Switching problem with MOSFET & ATTINY85

Thread Starter

Prashant Misra

Joined Feb 26, 2018
2
Hi,
I have this Laser device (LD) which works on 5V 800mA. This is a medical device which is suppose to work for 15 mins or
stop at a press of Switch. I got 3.7v Li-ion 2200 (4) connected and used ATTINY85 for timing and switching ON/OFF using lathing concept.
Once the pin 1 of ATTINY gets -ve it checks the status and starts 15 min counter OR stops the current process and goes to sleep till
button is pressed again. So far so good.
So when it wakes up it sends signal to MOSFET IRFZ44 (N Channel) Gate and Drains fwds the -ve to Buck-Boost to make 3.7v to 5V to power
up the LD (Problem) The device does not work.
If I connect battery to Buck boost directly, it does perfectly. Image attached for reference.

IMG_20180530_134553_Prashant Misra.jpg

Maybe this OR another way- I need a single button ON/OFF timer to power my LD device; to work for 15 Mins and switch off. Can be
stopped in middle with the same switch.
 

crutschow

Joined Mar 14, 2008
24,318
You need to look at the IRFZ44's data sheet.
It is a standard threshold MOSFET which requires 10V Vgs to fully turn on as shown in the data sheet clip below.
If the particular device you have has the maximum Vgs(th) of 4.0V it won't turn on at all with a Vgs of 3.7V.

You need a logic-level type MOSFET with a ≤2V max Vgs(th), which can fully turn on with a Vgs of 3.7V

upload_2018-5-30_7-30-6.png
 

BobTPH

Joined Jun 5, 2013
2,254
Look at the datasheet for IRFZ44. What voltage is required on the gate to turn it on? How much current can it pass with 3.3V on the gate?

Edit: Ninja'ed by crutcshow.

Bob
 

DECELL

Joined Apr 23, 2018
96
You could us a dual to get half your on resistance at 3V eg Si4946BEY.
Look at buck-boost controllers that use a few uA in sleep mode and get rid of the switch.
 

Thread Starter

Prashant Misra

Joined Feb 26, 2018
2
You need to look at the IRFZ44's data sheet.
It is a standard threshold MOSFET which requires 10V Vgs to fully turn on as shown in the data sheet clip below.
If the particular device you have has the maximum Vgs(th) of 4.0V it won't turn on at all with a Vgs of 3.7V.

You need a logic-level type MOSFET with a ≤2V max Vgs(th), which can fully turn on with a Vgs of 3.7V

View attachment 153389
Thank you, I did not check Vgs(th).

I am checking for IPD088N063N INFINEON and Vishay IRLZ14PBF.
Ordered both, let me check which runs well.

You could us a dual to get half your on resistance at 3V eg Si4946BEY.
Look at buck-boost controllers that use a few uA in sleep mode and get rid of the switch.
Thank you, I am a bit novice on this usage, please let me know as to how to get the same done
 

BobTPH

Joined Jun 5, 2013
2,254
And, for future reference, Vgsth is not what you should check to see if your gate voltage is high enough. Vgsth is where the MOSFET just barely turns on. Typically, you want to check the voltage(s) at which RDSon is specified. Using that voltage guarantees a certain ON resistance. If they specify RDSon at 10V and nothing else, you can be pretty sure that it will be way higher at 5V.

Bob
 
Last edited:

DECELL

Joined Apr 23, 2018
96
Can I be a AAC Fanatic- what does it stand for? I've googled and I am none the wiser.
Autoclaved Aerated Concrete?
American Addiction Centers?
Alarmed Anxious Constipated?
Ants Against China?
 

crutschow

Joined Mar 14, 2008
24,318
Can I be a AAC Fanatic- what does it stand for? I've googled and I am none the wiser.
Autoclaved Aerated Concrete?
American Addiction Centers?
Alarmed Anxious Constipated?
Ants Against China?
I'm not sure if you are serious but, if you are, look at the title of this forum. :rolleyes:
 
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