Fully saturating transistor!!!!

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Makaram

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Hey guys running a DC motor off 9.6v bat, using bd681g darlington transistor as a switch off a 555timer IC.
First question is whats the voltage i use to calculate the saturation current for the Base? e.g. do i use 9V or some IC related voltage (transistor max current is 4A)? and if a transistor isnt saturated does it mean that its providing more resistance leading to heat??????
 
Well, darlingtons have 2 transistors & they cannot both saturate so the VCE will be kinda high. The important info from the datasheet is shown in the attached image. So, the VBE is 2.5V when on, and the HFE is >750 A/A. So if the 555 output voltage is 8V and you want at least 4A to flow, the resistor value would be found from (8 - 2.5)/R*750 = 4, or R = (8-2.5)*750/2 = 4k; a resistor value less than that should be fine. VCE is going to be >3V @ 4A.

You might want to have a look at using a MOSFET instead as it's not hard to find one that will have less voltage drop across it in your application.
 

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You didn't show a schematic so we don't know if the darlington is used as a useless common-collector (emitter-follower) or as a proper common-emitter.

You don't say the max current of the motor so we are just guessing that it is the 1.5A listed with spec's in the datasheet for the darlington.
The max base-emitter voltage is 2.5V and the saturated gain is a minimum of 50 (30mA base current).

With a 9.6V supply the output high of an ordinary 555 is about 8.2V.

Then Ohm's Law calculates the series base resistor as (8.2V - 2.5V)/30mA= 190 ohms. The resistor value should be reduced if the battery voltage drops less than 9.6V.

The max saturated voltage drop at 1.5A is 2.5V so it will dissipate 1.5A x 2.5V= 3.75W and will need a heatsink.
 
Yes, a darlington is a bad choice, use a small and large transistor, and make a four pin 'darlington' to cure most of it's shortcomings.
 

Thanks for the reply. Confused how 30mA will only allow 1.5A while 30mA*750Hfe = 30A???????????
 
Hfe does not get it into saturation - only close. If you check the data-sheet Vce is 3 volts @ 1.5 amps and a base drive of only 2 ma.. The other 28ma. is used to pick up the other .5 volts of Vce. It's not a real hot transistor, but the price is right.
 
Makaram said:
Thanks for the reply. Confused how 30mA will only allow 1.5A while 30mA*750Hfe = 30A???????????
Click to expand...
The hfe may reduce in saturation. There's not much difference in the quoted VCE between where 30mA is being applied and where the hfe is 750 (therefore 3mA IB) - it's VCE=2.8V vs. VCE=3V according to the data. Anyway, as everyone's said, it might be better to use something other than a darlington; e.g. buffered transistor (like darlington but collectors aren't connected), or a mosfet due to the fact you're using a large current and a low voltage.

A suitable bjt in saturation might give you 0.5-1.5V VCE @ 4A and a mosfet (RDSON = 50mR) might exhibit 0.2V (VDS) @ 4A.
 
Isnt the whole idea of darlingtons to increase the current capabilities while maintaining amplification?
 
Makaram said:
Isnt the whole idea of darlingtons to increase the current capabilities while maintaining amplification?
Click to expand...
The datasheet you posted for the darlington shows that its minimum hFE is 750 only when it has a collector to emitter voltage of at least 3V so it is not saturated. Then it is a current amplifier.
The datasheet also shows that it saturates with a fairly high max saturation voltage only when its base current is 30mA when its collector current is 1.5A which is a current gain of only 50. Then it is a saturated switch and not a current amplifier.
 
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