A simple 7805 regulator circuits are flaky

[dxpwny]

Simple 7805 regulator circuit is flaky

So a year ago I plugged a 5 volt AC adapter into my project and blew it up. I had been using adapters with positive in the center ... and mistakenly used one that had negative in the center. Blew a $20 micro.

So ... I decided all my future projects would be designed with a bridge rectifier right after the power adapter. The bridge output would be connected to a 7805 and then I could use any adapter with either polarity (provided the voltage output was above about 6.8 V).

Made several projects with that basic power module design. When I want to use the project, just grab a 9 or 12 V adapter, not worrying about polarity, and all was well.

I just built a few more modules ... and low and behold I get one that worked fine with a particular polarity ... but not the other polarity.

I tried several adapters ... only ones with that one polarity would work. I figured it was the bridge rectifier that had to be bad.

Replaced the thing ... and ... now the other polarity works, but not the one that worked before. Again, I tried several AC adapters.

My design is damn simple. Power jack pins connect to AC inputs of bridge rectifier. DC output of bridge tied to a ~10 uF cap and the 7805 input. The 7805 output has a 1 uF cap on it. Both electrolytics - and polarity is correct.

Is there any other reasonable explanation - other then a batch of flaky bridge rectifiers ?

My guess as to what's going on is that all 4 pins of the bridge rectifier a fine inside - couldn't work other wise. But inside one of the diodes is bad in each bridge I used that resulted in only one adapter polarity working.

I couldn't think of a better way to supply power to my projects (almost always 5V TTL circuits) and not have to worry about the polarity of the power adapters.

 

[KlausST]

Re: Simple 7805 regulator circuit is flaky

Hi,

I was designing electronics for many years now, never had such a problem.

******
Instead of writing a lot of text a schematic, a photo of the pcb, measurement values like voltage and current ... is way more informative.

****
There are a view values you give that can be reviewed:
You said 6.8V of input voltage should work. 6.8V is definitely out of specification. A bridge rectifier drops about 1.4V (depending on type and current, both informations we don't know), and an 7805 needs about 7V input voltage to work correctely.
This means you need at least 8.4V of input voltage.
"It is not working" can be because of a defective part, or simply too low input voltage. A measurement could tell you.

If you look into the rectifiers datasheet you can see voltage rating, rms current rating and pulse current rating.
Check if all three values must meet your specifications.
I personally don't expect a bad batch of rectifiers, they are usually 100% tested during production.

Combined with bulk capacitors you should use ceramics capacitors.

Use overvoltage protectors like MOVs. Maybe input current limiters.
Use power supplies with datasheets. Give informations on the supplies: are they AC, rectified, rectified and filtered, or stabilized?

Klaus

 

[betwixt]

Re: Simple 7805 regulator circuit is flaky

Klaus is right, what you are missing is how the bridge rectifier works. You are quite right to use one in that application, providing the adapter output is isolated, if one side of the adapter is wired elsewhere, for example to a mains Earth pin, it will not work as expected. The problem is that whichever way you wire it, inside the bridge there are two diodes in series with the supply . If you swap the input wires, the other pair of diodes are in series instead. Each diode is dropping ~0.7V (can be more in some cases) so you have lost at least 1.4V of necessary overhead in the input voltage. The reason it works one way and not the other can only be down to variation in the diode voltage drop, both in the internal diodes and from one bridge rectifier to another. 7805s ratings are for a 'typical' minimum voltage drop of 2V but it could be a little higher. With the bridge I would suggest you use an input voltage of no less than 9V to be sure of operation. Measure the voltage under load, do not rely on what the adapter says on the label!

Brian.

 

[udhay_cit]

Re: Simple 7805 regulator circuit is flaky

Can you measure the input & output voltage of the 7805 with no load condition? If you give insufficient voltage to the regulator, it will will give you output voltage below 5V (with no load condition). Can you measure that?

Udhay

 

[dxpwny]

Re: Simple 7805 regulator circuit is flaky

That is not the issue. The issue is 9 VDC and 12VDC AC adapters that use positive center power jacks work, those with negative center do not. Or visa versa. Change the bridge the problem goes away. Change the bridge the problem reverses polarity. The problem is not insufficient voltage to the regulator.

I have now tried with multiple 12 VDC adapters. Some may have put to much emphasis on my statement "provided the voltage output was above about 6.8 V". Testing done under load and not. No difference.

Either polarity should work due to the bridge rectifier. It does not.

I Sorry I did not think of this before. Replaced AC adapter with a BK Precision 1686 power supply. SAME issue. Polarity X works, polarity Y does not. Under load and not. Input from BK supply 15Vdc.

Does anyone think it can be anything but the bridge that is bad ?

Thanks for your insight.

 

[KlausST]

Re: Simple 7805 regulator circuit is flaky

Hi,

Does anyone think it can be anything but the bridge that is bad ?

With your informations we only can guess. If you think assumptions help you, then it's OK.

But if you need real help, you should give us the requested informations.

Klaus

 

[betwixt]

Re: Simple 7805 regulator circuit is flaky

It's the 6.8V you mentioned that cast doubt on the voltage as it needs more than that before a 7805 even without the bridge present.

We are assuming you are using DC adapters as you didn't mention RMS or AC voltages and you did specify the polarity at the plug. If those adapters are AC, the input capacitor is woefully inadequate. That doesn't explain why the precision PSU didn't work though.

When you say 'flaky' what exactly do you mean?
What voltage are you actually getting out of the 7805 and is it stable or not?
Are the input and output capacitors VERY close to the 7805, the data sheets suggest within 1" (~25mm)?
What type of bridge rectifiers are you using? rating?

Bridge rectifier can be bad but I only recall replacing one with a single diode fault once in nearly 50 years - used properly they are extremely reliable.

Brian.

 

[KlausST]

Re: Simple 7805 regulator circuit is flaky

Hi,

used properly they are extremely reliable.

That's my experience, too.

But what do we know? Maybe voltage causes the problem, maybe current causes the problem...
We simply don't know device specifications, nor application circuit.

Klaus

 

[schmitt trigger]

Re: Simple 7805 regulator circuit is flaky

a schematic of your circuit, with components values and fully annotated with actual voltage readings, is better than a 1000 word description

 

[samson A.]

Re: Simple 7805 regulator circuit is flaky

Yeah seen that before. One damaged pin or a damaged diode within the bridge rectifier is certainly the cause. Change your rectifier friend

 

[dxpwny]

Re: Simple 7805 regulator circuit is flaky

Thanks everyone. I think it would have to be a diode that is bad inside the bridge and not a damages pin on the rectifier that is connected internally.

Perhaps it as my soldering ? With the first few I made I had trouble getting solder to wrap around the rectifier pins - they were somewhat corroded. Then I used a metal brush on a Dremmle tool to clean them up.