milliamp matching

Reply

  #1  
Old 05-23-18, 05:43 PM
Member
Thread Starter
Join Date: May 2018
Location: USA
Posts: 2
Received 0 Votes on 0 Posts
milliamp matching

I have a similar situation of an earlier (DC volt) post.
I have an old 5W 12V telephone transformer that puts out 200 milliamps. I have a new inexpensive two-inch DC 12V cooling fan that rated at 120 milliamps. Seems to me the simplest solution for me is to just add a second fan (which would be useful in my application) wired in parallel. I don't want to burn out the fan and extra air is 100% ok with me. I would like to avoid using resister(s) soak up the excess current.
 
Sponsored Links
  #2  
Old 05-23-18, 06:00 PM
Join Date: Jan 2009
Location: Trumbull, CT, USA
Posts: 422
Received 5 Votes on 5 Posts
Make sure it is in fact DC as a lot of those phone power supplies are AC-AC and not AC-DC Also a 200MA supply is fine as the 120MA fans will only take what they need (You have 200MA available) so you are OK with that
 
  #3  
Old 05-23-18, 06:04 PM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Northern NJ - USA
Posts: 54,648
Received 517 Votes on 487 Posts
Make sure your old power supply is DC. Many of those put out AC.
 
  #4  
Old 05-23-18, 08:27 PM
Member
Thread Starter
Join Date: May 2018
Location: USA
Posts: 2
Received 0 Votes on 0 Posts
Thanks for the quick responses.

How often to you found a transformer that puts out DC when on the back of the transformer it says:
INPUT: AC120V
OUTPUT: 12VDC 200mA
?

I have quite a few AC120V to 12VDC (about seven). As some have said on this site, the actual DCV number may not be what the label says. One that I checked said (multimeter) the output was 18V not the label's 6V.

I go back to my original question. If I use the supply that puts out a bit too much current (compared to the other "12VDC" transformers I have), should I wire a second fan in series or parallel? I like the idea of two fans in case one fails.

The fan is a Raidmax 60mm RF-6015-M It will be on a timer.
 
  #5  
Old 05-23-18, 08:31 PM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Northern NJ - USA
Posts: 54,648
Received 517 Votes on 487 Posts
Your fan requires 12vdc. You need to insure that your power supply has more current available than the fan requires.... it does.... you are good. If the power supply was of a smaller current then the fan.... it would run warm.

The fan only takes the current from the supply that it needs.

Most of those wall warts are not regulated so it's very common that they test several volts higher.
Once they are loaded..... something connected.... the voltage usually comes down to about what's listed.

Many devices use an AC output wall wart and keep the DC conversion parts in the device. This cuts down on heat and cost of the adapter.
 
  #6  
Old 05-24-18, 02:33 AM
Gunguy45's Avatar
Super Moderator
Join Date: Dec 2007
Location: USA
Posts: 21,119
Received 2 Votes on 2 Posts
I think your real question is being missed. If you put 2 fans in parallel that each require 120ma...then you need a supply that can put out at least 240ma and preferably more like 300 or even 400 to prevent over heating.

As was stated...a power supply doesn't really "put out" anything...not like a garden hose turned on full filling up a bucket. Think of the power supply as a pond and the fan as a pump. If the pump is on and running full speed, it can only draw so much out of the pond, even though the pond can supply far more. Add another pump and the pond has to be big enough to supply water to both without dropping the level too much.


I remember being told you can't jump a motorcycle with a car, because the car system puts out too much and could burn out the bikes electrical system and even blow up the battery. Luckily I was an Electronics Technician and knew it to just be bunk.
 
  #7  
Old 05-24-18, 06:40 AM
Member
Join Date: Oct 2015
Location: Ct.,USA
Posts: 1,342
Received 45 Votes on 39 Posts
AC to DC power supplies (PS) are rated in watts. Ideally watts in equals watts out. It never happens due to heat losses and impedances. In addition the DC output has additional restrictions. While there are infinite values of DC currents times DC voltage that equal the output watts, the available DC voltages and currents are limited by design. A 100 watt, 25V DC output PS is limited to 4A DC even if the DC output is adjustable. You are exceeding a rated limit if you decrease the DC voltage to 10 V DC and increase the load to 10 A DC even though DC voltage times DC current equals 100 watt. The PS will fail.
 
Reply

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: