Base saturation for TIP102 transistor as switch?

Discussion in 'General Electronics Chat' started by Nicholas, Nov 30, 2012.

1. Nicholas Thread Starter AAC Fanatic!

Mar 24, 2005
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1
Hi all!

It's been a long time since I've been here last, I plan to come by more
often

If I have a TIP 102 transistor that I just want to use at maximum(as
a switch), how much should the base voltage be at? And do the amperage
matter?

Let's say I use a microcontroller that has 5V 30mA output. I *think*
that 2V is maximum for the base input at the TIP 102, if this is so, should
I just cut it down with a resistor?

(The TIP 102 will switch 24V at around 5A in this application, if this is
important)

Thanks!

Nicholas

2. Audioguru Expert

Dec 20, 2007
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1,185
A TIP102 will switch on 5A fairly well when its base current is about 25mA. Its saturation voltage loss has a max of about 2.25V at 5A so it will need a medium size heatsink.
Its base voltage will be about 1.6V to 2V so limit the base current with a series resistor.
You will not know the output voltage of the micro when its current is 25mA, it might be 3.5V when its supply is 5V. Then the series base resistor is about 68 ohms.

3. GopherT AAC Fanatic!

Nov 23, 2012
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6,786
You have a lot going on here.
The TIP102 is a bipolar darlington transistor. Therefore, the input voltage is not the key to driving a 4 amp load, the input current is the key.

The base to emitter will be something between 1.2 volts (not specified on datasheet) to 2.5 volts (max on datasheet). Therefore, assume the worst case and assume you have 5V from your uController to the TIP102. Then you will have a voltage drop of 5-1.2V = 3.8V (key value1)

You must also limit the output current from the microcontroller (uController) to 25mAmps (0.025 amps) max (use 0.020 amps for some safety zone). This means you must have a resistor between the transistor and uController of 3.8V/0.020A = 180 ohms.

Assume you have a current gain of at least 200 for TIP102. Then your input of 0.020A x 200 (Gain) = 4 amps. You will do fine.

The current limiting input resistor of 180 should be all you need. The only problem is if your Vbe voltage (base to emitter voltage to start the transistor) is really closer to the max on the datasheet than to the average for a darlington. In that case, the driving voltage from your uController will only be 5 - 2.5V = 2.5. In that case, the driving current will be reduced and may not approach your desired 4 amps. Lets check....

Current into your Darlington TIP102 will only be 2.5V/180 = 0.014 amps. Currnet gain (hfe) is actually listed in the ranges of 200-1000 so you should be ok. Best way is to test the turn-on voltage for your darlington using a simple test circuit - see Vol III of books on top of this page if needed.

Just make sure you connect emitter of the TIP102 to ground and the motor between the collector and postive voltage rail. Dont forget the 180 ohm resistor. Cheers.

4. crutschow Expert

Mar 14, 2008
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To avoid the high power loss of the TIP102 Darlington when ON you could use a logic-level MOSFET. With a high current, low on-resistance MOSFET the dissipation could be kept below a watt which wouldn't require a heatsink.

5. GopherT AAC Fanatic!

Nov 23, 2012
8,025
6,786
Audio Guru beat me to the punch again while I was writing!

Cheers AG!

6. Nicholas Thread Starter AAC Fanatic!

Mar 24, 2005
135
1
Wow, thanks! Very nice of you, and very in-depth!

So, if I should use a MOSFET instead, which one would suit me nicely? The 4-5A might change later on

Also, the application will only be on a short time, like ON, then off. Maybe 1 sec
at a time.

Thanks again!

Nicholas

7. crutschow Expert

Mar 14, 2008
20,283
5,739
Just Google logic-level MOSFETs and select one with the voltage about 150% of the maximum voltage you have and at least a 10A rating. You can also search in suppliers such as DigiKey or Mouser to see what's readily available.

If the duty-cycle is sufficiently low than you may not need a heat-sink with the TIP102. Of course you will still lose a couple volts across the load due the its voltage drop.

8. Nicholas Thread Starter AAC Fanatic!

Mar 24, 2005
135
1
Hi, I think the IRF530 MOSFET is right for me then!

So, if that gets 5V on the gate, would that mean it opens completely/saturates?

If my output from the microcontroller is 5V, it would be great if that was right for
maximum throughput

Thanks!

9. Audioguru Expert

Dec 20, 2007
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1,185
No.
The IRF530 is an ordinary Mosfet that needs 10V gate to source to completely turn on. A door or window opens, but A Mosfet turns on.

You were recommended to use a logic level Mosfet.
An IRL530 turns on very well with 4V or 5V gate to source.

10. Nicholas Thread Starter AAC Fanatic!

Mar 24, 2005
135
1
Ok, I think I'm following now!

How about an IRLZ44? That is stated to have full saturation at 5V, yes?

Thanks!

11. Audioguru Expert

Dec 20, 2007
10,612
1,185
Maybe.
It has a high gate capacitance that takes a fairly high current to switch it on and off quickly. If it switches many times slowly then it gets very hot.