Koford (Slot Car) Bench Power Supply Project

[Gryd3]

However, you did say determining the ideal value resistor would be difficult?

I think if I could get the input voltage to the meter to be @ ~5v via a resistor on the capacitor between the meter, it would be perfect...

-TheChad

Yes, as it is directly proportional to the current draw of the meter...
So for sake of argument, the meter draws anywhere from 10mA to 150mA (purely made up, but LEDs add up quickly)
If you want to drop the voltage down from 19V to 8V, you would need a resistor that drops 11V @ 150mA (~73Ω.)
But what if the meter is drawing less current? That 73Ω will barely drop a volt.
The reason calculating this is difficult, is because many devices have a dynamic current draw that changes during operation... This change will directly affect the resulting voltage drop across the resistor which could result in the voltage dropping too much, or not enough.
This is a very simple solution mind you, and could work, but it is far from ideal and requires experimentation or knowing the current draw characteristics of the meter.

While you tinker, can you please elaborate on the wiring for the meter... or post a diagram/pic of it?

 

[TheChad]

Okay, Here are some pictures of the (almost) complete project!

The only thing I have left to do, is to figure out what to do with the power to the volt/amp meter.. Otherwise, everything else is done!

Thank you EVERYONE for all the help! You all have been such a great help to me! This has been a fun project!

-TheChad

 

[TheChad]

Yes, as it is directly proportional to the current draw of the meter...
So for sake of argument, the meter draws anywhere from 10mA to 150mA (purely made up, but LEDs add up quickly)
If you want to drop the voltage down from 19V to 8V, you would need a resistor that drops 11V @ 150mA (~73Ω.)
But what if the meter is drawing less current? That 73Ω will barely drop a volt.
The reason calculating this is difficult, is because many devices have a dynamic current draw that changes during operation... This change will directly affect the resulting voltage drop across the resistor which could result in the voltage dropping too much, or not enough.
This is a very simple solution mind you, and could work, but it is far from ideal and requires experimentation or knowing the current draw characteristics of the meter.

While you tinker, can you please elaborate on the wiring for the meter... or post a diagram/pic of it?

Here's a link to the eBay auction of the meter I bought: http://www.ebay.com/itm/261591441659?_trksid=p2059210.m2749.l2649&ssPageName=STRK:MEBIDX:IT

It has pictures of the meter and connections... As you can see, there is a wire harness with 3 wires, Red/Black/Blue. Those are the load wires that tell the volt/amp meter what to display, then there is a 2 wire harness (small wires) those are the power wires that power up the meter display.

I used the old 1k potentiometer to do some testing.... With the 1k pot @ 995Ω setting, the voltage output was 6.20v, which would be acceptable... I then turned the power supply up threw the range 1.2-12.6v, and the voltage after the 1k pot measured from the 6.2v - 8.2v.. Which would still be fine... But I shut off the power supply and it still took quite awhile to drain off the capacitor enough to drop the voltage down enough to power off the meter...

Also, while running the power supply threw the works, I notice another issue... If I shut the main power switch off with out turning the power adjust knob off, there will still be power to the test leads until the capacitor drains off... With the knob shut off, It breaks the connection to the lead's and has 0 volts.

This is all with no load connected.

Thanks,

-TheChad

 

[TheChad]

Just a thought.... I wonder if there is such a thing as a rocker switch that has 2 separate circuits?

Currently the main power rocker switch just breaks the A/C feed from the wall.. If there is such a rocker switch that has 2 separate circuits, I could break the connection between the capacitor and the meter, which would instantly shut the meter off.. The capacitor will drain itself in 30 seconds, and the problem is solved.....

-TheChad

 

[(*steve*)]

I wonder if there is such a thing as a rocker switch that has 2 separate circuits?

Yes there are, but I would recommend you don't use the same switch for both mains and low voltage circuits.

You may be better off using a relay.

 

[Gryd3]

Just a thought.... I wonder if there is such a thing as a rocker switch that has 2 separate circuits?

Currently the main power rocker switch just breaks the A/C feed from the wall.. If there is such a rocker switch that has 2 separate circuits, I could break the connection between the capacitor and the meter, which would instantly shut the meter off.. The capacitor will drain itself in 30 seconds, and the problem is solved.....

-TheChad

There absolutely is.
Its abbreviated DPST or DPDT for Double Pole, (Single/Double) Throw.
I did not mention this, because it would involve connecting a high voltage mains line to one side, and low voltage to the other. It's usually best to isolate these things to prevent shorts between high/low voltage... you don't want a faulty switch to send 120+ Volts to the voltage input on your meter if it's only rated up to 30

 

[TheChad]

There absolutely is.
Its abbreviated DPST or DPDT for Double Pole, (Single/Double) Throw.
I did not mention this, because it would involve connecting a high voltage mains line to one side, and low voltage to the other. It's usually best to isolate these things to prevent shorts between high/low voltage... you don't want a faulty switch to send 120+ Volts to the voltage input on your meter if it's only rated up to 30

Well, that and the meter is DC and the main's are AC....

I was reading about the DPST and it says they are isolated from each other... so even if there was a short inside the switch, the 2 circuits shouldn't short to each other?

-TheChad

 

[Gryd3]

Well, that and the meter is DC and the main's are AC....

