Incorrect voltage reading from bench supply

[windmill john]

I have just picked up an Irwin EJ1001 bench top power supply.

I have had Schorsch’s old battery under my bench for 5 years; took it to the tip yesterday.
I occasionally used it to test bulbs, wires etc.

This Irwin was free, something I’ve wanted for years but couldn’t justify buying one, as it is only for very occasional testing.

Anyhow, it may be faulty!

Assuming my multimeter battery is good, the Irwin is giving me a reading tenth of what it should be; yes I’m in the right scale you cheeky buggers

So with the Irwin set to 12V, my meter shows 1.2V
With my meter set to 2V and the Irwin on 2V, I get out of range.

Anything simple I can check? Be gentle, my circuit board testing is, erm.... limited.

Thanks

John

[bwprice100]

Check you multi meter against a new PP9 battery as a starting point.

I hope this helps

Brian

Sent by me using tapety talky

[windmill john]

i had just got from work and banged the meter on. I’ll try another meter tomorrow, Irwin now in the shed, cat on lap, can’t move!

[Galactic Greyhound]

Hi John,

Voltmeters are very high resistance devices.

If you just switched the bench power supply to 12v output and clipped your Voltmeter across the output terminals then, depending on how the power unit is designed, it may not switch on because it will see the very high resistance i.e. almost an open circuit, and there will be negligible current flow.

Try connecting a 12v filament bulb (your Test Lamp? 12W = 1 Amp draw) across the power supply output and see if it lights up.
You can then connect your Voltmeter across the bulb/output terminals to see what the voltage is.

I know the problem with the cat on lap as well!

[windmill john]

Thanks Ced, I’ll see if I can get in the shed tomorrow night and check.

The other cat is on my lap now! I’m trying to get to bed!

0CB26F39-681E-4335-AF5B-038AA5723CF8.jpeg

[george baker]

Hi
Yep, some psu's only stabilize under load, some pc psu's wont give good 12v unless their 5v output is being loaded

Cats, bah!

George

[P-K]

george baker said :-
Hi
Yep, some psu's only stabilize under load, some pc psu's wont give good 12v unless their 5v output is being loaded

Indeed George. Dead right.
That's because a PC uses a switch mode power supply (no transformer).
A swine to fault find.

I'd guess the one in question is a linear power supply.
But you're right again George in that some require a few milliamps current flow before the voltage control circuit operates.

[windmill john]

Well thanks all.

Okay, here we go. The 12 volt position does not work; that’s handy!

Bulb lit at the 2, 4, 6 and 8 Volt settings with obvious brightness increases; just not the 12V setting.

My lamp was actually a panel bulb of 2 watt rating.

Readings:

At 8 V, 7.4V
At 6 V, 5.99V

Great, so all I need to do now is buy an old bike with 6 volt electrics and I’m laughing

9621DAEB-3195-4834-A9DD-4AAC5DD6830C.jpeg

[Galactic Greyhound]

Maybe that's why it was free!

Might be worth opening up and checking that the switch is not faulty.

[windmill john]

As it was free, I have no qualms of popping inside and checking all.

Thanks Ced.

John

[windmill john]

Yes P-K it is a linear power supply, just looked inside. Big transformer and the rest was a very simple circuit board.

I thought I’d have to order a rotary potentiometer, but it turned out simple and not needed.

It looks like it has been used on 6 volt for so long, the rotor was a bit gungy.
A bit of constant twiddling brought 12 volts back on line, all good.

Thank guys

BTW, I only know it is a linear power supply as I had recently done a bit of reading about the difference between switch mode and linear power supplies. I’d like to get more into the understanding of circuits.

[Galactic Greyhound]

Hi John,

See the following - lots more on YouTube!

Linear Mode Power Supplies:
https://youtu.be/brB1sZyJPls


Switched Mode Power Supplies:
https://youtu.be/nbKwUSH9NiY

[windmill john]

I did watch the video by the post apocalyptic inventor a while back...... well, he is German and so are my bikes

[P-K]

I used to teach some of this electronics stuff (and Computer Science) which is why / how I know.
I'm not into running an on-line tutorial, but for the odd one off answer, ask away.

[windmill john]

I think the hardest thing about learning electronics is why?

Like most things, you need a project.

So let’s say it suggest making a door buzzer, a moisture sensor etc. As you can buy one very cheaply, or more importantly you don’t need to make one, this is where I struggle.

Thinking of a useful reason to learn is the hardest.

Or, doing it in reverse. I once bought an old radio to repair. This was silly the other way because I didn’t know how to circuit test.
If I tested a resistance in part of the circuit for example, I didn’t know if the resistance showing was from the part I was testing or some other part of the circuit!

[george baker]

Hi John,
WHY? Is true about all learning unless you are on a course, Then the poor teacher has to "make you have it"



George