Adjustable transistor load

Guest
Hello I have started a project to use a high powered transistor, to
draw an adjustable load from a power supply, (AC-DC Power supply)Id
ideally like to create a circuit that can load a supply of about 100W
from 3-60V, I am trying to keep the circuitry as simple as possible to
minimise EMC noise. Any websites or circuit suggestions would be very
much appreciated!
Thanks Dan
 
<your.mum@ntlworld.com> wrote in message
news:1110373629.451238.137270@o13g2000cwo.googlegroups.com...
Hello I have started a project to use a high powered transistor, to
draw an adjustable load from a power supply, (AC-DC Power supply)Id
ideally like to create a circuit that can load a supply of about 100W
from 3-60V, I am trying to keep the circuitry as simple as possible to
minimise EMC noise. Any websites or circuit suggestions would be very
much appreciated!
Click to expand...
How about a bridge rectifier with the transistor as a variable active load
across the output?

Leon
--
Leon Heller, G1HSM
http://www.geocities.com/leon_heller
 
I am a fairly intermediate experienced technician, an apprenticeship in
england. Could you talk me briefly how that would work? I have dealt
with Bridge rectifiers before as tyhey are used in our power supply
circuits and produces a continuous positive trace. I have used a LM317
voltage regulator to control the transistor. as I am dealing with high
power transistors they are getting rather hot. I am using a large
heatsink but now need to include some feedback to counteract the losses
due to heat. Thank you for the alternative idea though its something
to include in my project.

Dan
 
your.mum@ntlworld.com wrote:
Hello I have started a project to use a high powered transistor, to
draw an adjustable load from a power supply, (AC-DC Power supply)Id
ideally like to create a circuit that can load a supply of about 100W
from 3-60V, I am trying to keep the circuitry as simple as possible to
minimise EMC noise. Any websites or circuit suggestions would be very
much appreciated!
Click to expand...
Do you mean a supply with a dc output, and you want a controllable
current sink?
You can make a controllable current sink with an op-amp and a power
transistor. Connect the opamp output into the base of the transistor
(through an appropriate resistor), and connect the emitter of the
transistor to ground through a current sensing resistor (say, 0.1 ohm).
Then hook the junction of the emitter and the resistor to the inverting
input of the opamp, and apply a voltage to the non-inverting input as
your demand, or current-setting input. The opamp output will increase
until the current sense resistor has the same voltage as the
non-inverting input. So, 1 volt in for 10 amps out, 0.1 volt for 1 amp.
It should also work with a power fet as the current sink, mutatis
mutandis. You should take steps, like the opamp output resistor and
shunt diodes, to protect the opamp from excessively high voltages.

--
john
 
your.mum@ntlworld.com wrote:
Hello I have started a project to use a high powered transistor, to
draw an adjustable load from a power supply, (AC-DC Power supply)Id
ideally like to create a circuit that can load a supply of about 100W
from 3-60V, I am trying to keep the circuitry as simple as possible to
minimise EMC noise. Any websites or circuit suggestions would be very
much appreciated!
Thanks Dan
Click to expand...
As always, depends on what you're trying to accomplish.
What's your definition of load? Watts is the only unit of measure you
mentioned. Do you want constant wattage load? Constant current load?
Constant resistance load? Other kind of load?

"AC-DC Power supply"???? Do you want to load an AC source? Depending
on what kind of load characteristics you want, this can become complex.

What about transient response. Do you change the load? How fast does
it have to react? Does the source change? How fast does the load have
to react? What level of accuracy/repeatability do you need? What do
you want to happen when some limit parameter is exceeded? Like under 3V
or over 60V? Fuse blows? Source voltage has AC component? Why do you
want to minimize EMC noise? Minimize doesn't help much. If it matters,
it has a number(s) and a procedure to measure it. What are the numbers?

FWIW, for DC power supply testing, I use a transient load that can slew
almost an amp/nanosecond. Turns up all sorts of power supply layout and
transient problems. For simple fixed current load, a Dynaload brand
linear load works.

