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Topic: Can I use a transistor to turn on and off positive voltage? (Read 3206 times) previous topic - next topic

CrossRoads

Don't use TIP120, that has a dual transistor output, you just want a single transistor to minimize the voltage across the button when the transistor is turned on.

Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

pederw


pederw

Don't use TIP120, that has a dual transistor output, you just want a single transistor to minimize the voltage across the button when the transistor is turned on.


Thanks ! What do you mean about single and dual transistor output ?
Learning a lot today ! :D

CrossRoads

See the datasheet
https://www.fairchildsemi.com/datasheets/TI/TIP122.pdf
Schematic on page 1 shows dual transistor arrangement - good for high current driving, poor for logic use.
Vce(sat) on page 2 shows Vce of at least 2V - not good if you are trying to pull a 1.7V signal low.

A Low on resistance (low Rds) N-channel MOSFET would be better, it would have very low voltage drop so the 1.7V would be really low when switched.
http://www.digikey.com/product-detail/en/alpha-omega-semiconductor-inc/AOI510/785-1487-1-ND/3603498
Same for P-channel MOSFET for the high side
https://www.digikey.com/products/en/discrete-semiconductor-products/transistors-fets-mosfets-single/278?k=p-channel+mosfet&k=&pkeyword=p-channel+mosfet&pv1993=24&FV=fff40015%2Cfff8007d%2Cffe00116%2C1140050&mnonly=0&newproducts=0&ColumnSort=0&page=1&stock=1&quantity=0&ptm=0&fid=0&pageSize=25
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

CrossRoads

MOSFET uses voltage levels to turn on and off, very little input current, like 1microAmp. Outputs can be found with very low Rds (Resistance from drain to source) making them look very much like a mechanical switch.
Power dissipated in the device is then related to current flow, P = I*I*R, so for high current, low Rds is very beneficial.
1A*1A*.010 ohm = .01W.

Bipolar Junction Transistor (BJT, NPN type and PNP type) need current into the base to let current flow from collector to emitter, with a more or less fixed voltage across C to E (Vce) of 0.5 to 0.7V.
Since Vce doesn't change, power dissipated by the device can be high. P=I*V, so 1A*.5ohm = 0.5W.  Big difference.

In this application tho, where the current flow is low, but the voltages are also low, the low Rds of the MOSFETs makes more sense to use so the voltage swings can look more like the switch contacts you are duplicating.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

pederw

Okei, think I understand.
Summary;
BJT and MOSFETS are tow type transistors.
BJT er better for high current, but have a little voltage drop. Use current to ''turn on and off''
MOSFETS are better for low current with little or nothing voltage drop(?). Use voltage turn on and off.

Questions.
1. Does MOSFETS exist in PNP and NPN to or is it only one type ?
2. Do you activate and use MOSFETS the same as BJT? Only voltage/current different ?
3. Is TIP120 a BJT ?
4. Witch MOSFETS do you recommend for me? May be useful for other projects as well. Want both types, NPN and PNP if that exist.

Thank you CrossRoads :)
Found this project very interesting now when this is a little outside my electronic knowledge.

Grumpy_Mike

Quote
1. Does MOSFETS exist in PNP and NPN to or is it only one type ?
Yes they are called N-channel FETs and P-channel FETs

Quote
2. Do you activate and use MOSFETS the same as BJT?
Sort of but the base ( or gate as it is called on a FET ) resistor should be small just protect the Arduino around 100R should do. It takes virtually no current.

Quote
3. Is TIP120 a BJT ?
Yes but it is a darlington pair not a single transistor.

Quote
4. Witch MOSFETS do you recommend for me?
There are many and the best is to find what you can get round where you live. Look for a "Logic level" FET.


dcrash36

just as a quick transistor run through for you, BJTs, which is what the TIP series is, are amplifiers not switches, FETs are switches, with a PNP you must pull current from the base in order to pull the collector up close to the positive rail, PNPs are high side drivers, base must be roughly 0.7V below the emitter in order to conduct, with an NPN you must inject current into the base in order to get the collector to pull down near the gnd rail, these are low side drivers, base must be roughly 0.7V above the emitter in order to conduct....the amount of base current you choose to push or pull is directly related to the collector current by the gain in the datasheet for the part you are working with

PNP goes between load and positive rail, NPN goes between load and gnd rail

pederw

MOSFET uses voltage levels to turn on and off, very little input current, like 1microAmp. Outputs can be found with very low Rds (Resistance from drain to source) making them look very much like a mechanical switch.
Power dissipated in the device is then related to current flow, P = I*I*R, so for high current, low Rds is very beneficial.
1A*1A*.010 ohm = .01W.

Bipolar Junction Transistor (BJT, NPN type and PNP type) need current into the base to let current flow from collector to emitter, with a more or less fixed voltage across C to E (Vce) of 0.5 to 0.7V.
Since Vce doesn't change, power dissipated by the device can be high. P=I*V, so 1A*.5ohm = 0.5W.  Big difference.

In this application tho, where the current flow is low, but the voltages are also low, the low Rds of the MOSFETs makes more sense to use so the voltage swings can look more like the switch contacts you are duplicating.
Witch MOSFETS do you recommend for me? May be useful for other projects as well. Want both types, N channel and P channel.

pederw

Is JFET, FET and MOSFET the same but just different names ?

Grumpy_Mike

Is JFET, FET and MOSFET the same but just different names ?
No.

You need to look for a logic level FET. This is the second time you have been told this by me.

pederw

No.

You need to look for a logic level FET. This is the second time you have been told this by me.
Yeah I know that, just wondered if someone could tell the difference.

Grumpy_Mike

It will say "LOGIC LEVEL" in the data sheet.

Failing that look at the on resistance from source to drain in the list of parameters. It will give a value in ohms, or fractions of an ohm. Then look at the test conditions for this parameter it will give you the gate voltage. If this is 5V or under it is a logic level FET, if it is 10V then it is not.
Do not be fooled by the gate threshold voltage, that is nothing to do with what you are looking for.

allanhurst

https://en.wikipedia.org/wiki/Transistor
And for a suitable circuit, see the attached

pederw suggested it before

regards
Allan

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