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'[EE] inexpensive adjustable 0-12A current sink?'
2011\09\30@130150 by doug metzler

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I'm thinking something benchtop with an analog ammeter and a big set
dial that I can use to test FET heat under different load conditions.

I did some googling but I only found babies - 0-1A.

Do you guys know of anything that might fit the bill?  Unfortunately
cost is a large consideration right now, but high accuracy is not
necessary.

for my purposes I'll be testing with 15VDC

Thanks,

Doug

2011\09\30@133018 by Mike Hord

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Well, bear in mind that you can parallel several FETs to share the heat
dissipation.

15A is a lot, though, at 15VDC. That's, what, 225W? I made one recently to
do
markedly less (only 2A), using a couple of our bigger heat sinks along with
a
couple of FETs:
http://www.sparkfun.com/products/10213
http://www.sparkfun.com/products/9576

There should be a couple of tutorials hitting our website soonish- one
covering the
heat sinking needs and another covering the actual design of the circuit.
When they
hit I'll post to this thread.

MikeH

On Fri, Sep 30, 2011 at 11:01 AM, doug metzler <spam_OUTdoug.metzlerTakeThisOuTspamgmail.com>wrote:

{Quote hidden}

>

2011\09\30@133054 by jim

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Use a couple of 2N3055 Bipolar Power Transistors on a good heatsink.
Put the Collector on the PLUS and the Emitter on the GND, and Bias the  BASE relative to the Emitter.  With an Ammeter in series with the load,

you can tell when you have reached the 12A level.  The transistors will get warm, but they can handle it. These
transistors  can handle 15 amps continuous.  You could get by with just one if the
load
isn't left on for a long period of time.

Regards,

Jim

> ---{Original Message removed}

2011\09\30@133604 by M.L.

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On Fri, Sep 30, 2011 at 1:30 PM,  <.....jimKILLspamspam@spam@jpes.com> wrote:
>
>  Use a couple of 2N3055 Bipolar Power Transistors on a good heatsink.
>  Put the Collector on the PLUS and the Emitter on the GND, and Bias the
>  BASE relative to the Emitter.  With an Ammeter in series with the load,
>
>  you can tell when you have reached the 12A level.
>  The transistors will get warm, but they can handle it. These
> transistors
>  can handle 15 amps continuous.  You could get by with just one if the
> load
>  isn't left on for a long period of time.
>
>  Regards,
>
>  Jim
>

I did just this recently. I made a current feedback circuit using an
opamp to control the base though.
If I were going to do it for real I would use a darlington PNP to
simplify the base drive requirements.

At several amps, bipolar transistor current gain drops off pretty fast.

--
Martin K.

2011\09\30@133618 by John Ferrell

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On 9/30/2011 1:01 PM, doug metzler wrote:
> I'm thinking something benchtop with an analog ammeter and a big set
> dial that I can use to test FET heat under different load conditions.
>
> I did some googling but I only found babies - 0-1A.
>
> Do you guys know of anything that might fit the bill?  Unfortunately
> cost is a large consideration right now, but high accuracy is not
> necessary.
>
> for my purposes I'll be testing with 15VDC
>
> Thanks,
>
> DougM
I have used Automotive head lamps in the past. They are kind of rubbery as far as controlling them but they served my purpose.

Also, a piece of wire works well if you monitor the temperature as you go.  Your 12v X 12A = 144 watts, not too hard to dissipate.


