P-channel resistor values - Page 1
Forum section: electronics / beginners

author: kozlojak
author_rank: Contributor
timestamp: June 19, 2013, 11:41:48 am
content: when using a circuit like this one(not my image , I am using a 6V source) what is the formula to calculate the resistor values? I don't seem to be getting enough current to my load so I think I may have bad resistor values.

author: amyk
author_rank: Super Contributor
timestamp: June 19, 2013, 11:57:12 am
content: What's Vgs?

author: BravoV
author_rank: Super Contributor
timestamp: June 19, 2013, 12:04:24 pm
content: Looking at this circuit powered from 6 volt source and the results, its very unlikely he is using a logic level mosfet.To op, what is your mosfet ? If its common jelly bean power mosfet, the gate must be driven quite hard near 10 volt difference between source and gate pin, your 6 volt might not be hard enough to turn it on properly.If possible, use logic level mosfet.

author: kozlojak
author_rank: Contributor
timestamp: June 19, 2013, 12:27:23 pm
content: I am using the IRLML5103http://www.irf.com/product-info/datasheets/data/irlml5103pbf.pdf

author: Unknown
author_rank: Unknown
timestamp: June 19, 2013, 01:03:58 pm
content: You would need to replace the transistor base resistor with a 1K value as minim or less, but not zero ohms, due the base/emitter junction conduction is reducing the bias voltage to the power FET gate... the transistor needs to be saturated to turn-on the FET.Ideally, a -negative gate voltage would saturate the FET, for low ohm S/D conduction.

author: kozlojak
author_rank: Contributor
timestamp: June 19, 2013, 02:09:43 pm
content: I change the transistor base resistor to 1k but was still having the same problem.I think the problem is the lm317 that the mosfet was feeding into, I was trying to get 150ma constant current and was only getting about 120ma on 5.5v left in the batteries, I was getting 5.1v into the regulator, and 3.4 out.when I used a psu to get 6v on the input I was able to get up to 130ma so I think it still needs more power due to the drop out.I am going to try a drop in replacement for the LM317 -http://www.st.com/web/en/resource/technical/document/datasheet/CD00000544.pdfif that does not work I will need to look into using some different batteries.

author: edavid
author_rank: Super Contributor
timestamp: June 19, 2013, 03:56:07 pm
content: Quote from: vlf3 on June 19, 2013, 01:03:58 pmI just want to mention that the above is not right. As long as the base current is more than the collector current divided by beta, the transistor will be saturated. So, any base resistor up to say 50 times the collector resistor is fine.

author: edavid
author_rank: Super Contributor
timestamp: June 19, 2013, 04:00:16 pm
content: Quote from: kozlojak on June 19, 2013, 02:09:43 pmThe regulator is not really a good approach. You should replace the P MOSFET with a simple PNP current source, using 2 diodes to set the base voltage. This will get you to about 1.4V dropout.

author: Mr Smiley
author_rank: Frequent Contributor
timestamp: June 19, 2013, 04:07:18 pm
content: Is there any chance of having one of the above smileys changed for a crystal ball.Then every new topic can just have the crystal ballwas the 317 mentioned in the original postThis is the problem, the whole problem and nothing but the problem "Crystal Ball smiley" honest

author: Unknown
author_rank: Unknown
timestamp: June 19, 2013, 06:33:22 pm
content: Indeed, a Crystal Ball is the answer...