I was reading about the DPST and it says they are isolated from each other... so even if there was a short inside the switch, the 2 circuits shouldn't short to each other?

-TheChad

It's still risky and a bad practice.
I won't say it won't work, but I would never advise doing it.

 

[TheChad]

Yes there are, but I would recommend you don't use the same switch for both mains and low voltage circuits.

You may be better off using a relay.

I thought about a relay, but I don't think I have room for a relay.... I pretty much used up all available space...

I have been looking/reading looking for any type of electronic relay/switch, but not finding much solutions...

-TheChad

 

[Gryd3]

A relay can be quite small. Although I agree space is limited.
It does sound like the most ideal solution at the moment.

 

[TheChad]

It's still risky and a bad practice.
I won't say it won't work, but I would never advise doing it.

Fair enough.... The best option would be a switch potentiometer with 2 separate switches... But I haven't found one of those.... My current Switch potentiometer, the switch is connected between the + out from the DC/DC Converter and the + in on the volt meter / the + test lead, so when the potentiometer is switched "Off" it show's 0.00v on the meter and cuts the voltage to the test leads.... Otherwise it shows 1.2v on the meter and there is 1.2v on the test leads....

If the DC/DC Converter went all the way down to 0v instead of 1.2v, then I could use the switch on the potentiometer!

It always has to be difficult!

-TheChad

 

[TheChad]

So what relay would you guys suggest?

Relay's have to be activated by something right? How would I activate the relay?

-TheChad

 

[(*steve*)]

The relay could be operated by the AC from the secondary of the transformer. You would need a transformer with an AC coil. The contacts would need to be rated for something comfortably in excess of the maximum current your power supply is capable of.

You will use the "normally open" contacts.

 

[TheChad]

The relay could be operated by the AC from the secondary of the transformer. You would need a transformer with an AC coil. The contacts would need to be rated for something comfortably in excess of the maximum current your power supply is capable of.

You will use the "normally open" contacts.

I don't quite understand what you are saying? Operated by the center lead on the transformer? That lead was cut off flush as I never used it..... If that's what you are talking about?

I assume you meant a relay with an AC coil?

-TheChad

 

[Gryd3]

TheChad, he meant secondary not center

The Primary is the 120V side, the Secondary is the ~19-20V side before the rectifier. This voltage can turn a relay on/off, and because it is before the rectifier, the capacitor will not back-feed and keep it powered. So as soon as the transformer looses power, the rely switched over and the meter goes out.

 

[TheChad]

TheChad, he meant secondary not center

The Primary is the 120V side, the Secondary is the ~19-20V side before the rectifier. This voltage can turn a relay on/off, and because it is before the rectifier, the capacitor will not back-feed and keep it powered. So as soon as the transformer looses power, the rely switched over and the meter goes out.

10-4, so I would connect the relay to the output of the transformer before going into the rectifier. Got it.

Soo the AC feed with act as the switching feed only and the meter would be connected between the terminals that open/close with the voltage from the AC?

So can you guys suggest a relay? Will the relay automatically be closed any time there is power from the AC? in other words the amount of voltage wont matter?

Thanks!

-TheChad

 

[Gryd3]

Think of it is different stages of your circuit:
Stage 1) High voltage AC into the transformer primary with a switch on the front panel to cut power
Stage 2) Low voltage AC out of the transformer that gets fed directly to the rectifier.
** You will connect the relay here. So that the output for the transformer branches off and powers both, the rectifier and the relay at the same time. This is not to be connected in series, but in parallel so that the relay does not impede current flow into the rectifier.
Stage 3) Low voltage DC out of the rectifier that can either be completely switched by the relay, or have a branch to the meter that is controlled by the relay.
Stage 4) Voltage Regulator takes rough Low voltage DC and regulates it based on potentiometer setting.

So Stage 3, and 4 is where you want to mess with. Do you have any local suppliers you could buy a relay from?
I can't personally recommend one, as I have only ever used relays in a DC configuration, and although I am confident enough to try to trigger one with AC, I am not confident enough to recommend you grab any ol' relay to do this. Hopefully @(*steve*) has a good product in mind for you.

 

[(*steve*)]

The Chad, remind me again:

1. The secondary voltage of the transformer
2. The maximum output current
3. Your preferred supplier

And I'll try to look something up for you.

 

[TheChad]

The Chad, remind me again:

1. The secondary voltage of the transformer
2. The maximum output current
3. Your preferred supplier

And I'll try to look something up for you.

1. 12.6v (rated), but measures 14.8 (no load)
2. 8A
3. Radio Shack (only local supplier) or Mouser

Is there such a thing as an electronic relay that doesn't have the *click* of the magnetic relay and maybe smaller?

Thanks!

-TheChad

 

[(*steve*)]

1. 12.6v (rated), but measures 14.8 (no load)
2. 8A
3. Radio Shack (only local supplier) or Mouser

This would be appropriate.

Note that I am not certain why you need this relay. I think it's to stop the voltage slowly dcaying at the output when the power is removed. I don't think you need to worry about this, but if you really do, this is an appropriate relay to switch the output off when the power is removed.

Is there such a thing as an electronic relay that doesn't have the *click* of the magnetic relay and maybe smaller?

Yes to the first part, but it would be larger.

The click of the relay is likely to be almost simultaneous with the click of the power switch.