Fitting a ammeter after the change over switch?

[Penlan]

2 Ammeters arrived today from China?, both ordered as centre zero up to 1 amp either way.
One as above t'other sweep zero to 1 amp only...
I've been offered a £2 refund?
Looking on Ebay, this supplier seemed to be the only one for these ammeters, all under different names, but John Sun's is the final address for all of them.
 

Edited June 8, 2021 by Penlan

[Grovenor]

On 03/06/2021 at 12:12, cliff park said:

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.

You can connect an ammeter via a bridge rectifier and it will work perfectly well as an ammeter, reading current in either direction. The only effect is to lower the maximum voltage at the motor by 1.4 volts. This would not usually be an issue but if it is then raise the supply voltage to suit.

[AndyID]

5 hours ago, Grovenor said:

You can connect an ammeter via a bridge rectifier and it will work perfectly well as an ammeter, reading current in either direction. The only effect is to lower the maximum voltage at the motor by 1.4 volts. This would not usually be an issue but if it is then raise the supply voltage to suit.

 

The bridge rectifier better be rated well above the maximum current the controller can deliver, the voltage drop increases with current and it's going to be a good bit more than 1.4 volts.

[Penlan]

The Combi delivers < 12V.
The bespoke controllers I have from some 40+ years ago are delivering < 16V, both types from a 18VAC supply.
Having had a couple of problems, though unrelated to the electronics, with my own controllers and the person I'm doing the control panel for has requested Combi's + the Ammeters, I thought I would try the Combi's, especially as I now have 5 at an average cost of £15 each.  That loss of < 4V between my old controllers and the Combi's means my loco's have a reduced top speed, in fact I might almost have to reduce the gear ratio's to get a decent speed from the loco's.
Swings and roundabouts comes to mind.
I'm putting my Combi electronics into my own boxes (hand held controllers) and they plug, via looooong walk-a-bout leads, into boxes with the Ammeter, plus a visible 12V, 21W car brake light bulb (BAY9S) in circuit to protect (a bit) against shortages. I'm aware there's a cut-out in the Combi circuit, but old fashioned Belt & Braces, and yes, somewhere at home I do have one of those controllers with brass studs and bits of resistance wire between each stud etc., Oh the joys of old style '0' gauge .
Meanwhile, I'm getting there, both for the friend and myself.
I must say I'm very grateful for all the guidance I've received on this posting, thank you.

Edited June 8, 2021 by Penlan

[Grovenor]

4 hours ago, AndyID said:

 

The bridge rectifier better be rated well above the maximum current the controller can deliver, the voltage drop increases with current and it's going to be a good bit more than 1.4 volts.

The volt drop in a diode is pretty constant at 0.7V irrespective of current which is why they are often used in preference to resistors for dropping voltage. With the bridge there are 2 diodes in series. It will be close to 1.4V, not a good bit more. And there is no problem getting a rectifier with adequate rating, could just be the same as the one in the controller.

[AndyID]

8 hours ago, Grovenor said:

The volt drop in a diode is pretty constant at 0.7V irrespective of current which is why they are often used in preference to resistors for dropping voltage. With the bridge there are 2 diodes in series. It will be close to 1.4V, not a good bit more. And there is no problem getting a rectifier with adequate rating, could just be the same as the one in the controller.

 

Here's one diode in a 4 amp bridge dropping 0.77 volts at 380 mA. (At low current the drop is 0.70 volts.)

 

 

 

The drop increases to 0.835 volts at 1 amp and .866 volts at 2 amps.

 

 

Edited June 9, 2021 by AndyID
Correction

[Crosland]

16 hours ago, Grovenor said:

The volt drop in a diode is pretty constant at 0.7V 

 

A common mis-conception.

 

Look at any standard silicon diode data sheet and you will see that the forward voltage is nearer 1V at the maximum rated current. E.g. https://www.vishay.com/docs/88503/1n4001.pdf for the common 1N400x series or https://www.vishay.com/docs/88516/1n5400.pdf for the iN540x 3A diode.

[Grovenor]

But much more constant than using a dropper resistor and perfectly Ok for the application under discussion.

[AndyID]

44 minutes ago, Grovenor said:

perfectly Ok for the application under discussion.

 

We're going to have to agree to disagree on that  

[kevinlms]

On 10/06/2021 at 01:32, Grovenor said:

But much more constant than using a dropper resistor and perfectly Ok for the application under discussion.

I have to agree, because if you read the OP (a novel approach?), it refers to a 1 Amp ammeter, so any increase in voltage drop over 1 Amp is irrelevant.