# DC and AC Amps difference?

Discussion in 'The Projects Forum' started by bowlingo, Mar 8, 2012.

1. ### bowlingo Thread Starter Member

Jun 29, 2011
162
0
Hi all,

I have a 5A 12V DC load that is supplied via a 5A 12V transformer. Someone has told me I can switch this via a 230V 3A relay i.e I break the 230V live on the supply side of the transformer. Is this correct? I always thought AC current was the same as DC current?

Thanks

2. ### mcgyvr AAC Fanatic!

Oct 15, 2009
4,791
976
They are wrong.
Amps are amps and in fact DC is actually harder to break due to it not having a zero crossing point like AC does.
So a relay might be rated like this 6A at 120VDC or 28VDC

Last edited: Mar 8, 2012
3. ### Paul Kerry Member

Jan 9, 2012
36
4
Your load equates to 12V x 5A = 60 watts, so the current on the primary (230V) of your transformer will be 261 mA. What you have been told is correct, but if you have a reasonable amount of smoothing on your 12 V DC your load will not power off immediately.

Regards
Paul

4. ### praondevou AAC Fanatic!

Jul 9, 2011
2,936
489

It's absolutely not. If you think it is you will get yourself into trouble, sooner or later.

5. ### bowlingo Thread Starter Member

Jun 29, 2011
162
0
Thanks Paul...starting to make sense now 60 / 230 = 0.261 (261mA) this is the correct ohms law calculation?

"but if you have a reasonable amount of smoothing on your 12 V DC your load will not power off immediately" Can you explain this please?

6. ### praondevou AAC Fanatic!

Jul 9, 2011
2,936
489
Because the smoothing capaicitor will provide current through your load,i.e. it discharges through it. The time it takes to discharge the capacitor depends on the load resistance and capacitor size.

7. ### mcgyvr AAC Fanatic!

Oct 15, 2009
4,791
976
wow I totally missed the last sentence about using the relay on the 230V side of the transformer.. It will work there no problem but you cannot use the relay on the 12V side.