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nightdesigns
Gone missing, back soon
Premium
join:2002-05-31
AZ
Reviews:
·CenturyLink

DC Voltage drop under load

I was playing around with a 12v 1 amp device and the power supply with no load reads the proper > 12v. Once the load is applied, the voltage drops to the 8-9 volt range.

I'm trying to wrap my head around what I am seeing. I believe the device is actually still getting 12 volts so what is my meter reading?

I split off the wires just before the barrel connection and took the voltage reading from there.

Thanks
--
This Space for Rent...



Tursiops_G
Technoid
Premium,MVM
join:2002-02-06
Norwalk, CT
kudos:1

That sounds about right for an Unregulated Linear power supply (transformer/rectifier) when loaded down at or near it's max rating.

If it's a Switching power supply (SMPS), then it's either damaged, or Severely overloaded.
--
If You're Unsure, "RTFM"... If You're SURE, "RTFM" Anyway.



aurgathor

join:2002-12-01
Lynnwood, WA
kudos:1
reply to nightdesigns

Another, though somewhat less likely possibility is that you're using wimpy or damaged leads, and there is a significant voltage drop in the leads.

If you have 2 multimeters, it should be very easy to figure out what's going on.
--
.sig



SmokChsr
Who let the magic smoke out?
Premium
join:2006-03-17
Saint Augustine, FL

said by aurgathor:

If you have 2 multimeters, it should be very easy to figure out what's going on.

Or apply the load and measure at the plug, then measure close to the supply and compare.

Also this may be a symptom of a bad capacitor in the supply. If you have a 1000uf cap, and place it across your meter leads while measuring and you see the voltage come up, that is likely the problem.

lutful
... of ideas
Premium
join:2005-06-16
Ottawa, ON
kudos:1
reply to nightdesigns

Click for full size
Probably a combination of high resistance cable and current draw quite close to 1A.

I am taking this opportunity to highlight the LT4180 IC from Linear Technologies which can be used in new power supply designs to avoid such problems.


SparkChaser
Premium
join:2000-06-06
Downingtown, PA
kudos:3
Reviews:
·Verizon FiOS

1 edit
reply to nightdesigns

If I read correctly you have a device that requires 12V 1A. You are connecting it to a PS. What is that PS and what is it marked as?

Edit: Are you sure you're drawing 1A?
Best way to check the supply is a simple resistive load. e.g. resistor, lamp



tenper

@172.56.27.x
reply to nightdesigns

you can expect a simple power supply to drop ~10% at full load. many are designed to be 10% high under no load, e.g.
12. + 1.2, or 13.2 volts, in your example. that's so the loaded voltage will be as specified. if you are reading 8 volts under load another possibility is that your 1 amp load device
cis really pulling 2 amps or more, overloading the power supply. you can check that by measuring the amperes in series with the load.



cowboyro
Premium
join:2000-10-11
Shelton, CT
reply to nightdesigns

Measure the voltage on the AC range. If you see more than 0.5V under load then you are having a "rectifying" issue - either diodes in the bridge gone bad or bad/too small capacitor(s). Or the power supply is simply an undersized junk...