Resistor in series with voltage regulator

beard_the_barman's picture


Joined: Apr 24, 2020

Replies: 0
Resistor in series with voltage regulator
April 24, 2020 - 12:01pm

I need to reduce the voltage at the input of the L7812 in order to reduce the power dissipation on this component. I am using as power supply a 18Vac transformer, 8A bridge rectifier and 3x 4700uF/35V. The minimum average voltage at the output of the filter capacitor (3x 4700uF) when using the soldering station is about 20V. 

I think about a resistor connected after D1 (in series with D1, on the K of D1) in order to reduce the voltage at the input of the 7812. The problem is that I cannot mount the regulators on a bigger heatsink, because I don't have any more space in the case and this is why I am asking about introducing a resistor.

I calculated that using a 33R/2W resistor should work. The average current through 7812 is about 0.12A.

I am waiting for your opinion. Thank you. 



Sourav Gupta's picture


Joined: Feb 12, 2018

Replies: 696

The resistor will start to dissipate heat and the transformer will unable to provide proper current during full load condition of the 7812. It is a really bad design tacticts and even the resistor will burn out after several days or hours.

At this situation, the best way is to change the transformer and use some thing like 15VAC output with the same rating. The transformer size will be small, the price will be lesser.

jaksonlee's picture


Joined: Nov 07, 2019

Replies: 124

The voltage drop of the resistor is result of a voltage divider between the resistor and the linear regulator. If the input voltage is 17v and the Linear voltage regulator is a 5v regulator, then that leaves 12 to spread between the resistor and the LVO. The LVO can said to have the resistance of its load.

Technical Shahzad's picture


Joined: May 23, 2020

Replies: 48

If there is only the issue of space, then you can use SMD type of regulators available in market, you can check from your local electronics market for getting 7812 regulator in SMD version. Other thing you can use a zener diode of your required rating and high power. The resistor will work in start but with passage of time its accuracy will reduce and after some time it'll burn out, which may affect your process. Here I would like to suggest you to get connect with the Sierra Software Ltd. platform technical support team for getting better assistance regarding your queries and project support.