Simple shunt regulator issue

Hvlemmi

Member
2014-07-16 11:21 pm
Hello,

I am building a shunt regulator and I have encountered an issue with it. Let me first give the proper information before I go into the details and the issue;

https://cdn.xingosoftware.com/audioxpress/images/fetch/dpr_1,w_765,h_574,c_fit/https%3A%2F%2Fwww.audioxpress.com%2Fassets%2Fupload%2Fimages%2F1%2F20180624192648_Figure3-WaagboShuntorNot.jpg

I am building the top regulator named 'Simple Shunt Regulator'. Since I wasn't able to source the 2sJ200/2sJ201, I have used the IRFP9240.

I am using a resistor as load (I tried values between 3 and 100 ohm ).

I am using a 5v1 Zener.



Before the regulator I have used a bridge rectification and a 3300uf cap.

When I am using the 27r and test the output voltage, I get 0.3V. When I changed the 27r to 5r1 I get 0.5v. It's a change, but obviously not what I have in mind. In both situations the resistor becomes pretty hot.

I imagine it could be the load being too weak, but I thought it would be good to have a few external eyes taking a peek on what I'm doing wrong. I am looking forward and obviously am thankful for external feedback on this issue.
 

arpagon

Member
2011-10-29 12:25 pm
IRFP9240 is quiet different than 2sJ200 but it's not the case at the moment. You should measure the voltage at the output of the bridge rectifier first without load and then when it is loaded with the regulator.
The resistor in the input of regulator (27R/5W on schematic) defines the overall current drawn from the rectifier but in this so simple schematic the current depends on the regulator's input voltage and can be calculated as follows:
I = (Uin - Uout)/R
 

Hvlemmi

Member
2014-07-16 11:21 pm
Thank you for the reply!


The transformer I use produces 14v unloaded.
With the load (and input resistor value of 5R1) I get 8.3V on rectification and I measure 0.07V at the shunt output.
With the load (input resistor of 27R) I get 14V and I measure something like 0.5v on the shunt output.
Am I simply getting the load resistor wrong? I want to set the shunt on 1000-1500mA for LT3045 which accepts for 5-15v so my target window is like 5-10v.
I did try to calculate the resistance hence I came out on 5R, but obviously it doesn’t come out that way. Am I getting the load resistance wrong?
 

Hvlemmi

Member
2014-07-16 11:21 pm
Mark, thanks. I have searched the net and found these as possible replacements. What can you recommend as a proper alternative?

Jan, thanks for the reply. I've did some reading on things you did before I started building the shunt.
I am getting a voltage of 0.74/0.75 across the zener. I am using a 5.1V zener, the JANTXV1N751A to be precise. I have triple-checked the direction, and it is definitely correct. Could I have possibly pan-fried the zener while I ran it too hot?
 
Some problems with this circuit:
1. There is nothing to limit the (transient) Zener current and the BC549 and zener has probably been destroyed as a result.
2. The vto of the IRFP9240 MOSFET is about 4 Volts so it is not going to conduct/ shunt with less than ~5V.
But I would guess that you also have wiring errors as well, such as a backwards zener.

Add a resistor to the base of the BC549, test all parts are still good, use a DMOS or "lateral" FET or BJT.
 
Last edited:

Hvlemmi

Member
2014-07-16 11:21 pm
Some problems with this circuit:
1. There is nothing to limit the (transient) Zener current and the BC549 and zener has probably been destroyed as a result.
2. The vto of the IRFP9240 MOSFET is about 4 Volts so it is not going to conduct/ shunt with less than ~5V.
But I would guess that you also have wiring errors as well, such as a backwards zener.

Add a resistor to the base of the BC549, test all parts are still good, use a DMOS or "lateral" FET or BJT.

Thanks, what would the value of this base resistor be?
On the point of the mosfet, what would you recommend? A different mosfet, if so which one?

The biggest wiring issues definitely are to find in the top of my head, I'm doing my best to learn and rearrange that wiring though.


Edit: I have a spare BC549, also a spare zener (perhaps its a good idea to get some extra, to keep the good parts out until I get everything to work).
 
Last edited:
Thanks, what would the value of this base resistor be?
On the point of the mosfet, what would you recommend? A different mosfet, if so which one?

The biggest wiring issues definitely are to find in the top of my head, I'm doing my best to learn and rearrange that wiring though.


Edit: I have a spare BC549, also a spare zener (perhaps its a good idea to get some extra, to keep the good parts out until I get everything to work).

A base resistor of about 100 Ohms would limit the fault condition current to about 100mA.
The FET is a problem because "lateral" FETs with low gate voltages are going away. A popular number was 2SJ162. I looked in the selector guides at onsemi and the only such parts where small smt. You may be best to switch to a BJT or rearrange the circuit so that ~10VDC is available to the gate. You should also consider that N channel devices are a better variety, cheaper, and more available, so it would be better to invert the shunt to use a PNP and a N-channel FET. A big part of designing electronic circuits is knowing and working with common available parts.

But I think as a beginner you should not try to make anything special like your shunt regulator. There are plenty of simple 5V regulators.
 
The series R is 27R. A 3R load will never give more Vout than ~1/10th of Vin...
A 103R load will max give ~3/4th of Vin as Vout.
You should initially check this out without any load or with a very light load like 1k.

Basic check: if you disconnect everything after the zener and bias resistor, what do you then see across the zener?

Jan