Question About Amps - Page 1
Forum section: electronics / beginners

author: UnkleBob
author_rank: Newbie
timestamp: November 21, 2016, 02:45:46 am
content: 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

author: Brumby
author_rank: Unknown
timestamp: November 21, 2016, 02:57:27 am
content: Your understanding is correct.Quote from: UnkleBob on November 21, 2016, 02:45:46 amAssuming the voltage of the transformer and motor match, 1A.A transformer rated at 24V 5A will becapableof supplying currentup to5A without being overloaded. Theactual current deliveredwill be determined bythe 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?

author: Marinated
author_rank: Contributor
timestamp: November 21, 2016, 03:01:51 am
content: 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.

author: John Coloccia
author_rank: Super Contributor
timestamp: November 21, 2016, 03:07:06 am
content: 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 backwardsReally 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."

author: Brumby
author_rank: Unknown
timestamp: November 21, 2016, 03:10:05 am
content: 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 therewasa 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.

author: Brumby
author_rank: Unknown
timestamp: November 21, 2016, 03:14:01 am
content: Quote from: John Coloccia on November 21, 2016, 03:07:06 amNot 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.

author: UnkleBob
author_rank: Newbie
timestamp: November 21, 2016, 02:27:09 pm
content: 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 3AOutput: 0-30Vdc 2mAh-3Aif 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=10KC8=330P (331)R7=0.47R 5W C9=100P (101) R8?R11=27K D1,D2,D3,D4=1N5408 R9?R19=2.2K D5,D6=1N4148R10=270K D7,D8=5.1V ZD R12?R18=56K D9,D10=1N4148R14=1.5K D11=1N4004R15,R16=1K Q1=9014R17=33R Q2=2SD882R22=3.9K Q3=9015RV1=100K 3296W Q4=2SD1047P1,P2=10K B U1,U2,U3=TL081C1=3300uF/50V D12=LEDC2,C3=47uF/50V

author: rstofer
author_rank: Super Contributor
timestamp: November 21, 2016, 03:26:54 pm
content: Quote from: UnkleBob on November 21, 2016, 02:27:09 pmI 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.

author: raspberrypi
author_rank: Frequent Contributor
timestamp: November 21, 2016, 10:02:37 pm
content: I always think of it this way:Volts : Always has to matchAmps (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 internetIs there a mu and other symbols?

author: Ysjoelfir
author_rank: Frequent Contributor
timestamp: November 22, 2016, 10:34:44 am
content: 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.

author: rolycat
author_rank: Super Contributor
timestamp: November 22, 2016, 11:40:07 am
content: This design has been discussed several times on the forum,hereis a recent thread.The link in that thread is dead, butthisis 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.

author: Ysjoelfir
author_rank: Frequent Contributor
timestamp: November 22, 2016, 02:03:10 pm
content: Quote from: rolycat on November 22, 2016, 11:40:07 amThanks, 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.QuoteThanks for that, too.QuoteShould be obvious

author: Kleinstein
author_rank: Super Contributor
timestamp: November 22, 2016, 04:47:12 pm
content: 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.

author: UnkleBob
author_rank: Newbie
timestamp: November 23, 2016, 04:33:49 pm
content: Quote from: Kleinstein on November 22, 2016, 04:47:12 pmyeah should have done that, blew the opamps ect ect.. but hey this was expectable with the chinese junk!thx everybody for the info!

author: CraigHB
author_rank: Regular Contributor
timestamp: November 23, 2016, 04:51:41 pm
content: Quote from: Ysjoelfir on November 22, 2016, 10:34:44 amThe 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

author: UnkleBob
author_rank: Newbie
timestamp: November 24, 2016, 01:16:05 pm
content: Quote from: Ysjoelfir on November 22, 2016, 10:34:44 amhttps://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-zj6qB6AIMthis is the vendor i've got it from, enjoy!