×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Are you an
Engineering professional?
Join Eng-Tips Forums!
• Talk With Other Members
• Be Notified Of Responses
• Keyword Search
Favorite Forums
• Automated Signatures
• Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

#### Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

# How much heat is given off from a power supply

## How much heat is given off from a power supply

(OP)
I am trying to calculate the internal temp of an enclosure. If I use a 120vac/24vdc 25 watt(full rated capacity) power supply, but only use 100mA of the available output current, how much heat will be radiated from the supply.

### RE: How much heat is given off from a power supply

Measure input winding direct current resistance Ri
Measure output winding direct current resistance Ro
Measure input current Ii
Output current is 0.1A = Io
Pin = Ri x Ii**2 in watts
Pout = Ro x Io**2 in watts
1 Watt = 1 Joule/second = 0.2388 Calories/Seconds = 9.478 x 10**-4 BTU Intenational/second
where Calories are IT (international)

### RE: How much heat is given off from a power supply

(OP)
Thank you very much jbartos.
The power supply in question is a low cost switcher manufactured overseas, the stateside distributor does not have the DC Ri and Ro.  I will order a sample and measure the values myself.  For preliminary design purposes, do you think it is safe to assume the power lost (given to heat) will be < 5W?

### RE: How much heat is given off from a power supply

I am sorry that I misinterpreted your posting for a practical implementation. Heat dissipation of electrical loads and equipment are often available in catalogs or from manufacturers. If the circuit is known, the heat dissipatioin may be estimated/calculated/analyzed.
1. Distributor name and contact or
2. Manufacturer name and contact and
3. Efficiency of the unit
Some enclosure manufacturers may support electrical equipment heat dissipation data to increase the sale of their enclosures.
Often, the efficiency may provide a solution for the equipment heat dissipation. That power supply may have 65% efficiency only. This would imply the higher power dissipation than 5W at full rating. Then, one would need a curve of heat dissipation or efficiency curve versus the output current.

### RE: How much heat is given off from a power supply

(OP)
Manufacture is "Meanwell", model# S-25-24.  Distributor is Jameco. Jameco (Cust. service/sales) is of no help.
The spec sheet does include efficiency (80%), but the efficiency is given @ 100% load.  Because I will at most use only 10% of PS output capacity, I discounted using the efficiency in my estimation.  I do not have a good feel for efficiency curves of these types of circuits.  At such a low level of usage, is it possible to predict if the efficiency is likely to increase or decrease?
Also, there is another variable: PS will operate an absolute encoder; current draw will vary 20mA-100mA depending on usage and duty cycle of the equipment.
As you can see there are a lot of dynamics to this application, I need to provide a supplemental anti-condensating heater to the controls enclosure.  My question regarding the heat generated by the PS is needed only to gauge how much additional heat I will need to supply.

### RE: How much heat is given off from a power supply

Suggestion:
Try to contact the manufacturer at info@meanwell.com
as a potentially accurate source of information.

### RE: How much heat is given off from a power supply

David Tung from Meanwell was kind enough to provide the following:
1. Heat dissipation at 0.1 A (at 115VAC) output current is 2.7 Watts obtained by measurements
2. Efficiency at 0.1 A (at 115VAC) output current is 46.9% obtained over measurements and calculations.
A copy of the David Tung email is available.

### RE: How much heat is given off from a power supply

(OP)
Thank you jbartos.
I would greatly appriciate it if you would forward the Meanwell info to me. Email: joeskrab@stratos.net

#### Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

#### Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

#### Resources

Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a partâ€™s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now

Close Box

# Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

• Talk To Other Members
• Notification Of Responses To Questions
• Favorite Forums One Click Access
• Keyword Search Of All Posts, And More...

Register now while it's still free!