Question About Amps

[UnkleBob]

Wel hello there, i'm new to this forum, real big fan of the EEV channel.

i was experimenting with a DIY power suply that needs 24V 3A AC.
the transformer i have here is SEC. 24V 5A AC.

thought well i will use this, and booom :0. project blew right up in my face... i double checked the circuits, no wrong connections.
so my question is could the extra 2 amps have blown my PCB?

i always thought that amps weren't that big o deal. that more amps didn't matter that much.
For example if i have 1 transformer of  say 2A, and i want to connect a motor of 1A the motor will have 1A or 2A going throug it?


another example my pc at home uses 300 wats, 230V wich wont come near the 16A that we have behind the socket in the wall, and my pc wont blow up.


can someone help me out on this one, really bugging my head

[Brumby]

Your understanding is correct.

i always thought that amps weren't that big o deal. that more amps didn't matter that much.
For example if i have 1 transformer of  say 2A, and i want to connect a motor of 1A the motor will have 1A or 2A going throug it?

Assuming the voltage of the transformer and motor match, 1A.

A transformer rated at 24V 5A will be capable of supplying current up to 5A without being overloaded.  The actual current delivered will be determined by the circuit connected to it.  If you have a circuit that only draws 100mA, then that is all the transformer will deliver.  If you wanted to, you could run 50 of these circuits off the one transformer.

The only (minor) reservation is that because of its internal resistance, the voltage on the secondary will be higher at light loads than it will be at full load.  Exactly what this difference will be can vary, but I would not expect there to much difference between a transformer rated at 3A and one rated at 5A - and certainly not enough to 'blow up' a circuit.


One question - what is the primary voltage specification - and does this match the supply?

[Marinated]

Could you give links to datasheets or product pages for the power supply and transformer? Also, could you be more specific than "blew up"? For example, have you tested power supply components to find what if anything was damaged? This info could help in providing a more specific answer.

[John Coloccia]

The current rating of a supply is telling you what it can nominally provide, i.e. it can provide UP TO 5A. If your circuit only draws .001mA, then you will get .001mA. Your circuit blew up for some other reason.

Maybe you can give more details on the DIY power supply you built? Your own design? Kit? What do you mean by "blow up?" Smoke? MOSFETs flying apart? Caps blowing up? Here's a couple of common things that can cause spectacular results:

-installing polarized caps backwards
-installing MOSFETS incorrectly
-confusing resistor values. For example, installing a 100ohm resistor instead of a 1k or 10k
-installing diodes backwards

Really it could be anything, but the stuff I listed is very easy to do, we've pretty much all done it at least once (usually more!), and we've all had the experience of:

"Hmmmm...I wonder what that smell is?"

*BOOOOM*

"Oh, that's what it was."

[Brumby]

The question was about AMPS - and that has been answered (hopefully to the OPs understanding).

The question as to what went wrong for it to blow up is something different ... and that's an adventure in itself.

First thoughts:
 - transformer has 110V primary and was connected to 240V
 - component fitted incorrectly
 - solder bridge
 - faulty component
 - incorrect component
 - that there was a wiring error (I don't mean to impugn, just suggesting revisiting this)

Knowing what actually released the magic smoke would be helpful in progressing this avenue.

[Brumby]

-confusing resistor values. For example, installing a 100ohm resistor instead of a 1k or 10k

Not hard - especially with red and brown bands on the blue-bodied metal film resistors.  I've been reading resistor bands for a few decades now and I sometimes pull out my meter to double check.

[UnkleBob]

thanks guys for the answer, i finally got confirmed whit what i always had in my mind.


well for the blowing up part, it was a faulty diode in my bridge rectifier, which allowed to destroy more.. i think... havent tested much further in the circuit.

the project is a experiment from, yes china! as i have a variable bench supply (really old one) i wanted to test how bad is this DIY China stuff might be... (not bad actually 1% resistors came with the kit, quality through hole PCB, and if you replace the dodgy looking components (like the transistors and pots) for quality ones you end up with a not the best but semi good project!)

Specs:

 input:     24Vac 3A
 Output:  0-30Vdc 2mAh-3A



if you guys want more and help give me a heads up so i can provide more data !
for now i'm going in with the old multimeter and gut feeling! (as i still lack a scope!)

