DC source that outputs a constant current also the useful load is variable

Thread Starter

simplex1

Joined May 2, 2024
11
Fig. 1 shows, in a simplified way, a DC power source that unfortunately can only work if the current drawn from it is constant and has a given value "I", otherwise this source will wear quickly due to its construction.

This source has to feed a useful load that varies in time. I guess that, in order to keep the current "I" constant, I have to use a microcontroller controlled dump load that, based on the measured current drawn by the useful load, will absorb such a current as to keep "I" constant.

Question: Can you suggest a schematic for the dump load?

Variable load - constant current.png

Fig. 1. Variable useful load and DC power source that only works at a constant voltage and current.
 

LowQCab

Joined Nov 6, 2012
4,312
I've never heard of a "Dump-Load" ........

A Constant-Current-Regulator is an easy job.

Some designs are more efficient than others.

If You would describe, in detail, your secret "Useful-Loads" and the surrounding circumstances
You are more likely to get a reply that suits your situation the best.
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Thread Starter

simplex1

Joined May 2, 2024
11
The power source is a helicopter gas turbine (that has to work at constant speed/torque otherwise it wears quickly) that through a gearbox turns a high frequency electric generator. An AC to DC power converter outputs DC power that is taken by an ensemble of electric motors which makes the useful load.

However, please disregard this complication. Consider that the power source is like a battery obliged to deliver a constant current/voltage. Also consider that the battery is of reduced power like say 100 - 500 W.
 

WBahn

Joined Mar 31, 2012
30,303
Fig. 1 shows, in a simplified way, a DC power source that unfortunately can only work if the current drawn from it is constant and has a given value "I", otherwise this source will wear quickly due to its construction.

This source has to feed a useful load that varies in time. I guess that, in order to keep the current "I" constant, I have to use a microcontroller controlled dump load that, based on the measured current drawn by the useful load, will absorb such a current as to keep "I" constant.

Question: Can you suggest a schematic for the dump load?

View attachment 321483

Fig. 1. Variable useful load and DC power source that only works at a constant voltage and current.
This sounds like a highly-contrived problem, which leads me to suspect that it is some kind of school assignment. If that's the case, then highly-contrived problems are typical and fine. But let us know if that's the case, otherwise you are going to get all kinds of pushback about the premise of the problem. Also, if it's an assignment, you need to show your best effort thus far. What are your thoughts? What are the limits of what is even possible?
 

schmitt trigger

Joined Jul 12, 2010
961
Let me see if I understand, you would like to maintain a constant current loading on your power supply, regardless of the amount of power that the electrical motors are consuming?
 

Thread Starter

simplex1

Joined May 2, 2024
11
This is no homework. I just want to build a scaled down model of the real size device. I do not have a helicopter turbine - generator - power converter group and I will simulate it with a battery.

I suspect that the dump load has to be an RLC circuit connected and disconnected from the battery by an IGBT driven by a PWM signal generated by a microcontroller, but I do not know the exact schematic of this circuit. Once I have such a circuit I can likely dimension it for a given power.
 

Thread Starter

simplex1

Joined May 2, 2024
11
Why does it have to be microcontroller controlled? Why can't it just be an analog circuit?
I am familiar with Arduino boards and Atmega microcontrollers. It could be an analog circuit as you say. I do not know.

I thought of a microcontroller because it will allow me, amongst other things, to dynamically set the value of the current "I", if I see that for a long period of time the power dissipated by the dump load is too big.
 

AnalogKid

Joined Aug 1, 2013
11,209
The circuit structure you are looking for is called a shunt regulator. Usually, it is set up to provide a constant voltage across the load. In your case you need to insert a small resistance in series with the load to sense the current, then use that signal as an input to the regulator circuit.

ak
 

WBahn

Joined Mar 31, 2012
30,303
I am familiar with Arduino boards and Atmega microcontrollers. It could be an analog circuit as you say. I do not know.

I thought of a microcontroller because it will allow me, amongst other things, to dynamically set the value of the current "I", if I see that for a long period of time the power dissipated by the dump load is too big.
But if you change the current, doesn't that defeat the entire stated purpose, namely that if the generator produces anything other than a specific current that it will be quickly damaged?

My first thought is to put a current sense resistor in the bottom trace (so that it has all of the current flowing through it, but the load and the shunt current) and then use that to control a PNP-based current mirror that holds the voltage across the shunt resistor at one diode drop at the desired total current.

Depending on voltage, power level, and range of compliance required, thermal management might be quite a challenge.
 

