circuit that limit the voltage to 14 V

[yassin.kraouch]

Hello, Please i am blocked on this situation : i have an input voltage between 3 and 20 V, i want that the voltage do not exceed 14 V, have you an idea on how to do this ?

 

[WimRFP]

Please provide some additional info. Think of: minimum clamping/clipping voltage, voltage drop when input voltage is below clamping voltage, required load current, etc.

 

[yassin.kraouch]

required current is 500 mA, just i have to keep the voltage to not exceed 14 V, and below 14 V the voltage is the same as input, if there will be a lottle dropout voltage it is not problem,

 

[sumeetpanse]

if it is DC voltage you can use IC LM7814.it will regulate your voltage upto 14 V.if you are designing protect circuitry you can use fuse of 14 V specification.

 

[js]

use LM317 for example (or any other linear regulator)
example
https://www.eleccircuit.com/lm338-lm350-lm317-voltage-regulator-calculator/
and set to 14V output voltage!
If input voltage is bellow 14V also output voltage will go down....

 

[yassin.kraouch]

is there any thermal risk, if the voltage is 20 V or 24 V ?

 

[js]

is there any thermal risk, if the voltage is 20 V or 24 V ?

Yes thermal load on linear regulator is:

P=(Vin - 14) / Iout
If output voltage is 14V
You need cooling....

 

[IanP]

Hello, Please i am blocked on this situation : i have an input voltage between 3 and 20 V, i want that the voltage do not exceed 14 V, have you an idea on how to do this ?

Voltage regulators such as 78XX, 317 and similar, can not work properly with input voltages lower than V[out]. In fact, most of them will require that the input voltage is about 3V higher than the output voltage.
What you can consider for applications where the input voltage can go as low as 3V is a shunt (or parallel) regulator build of a Zener diode, Power Transistor mounted on a Heatsink and a small properly rated series resistor, as shown on the attached picture.
More info here:
https://www.circuitstoday.com/zener-controlled-transistor-voltage-regulators
:wink:
IanP

 

[raiya_23]

how about using a zener diode?

 

[yassin.kraouch]

Voltage regulators such as 78XX, 317 and similar, can not work properly with input voltages lower than V[out]. In fact, most of them will require that the input voltage is about 3V higher than the output voltage.
What you can consider for applications where the input voltage can go as low as 3V is a shunt (or parallel) regulator build of a Zener diode, Power Transistor mounted on a Heatsink and a small properly rated series resistor, as shown on the attached picture.
More info here:
https://www.circuitstoday.com/zener-controlled-transistor-voltage-regulators
:wink:
IanP

The problem is that i need 500mA and i am worried about heat problem, really i am blocked, my idea is to use two output transistor in parallel but i am worried of the current imbalance between two transistor :/

 

[zubusha]

Check This It's Made For Personal Use,,

 

[jeffrey samuel]

Regulator coupled with heat sinks are a good solution

I have tried it out out but seen so in many designs

Do correct me if I am wrong

 

[WimRFP]

If dissipation isn't a problem a linear high power low drop out series regulator can do the job. These regulators use a PNP pass transistor avoiding the BE voltage drop. You need to study the datasheets (and do experiments) to figure out how the regulator behaves with low input voltage.

Are you familiar with electronic design?

 

[yassin.kraouch]

yes i am not an expert in electronis, but i can do some hobby circuit alone, have you an idea on how to do this ?

 

[zubusha]

Why Don't You Use MC33063 , Buy Any Car Mobile Charger, Change The Rsistor Value To Get 14v...

 

[picgak]

Hi yassin.kraouch,
Your problem is very simple, Use a series pass NPN transistor (say 2n3055) connect a zener diode available say 14.6v or below 14.6v between the base and -ve, put a resistor from collector to base near 470 ohm, you will get the required voltage from the emitter. Hope your problem is solved, dont forget to click the helpedme button below.
regards ani

- - - Updated - - -

Hi yassin.kraouch,
I forgot to tell you please connect the varying +ve voltage to the collector of 2n3055

 

[yassin.kraouch]

Hi yassin.kraouch,
Your problem is very simple, Use a series pass NPN transistor (say 2n3055) connect a zener diode available say 14.6v or below 14.6v between the base and -ve, put a resistor from collector to base near 470 ohm, you will get the required voltage from the emitter. Hope your problem is solved, dont forget to click the helpedme button below.
regards ani

- - - Updated - - -

Hi yassin.kraouch,
I forgot to tell you please connect the varying +ve voltage to the collector of 2n3055

Dear picgak, yes this is the solution i am trying to validate now, but there is many issues, available space, i have just 40mmx10 mm, also if i used MJD31CT4, but there is some thermal risk if the ambient temperature is 85°, i thought on putting two pass transistor in parallel but, i am worried about the imbalance current between the two pass transistor

 

[WimRFP]

First guess, dissipating 3W in 40mm*10mm PCB at 85 degrees ambient will result in too high junction temperature for Silicon. If the load has large capacitance on board, even the inrush peak may damage/destroy the transitor(s), especially after cycling power under worst case situation.

Using more smaller transistors (in DPAK ?) spread over the PCB area may result in acceptable junction temperature. It may also save you from second breakdown issues (check the DC SOA). If you can accept around 0.7V drop, you can use PNP transistors with current sharing resistors. 300mV across each resistor is sufficient. You can buy so called "low VCEsat" transistors that I used for similar applications (Diodes/zetex, NXP (former Philips), etc). Your MJD31/ MJD32 looks good, only the hfe is low. Maybe you need two of them.

If the dissipation is a problem, you have to go to a switching solution.

 

[yassin.kraouch]

First guess, dissipating 3W in 40mm*10mm PCB at 85 degrees ambient will result in too high junction temperature for Silicon. If the load has large capacitance on board, even the inrush peak may damage/destroy the transitor(s), especially after cycling power under worst case situation.

Using more smaller transistors (in DPAK ?) spread over the PCB area may result in acceptable junction temperature. It may also save you from second breakdown issues (check the DC SOA). If you can accept around 0.7V drop, you can use PNP transistors with current sharing resistors. 300mV across each resistor is sufficient. You can buy so called "low VCEsat" transistors that I used for similar applications (Diodes/zetex, NXP (former Philips), etc). Your MJD31/ MJD32 looks good, only the hfe is low. Maybe you need two of them.

If the dissipation is a problem, you have to go to a switching solution.

what do you think about parallel transistor solution ? will this cause a current imbalance ?

 

[WimRFP]

Using more smaller transistors (in DPAK ?) in parallel spread over the PCB area will reduce junction temperature. When you use emitter resistors and accept about 300mV additional drop, you don't have to worry about current imbalance.

So when you can accept some voltage drop, current imbalance isn't a problem.