Your specs are challenging. 100W @ 3V is 33A.
Whatever you do, you have to dissipate the heat somewhere.
The 3V limit means that you can't do much with resistors unless you
switch them which affects transient response.
The 60V limit means that you have to be careful about secondary
breakdown if you use bipolar transistors. Plan on using MANY more
transistors than you'd think from the wattage requirement. Yes, I know
that 100W @ 60V is not much current, but if you count on this, make sure
you include electronic protection to make sure you don't have high
current transients at high voltage that blast your transistors. It's
probably cheaper and more reliable to use more transistors than to
employ more sophisticated protection. And when you think about it,
you're more likely to want more than 1.6A at 60V anyway. And put a
circuit breaker to protect against catastrophic failure. Board layout
or wiring paths matter! You have to make sure the current goes where
you intend.

Once built a charge dump load for a wind generator. Used a PIC with
a single 0-11Amp switching load and saturated FETs to switch
in BIG resistors to change ranges 10-20A, 20-30A etc. Just measured the
voltage and ramped the PWM up and down to keep the voltage below the
target. Fixed voltage made it easier.
Transient response was horrible, but ok for this application.

All depends on what you're trying to accomplish. 95% of the design is
trivial. It's the last 5% that nobody told you about that is gonna bite
you in the...anybody who tells you it's 100% trivial or that you only
need an op-amp and a transistor has never built one.
One size does not fit all unless you have unlimited funds.
More complete specification can simplify your task...assuming you really
don't need much.
mike

--
Return address is VALID but some sites block emails
with links. Delete this sig when replying.
..
Wanted, PCMCIA SCSI Card for HP m820 CDRW.
FS 500MHz Tek DSOscilloscope TDS540 Make Offer
Wanted, 12.1" LCD for Gateway Solo 5300. Samsung LT121SU-121
Bunch of stuff For Sale and Wanted at the link below.
MAKE THE OBVIOUS CHANGES TO THE LINK
ht<removethis>tp://www.geocities.com/SiliconValley/Monitor/4710/
 
<your.mum@ntlworld.com> wrote in message
news:1110373629.451238.137270@o13g2000cwo.googlegroups.com...
Hello I have started a project to use a high powered transistor, to
draw an adjustable load from a power supply, (AC-DC Power supply)Id
ideally like to create a circuit that can load a supply of about 100W
from 3-60V, I am trying to keep the circuitry as simple as possible to
minimise EMC noise. Any websites or circuit suggestions would be very
much appreciated!
Thanks Dan
Click to expand...
Here are a few ideas for design from magazines that will get you started in
a path that may get you what you want. You didn't say whether your load
should be constant power, constant current, constant resistance, etc. Those
types of loads require different techniques to get you there, so maybe if
you could be more specific about how you want to test the power supplies,
maybe someone can be more specific with a solution for you.

http://www.elecdesign.com/Articles/Index.cfm?AD=1&ArticleID=6415
http://www.elecdesign.com/Articles/Index.cfm?AD=1&ArticleID=4056
http://www.edn.com/archives/1998/031398/06di.htm

Additionally, ISTR an article in Nuts & Volts magazine that described an
active power supply load.. I'll see if I can dig it up and scan it if
you're interested. I think it was a constant resistance load.

--
Dave M
MasonDG44 at comcast dot net (Just subsitute the appropriate characters in
the address)

Never take a laxative and a sleeping pill at the same time!!
 
Hi I work in a power supply design business. When we test for emc
radiation we connect the units outputs to sets of resistive loads, to
draw the correct amount of power from each of the outputs. I am trying
to build a circuit that will achieve the same but can be used on a
wider variety of outputs. once the circuit is connected it is left
running. no transients are applied. I was hopin this would be a
simple project but have become lost along the way. Thank you for your
help Dan
 
<your.mum@ntlworld.com> schreef in bericht
news:1110373629.451238.137270@o13g2000cwo.googlegroups.com...
Hello I have started a project to use a high powered transistor, to
draw an adjustable load from a power supply, (AC-DC Power supply)Id
ideally like to create a circuit that can load a supply of about 100W
from 3-60V, I am trying to keep the circuitry as simple as possible to
minimise EMC noise. Any websites or circuit suggestions would be very
much appreciated!
Thanks Dan
Click to expand...
Dan,

Elector ever published an active controllable load. Bur it's some time ago
and I don't have it at my hands at the moment.

petrus bitbyter
 
You must log in or register to reply here.

Welcome to EDABoard.com

Sponsor

Back
Top