Switch transistor with load connected to the ground - Page 1
Forum section: electronics / projects

author: dansku
author_rank: Regular Contributor
timestamp: January 15, 2016, 05:03:58 pm
content: Hello guys,I have a 3v3 microcontroller and I need to switch a 12V load that needs to be connected to the GND.My main problem is that the load needs to be connected to the ground.I have been struggling a bit to get the circuit right, so far I got the one attached.Can you guys give me a hand to get it right? I am using a 2N2222 connected to the µC and a NTD2955 to drive the load.Any help would be awesome!Thank you

author: Ian.M
author_rank: Super Contributor
timestamp: January 15, 2016, 05:54:45 pm
content: The 2N2222 with less than 0.4mA base current doesn't turn on hard enough to pull the gate low against the 47R pullup, so make R1 1K. You may have cooked the 2N2222, so better change that as well.

author: andybarrett1
author_rank: Regular Contributor
timestamp: January 15, 2016, 08:40:29 pm
content: I use this….BRAndy

author: dansku
author_rank: Regular Contributor
timestamp: January 15, 2016, 09:20:51 pm
content: Quote from: andybarrett1 on January 15, 2016, 08:40:29 pmThank you very much for the circuit.Do you know any smd equivalent for the components?

author: dadler
author_rank: Unknown
timestamp: January 15, 2016, 09:56:12 pm
content: http://www.mouser.com/ProductDetail/Fairchild-Semiconductor/MMBT3904/?qs=sGAEpiMZZMvAvBNgSS9Lqpg%252bOuqhHUdo

author: dansku
author_rank: Regular Contributor
timestamp: January 15, 2016, 10:00:09 pm
content: What kind of parameters do I need to check in order to find a smd version of the mosfet as well?thank youQuote from: dadler on January 15, 2016, 09:56:12 pm

author: mikerj
author_rank: Super Contributor
timestamp: January 16, 2016, 10:08:45 am
content: Why is R1 such a ridiculously low value? You will be dissipating ~3watts in this resistor with the MOSFET switched on.If it's because you need very high switching speeds, thn considering using an active pull-up rather than a resistor, or just use a MOSFET driver.If switching speed isn't a problem (i.e. you aren't switching the load on and off at high frequency), then you should be able to make R1 much higher in value.

author: andybarrett1
author_rank: Regular Contributor
timestamp: January 16, 2016, 04:20:14 pm
content: Ipak smdhttp://uk.farnell.com/fairchild-semiconductor/fqu11p06tu/p-channel-mosfet-60v-9-4a-ipak/dp/1783413Lots of amps still needs big heat sink though !!!

author: langwadt
author_rank: Super Contributor
timestamp: January 16, 2016, 04:41:48 pm
content: much simpler to use one of the numerous smart high side switches like fpf2702, auir3315 to name a few, and they add protecting against shorts and over temp

author: Audioguru
author_rank: Super Contributor
timestamp: January 16, 2016, 04:43:12 pm
content: Do you know what you did wrong?1) What is 7.5V?? The collector of the 2N2222 transistor goes from +12V when it is turned off to +0.2V when it is turned on.2) How much current does the gate of the Mosfet use?? NOTHING! Then why use 47 ohms which will have a current of 11.8V/47= 251mA (!) if the 2N2222 transistor had enough base current to conduct that much.Yes, the 47 ohm resistor can be 10k ohms and have a current of only 11.8V/10k= 1.2mA and the base resistor must have a current of at least 1.2mA/10= 0.12mA. Then the base resistor value is (3V - 0.7V)/0.12mA= 19k ohms, use 18k.

author: Ian.M
author_rank: Super Contributor
timestamp: January 16, 2016, 04:54:15 pm
content: You also have to consider turn-off speed and the power dissipated in the MOSFET during turnoff. Too high avalue pullup resistor on the MOSFET gate will result in device destruction turning off a high current load due to the MOSFET being in the linear region for the time it takes to discharge the gate to below the gate threshold voltage. The higher the voltage being switched, the worse the situation is due to the Miller capacitance.

author: dansku
author_rank: Regular Contributor
timestamp: January 21, 2016, 02:12:38 pm
content: Quote from: langwadt on January 16, 2016, 04:41:48 pmSo in the end I am using this chip:http://uk.rs-online.com/web/p/current-limit-switches/8088972/It worked fine, and only 3 external resistors (like shown on the datasheet).Thanks everyone for the help!