Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

   Home   Help Search  
Pages: Prev 1 [2]   Go Down
  Print  
Author Topic: How to "test" a power supply?  (Read 3881 times)
W6EM
Member

Posts: 791




Ignore
« Reply #15 on: December 07, 2012, 07:06:20 PM »

You could try a hunk of 18AWG nichrome wire.  At about .4 ohms per foot, it might do the trick.  It's going to get hot, so you'd need a high temperature insulated form of some sort.  Perhaps, a glass bottle or test tube that you could use to wrap it around.  While a spent coke bottle might work, it's not pyrex, so be careful.

Here's a link to some on eBay:http://www.ebay.com/itm/Nichrome-resistance-wire-18-AWG-gage-10-feet-foam-cutting-sealing-heating-/320951837720?pt=LH_DefaultDomain_0&hash=item4aba386418

You won't be able to solder to it, so you'll have to use some type of clamp arrangement on each end to connect to it.  You could adjust the position of the clamps to get the desired load resistance and load current.

Lee
Logged
K8AXW
Member

Posts: 3825




Ignore
« Reply #16 on: December 07, 2012, 08:44:00 PM »

Be aware of the characteristic or problem with using wire for a power supply dummy load.  The resistance changes with heat.  Unlike a light bulb, wire will continue to change resistance until it is destroyed.  It would be good to monitor the current and keep the test brief.

Nichrome ribbon from a heater is good..... or just use an electric heater.

If you intend to test your power supply(s) periodically, it might be a good investment to build the variable dummy load for power supplies.  They're inexpensive to build.
Logged
K5LXP
Member

Posts: 4479


WWW

Ignore
« Reply #17 on: December 07, 2012, 09:02:52 PM »

The problem with "free air" loads made with wire is the resistance and thus current is going to be highly dependent on the temperature of the wire.  When you first clamp on the load the resistance will be low but will very quickly rise and reach an equilibrium point between electric power in and power radiated out as heat.  Even water cooled ones will show a resistance change as the water surrounding the wire warms.   At the power levels we're talking about here (20A@14V) there's a fair amount of heat to shed.  Some means of holding the wire and providing either forced or convected air past it to will be needed to get to where a constant load is possible.  I can't think of too many ways to create a passive load that won't be subject to some degree of temperature coefficient so thermal management is going to be the thing to figure out no matter what means of resistive element is used.

My EV battery pack load is two 3000W 220V water heater elements in parallel mounted in a steel pipe with hose fittings on the ends.  I connect a garden hose to cool it and can actually adjust the current a bit by changing the water flow and thus element temperature.


Mark K5LXP
Albuquerque, NM
Logged
KE3WD
Member

Posts: 5689




Ignore
« Reply #18 on: December 08, 2012, 06:46:26 AM »

Temperature Coefficient is the reason I poured that Portland Cement around the wire resistor. 

Increases the thermal mass while providing very good thermal contact with the wire. 

That is the principal behind the "Ceramic" wirewound resistor. 


73
Logged
K0JEG
Member

Posts: 658




Ignore
« Reply #19 on: December 09, 2012, 12:14:37 PM »

Somewhat off topic, what is the general opinion of using a 100A battery tester to check station backup batteries? I have an old telecom gell cell (http://www.mkbattery.com/images/E31%20SLD%20G.pdf) that will run my entire station for hours if needed, but I rarely run it down much. Once in a great while I'll throw the battery tester on to make sure it still holds a charge, but I'm sure there's a better way.

It's basically a resistive load with a volt meter and switch to cut in the load, made for automotive SLA batteries.
Logged
K5LXP
Member

Posts: 4479


WWW

Ignore
« Reply #20 on: December 09, 2012, 02:50:25 PM »

An automotive tester 10-second test is good for finding out the cranking current of a battery, but not so much for determining the merit of a storage battery like the gel cell you have. 

Ideally, you'd test the battery at the currents you actually expect to use it.  For standby ham use, that might be the receive current of a radio or two and maybe some low voltage lighting.  You can readily duplicate this kind of load with lamps or even the actual devices, and see how long the battery can supply power down to 10.5V or when the equipment stops working, whichever comes first.

If you want to test the battery against rated spec, use a C/20 load, in this case 5 amps down to 10.5V.  This battery should be able to supply 5A for 20 hours. 


