Fitting a ammeter after the change over switch?

[Penlan]

I have been asked to build a couple of control panels incorporating cables to 'Combi' hand held controllers, and to include on the panels an ammeter for each controller, so the said person can see if the loco's etc., are drawing a sensible electric load, and are 'live' in section.
The accepted practice would be to put the ammeter in circuit before the change over switch, but obviously the circuitry is all self contained (and difficult to get at) in the Combi hand held controller, even when I've 'de-constructed' them?
I see you can get ammeters with a + / - reading (centre zero), though they all seem to come from China with a long delivery period and are a deep depth into the panel, unlike some UK ones I've had in the past.  Would the + / - ones give me what I'm looking for after the direction switch?
Or is there a 'cunning plan' to install an ordinary 0 - 1amp (etc.,) after the direction switch.

 

[jpendle]

Hi,

 

A center zero ammeter will work as they are designed to measure current flow regardless of the voltage polarity of the circuit.

I can't think of a "cunning plan" option.

 

Regards,

 

John P

[kevinlms]

The disadvantage with a centre zero one, is that the scale is smaller and harder to read. I would prefer a larger scale version every time, so working out a way to wire BEFORE the direction switch, would be my choice.

Edited June 3, 2021 by kevinlms
Wrong info

[kitpw]

If up to 1amp is ok, then this should do the job...https://www.digikey.co.uk/product-detail/en/adafruit-industries-llc/4404/1528-4404-ND/11503233.

I have one of the Adafruit centre zero voltmeters from Digikey and, as it happens, was looking the other day to see if they have a centre zero ammeter so that it too works in both directions - which they do!

 

Kit PW

A 1920s 7mm terminus layout: Swan Hill - https://www.rmweb.co.uk/community/index.php?/blogs/blog/2502-swan-hill/

[Crosland]

What power supply does the combi have? If it's DC then the ammeter can go on the input.

[Porcy Mane]

4 hours ago, Penlan said:

The accepted practice would be to put the ammeter in circuit before the change over switch, but obviously the circuitry is all self contained (and difficult to get at) in the Combi hand held controller, even when I've 'de-constructed' them?

 

I managed to break the track with a scrawker on the controller side of the Forward/Off/Reverse switch. Then drilled two holes into the PCB track either side of the break, to solder the ammeter wires into. The PCB tracks are about 2mm wide. This was with a Gaugemaster 100 which I suspect uses the same PCB as the Combi.

Pic shows where I made the connections.  (arrowed) Shouldn't be too difficult to use some flexible six core to mount ammeters remotely in an external panel.

 

 

I've used loads of the Chinese sourced ammeters in the 91C4 package. Sometimes they have arrived next day if they are in stock at the UK s importers round Heathrow way.

https://goo.gl/maps/fJdJsqYq7cgBYDk39

 

Here's a pair currently on Ebay:

 

https://www.ebay.co.uk/itm/234003552461?hash=item367bb2d8cd:g:aP0AAOSwiW1gmUvq

 

The readings compare well with my regularly calibrated Fluke meters.

 

21 minutes ago, Crosland said:

What power supply does the combi have?

 

16v AC.

Edited May 29, 2021 by Porcy Mane
Grammar (& it's still crap!)

[Crosland]

1 hour ago, Porcy Mane said:

16v AC.

 

You can use an AC ammeter in the supply feed, in that case.

[Porcy Mane]

23 minutes ago, Crosland said:

 

You can use an AC ammeter in the supply feed, in that case.

 

Considerably harder to find an AC ammeter of the correct range / suitable size / attractive price.

[Penlan]

2 hours ago, Crosland said:

What power supply does the combi have? If it's DC then the ammeter can go on the input.

AC, the rectifier is tucked away between a heat sink and the 'main' board.

[Penlan]

Porcy Mane, many thanks, that looks the way to go for a de-constructed Combi.
I shall also buy in a couple of centre zero's mentioned by kitpw, they look useful
for this particular project.
.
Many thanks to you all for your help and observations, much appreciated.

[Porcy Mane]

14 hours ago, Penlan said:

I shall also buy in a couple of centre zero's mentioned by kitpw,

 

 A better photograph. It shows were I connected my ammeter & the associated break in the track.  Should you wish to return the controller to standard spec. it's just a matter of filling in the track break  with solder.

 

If you connect the ammeter this way you will need a full scale deflection meter as it is before the forward/reverse switch. A centre off meter would only deflect one way.

 

 

With the latest generation of low power consumption motor/g'box combo's I've found you need all the deflection a full scale meter gives to register at loco start.

 

If I remember correctly the voltmeter was soldered into existing holes in the PCB.

 

P

 

 

[Penlan]

Porcy Mane, Thank you.
I have an old Scalespeed controller in a separate box I use for test purposes, and on the face there's a also couple of (small) sockets for a Multimeter to plug into with a switch for either Volts or Ammeter testing.
Had that a few years (40+ ?) now.

[AndyID]

On 29/05/2021 at 06:05, Penlan said:

Or is there a 'cunning plan' to install an ordinary 0 - 1amp (etc.,) after the direction switch.

 

 

If you don't mind going overboard you could put a very low resistance in series with the output after the switch, amplify the voltage-drop with op-amps and use their outputs to drive a voltmeter with a suitably altered legend.

 

But cutting the trace does sound a lot simpler

[Penlan]

Not really relevant, but I wonder why Combi's have a RED LED to show that the power is being delivered to the track.
The LED gets brighter the more voltage going through. 
'Red' to me is 'danger', a short or similar, I find these LED's distracting.
On my own hand held bespoke walk-a-bout controllers, I have a small green LED to show there's power to it - it doesn't vary, it source is straight from the input voltage (circa 15v DC).
 

