I have an LED current source circuit using 2N2907 transistors driven from a 595 shift register that will source 35mA. This will give me 350mA of drive, but I need more like 940mA. Will a P-channel MOSFET do this? I want individual transistors, SMT is fine. I could live with less then 940mA, but 350mA is too little. Please recommend a transistor replacement that will give me more drive. Alternatively recommend a very inexpensive replacement that will give me 7 lines of ~940mA ( only one one used at a time ) switchable from logic levels ( +-35mA max ).

Thanks!

 

PNP Transistor

 

A more powerful transistor needs more base current. Where will it come from?
Use a PNP power darlington transistor.

 

Will a P-channel MOSFET do this?

Yes, you can use a logic level gate p-channel MOSFET. Hard to recommend something. On digikey there are still 15 pages of MOSFETs when I narrowed down the search to 950mA to 3A transistors starting at prices of 43 cents for a single unit.

 

One of the big problems is that we have NO idea where you are. If you are in the USA, you can obtain a huge variety of transistors and MOSFETs. If you are in a third-world country, your selection will be extremely limited.

You should put your general location in your profile. Country and state, province or region is fine - or just country if you don't wish to be overly specific.

 

People who don't show where they live are on the Moon where they can't buy ANYTHING.

Many poor students ask for cheap (cheep) parts but forget about their poor quality.

 

Sorry, I am in California. I never used a discrete MOSFET before, so I don't know how to select one, or what the 'supporting circuit' looks like. Do I need a 'base resistor' for the MOSFET, what value, etc. Is there one with the same package as the 2N2907 ( TO92 )?

Also I don't want something of poor quality, just the cheapest price to do what I need. A max chip is way way out of the question...

 

Well , most of them are SMD. Through hole components you are looking for have a current rating of less than 280mA or more than 5A on digikey. The problem with all online retailers is shipping cost, you may be better off with your local retailer... if you have one in town.

A gate resistor is a good idea since the maximum output current of the HC595 from TI for example is 20mA. But unless you are planning to drive the Mosfet with high frequency signals it will probably also work with no gate resistor. (the gate acts as a capacitor which you charge and discharge,so there is only a short current pulse). I like using components inside their specs so I don't need to bother about unexpected behaviour, for 5V the "correct" min value in this case would be 250R.

Using a MOSFET is easier as a BJT, forget about base current and DC gain, it's commonly used as a simple ON/OFF switch.
The "supporting circuit " for a sourcing p-channel MOSFET looks like this:

 

Thanks for the MOSFET circuit. I have a few questions about it:

* "Current continuous drain" from digikey, is this how much current it will source?

* I am modulating this at aprox 1Mhz, is this considered 'high frequency'?

* I don't follow your math on the current limiting resistor. Can you show me how you got your numbers.

I mocked up my circuit below, you mind having a look, am I doing this right? If I make sure the current through my LED current limiting resistors don't add up to more than the max drain on the sinc register or the max continuous drain on the MOSFET, then I am OK, right?

Thanks again!

 

Thanks for the MOSFET circuit. I have a few questions about it:

* "Current continuous drain" from digikey, is this how much current it will source?

Yes , it changes with temperature, though.

* I am modulating this at aprox 1Mhz, is this considered 'high frequency'?

1MHz ??? Is this correct? That's indeed high frequency. What's the reason for this?

* I don't follow your math on the current limiting resistor. Can you show me how you got your numbers.

5V/20mA = 250R

I mocked up my circuit below, you mind having a look, am I doing this right? If I make sure the current through my LED current limiting resistors don't add up to more than the max drain on the sinc register or the max continuous drain on the MOSFET, then I am OK, right?

you forgot to attach the diagram

 

The 1mhz was a highball estimate. I need to multiplex 7 banks for LEDs. I was planning on doing that from a timer interrupt on an attiny88. I have not actually figured what the actual frequency will be, much slower than 1Mhz I am sure. What should I use as my upper limit on frequency?

I don't know why my diagram did not show up for you, it looks fine for me... The image is stored at:

https://vzcat.com/mosfet.jpg

 

My two browser blocked the image and the link you provided gives me an "invalid certificate" warning. Please just attach the picture you have. Click on "attachments" , choose the file and upload it.

Use png or jpeg file extensions preferably.

Thanks