Fried PSU for 12v cb radio base station conversion

[roughshawd]

Last night I tried to fix my PSU and it had a broken connection to the external transistor that I think shorted out. The diodes connection cooked the board as the pic shows. I want to try and rebuild it.

 

[Martaine2005]

Clearly there is a cracked diode.
But what does your fault finding say?.
Is the output transistor shorted?
Is the diode shorted?.
It’s a pretty simple power supply so you should get it working again.
Edit: is this a typical 13.8V linear supply?.

 

[kellys_eye]

The wire for the main regulating (bypass) transistor is missing the emitter connection.

 

[roughshawd]

How do I test the transistor to see if it's ok?

 

[kellys_eye]

Use a transistor tester or Google how to use your testmeter diode check function to test the individual junctions.

 

[roughshawd]

After testing the transformer I think it's ok. Center tapped 37vac at 19vac... Nominal.
So on to the build. I will need a 6a draw at high volume transmit, so I want to target a 6a output, even if I have to build a circuit that is TSR until required. The requirements are .5a standby, 1.5a receive, 6a transmit . Must have 6a.

The original PSU used a 35v uf2200 capacitor, I am researching the components, and there was a clear orange either zener or something. The one in the pic has no ID numbers.. it is not shorted...

.'

 

[kellys_eye]

The transformer secondary voltage seems rather high for a 12V PSU as, at a draw of 6A, you will need to dissipate around 75W of heat in the regulating transistor. Perfectly feasible of course but depends on the efficiency of your heatsink.

The most basic 12V regulator at 10A can be made using the LM317 and 2N3055 transistor - Google some schematics.

 

[roughshawd]

I did some homework. The LM317 is s 1.2a component. I am kind of sketchy about this, seems like everytime someone tells me of this great little thing that is a fix all, I am just to worried about it all coming together. I found some 6a 40v diodes, that should make DC and not fry when I try to transmit. Question now is whether to make a bridge or just use 2. I learned that the 4 component bridge is the only way to guarantee, you will get DC from a transformer.

 

[Martaine2005]

I found some 6a 40v diodes, that should make DC

I thought you required 6A for transmitting?. If so, use a 10A minimum. You can’t/shouldn’t run components at their max. Have headroom to run them cooler, more efficient and last longer.

 

[bertus]

Hello,

Here are some examples for a bypass transistor for a regulator chip:

In the example the LM78XX is used, but a LM317 with the correct resistors will also work.

Bertus

 

[roughshawd]

All right!! Ok I have lots of saved spares components like these dual diode packs, that are just right for what I want. D83-004. I will need a 4 or 5 watt discharge resistor, but I am a bit stupid as to the charging cap. The original was a 35v 2200uf, but that supply didn't supply enough current. I need 6a , and I don't want to be too frugal when the button gets pushed and the draw goes from 1.5 to 6! EMITTER LINE RECONNECTED.

 

[roughshawd]

These packs..
But they are dual diode output packs, and I don't have the matching input packs like |>|<| and I don't have the |<|>|...
They are 6a so I need to use them.

 

[roughshawd]

I haven't been able to find the datasheet on this diode pack...

 

[Martaine2005]

Did you search for : C25M-02C?

 

[bertus]

Hello,

I often use the KBPC bridge rectifiers.
I have attached the 25 and 50 Amp datasheets.

I have also attached the datasheet of your diode.

Here is also info on the TO-3 powertransistor pinout:


Bertus

 

[roughshawd]

Old PSU ain't getting it. My other PSU has switch and fuse issues so that's the focus now. The switch broke. Can I use this one instead..it's Already installed, but I don't know what to do with the acc for the light I. The old switch. The cap was wired directly to the brown fuse wire. That cap goes to the green ground wire on the AC plug. The acc send to the switch, is on the other side of the transformer past the switch

 

[roughshawd]

It's hot... It's pumping out 12v. I forgot I had this one!!!!

 

[roughshawd]

It's working... It's working!!!