-- John Ferrell W8CCW

2011\09\30@134151 by V G

picon face
On Fri, Sep 30, 2011 at 1:01 PM, doug metzler <doug.metzlerspamKILLspamgmail.com>wrote:

> I'm thinking something benchtop with an analog ammeter and a big set
> dial that I can use to test FET heat under different load conditions.
>
> I did some googling but I only found babies - 0-1A.
>
> Do you guys know of anything that might fit the bill?  Unfortunately
> cost is a large consideration right now, but high accuracy is not
> necessary.
>
> for my purposes I'll be testing with 15VDC
>

I designed this a few weeks ago:

http://solar-blogg.blogspot.com/2011/09/constant-current-sourcesink.html

You can add as many MOSFETs as you like. Can sink a LOT of current. Play
with the simulation yourself. Let me know if you want the LTSpice simulation
file

2011\09\30@135811 by doug metzler

picon face
I like the idea of nichrome wire on a very large heatsink with a
series of FETS - I've got a bunch of IRF530's in stock - I'll drive
those against the nichrome on the same large heatsink (I've got a
stack of old CPU sinks around)

Then just run down to the auto parts store and get an ammeter - sounds
like a great little weekend project!

So no inexpensive off-the-shelf boxes that you guys know of?

Thanks!

DougM

On Fri, Sep 30, 2011 at 10:40 AM, V G <.....x.solarwind.xKILLspamspam.....gmail.com> wrote:
{Quote hidden}

>

2011\09\30@145655 by Steve Smith

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We use Torkel 840's.... they do about 10Kw programmable form about 10v to
220v dc..!

Steve

-----Original Message-----
From: piclist-bouncesspamspam_OUTmit.edu [@spam@piclist-bouncesKILLspamspammit.edu] On Behalf Of
doug metzler
Sent: 30 September 2011 18:02
To: Microcontroller discussion list - Public.
Subject: [EE] inexpensive adjustable 0-12A current sink?

I'm thinking something benchtop with an analog ammeter and a big set
dial that I can use to test FET heat under different load conditions.

I did some googling but I only found babies - 0-1A.

Do you guys know of anything that might fit the bill?  Unfortunately
cost is a large consideration right now, but high accuracy is not
necessary.

for my purposes I'll be testing with 15VDC

Thanks,

Doug

2011\09\30@152636 by Spehro Pefhany

picon face
At 01:58 PM 30/09/2011, you wrote:
>I like the idea of nichrome wire on a very large heatsink with a
>series of FETS - I've got a bunch of IRF530's in stock - I'll drive
>those against the nichrome on the same large heatsink (I've got a
>stack of old CPU sinks around)
>
>Then just run down to the auto parts store and get an ammeter - sounds
>like a great little weekend project!
>
>So no inexpensive off-the-shelf boxes that you guys know of?
>
>Thanks!
>
>DougM

Depends on what you mean by 'inexpensive', but such boxes
start at around $400 (maybe less). We have a nice little 300W digital
one that cost less than $800- it can simulate constant current,
constant power, constant resistance, you can program it remotely
to look at PSU transient responses etc,

Cheap like borscht- we're mostly done with fiddling around with power
resistors, heatsinks, MOSFETs and such like every time we want to
test a battery or reasonably hefty power supply.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
KILLspamspeffKILLspamspaminterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

2011\09\30@152904 by Roy

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Hi VG,

Can you either post or send LT Spice file to RemoveMEroy.hopkinsTakeThisOuTspamxtra.co.nz

Thanks from Roy

-----Original Message-----
From: spamBeGonepiclist-bouncesspamBeGonespammit.edu [TakeThisOuTpiclist-bouncesEraseMEspamspam_OUTmit.edu] On Behalf Of
V G
Sent: 1 October 2011 6:41 a.m.
To: Microcontroller discussion list - Public.
Subject: Re: [EE] inexpensive adjustable 0-12A current sink?

On Fri, Sep 30, 2011 at 1:01 PM, doug metzler <RemoveMEdoug.metzlerspamTakeThisOuTgmail.com>wrote:

{Quote hidden}

I designed this a few weeks ago:

http://solar-blogg.blogspot.com/2011/09/constant-current-sourcesink.html

You can add as many MOSFETs as you like. Can sink a LOT of current. Play
with the simulation yourself. Let me know if you want the LTSpice simulation
file

2011\09\30@155539 by doug metzler

picon face
By inexpensive I was thinking about $100.