Mark K5LXP
Albuquerque, NM
Logged
W6EM
Member

Posts: 791




Ignore
« Reply #21 on: December 09, 2012, 04:06:04 PM »

Be aware of the characteristic or problem with using wire for a power supply dummy load.  The resistance changes with heat.  Unlike a light bulb, wire will continue to change resistance until it is destroyed.

Your assumption is based on an assumption of no thermal equilibrium where energy in is equal to energy conducted, convected, or radiated away.  Also, in a light bulb, no oxygen is present to "burn" the tungsten filament.  Crack the glass and let some air in, and voila, no more wire.

Quote
Nichrome ribbon from a heater is good..... or just use an electric heater.


Nichrome wire/ribbon will take some tinkering.  Perhaps, a series of taps to achieve desired equilibrium current, or switching of series/parallel lengths.  No, an electric heater, unless it's a 12V heater, will be of limited use.  What's 1200W at 120V isn't at 12V.  You could, however, scavenge the nichrome from one to "roll your own."
Logged
KE3WD
Member

Posts: 5689




Ignore
« Reply #22 on: December 09, 2012, 05:42:19 PM »

No, an electric heater, unless it's a 12V heater, will be of limited use.  What's 1200W at 120V isn't at 12V.  You could, however, scavenge the nichrome from one to "roll your own."

As we implemented (discovered?) at that old UPS design lab, ten of those oil filled radiators connected in parallel made a dandy 12V load that could handle 12KW or more.  Not that we had anything that could load them that hard. 

Why did we do it that way? 

Pennies on the dollar as compared with off shelf lab grade stuff.  YMMV


74
Logged
KA5IPF
Member

Posts: 999


WWW

Ignore
« Reply #23 on: December 09, 2012, 06:28:11 PM »

Google "Rex Rheostat" and see what comes up. I have an older one that is 2 1.6 ohm rheostats in one package. They go down to .4 ohms. The rating is either 22 or 44 amps depending on how you hook them up. Only problem is the min is abt 8 amps. I use it to test supplies with an old telephone company ammeter inline. It will make up to a 50A supply groan....

Or at a place I worked at we had a large rats nest (think 2-3 ft in diameter) of 18ga wire in a corner for testing supplies.

You might get lucky and find one of the Rex's.

Clif
Logged
K8AXW
Member

Posts: 3825




Ignore
« Reply #24 on: December 10, 2012, 09:12:10 AM »

It shouldn't be a big deal to connect a 13.8V power supply to an electric space heater and find the necessary resistance point to give you the current you need. (Not wall or baseboard heaters)

Connect one lead of the power supply to the end of the heating element and then tap down on the element with a clip lead and alligator clip heavy enough to carry the current.  Since these are short run it won't be necessary to use heavy wire or clips.

Space heaters with the flat nichchrome ribbion wire would be more suitable than the heaters with round nichrome elements.
Logged
KE3WD
Member

Posts: 5689




Ignore
« Reply #25 on: December 10, 2012, 09:28:46 AM »

It shouldn't be a big deal to connect a 13.8V power supply to an electric space heater and find the necessary resistance point to give you the current you need. (Not wall or baseboard heaters)

Connect one lead of the power supply to the end of the heating element and then tap down on the element with a clip lead and alligator clip heavy enough to carry the current.  Since these are short run it won't be necessary to use heavy wire or clips.

Space heaters with the flat nichchrome ribbion wire would be more suitable than the heaters with round nichrome elements.

The oil filled units use an element built much like the elements on an electric range. 

The actual heating wire is inside a ceramic coaxial surround and that ceramic is then covered by a metallic outer shield. 

Tapping into that would prove difficult to implement and likely would destroy the element. 

There is another old trick for building your own load resistances and that involves the use of blued steel strap as found on shipping containers.  But these days that stuff is getting harder and harder to find as they now use some sort of composite in a lot of cases. 

If you can find blued steel strapping, though, it can be put together in parallel as well as just simply measuring the resistance of one length.  The trick here is to know that you have to sand or file off the bluing where you bolt our electrical connections.  A few porcelain standoff insulators mounted such that you can zigzag the strap compacts the thing nicely.  These can be air cooled, or can also be embedded inside cement. 


73
Logged
Pages: Prev 1 [2]   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!