Electronic Load For Checking Power Supplies

[Joe]

"Electronic Load For Checking Power Supplies" - this project appeared in
Silicon Chip May 1990. If someone has that particular magazine, could they
tell me please, whether this load circuit is self powered by the power
supply on test, or whether it needs a battery/power supply to run it.

I'd like to know that it is self powered before I send off for a reprint to
Silicon Chip.

Thanks for your help.

Joe.

 

[Steve]

"Electronic Load For Checking Power Supplies" - this project appeared in
Silicon Chip May 1990. If someone has that particular magazine, could they
tell me please, whether this load circuit is self powered by the power
supply on test, or whether it needs a battery/power supply to run it.

There was also a 50W one in Sep 2002, from memory this one could be self powered.

nifty

 

[Joe]

"Joe" <[email protected]> wrote in message

There was also a 50W one in Sep 2002, from memory this one could be self powered.
nifty

Yes Steve I have that issue, but unfortunately it needs a 9 volt battery or
power supply to run it. The current is only about 390 microamps but I have
no idea how long an alkaline 9 volt battery would last and what happens when
the battery gets low. Does it just stop working or does the display show an
inaccurate reading for instance.

I have to build a few of these for work and I'm sure the other guys would
never remember to check the battery!

Thanks for your reply.

 

[Joe]

"Joe" <[email protected]> wrote in message

Greetings Joe.
I have that issue, and I know someone who built one.
He is very happy with it.

The unit is powered from the battery under test.
A meter shows the current drain.
A multiturn pot controls the base current to 2 2N3055 transistors.
Each of these has 7 3.3 ohm 5W resistors in par. to ground.

Bye.
Russell Griffiths

Very good Russell, that sounds very much like one at we have work that
someone made up years ago.

It has 7 x 3.ohm resistors in the emitters to ground, and the bases of the
transistors are connected to the centre of a resistive voltage divider
across the battery under test. The divider consists of a 10k multi-turn pot
and two fixed resistors. A 3 or 5 amp ammeter is in series with the
collectors.

Is that similar to your circuit?

Yes it does work well, but it has the disadvantage, that as the voltage on
the battery on test drops, the current also drops, so that at 12 volts with
1 amp drain, the current falls to 600 to 700 milliamps when the voltage
drops to 9 volts.

I need to build some dummy loads for my workplace that are constant current,
regardless of voltage. The circuit in Silicon Chip Sep 2002 does just this
by monitoring the voltage across the load resistors and then controlling the
output transistor (FET). Unfortunately no supplier I could find has this as
a kit, and secondly it needs a separate supply to run it which makes it a
bit messy for a commercial application.

I do have a circuit from the late 1980's that appeared in Electronics Today
which also monitors the voltage drop, so it should work for me as a constant
current load, and it is self powered as well.

Thanks for your help.

 

[Franc Zabkar]

Very good Russell, that sounds very much like one at we have work that
someone made up years ago.

It has 7 x 3.ohm resistors in the emitters to ground, and the bases of the
transistors are connected to the centre of a resistive voltage divider
across the battery under test. The divider consists of a 10k multi-turn pot
and two fixed resistors. A 3 or 5 amp ammeter is in series with the
collectors.

Is that similar to your circuit?

Yes it does work well, but it has the disadvantage, that as the voltage on
the battery on test drops, the current also drops, so that at 12 volts with
1 amp drain, the current falls to 600 to 700 milliamps when the voltage
drops to 9 volts.

Why not feed the divider with a zener regulated source, or a 5V
3-terminal regulator?

I need to build some dummy loads for my workplace that are constant current,
regardless of voltage. The circuit in Silicon Chip Sep 2002 does just this
by monitoring the voltage across the load resistors and then controlling the
output transistor (FET). Unfortunately no supplier I could find has this as
a kit, and secondly it needs a separate supply to run it which makes it a
bit messy for a commercial application.

I do have a circuit from the late 1980's that appeared in Electronics Today
which also monitors the voltage drop, so it should work for me as a constant
current load, and it is self powered as well.

Thanks for your help.

- Franc Zabkar

 

[tony]

Hi,

Can you get the source to recharge the internal battery to take the
place of an internal Alkaline?? Or if the current draw is that small,
how about a large capacitor??

Tony

 

[Joe]

Hi,

Can you get the source to recharge the internal battery to take the
place of an internal Alkaline?? Or if the current draw is that small,
how about a large capacitor??

Tony

The current draw from the battery in the Silicon Chip Sept 2002 is 0.39ma
(from memory) so an alkaline 9 volt battery should last months. More than
enough for a home hobbist but not for enough for a commercial application.

The article doesn't specify what voltage is required for the ICs in that
circuit.

I'm going to use the simpler circuit of a 2N3055, load resistor with a 5
volt regulator to control the variable bias on the base. I've tried it and
its stable enough for my purpose.

Joe