Thread Starter

simplex1

Joined May 2, 2024
11
AnalogKid,

I understand that you suggest something like this:

A shunt regulator detects output voltage variation via external resistors by using an error amplifier and controls a transistor connected in parallel to the load to keep the output voltage constant. In other words, the characteristic of a shunt regulator is to minimize current variation of power supply (operation in the direction of minimizing variations) even if the load fluctuates. However, as a basic limitation of shunt regulators, they can not tolerate more fluctuations of current flowing in the load than the difference between the min. level and the max. level of the current flowing in the transistor.
The basic operation of a shunt regulator is described below using a simplified circuit.
When the output voltage drops due to a variation of the load, the voltage applied to resistors R1 and R2 drops. This then lowers the voltage of V1, which is the output voltage divided by R1 and R2. In other words, the input voltage of the error amplifier's non-inverted pin (+) is also lowered (below the internal reference voltage). As a result, the error amplifier causes the voltage applied to the base of transistor TR to drop, which suppresses the current flowing to the TR collector. This in turn raises the output voltage, stabilizing it.

Conversely, when the output voltage rises due to a variation of the load, V1 also rises, causing the error amplifier to raise the voltage of the TR base. This in turn increases the current flowing to the TR collector, which lowers the output voltage, stabilizing it. In other words, the shunt regulator operates to ensure that V1 is always equivalent to the internal reference voltage.
Source: https://en-support.renesas.com/knowledgeBase/16981013
 

Thread Starter

simplex1

Joined May 2, 2024
11
But if you change the current, doesn't that defeat the entire stated purpose, namely that if the generator produces anything other than a specific current that it will be quickly damaged?
Yes, you are right. However, in the case of the real helicopter gas turbine, there is no problem if the power generated is changed each say 10 minutes. The rapid wear appears when you change the above mentioned power each few seconds.
 

LowQCab

Joined Nov 6, 2012
4,312
After all these Posts,
I still don't know what the problem is that You are trying to solve.

You have a Model of a Helicopter,
OK, Fine.
What type of engine does it have if it doesn't have the usual Geared-Shaft-Output-Turbine ?

What is the average RPM of the Power-Take-Off that drives the Alternator ?

What is the complete, detailed description of the Alternator ?

Does it even have an Alternator ?,
or are all the Electrical-Accessories is this Model 100% Battery-Powered ?

If no Alternator, and 100% Battery-Power for the Accessories,
then there is no quandary here.
You need a Battery-Pack that produces at least ~5-Volts higher than the Voltage required by the Accessories,
when the Battery is in a fully discharged state,
and then a "Buck-Converter" Voltage-Regulator to keep the Voltage at a level that
the Accessories expect to be fed.

If You have a Permanent-Magnet, Unregulated-Alternator,
a different approach to Voltage-Regulation is required,
which can be very complex but efficient, or
very simple and crude, and very inefficient.

If You have a "Regulated-Alternator", with a controlled "Field-Winding" and Regulator,
then there should be no issues.

There are no other scenarios that I can imagine.

Please directly, and completely, answer the previous questions in this post
so that we all can get on to the same page.

It seems that You keep referring to "supposed" problems that just don't exist, or apply in this case.
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Thread Starter

simplex1

Joined May 2, 2024
11
Maybe the schematic in Fig. 2 is more clear. Basically what I want is that DC to DC convertor (preferably a high power one).

Drawing1.jpg

Fig. 2. The same configuration as in Fig. 1 above but with the Dump Load replaced by a DC to DC convertor with a fix dissipation resistor R connected at its output.
 

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MisterBill2

Joined Jan 23, 2018
19,473
One thing to consider at the very start is that a battery power source is vastly different from a turbine driven generator. A battery voltage will stay constant, or very close to constant, even at no load. so there is no need at all to keep a battery loaded to hold the voltage constant. If you have a 24 volt battery pack feeding many amps to a group of lift motors and suddenly switch all the motors off, the battery voltage will remain at 24 volts. This is totally different from an alternator or generator system in every aspect.

What this means is that the anticipated problem does not exist.
 

Thread Starter

simplex1

Joined May 2, 2024
11
What this means is that the anticipated problem does not exist.
I have already stated in my first post that what I shown as a battery is not a battery but a complicated DC power source that can only work well if the current extracted from it is constant. After that I explained what this DC source is, in reality.
 
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Thread Starter

simplex1

Joined May 2, 2024
11
Regulation to hold “I”=constant using op-amp:

The “I” is set with potentiometer.
A current throug T1 is always added to I2 to maintain I=constant.
Thank you for the schematic. Can you give an estimate of the level of power (watts, tens - hundreds of watts, more) that can be dissipated with that circuit?
 
Thank you for the schematic. Can you give an estimate of the level of power (watts, tens - hundreds of watts, more) that can be dissipated with that circuit?
It all depends what transistor and heatsink you use.
With 2n3055 in TO3 pakage and medium heatsink you can roughly count with 50W.
If 10pcs of it are connected parallel you are at 500W (a small driving transistors will be needed also).
 
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