VG, I am going to base my design on your schematic - I'm just modelling it now.

Thank you very much you are running significant beer credits :-)

DougM

On Fri, Sep 30, 2011 at 12:29 PM, Spehro Pefhany <speffEraseMEspam.....interlog.com> wrote:
{Quote hidden}

>

2011\09\30@190132 by RussellMc

face picon face
> I like the idea of nichrome wire on a very large heatsink with a
> series of FETS - I've got a bunch of IRF530's in stock - I'll drive
> those against the nichrome on the same large heatsink (I've got a
> stack of old CPU sinks around)

If you don't mind having mean current equal to target current  rather
than steady current you can use PWM with a resistor load. That plus a
suitable FET gives very little dissipation in the FET.

You can use air wound nichrome wire as the load and so need no
heatsinking at all for the resistor.

If you must have steady current, add a suitable series inductor and a
catch diode and you have a buck converter.

I have extensively  used PWM + an air wound  Nichrome resistor +
MOSFET up to 5000 Watts with good effect. This was the load for an
alternator that was DC rectified and then had suitable filter
capacitors added so the capacitors saw the PWM ripple while the
alternator saw the relatively steady capacitor voltage as load. Worked
very well.

Nichrome is normally encountered as either tape in toaster elements or
as a relatively thin wire for AC mains use. This is usable at low
voltage with suitable numbers of strands in parallel. However, much
thicker wire is available from electrical suppliers allowing the
construction of robust high power load resistors at lower voltages.
Wire can be wound in a spiral and supported occasionally to make an
open wire resistor.


           Russell McMaho


'[EE] inexpensive adjustable 0-12A current sink?'
2011\10\01@091713 by Spehro Pefhany
picon face
At 07:00 PM 9/30/2011, you wrote:
>Nichrome is normally encountered as either tape in toaster elements or
>  as a relatively thin wire for AC mains use. This is usable at low
>voltage with suitable numbers of strands in parallel. However, much
>thicker wire is available from electrical suppliers allowing the
>construction of robust high power load resistors at lower voltages.
>Wire can be wound in a spiral and supported occasionally to make an
>open wire resistor.

If you happen to have some type K or E thermocouple wire kicking around,
the Chromel '+' wire is pretty close to Nichrome (90Ni10Cr vs. 80Ni20Cr).
The Constantan wire in J type is also useful as it has a low tempco.
All are non-magnetic.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
RemoveMEspeffEraseMEspamEraseMEinterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

2011\10\01@092357 by Electron

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I think it's easy at surplus locations to find batches of power resistors.
E.g. I got one (almost for free) containing one thousand 5W 100ohm resistors.
I still haven't found the time, but I'd like to make a programmable sink
with them, some mosfets and a PIC.

Basicly I'd first select the resistors to get the best precision possible,
then I'd group some in power of 2's, and (through mosfets) toggle them
with the PIC.

i.e. like PIC bit 0 turns on mosfet 0 which puts to ground a 400 ohm
resistor (just 4 of them in series). PIC bit 1 turns on mosfet 1 which
puts to ground 200 ohm of power resitors. PIC bit 2 turns on mosfet 2
which puts to ground 100 ohm. PIC bit 3 turns mosfet 3 which puts to
ground 50 ohm (2 100 ohm resistors in parallel), on And so on, for say
12 bits.

Then the PIC would measure current, and activate the power resistors
network accordingly to cause the wanted current or power sink.

Greets,
Mario


At 01.00 2011.10.01, you wrote:
{Quote hidden}

>

2011\10\01@122408 by doug metzler

picon face
Two more great ideas - there is an electronics surplus place in town -
I was thinking of going there to see if they had a suitable ammeter so
I'll pick up some big wirewound resistors as well.

and the second idea is to put a current sense resistor on it - even
though I don't need it for the rough measurements I'm making now later
on I could add a microcontroller as a feedback mechanism.  Then of
course using a system similar to Mario's (digital potentiometer?) make
it programmable.

