Continue to Site

Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Register Log in

Actual realistic power rating of power essitor?

Status
Not open for further replies.
G

grizedale

Advanced Member level 3
Joined
Jun 13, 2011
Messages
838
Helped
17
Reputation
34
Reaction score
17
Trophy points
1,298
Activity points
8,804
Actual realistic power rating of power resistor?

Hello

We are doing a 90-265VAC, 330W flyback with no PFC stage for guitar amplifier supply (Class D).

It is single switch flyback, so with a 1% leakage transformer, we will get 9W of loss in the primary clamp at max load/min vin.

Therefore we wish to put two of these 5W resistors into the RCD primary RCD clamp

**broken link removed**

...they say they can handle 5W, but does that 5W rating depend on this resistor being soldered into a large thick copper plane to remove the heat?

....or could this resistor dissipate 5W continuously even if soldered to a minimum footprint?
 
Last edited:

I have previously used this resistor form in a number of projects. They are disspating the power by surface convection like other ceramic wirewound resistors. The surface gets rather hot.

You can read from the derating curve, that they are assuming a surface temperature near to 250 °C. Connection traces of sufficient width should be present to avoid self-desolder of the part. You should reserve sufficient space around the resistors to allow free convective air circulation and avoid overheating of near-by parts.
 

To emphasize what FvM says, these resistors will get very hot at 5W. Recently I tried using some 5W surface mount resistors, and found that while the resistors could withstand 5W, when they got near that limit the plastic on a nearby connector (which connects to a thick trace directly to a lead of the resistor) would start melting. The solder on the resistors could also melt, but I used glue underneath the resistors to hold them in place when this happens. So definitely give thought to how other components are going to feel about that resistor getting to >200C.
 

You can consider using a heatsink to make sure the resistors don't get too hot. In some commercial power supplies where such large resistors are used, I've seen them being attached to a heatsink.
 

Heatsink is an option. The ceramic case power resistors are designed to work without it, but a hetasink of course reduces the surface tempreature. As a drawback, you need to account for the additional power in the heatsink thermal resistance calculation.
 

Status
Not open for further replies.

Similar threads

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Back
Top