How to limit current

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Ok, thank you for explaining that, I think I'm finally understanding this. The Vce is what determines where the transistor is saturated or not, the lower it is the more saturated the transistor is.

How do we go about determining the maximum hFE a transistor has?
I will use PN100 NPN and MPS751 PNP as an example: https://www.electro-tech-online.com/custompdfs/2010/05/PN100PDF.pdf and https://www.electro-tech-online.com/custompdfs/2010/05/MPS650-DPDF.pdf
PNP: This transistor is saturated, the Vbe max is 1.2V.
NPN: The collector will receive 10.8V so it's Vce will have a minimum of 1.2V (it could have a bit less Vce e.g. 1V than this if it's more of a typical transistor). From the hFE section, I see it says Ic = 100mA, Vce = 1V, hFE = 100 Min, but there is no maximum, why is this?
Do we use the hFE graph to approximate the maximum hFE even though it's for a Vce of 5V? E.g. 100mA Ic = 250 hFE so we might add 50 more to that to be safe to have 300 hFE?
Or should we just look at the hFE section, look at the maximum under any conditions (600 hFE in this case) and use that as the max hFE?

Thanks!
 
It is the minimum hFE you should look at. For a switch transistor the base current must be 1/10th the collector current regardless of the hFE.
Since every transistor has a different hFE (even with the same part number) then hFE cannot be used to limit collector current.
 
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You look at the minimum hFE when you want to know if you will always have enough current.

Most designs are designed to work properly even with a very high hFE. As you noticed some transistors don't even have a maximum specified. I am lazy and usually make my circuits so they work with the minimum hFE and would also work if hFE was infinite.

Use a resistor to limit the collector current.
 
Ok, so the minimum hFE is 100. I go back to my design, take out the NPN Q1 resistor, put in a 44Ohm resistor in between Q1 emitter and ground, then the hFE can vary all it likes but it won't change the current by too much. The resistor needs to be rated at 0.4W (Q1 emitter is 4.3V), so grab a 1W one to be safe. Is there anything that's an issue with the design now?

**broken link removed**
 
Base-emitter resistors are missing at Q2 and Q4 to turn them off quickly.
Your schematic has them slowly floating off which might take all day.
 
Hmm, are you saying that because of the motor will keep it going for a while or is it because of another reason?
I will put in the 4 diodes like in one of my previous circuits to combat back EMF, will this also help to turn them off quickly?
 
 
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Just a quick question, if I had a 12v battery source and used a 6v motor, is it right to say that the PNP transistor would have a Vce of about 6v, so it's not saturated any more, thus the whole circuit would change and if left as it was there is a good chance the PNP transistor would blow?
 

hi
The Vce sat is when the transistor is hard ON, not 6V.
The transistor if driven enough base current would saturate and if the current exceeded the transistor rating it would cook and die, if was able to carry the current, most likely the 6V motor would smoke!!
 
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Thanks Eric .

Just another example (I like examples), Q1 NPN has a collector power dissipation of 1W and I'm focusing on this transistor only. Assume I've connected a 12V device. The Vce for this would be around 7V at best (when hFE is 990, Ic is 97mA just for fun). 1W / 7V = 0.142A or 142mA so I'm within the thermal limit.
Now if my battery was 16V and I have a 16V device, Vce is at best 11V which means the max current can only be 0.09A / 90mA which would mean the transistor would overheat and eventually die?
 
The datasheet for every transistor lists its max allowed temperature.
The minimum and maximum hFE are also listed.
The max saturation voltage loss is listed when its base current is 1/10th the collector current.
Then you can simply calculate how much power is wasted making heat.
 
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