# NMOS - NPN current limiter circuit

#### ssetiawan

Joined Nov 23, 2020
9
Hi, I am having a hard time trying to analyze the circuit below:

It is said that the output (9V_currentLImited) is well 9V with a 30mA current limitation. capacitors C2, C3 and C4 are electrolytic capacitors while C1 is the non-electrolytic type. V1 is a 9V voltage source.

What I have tried to understand so far is that the NMOS (M1) source is supplying a slightly less than 9V due to the property of NMOS which then powers the Base of Q2. But, I have a hard time trying to calculate where does the 30 mA current limitation comes from?

Next question, what does the capacitors take part in this circuit?

Thanks!

#### BobTPH

Joined Jun 5, 2013
5,748
30 mA x 22 Ohms is 0.66V which turns on the BJT, which turns off the MOSFET.

Bob

#### ericgibbs

Joined Jan 29, 2010
16,364
hi ss,
Is this a homework or college assignment question.?
E

#### ssetiawan

Joined Nov 23, 2020
9
hi ss,
Is this a homework or college assignment question.?
E
Hi Eric,

no it's a part of a schematic of a device actually..

#### ssetiawan

Joined Nov 23, 2020
9
30 mA x 22 Ohms is 0.66V which turns on the BJT, which turns off the MOSFET.

Bob
Hi Bob,

but if I wanted to change the 22 ohm resistor to 5 ohm for example, how can I analyze the current limiter output?

SS

#### ericgibbs

Joined Jan 29, 2010
16,364
hi ss,
OK, the 30mA limit only applies while charging the capacitor bank.
The current drawn from the charged cap bank is not limited, is this from a spark welding or etching unit.?

E

#### ronsimpson

Joined Oct 7, 2019
2,386
M1, the MOSFET cannot be turned on hard. The Gate turn on voltage must be subtracted from the 9V. I don't see how the output can be above 6V for that reason alone.

#### ssetiawan

Joined Nov 23, 2020
9
hi ss,
OK, the 30mA limit only applies while charging the capacitor bank.
The current drawn from the charged cap bank is not limited, is this from a spark welding or etching unit.?

E
Hi Eric,

This is a part of a muscle stimulation unit which is capable to give out up to 60mA. So there is a DC/DC converter connected from the output (9V_currentLimited) which provides the needed current..

But, I'm still kind of confused, how do we get this 30mA? How can I calculate it based on the circuit to get 30mA as the current limitation?

thanks!

#### Papabravo

Joined Feb 24, 2006
19,266
Hi Eric,

This is a part of a muscle stimulation unit which is capable to give out up to 60mA. So there is a DC/DC converter connected from the output (9V_currentLimited) which provides the needed current..

But, I'm still kind of confused, how do we get this 30mA? How can I calculate it based on the circuit to get 30mA as the current limitation?

thanks!
The current limit is set by asking what current through the resistor will produce a voltage across the resistor that will allow the NPN transistor to turn on. This voltage, which is the same for virtually ANY silicon NPN transistor, is 0.66 Volts. So,

$$R_{limit}\;=\;\cfrac{0.66 V}{I_{limit}}$$

In practice you would pick the closest 1% value for the resistor.

#### ericgibbs

Joined Jan 29, 2010
16,364
hi ss.
THis is your circuit powering up from zero time.
Note the 30mA limit.
E

#### ericgibbs

Joined Jan 29, 2010
16,364
hi ss,
This sim shows the effect of placing a 220 ohm load at 10secs for 5 second period, limits to ~30mA

E

#### ssetiawan

Joined Nov 23, 2020
9
hi ss.
THis is your circuit powering up from zero time.
Note the 30mA limit.
E
View attachment 264259
Hi Eric,

thank you for your help with the simulation! I never knew about the "startup" spice directive before, now I know. And also with Papabravo's explanation:

The current limit is set by asking what current through the resistor will produce a voltage across the resistor that will allow the NPN transistor to turn on. This voltage, which is the same for virtually ANY silicon NPN transistor, is 0.66 Volts. So,

$$R_{limit}\;=\;\cfrac{0.66 V}{I_{limit}}$$

In practice you would pick the closest 1% value for the resistor.
I think I understand how it works now.. Thank you all for the kind explanation and time to simulate it even!

Have a great day

/SS

#### ssetiawan

Joined Nov 23, 2020
9
M1, the MOSFET cannot be turned on hard. The Gate turn on voltage must be subtracted from the 9V. I don't see how the output can be above 6V for that reason alone.
According to the simulation, the output is around 7.8 volts. I always blindly assume that it has a 9V output.

Thank you for pointing that out!

/SS

#### ssetiawan

Joined Nov 23, 2020
9
hi ss,
This sim shows the effect of placing a 220 ohm load at 10secs for 5 second period, limits to ~30mA

E
View attachment 264262
Hi Eric,

A follow up question though.. This kind of means that we do not really need M1, am I right? It seems that M1 is only being passed by the 9V input?

/SS

#### ericgibbs

Joined Jan 29, 2010
16,364
hi,
Do you men like this.??
E

#### crutschow

Joined Mar 14, 2008
30,774
This kind of means that we do not really need M1, am I right?
No.
Why do you think it's in there if not needed?

M1 is what does the actual current limiting.
R1 and Q1 are what determine the value of the limit.

If you wanted to charge the capacitors to the full 9V, you could modify the circuit by using a P-MOSFET and a PNP transistor (below):

#### ci139

Joined Jul 11, 2016
1,873
going complex (simplifed) https://tinyurl.com/ybhkzv49 allows higher charging currents
PS! -- the circuit proposed is not so practical as there are 2 ... 3 independent reference voltages and 3 amp.-s . . . a slight drift at any of these may induce a large difference at output current limit value !!!! . . . so the practical v. of it requires revising the "sensitivity" + minimizing the no. of independent references to preferably a single-source bond ref.-set. !!!

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#### ssetiawan

Joined Nov 23, 2020
9
No.
Why do you think it's in there if not needed?

M1 is what does the actual current limiting.
R1 and Q1 are what determine the value of the limit.

If you wanted to charge the capacitors to the full 9V, you could modify the circuit by using a P-MOSFET and a PNP transistor (below):

View attachment 264271
Hi, I have tried to understand your circuit and your explanations of your circuit library (here). So what I understood is, as I am using an N-MOSFET and a NPN transistor, the analysis is that the NPN will detect the voltage drop of the base-emitter Voltage when it is lower than 0.66V. Due to that, the voltage across the M1 gate-source reduces and cut off the MOSFET at the current limit point, which is 30mA in this case then..

Am I understanding it correctly?

#### crutschow

Joined Mar 14, 2008
30,774
Am I understanding it correctly?
Basically.
But it's when the NPN base-emitter voltage is greater (not lower) than 0.66V that it starts to reduce the M1 gate-source voltage and limit the current.

#### crutschow

Joined Mar 14, 2008
30,774
Below is the simulation for the PNP, P-MOSFET circuit:

Note that the capacitor charges to the full 9V (green trace), not to about 7.5V as the NPN, N-MOSFET circuit does.

Also note the PNP emitter-base voltage during the charge (red trace).
For a 50k bias resistor, the emitter-base voltage at the current control point is about 600mV.

The PNP controls the P-MOSFET gate-source voltage (blue trace) to maintain the 30mA current limit (yellow trace) during the capacitor charge.

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