ballancing resisitors in bridge rectifier

[W8JI]

Attachment 65928
OK thanks for the great info,
This is a photograph of the PCB board I copied from a Collins transmitter, I understand that the quality of parts made today resistors and caps really are not needed. however I am trying to copy what was in this particular transmitter so it will look the same or pretty close to it.

the diodes are 10A 1000 Volt. I know this was probably bigger than original, but looks wise you can not really tell. It's what I had on the shelf. the caps are .01 @ 1000Volt.

so I am planning on selling these PCB boards and I want to include directions on how to select the correct values for the resistors.

Thanks

Rick

Rick,

Be a little careful about blanket all-encompassing suggestions.

You'll notice Ameritron uses caps across diodes in some, but not all, amplifiers. This is done for transient suppression and RF bypassing when needed. Some amps have no caps, and some do.

Keep in mind nearly all life problems created when using resistors is because people do not realize resistors have voltage limits. This voltage limit varies with the manufacturer and the particular resistor design, but it can be pretty low. When I looked at some stuff for one company many years ago, they had 250 volt rated 470k resistors across 1000 volt diodes. Ouch.

If a breakdown >1000 volt resistor with good reliability is used across a 1000 volt diode there is no life issue at all, but most people use resistors rated at half the diode voltage or less and use carbons. They are setting themselves up for problems.

Depending on what you put the diodes in, you still might need capacitors. Resistors are very unlikely however.


73 Tom

[K8ERV]

people do not realize resistors have voltage limits.

73 Tom

True, but hard to find good ratings. Microwave oven caps have bleeder resistors. I used an expensive HV resistor, then saw that Heath sold an oven kit using an Allen-Bradley 10-meg, 2-watt carbon. Across up to 6kv transient. How? AB rated the resistor at 750v. I contacted them and they said the Heath application was ok. I did a lot of testing and found it did work, so specified them in our (Tappan) ovens. Never had a problem. Strange.

Another thing that may not be well known, is that the ohm value of carbons can change with the applied voltage.

TOM K8ERV Montrose Colo

[KM1H]

FWIW, Mouser has 1N4007A diodes (1000 PIV 1A) in boxes of 1000 for about ten bucks a box (we used to pay $30/box, but they cut the price). All the HV stacks you'll ever need.

Those are OK for receiver circuits
With the cost of 1N5408's, 3A/1000V, under .10 they have become the standard choice for amp builders and retrofit of the old tiny little things that can only take so many surges before they go bang.

The next level is the 6A10, 6A/1000V, which can be had in 100 qty for .16 each; those are fine with many tubes with handles. Some manufactures list as a 6A100 so be sure to read the specs on the 6A10. The diodes in that Collins board look like 10A10's

There have been many discussions on caps across individual diodes in a string and the general consensus is that they can cause problems. The single cap across the full string handles the transient and the output cap takes it to ground. I retrofit many Ameritron and other amps that come in here with PS problems.

Having a .01 across a single diode or a packaged bridge in a LV circuit can eliminate switching noise in sensitive radios and audio equipment. A scope will tell you if there is noise being generated in other equipment.

Carl

[WA7PRC]

Keep in mind nearly all life problems created when using resistors is because people do not realize resistors have voltage limits.
...
Depending on what you put the diodes in, you still might need capacitors. Resistors are very unlikely however.

Some don't have a voltage rating at all. Gotta RTD (Read The Datasheet).

True, but hard to find good ratings.

It's not so tough to find them. I quickly found 'em to retrofit the bleeder & HV multiplier resistors for my SB-220... 100KΩ/3W and 4M7Ω/½W. Sometimes it's tedious to RTD.

With the cost of 1N5408's, 3A/1000V, under .10 ...

The next level is the 6A10, 6A/1000V, which can be had in 100 qty for .16 each ...

No matter the rectifier, they're doggone dirt cheap, compared to yesteryear. A few pennies difference would matter only in a high-volume usage situation... or if you're a certain cheapskate/skinflint/tightwad ham.

[W0BTU]

Some don't have a voltage rating at all. Gotta RTD (Read The Datasheet).

They might not have a rating supplied by the mfgr, but you better believe that they have voltage limits. :-)

[WA7PRC]

They might not have a rating supplied by the mfgr, but you better believe that they have voltage limits. :-)

Probably itty-bitty.

[W8JI]

There have been many discussions on caps across individual diodes in a string and the general consensus is that they can cause problems. The single cap across the full string handles the transient and the output cap takes it to ground. I retrofit many Ameritron and other amps that come in here with PS problems.

That's a waste of time and money, unless for some reason it makes you feel emotionally satisfied to redo something that absolutely never is a problem.

[KM1H]

It makes me feel better that extraneous caps are no longer potential problem areas plus it conforms to current PS design....except for Ameritrons stuck in the 70's mentality.

[W8JI]

It makes me feel better that extraneous caps are no longer potential problem areas plus it conforms to current PS design....except for Ameritrons stuck in the 70's mentality.

Seriously, there is very good reasons they are used. The reasons won't change because KM1H throws a few insults out. :-)

[W8JI]

And JI still likes to browbeat the world into believing he is as good an engineer as HE thinks he is. Some things never change.

There is no reason for saying things like that.

Those capacitors have NEVER caused a problem, and actually serve a couple useful functions. It's OK to not like them Carl, if that is in your blood, but there is absolutely no reason to remove them. All that does is cobble up people's equipment for no reason, and likely makes some things worse like EMI back into the mains at the same time.

They were put there for good reason as part of European compliance for noise back into the mains, and to reduce RF back into the diodes. It was far cheaper and far more effective than buying large HV bypass caps rated at 5 kV.

Just because you do not understand the reason does not always make it a bad idea Carl. It like the RF choke vs stub. The stub was a great suggestion, and did everything better. I can't think of a single reason why a choke would be better in a monoband 6 meter amp, except for physical space. Change is good Carl. :-)