[kevinlms]

33 minutes ago, Penlan said:

Not really relevant, but I wonder why Combi's have a RED LED to show that the power is being delivered to the track.
The LED gets brighter the more voltage going through. 
'Red' to me is 'danger', a short or similar, I find these LED's distracting.
On my own hand held bespoke walk-a-bout controllers, I have a small green LED to show there's power to it - it doesn't vary, it source is straight from the input voltage (circa 15v DC).
 

Red LEDs were the first available and are CHEAPER.

[Penlan]

Thankyou Kevinlms 

[NinOz]

My solution was to put a DC meter , actually a V&A digital meter, after the throttle direction switch and connect a reversing switch after the meter.  Throttle switch just stays in the one direction.

Both meter and switch mounted in a small box (which can be easily moved between DC throttles).

[ikcdab]

On 29/05/2021 at 16:36, kevinlms said:

The disadvantage with a centre zero one, is that the scale is smaller and harder to read. I would prefer a larger scale version every time, so working out a way to wire BEFORE the direction switch, would be my choice.

 

You could connect it AFTER the switch by wiring it via a bridge rectifier, but you would need to adjust the scale to correct the voltage drop across the diodes - 1.4 volts.

Can you explain this for a beginner? I have a digital panel ameter I want to do this with. I thought I would use the bridge rectifier method as I have some in stock. If I do this, what effect will it have on the accuracy of the ammeter?

[Nigelcliffe]

28 minutes ago, ikcdab said:

Quote

You could connect it AFTER the switch by wiring it via a bridge rectifier, but you would need to adjust the scale to correct the voltage drop across the diodes - 1.4 volts.

Can you explain this for a beginner? I have a digital panel ameter I want to do this with. I thought I would use the bridge rectifier method as I have some in stock. If I do this, what effect will it have on the accuracy of the ammeter?

 

Volts easy - volts over the output, and add the voltage drop over the diodes. 

 

Amps, not so sure how it could be made to work - except by adding another direction switch down-stream of the ammeter.  

 

 

- Nigel

[cliff park]

Don't forget that an ammeter is in series with the supply. So if you feed the ammeter through a bridge rectifier then the supply downstream of that will always be one way, you would need another reversing switch after the ammeter. Digital meter sounds the way to go. How about two 4mm sockets in one leg of the feed, plugging the meter in, in current mode, would read whichever way the current is flowing. You could then bridge the two sockets, short link of wire with a 4mm plug on each end, and remove the meter completely for use elsewhere when not required.

[Porcy Mane]

On 01/06/2021 at 12:04, Penlan said:

'Red' to me is 'danger', a short or similar, I find these LED's distracting.

 

"Man out of my own heart".  Maybe because the first serious railway book I bought was "Red For Danger". (Wymans stand PZ, Station 1969) but more probably because I worked in highly safety conscious industries were red always indicated whoa.

 

On the GM 100 it was a simple job to change the red led for a yellow low intensity flat topper that doesn't burn your eyeballs out at maximum output. I'm firmly a DCC man so automatically replace the red led for a green one on NCE PCP powercab connectors.

 

 

So for me It's always red for short circuit, or used with green as a route indicator.

 

54 minutes ago, cliff park said:

Digital meter sounds the way to go. How about two 4mm sockets in one leg of the feed, plugging the meter in, in current mode, would read whichever way the current is flowing.

 

I prefer digital read outs for accuracy but for some model railway applications I much prefer analogue meters.  Half decent digital multi-meters provide both with an analogue indication being indicated in the form of a bar (Ribbon) display under the main digital read out.

 

When testing locos under DC, I much prefer an old style meter as any mechanical or electrical "stickyness" can be observed more readily by a flicking needle over the constantly changing alpha/ numeric display of a digital readout.

 

P

[Steven B]

If you're fitting the ammeter after the direction/polarity changing switch then I see three options:

• Use an AC ammeter on the input to the controller (although will also measure the current draw of the controller circuitry - you may be able to zero the ammeter though)
• Use two ammeters, diode protected, one for each track polarity
• Use a centre 0 type meter. These are often used for battery charging circuits but are rated for 10A or greater. I believe some of the ones used for audio applications have a much smaller range (<1mA) but it should be possible to put a parallel shunt resistor to rescale the meter into the <1A scale.

 

 

Steven B.

Edited June 3, 2021 by Steven B

[kevinlms]

4 hours ago, ikcdab said:

Can you explain this for a beginner? I have a digital panel ameter I want to do this with. I thought I would use the bridge rectifier method as I have some in stock. If I do this, what effect will it have on the accuracy of the ammeter?

I think I'd better withdraw that comment. It isn't correct. Find a way of putting the ammeter BEFORE the switch.

 

Or do what NinOz has suggested.

Edited June 3, 2021 by kevinlms
More info

[AndyID]

The simplest solution is to use a "hot wire" ammeter. They don't care about current direction, AC or DC. The only snags are they are not linear, they are a bit slow to respond and nobody seems to make them anymore 

[AndyID]

Another way to do it with a basic DC ammeter is with a four-pole relay or two two-pole relays. The relay is only energised for one controller output polarity. The contacts are arranged to insert the ammeter so it only passes current in one direction.

 

The relay can be energized by a simple transistor circuit that can operate the relay even at very low output voltages. I'm assuming the relay and it's control circuit will derive power from the controller's AC output.