List of Components:
R1=2.2K  1W C4=0.1uF(104)
R2=82R C5=0.22uF(224)
R3=220R C6=100P (101)
R4=4.7K C7=10uF/50V R5?R6?R13?R20?
R21=10K
C8=330P (331)
R7=0.47R 5W C9=100P (101) R8?R11=27K D1,D2,D3,D4=1N5408 R9?R19=2.2K D5,D6=1N4148
R10=270K D7,D8=5.1V ZD R12?R18=56K D9,D10=1N4148
R14=1.5K D11=1N4004
R15,R16=1K Q1=9014
R17=33R Q2=2SD882
R22=3.9K Q3=9015
RV1=100K 3296W Q4=2SD1047
P1,P2=10K B U1,U2,U3=TL081
C1=3300uF/50V D12=LED 
C2,C3=47uF/50V

[rstofer]

if you guys want more and help give me a heads up so i can provide more data !
for now i'm going in with the old multimeter and gut feeling! (as i still lack a scope!)

I wouldn't expect any signals beyond the first bulk capacitor to be wiggling.  A scope is not particularly helpful when troubleshooting a power supply.  That kind of work favors a DMM.
Yes, it's nice to see that signals that should be flat are flat but you can do that by measuring AC which will indicate how much AC is riding on top of the expected DC values.  Expect some but not a lot.

[raspberrypi]

I always think of it this way:
Volts : Always has to match
Amps (Supply): equal or lower. Lower is better. This is your reserve and your circuit will only take what it needs, but better not to run it at 100%

1st useful smiley in the history of the internet 
Is there a mu and other symbols?

[Ysjoelfir]

I love building those cheap chinese DIY Kits, would you be so kind to provide a link where to buy that?

Regarding your fault in the design, if one of the rectifier diodes was dead, I suppose the caps were not happy about that and blew up? Since you said something blew up, is there anything visibly burnt or exploded? A Picture of your circuit (component and solder side) could also be helpful to provide aid in troubleshooting.

[rolycat]

This design has been discussed several times on the forum, here is a recent thread.

The link in that thread is dead, but this is a kit currently available for under $6. It can also be readily found on eBay (search for "DC Regulated Power Supply DIY Kit").

As has been noted, it is not really capable of 30V or 3A output.

[Ysjoelfir]

This design has been discussed several times on the forum, here is a recent thread.

Thanks, I don`t have every existing chinese circuit diagram in my head, so didn`t know that this particular one is the one in question.

The link in that thread is dead, but this is a kit currently available for under $6. It can also be readily found on eBay (search for "DC Regulated Power Supply DIY Kit").

Thanks for that, too.

As has been noted, it is not really capable of 30V or 3A output.

Should be obvious

[Kleinstein]

The obvious limitation of that circuit is that the TL081 OP is specified for up to 36 V operation. With a negative -5 V supply, this leaved at most 31 V for the positive side, which means no more than 22 V, better 20 V AC for the transformer to be one the safe side (e.g. stay inside the specs for that OP). Obviously not more 30 V output in this case.

So with a 24 V transformer there is a chance to blow the OPs due to too much voltage, but it sometimes works - at least for a while.

To get an 3 A DC current out, an transformer specified for 5 A (AC) is actually about the right size, as the RMS AC current is considerably higher than the DC current.

For a first test of such a circuit it is a good idea to have an incandescent lamp in series to limit the current, or use a lab supply to power it. 

[UnkleBob]

For a first test of such a circuit it is a good idea to have an incandescent lamp in series to limit the current, or use a lab supply to power it.

yeah should have done that, blew the opamps ect ect.. but hey this was expectable with the chinese junk!

thx everybody for the info!

[CraigHB]

I love building those cheap chinese DIY Kits, would you be so kind to provide a link where to buy that?

The China vendor below stocks a bunch of DIY electronics kits, don't know if they have that particular one, but they might.  They're a department store and I use them for various products.  They've been reliable;

http://www.banggood.com/Wholesale-Arduino-Compatible-Kits-and-DIY-Kits-c-3091.html

[UnkleBob]

I love building those cheap chinese DIY Kits, would you be so kind to provide a link where to buy that?

https://www.aliexpress.com/item/2016-NEW-Free-Shipping-Red-0-30V-2mA-3A-Continuously-Adjustable-DC-Regulated-Power-Supply-DIY/32660068947.html?spm=2114.13010608.0.0.KP486L&aff_platform=aaf&sk=zj6qB6AIM%3A&cpt=1479993273234&af=1000161000000000242022015&cn=aliexpress&cv=banner&dp=19TZ1JSkINZ9yl5&tp2=4ren&&aff_trace_key=66cd4b4c7f294ff49096851b1a4655b4-1479993273234-01368-zj6qB6AIM

this is the vendor i've got it from, enjoy!