Today, however, get the basics done.

Thanks all, keep the suggestions coming!

DougM

On Sat, Oct 1, 2011 at 6:23 AM, Electron <RemoveMEelectron2k4spam_OUTspamKILLspaminfinito.it> wrote:
{Quote hidden}

>>

2011\10\01@153552 by Dwayne Reid

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At 07:23 AM 10/1/2011, Electron wrote:

>I think it's easy at surplus locations to find batches of power resistors.
>E.g. I got one (almost for free) containing one thousand 5W 100ohm resistors.
>I still haven't found the time, but I'd like to make a programmable sink
>with them, some mosfets and a PIC.

Metal Oxide 5W resistors work out nicely - they range from completely non-inductive to somewhat (barely) inductive.

I tend to choose lots of higher-value resistors, then put them in parallel as needed.  Putting several of a resistor in series, then putting that series string in parallel with a smaller number of resistors limits the power dissipation to the weakest element, which can be as low as a single resistor.

For example, choosing 100R resistors would limit my load rating to about 22Vdc - not high enough for me.  A better choice would be 470R.  Or, in the case given by Electron (above), make each resistor out of several 100R resistors in series.  5- 100R resistors in series would make 500R @ 25W.  That's good up to about 113V continuous - usable for most anything that I might need.

Using all of those resistors in that fashion (200 strings of 5) would give you almost a full 8 bits of resolution, all the way from about 2R5 through 500R with a maximum working voltage of about 113V (continuous) and a maximum power rating of 5kW at the lowest resistance setting.

Eminently reasonable, actually.

One of the many advantages of choosing Metal Oxide resistors is their extremely high tolerance to severe overload without failing or changing resistance.  I've run them red hot for several hours just to see how they behaved and was pleasantly surprised at their stability.

dwayne

-- Dwayne Reid   <RemoveMEdwaynerTakeThisOuTspamspamplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax
http://www.trinity-electronics.com
Custom Electronics Design and Manufacturing

2011\10\01@232600 by doug metzler

picon face
Ok, I hit the surplus place this morning, got a 10A analog ammeter for
$10 and came up with 4@0.75r resistors with no wattage rating, but
they're big old canister things about 5/8 * 3/4 * 3" that the
gentleman very kindly gave me for $2.95 ea.  I hooked them up in
parallel.

I had some IRF1010's in stock 3 of which I bolted to an old CPU heat
sink and stuck a computer fan on it.

I was originally planning on running the FETS in the linear region but
I couldn't get a smooth enough ramp up via the potentiometer, so I
went to PWM and a small microcontroller, but probably could have done
it with a 555.

It works quite well though even with the big heat sink the fets get
surprisingly hot when I'm running it right at 10A (the resistors get
pretty warm).  But I cooked it for quite a while and nothing exploded,
so I think we're ok.

I also happened to have an ACS711 kicking around that I'd been meaning
to play with so I stuck that on and fed it back to the
micro-controller as an analog input since as the fets heat up the
current definitely creeps.  I'm not sure I'll go so far as to
implement a PID controller but I'll definitely code in a little
correction.

But on the whole for a Saturday afternoon not too bad.

Thank you for your help.

DougM


On Sat, Oct 1, 2011 at 12:35 PM, Dwayne Reid <EraseMEdwaynerspamspamspamBeGoneplanet.eon.net> wrote:
{Quote hidden}

>

2011\10\02@024930 by V G

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On Sat, Oct 1, 2011 at 11:26 PM, doug metzler <doug.metzlerSTOPspamspamspam_OUTgmail.com>wrote:

> I was originally planning on running the FETS in the linear region but
> I couldn't get a smooth enough ramp up via the potentiometer, so I
> went to PWM and a small microcontroller, but probably could have done
> it with a 555.
>

That region seems to be very tight. I don't know if you can do it with a
capacitor and PWM reliably. I think and opamp is the only way to go with an
analog feed into the opamp

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