Koford (Slot Car) Bench Power Supply Project

[Gryd3]

Okay, seems simple enough, how do we know/verify what size capacitor is sufficient?

-TheChad

Daven stated 1000uF/Amp is the rule of thumb.
So 20uF would work for a 20mA circuit.
You have a small 100uF spare though, so why not just pop it in and try?
It's bigger than the rule of thumb value, but is already pretty small, and it only needs to be big enough to hold a charge for a fraction of a second inbetween pulses.
The bigger cap you have already used in your supply is sized for the same thing... but is sized to provide multiple amps... so when you turn it off and the only thing drawing power is your meter... it stays on for a while.
I'm thinking this solution will run cooler and give you a more immediate response when you shut off your supply than trying to drain the big capacitor you have with a couple wire wound resistors

 

[TheChad]

I will go to Radio Shack tomarrow and pick up the diode and give it a try!

-TheChad

 

[(*steve*)]

Daven stated 1000uF/Amp is the rule of thumb.

That's for full wave rectification. You need at least twice that for half wave rectification.

You have a small 100uF spare though, so why not just pop it in and try?

For 20mA that sounds fine.

20mA into 100uF gives 200V/s. so in 1/50 sec, the voltage will fall by 4V. So your ripple will be about 4V.

 

[Gryd3]

That's for full wave rectification. You need at least twice that for half wave rectification.



For 20mA that sounds fine.

20mA into 100uF gives 200V/s. so in 1/50 sec, the voltage will fall by 4V. So your ripple will be about 4V.

Thank you for that Steve.
The difference between full and half-wave didn't cross my mind.

 

[TheChad]

Okay!

So I went and picked up the diode. I connected everything (with test leads) and took some measurements.

The voltage out of the center tap after rectification measures 8.61v, so with what (*steve*) said, the ripple is dropping down to ~4.61v, which is just over the 4.5v required for the meter, but not by much.

Going by (*steve*)'s information, I *Should* have a ~500uF capacitor? What would my ripple be then?

The meter turns on and off instantly with the main power switch. *Perfect*

Thanks,

-TheChad

 

[TheChad]

I said in the last post that the voltage after rectification measures 8.61v, I don't know if this makes any difference, but to correct that statement..

Voltage after rectification (no capacitor) measures ~7.32v, Voltage after rectification (w/capacitor) measures ~8.61v.

-TheChad

 

[(*steve*)]

Going by (*steve*)'s information, I *Should* have a ~500uF capacitor? What would my ripple be then?

500uF will give approx 1/5th the ripple of 100uF. So 4V down to just under 1V. Personally, I'd go a 220uF capacitor.

 

[Gryd3]

I said in the last post that the voltage after rectification measures 8.61v, I don't know if this makes any difference, but to correct that statement..

Voltage after rectification (no capacitor) measures ~7.32v, Voltage after rectification (w/capacitor) measures ~8.61v.

-TheChad

Without the capacitor, your meter is going to give you an 'average' reading.
When you connect a capacitor, it will give you a 'peak' rating... the exception here is if there is a load drawing from the capacitor at the same time.

 

[TheChad]

500uF will give approx 1/5th the ripple of 100uF. So 4V down to just under 1V. Personally, I'd go a 220uF capacitor.

Okay, well I went to Radio Shack and they don't have a 220uF capacitor.. They only had 100uF then jumped to 470uF..

How important is getting a 220uF capacitor vs using the 100uF?

I can order one if I need to..

I could install another diode on the negative terminal, which would create a full wave rectifier?

-TheChad

 

[Gryd3]

Okay, well I went to Radio Shack and they don't have a 220uF capacitor.. They only had 100uF then jumped to 470uF..

How important is getting a 220uF capacitor vs using the 100uF?

I can order one if I need to..

I could install another diode on the negative terminal, which would create a full wave rectifier?

-TheChad

4 Diodes are required to make a full-wave rectifier. That's what the 'bridge' is made of that is already in your power supply.
Due to simplicity and size, I had recommend using a single diode to build a half-wave.
As far as the capacitor is concerned, bigger values will hold a larger charge which will reduce the ripple. Of course, it will also increase the amount of time it will take to drain and allow the device to shut off.
Excessive ripple could cause a multitude of things, or nothing at all...
It depends on the device you are powering. Some devices are built to handle it, other aren't. It could cause inaccuracies with the meter, or it could cause the internal power supply to fail prematurely.
You can try the 470uF if you want, or you could put a second 100uF in parallel with the first. I think your just fine with what you have though unless someone can think of a reason not to.

 

[TheChad]

Okay, I guess I'll stick with the 100uF..

I just thought of something... I soldered the diode to the center tap on the transformer, then connected the positive terminal from the meter to the other end of the diode... But I connected the negative terminal from the meter to the negative terminal of the large capacitor....

The meter works with out a negative connected at all, we already determined the negative is common..

Do I need to change where I have negative connected?

Thanks,

-TheChad

 

[Gryd3]

Okay, I guess I'll stick with the 100uF..

I just thought of something... I soldered the diode to the center tap on the transformer, then connected the positive terminal from the meter to the other end of the diode... But I connected the negative terminal from the meter to the negative terminal of the large capacitor....

The meter works with out a negative connected at all, we already determined the negative is common..

Do I need to change where I have negative connected?

Thanks,

-TheChad

I thought we had determined the negative on the switching power regulator was common... I had thought the meter had a completely separate power supply compared to the positive and negative inputs for reading voltage and current...

