Diodes overheating

[bhvm]

Dear All,
I am using a Full wave bridge rectifier to converte Transformer Output to 24v DC. The Load is about 1 ampere (LED strip) Using an 25v 5000uF cap on output for smoothening.
The Diodes i am using Currently are 6A4 Ones


These are rated at 6Amps and 400V ........So i am sure they are enough for the load.

I was previously using IN5004...which were rated 3 Amp and they heated up like crazy. Now i went for these bigger ones, sure the heat has gone down, but does not feel good for 12 hour/day operation. They heat to "Ouch" levels within a few minutes. The transformer and LED strip are on Normal temperature. Ambient temps are about 30.C.

Any ideas why this is happening?

[budm]

Re: Diodes overheating

Can I see the circuits?

[bhvm]

Re: Diodes overheating

There you go!

[budm]

Re: Diodes overheating

The most Vf on the Diode will be about 1V so if you have 1A load, the power dissipation on the Diode should be about 1W, 4 Watts total for all 4 Diode. If you are drawing about 1A, the transformer has to be able to supply about 1.6A.
Did you check the current draw from the power supply both AC current and DC current?

[bhvm]

Re: Diodes overheating

The most Vf on the Diode will be about 1V so if you have 1A load, the power dissipation on the Diode should be about 1W, 4 Watts total for all 4 Diode. If you are drawing about 1A, the transformer has to be able to supply about 1.6A.
Did you check the current draw from the power supply both AC current and DC current?

Thanks for the input.
The transformer is rated at 1.5A......Its just normally Warm , Not HOT, So I think it is operating normally.

The DC current draw (LED strip) Is 0.98 Amperes at 22.5V (measured after capacitor)
The AC Current Draw is about 0.1 Amp at 240V.

I just noticed that the Datasheets mention Diode operating Temps till 175.C !!
Could it be that they are designed that way?

[budm]

Re: Diodes overheating

At about 1A you can have ambient up to 150c if you use 25 Sq. mm ground plain as shown in the derating page. So you are way within the range. You can make them run a little cooler if the you leave the legs long.

[bhvm]

Re: Diodes overheating

At about 1A you can have ambient up to 150c if you use 25 Sq. mm ground plain as shown in the derating page. So you are way within the range. You can make them run a little cooler if the you leave the legs long.

Thats the same Datasheets i linked in initial post.
Yes the legs are uncut however I don't have any PCB to mount them. Wire just soldered from the diodes.

So to conclude, I can safely run them this way and expect them to stay fine till say 100.C in my present configuration? They seem well within range. I assume *Ouch* temperatures are like 60~75.C?
Thanks for taking my worries off! I shall proceed with the project now!

[budm]

Re: Diodes overheating

You are OK, way within the range.
You can use 4 pennies and solder each one to the 4 junctions to relief some heat but make sure they are not touching any metal plate.
I used them for cheap heatsink.
Do you have those cheap infrared gun from Habor Freight?

[bhvm]

Re: Diodes overheating

You are OK, way within the range.
You can use 4 pennies and solder each one to the 4 junctions to relief some heat but make sure they are not touching any metal plate.
I used them for cheap heatsink.
Do you have those cheap infrared gun from Habor Freight?

I was about to dip the diodes in Thermal glue and put them on an slab of aluminium (The plastic casing, Not leads) ... Would that work?

And no, I don,t have any IR gun. I use thermal probe from my multimeter. I don't know what harbour Freight is...

[budm]

Re: Diodes overheating

As long as thermal glue can transfer the heat instead of trapping it, then you are OK, just check it with you thermal probe.
I forget that you are not in the US.

[bhvm]

Re: Diodes overheating

Sure mate... now where's the rep+ Button?

[mariushm]

Re: Diodes overheating

The datasheet says:

Maximum Average Forward Rectified Current 9.5mm lead length @ TA = 75°C (See Fig. 1) 6A

Your leads are basically less than that, and you're not using any heatsink, you have 4 diodes in the air, without heatsink, the heatsink is basically the leads.

So the graph in figure 4, page 2 applies to you.. sort of. You don't have any heatsink in the current state, so that heat would dissipate in the pcb copper through the leads, so you can just say the distance to the heatsink is 25mm, so your thermal resistance is 20 C /watt.

The voltage drop at 1-1.5 A would be 0.8v, so from there you can do the math and figure out that the temperature of the diode will go up about 30-35C above ambient temperature at such current load.

You should have no problem finding a few cm/inches of copper foil or maybe just a few cm of copper pipe that's used for water pipes - some plumbers should have such waste.
Attach the copper foil to a piece of food or plastic and you have the heatsink... or cut the pipe in half horizontally, flatten it and you have copper pads you can solder leads to.

[bhvm]

Re: Diodes overheating

Problem solved,The thread may be closed please.

[PeteS in CA]

Re: Diodes overheating

Since the diodes had cap after them, they only conducted at the peaks of the AC cycles, and probably 3X-10X the DC current.