it is around 2 to 3 volts, but with a spread. One problem is that the Vgs in a mosfet is not a well controlled parameter, and if you sort through a bin of BS170s, you would want to find the one that has the lowest Vgs value.

The ZTX1049 on the other hand is an NPN transistor (high gain, superbeta, e-line high power) and the comparable parameter is the base emitter voltage that will deliver 200 mA in the collector emitter path. That is always going to be very close to 0.6 volt. In the Applied Sensors application, the transistor is used as an emitter follower (common collector), so the pump is connected the emitter to power the pump at about 4 volts.

The same circuit (=source follower, common drain) is fine with the N-channel mosfet, but the source will end up at about 3 volts (=Vgs) below the gate voltage (=5 volts). So only 2 volts across the pump. But it could happen that you get a BS170 with Vgs=1 volt, and it would work just as well as the ZTX1049.

I think that in Process Control, the BS170 is used in a common source configuration with the load connected in the drain circuit, and the 0 to 5 volt transition from the Stamp has no trouble crossing that Vgs threshold even given the variations in the Vgs parameter.

Now, your assignment:
Take what you know from Process Control, and rewire the pump circuit so that you can use the BS170 in common source configuration instead of as a follower, and make a minor change in the program to adjust for the inversion of the sense of the logic.

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Tracy Allen
www.emesystems.com

LoopyByteloose

Generally, you may never get the same MOSfet or transistor that the original manufacturer used. I suspect they prefer to use oddball listings to prevent DIY repair. Since they buy in 1000s, the can buy with any lable or spec they want. In the retail market, a rather limited variety of MOSfets or transistors are available. And of course, the Europeans use different ones than the US.

So when in doubt, this is where you really should be using a spec sheet and your acquired engineering knowledge. There are some websites that offer equivalent listings - but double check anyway. Things like a protective diode may be missing.

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Ain't gadetry a wonderful thing?

aka G. Herzog [noparse][[/noparse] 黃鶴 ] in Taiwan

Leon

The ZTX1049A will handle much more current than the BS170. This is probably important in this application.

Leon

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Amateur radio callsign: G1HSM
Suzuki SV1000S motorcycle

Post Edited (Leon) : 6/28/2009 7:27:22 PM GMT

MichelB

Thank you very much all and Tracy Allen for explanation.