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# MOSFET MAX CURRENT FOR BSS123

#### DIAGO1000

##### Newbie level 6
The BSS123 MOSFET SAYS MAX id IS 170mA AT 100V. So what is the max current at 5v?

#### betwixt

##### Super Moderator
Staff member
170mA.

The specification is 170mA maximum continuous drain current AND a maximum drain voltage of 100V. They are limits of two different parameters but whatever voltage and current you run them at you still have to observe the power dissipation limit of Id * Vd.

Brian.

#### DIAGO1000

##### Newbie level 6
The problem I have the power Dissipation is 0.36W but the vd at 100V and current at 170ma doesn't equate

#### betwixt

##### Super Moderator
Staff member
The actual dissipation is Rds * Id ^2, the apparent resistance between drain and source multiplied by the drain current. It is basic Ohms Law.

Are you saying you have 100V across drain and source and it is passing 170mA, maximum for both, and you are only seeing 0.36W dissipation? Please show your schematic.

The limits are 100V at any current and 170mA at any voltage provided the dissipation doesn't exceed 0.36W.

Brian.

Last edited:

##### Super Moderator
Staff member
You can safely give the mosfet 100V power supply, but you must limit turning it on to the extent it dissipates 0.36 W max.
Or 3.6 mA.

With power supply 2.1 V, then you may turn it on so it passes .17A because that makes 0.36 W.

#### KlausST

##### Super Moderator
Staff member
Hi,

Most probably you have
* OFF: 100V, 0 mA
* ON: some mV, 170 mA
In both cases the power dissipation is low.

Mind:
* not the power supply voltage counts, but V_DS, which is the voltage between Drain and Source
* and drain current
For power dissipation, the actual drain current x the actual V_DS.

Klaus

#### FvM

##### Super Moderator
Staff member
The BSS123 MOSFET SAYS MAX id IS 170mA AT 100V.
No. Please refer to safe operation area figure. Max DC current at 100V is about 3.5 mA.

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