Diodes of 5V supply getting hot

[tubee]

Hello all
Busy with my CD660 CDP, removed not used parts, because i want I2S from 7210 to four dacs in non-os.

I changed the 8 rectifier diodes to BYV26A, 4 for 5V, 4 for +/- 15V and added a 1 ohm resistor before the diodes to the AC-connection.

Now the BYV's from the 5V supply get hot, measured the voltage over the 1 Ohm: 500mV so the current draw is 500 mA, quite a lot for 1 SAA7210, 1 Ram and 1 uP, isn't it.?

Compared to a 640 the diodes from 5V get only sligthtly warm after 5 minutes. The BYV's from +/-15V stay cool, also because there is no current draw.

Btw, i removed all the not-used parts: 2 LM833, one 4560D, a TDA1541, and SAA7220.

What have i overlooked?

[Extreme_Boky]

By removing 7220 you may have introduced oscillations in / around 7210. Try to measure individual currents for each of 3 chips and see which one is causing high current consumption. I do not expect more than 200mA total current, 7210 would be by far the highest.

Make sure there are no oscillations around BYV26A. The caps across them are not needed any more.

Good luck

Extreme_Boky

[tubee]

Thanks Extreme_Boky for these helpful comments!

We now get closer to the solution i think.

Suspected the power consumption of 7210 allready yesterday, because this chip was getting slightly warm, this without load, rest of chips kept cool.
Also discovered yesterday-evening: CDM-2 works more quitly with a tantalium cap as decoupling of 7210, so it could be indeed oscillations around 7210. Have replaced different decoupling caps allready, at 7805, Ram, 7210 and u Processor, exchanged to regular Philips lytic 47uF 50V.

Big 10000uF 16V cap is OK, 7805 seems too.

I will then remove the brown SMD ceramic caps. Also will measure current flow through added 1 ohm R in series of each chip.

[pozo1992]

Hi there,


did you check the Datasheet of this diode BYV26A? When you assume 0.5A per Diode and have a forward voltage of 2V!!according to datasheet you get a power dissipation of approx. 1W per device. that`s a lot. With a thermal resistance of 100K/W you get a diode temperature of roughly 100degrees Celsius which is definitely very hot.
This diode has been optimized for fast switching, so forward voltage is not the first concern.
What are the original diodes? Normal ones have a forward voltage of 0.8V which gives less than half the power and heat.

Hope this helps.

Regards, Jürgen

[Jocko Homo]

If you took out the '7220, where does the clock come from now????

Won't explain why the chip gets hot. It shouldn't. Not like the '7220 will.

Jocko

[tubee]

Hello Pozo1992

The BYV26A can have 1A, jast as the original 1N4002's.
I selected them for speed and price. (I want to use the more expensive HFA08TB60's from SCHURO.DE for another project.)
Faster diode is to reduce HF induced from the diode itself.

BTW, the 500mA current is measured across a 1 ohm resistor before the diodes, in series with the AC-connection.
The R should reduce HF spikes, reduces slew-rate a little, and the filtercaps after rectifier can filter out HF more easy.
With 9V AC over rectifier you get about 9 watts (500 mA *2 *9V AC power consumption) this is a lot. The BYV's get around 60 degrees(cannot hold my finger on)

[Jocko Homo]

I don't think "slew rate" is a proper term here. How 'bout "peak rectifier currents"?

Jocko

[Extreme_Boky]

It seems all the IC's are okay. 7210 gets mildly warm - that's okay as well. You should start looking elsewhere...

Check those tantalum capacitors - older ones used to short out.

If you can get the lab power supply, disconnect diodes (existing) +5 volt power supply, connect the laboratory +5V and crank-up the current until the faulty part starts either burning, smelling or catching fire (joke). This is the quickest way.

Extreme_Boky

[tubee]

If you took out the '7220, where does the clock come from now????

Hello Jocko: From a separate 11.2896 Mhz XO into Xin of 7210. Cd can only spin with a clock signal. And yes i know slew is not the right term here, but more a opamp-term.

The 7210 consumes 135mA, and dissipates 0.7 Watt then, allright i think for a 40 pin chip.

Found out that the diodes to the + line 7805 get hotter then the ones to the minus 7906, so i just got me a new 7805, maybe the reg. is oscillating with the BYV's? Wil try it, but not tonight, because today i became in the ** decade of life, and will have some or more beers.....

[seoman]

Gefeliciteerd!!!! Heb je abram gezien?

Remember that rectifier diods always have to be oversized by a minimum of 1,5!
The charge peaks are enormous compared to the avarage current wich is drawn from the capacitor.
So maybe the peak handling of the BYVs is less good than the 1N4xxx


Groetjes Simon