Ideally the meter should be connected like the picture a few posts back... but without knowing how it's wired internally, it may not make a difference considering it's working now... Just make sure the readings it is giving you is correct.

 

[TheChad]

I thought we had determined the negative on the switching power regulator was common... I had thought the meter had a completely separate power supply compared to the positive and negative inputs for reading voltage and current...

We did... The meter does have a completely seperate power than the inputs for reading voltage and current...

But connecting only the red (positive) to the power for the meter, powers the meter...

 

[Gryd3]

We did... The meter does have a completely seperate power than the inputs for reading voltage and current...

But connecting only the red (positive) to the power for the meter, powers the meter...

Hrm.. if that's the case, it would appear to have a common ground as well which is not cool...
You had stated that you originally connected the + of the meter to the center tap after the diode, and the negative went to the negative of the existing capacitor. I would leave your setup the way it is.
It would be nice to have the negative power lead for the meter connected as well before the bridge... but if the negative lead for the voltage test is common, or internally tied together, you may end up creating a short within your power supply.

Regardless though, you said you have it wired and it is working as desired.
Are you happy with the current setup?

 

[TheChad]

Hrm.. if that's the case, it would appear to have a common ground as well which is not cool...
You had stated that you originally connected the + of the meter to the center tap after the diode, and the negative went to the negative of the existing capacitor. I would leave your setup the way it is.
It would be nice to have the negative power lead for the meter connected as well before the bridge... but if the negative lead for the voltage test is common, or internally tied together, you may end up creating a short within your power supply.

Okay, I just tried connecting it the way you suggested... Center tap->Diode->Positive lead on meter | Other lead from transformer-> Negative lead on meter... Zero Volts.

When I check with the volt meter. Touching the other side of the diode and one of the other taps on the transformer I get 0V. However when I touch the negative lead on the volt meter to the negative lead on the large capacitor, I get 7.32v.

So something isn't right.... Am I suppose to have a diode between the other tap on the transformer and the negative terminal? (2 Diodes)

I did test again the 2 (+ & -) leads to power the volt/amp meter, the positive MUST be connected for the meter to turn on, however the meter turns on with ONLY the + lead connected....

So I guess the negative lead in the Amp/Volt meter harness is the common (negative) for the whole meter..

To test this, I disconnected the negative terminal from the DC/DC Converter that goes to the Negative (Common) terminal of the Amp/Volt meter and the meter does not power on with only the positive connected, when I plug the Negative back into the DC/DC converter or connect the negative from the power harness to the negative on the main capacitor or the negative terminal on the DC/DC converter, the meter powers on....

So I guess that shows that the negative (common) in the meter is all the same. While there is 2 wires, its the same circuit..

-TheChad

 

[Gryd3]

Okay, I just tried connecting it the way you suggested... Center tap->Diode->Positive lead on meter | Other lead from transformer-> Negative lead on meter... Zero Volts.

When I check with the volt meter. Touching the other side of the diode and one of the other taps on the transformer I get 0V. However when I touch the negative lead on the volt meter to the negative lead on the large capacitor, I get 7.32v.

So something isn't right.... Am I suppose to have a diode between the other tap on the transformer and the negative terminal? (2 Diodes)

I did test again the 2 (+ & -) leads to power the volt/amp meter, the positive MUST be connected for the meter to turn on, however the meter turns on with ONLY the + lead connected....

So I guess the negative lead in the Amp/Volt meter harness is the common (negative) for the whole meter..

To test this, I disconnected the negative terminal from the DC/DC Converter that goes to the Negative (Common) terminal of the Amp/Volt meter and the meter does not power on with only the positive connected, when I plug the Negative back into the DC/DC converter or connect the negative from the power harness to the negative on the main capacitor or the negative terminal on the DC/DC converter, the meter powers on....

So I guess that shows that the negative (common) in the meter is all the same. While there is 2 wires, its the same circuit..

-TheChad

You should not need two diodes to power your meter... and I don't like how they provide you with a separate wire for negative to power the meter, and a negative to measure with... You could cause some damage not knowing that... they should have been isolated from each other, but I guess you get what you pay for. Luckily, your project does not require the negatives to be at different potentials.
I'd leave the diode on the positive side of your meter and leave it how you currently have it working.

 

[TheChad]

You should not need two diodes to power your meter... and I don't like how they provide you with a separate wire for negative to power the meter, and a negative to measure with... You could cause some damage not knowing that... they should have been isolated from each other, but I guess you get what you pay for. Luckily, your project does not require the negatives to be at different potentials.
I'd leave the diode on the positive side of your meter and leave it how you currently have it working.

Why would I get a 0v reading when touching the center tap -> Diode and another leg from the transformer? What am I missing?

Was the diode suppose to be on the negative side?

-TheChad

 

[Gryd3]

Why would I get a 0v reading when touching the center tap -> Diode and another leg from the transformer? What am I missing?

Was the diode suppose to be on the negative side?

-TheChad

The diode is good where you have it. Its only there to prevent current flowing backward through the meter's power supply.
Did you have your meter set on DC or AC when you measured from the center tap to the other leg?

 

[TheChad]

The diode is good where you have it. Its only there to prevent current flowing backward through the meter's power supply.
Did you have your meter set on DC or AC when you measured from the center tap to the other leg?

DC

 

[TheChad]

TA-DA!

It's DONE! Took awhile to get there, but this project is complete!

Thank you all soo much for your help! I definitely couldn't have done all this with out you guys!