Cheap make before break SPDT switches?

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PaulMcC

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Anyone know where I can get a relatively cheap single pole double throw switch that is make before break? Needs to be able to handle about 20amps at 12v, and I'm not fussy about whether it is a toggle, push button, rotary switch or whatever.
Everything I can see that is relatively cheap seems to be BBM.

Thanks
 
would a rotary 1, 2, both, off battery selector switch be suitable. Obviously heavier than you need but MBB switching via both although BBM on those types that allow switching via Off

Not relatively cheap I suppose.
 
Thanks VicS. I had the same thought and actually have a spare 1-2-both switch but it is about 6 inches across and made of orange plastic. I may end up resorting to that but I currently hoping I can find something small suitable to fit on, or next to, the main bank of switches above the chart table.
 
PaulMcC said:
Thanks VicS. I had the same thought and actually have a spare 1-2-both switch but it is about 6 inches across and made of orange plastic. I may end up resorting to that but I currently hoping I can find something small suitable to fit on, or next to, the main bank of switches above the chart table.
Click to expand...

Out of interest why do you need a MBB switch.

I take it you have looked at RS.
 
Not precisely what you asked for but easy and cheap: you could fit a BBM SPDT, then right beside it you could fit a BBM DPST NO non latching switch. If you take the 3 wires that you have connected to the SPDT switch (let's call them common, A and B) and
(i) link the common to one side of both poles of the momentary DPDT,
(ii) link A to the other side of one of the poles of the NO SPDT switch, and
(iii) link B to the other side of the other pole of the NO SPDT switch

then if you briefly press and hold the NO SPDT switch to "make" it, while you operate the BBM SPDT switch, you will cause the BBM SPDT to function as if it were a MBB. The NO SPDT switch basically joins all three wires together when you press it

This requires whatever equipment you're dealing with to tolerate an overlap measure in seconds not milliseconds. and it is not foolproof if someone else uses your boat, but if only you use the boat and the MBB function isn't life-or-death, this will meet your needs. A pair of Carlings to do this would be £12 or so

BTW if you want to be fussy, Axon will etch any legend you want onto a Carling actuator and you can backlight them if you get the Carlings with LEDs. I have all mine backlit red with the legends permanently illuminate during night time sailing. Would be handy in your case, because you could have a custom LED backlit legend in the switch set up I describe above, eg "HOLD WHILE OPERATING XXX SWITCH". Each engraved actuator is less than a tenner and Axon are very nice guys to deal with. Note that you would need a NO DPST switch with the LED on the top side of the switch so that it back lights the legend all the time, instead of the (more common) switch with LED at bottom that illuminates the "on" window only when the switch is pressed. However, Carling stock such a switch and I bought some a few weeks ago. I always overstock so I'll mail you one FOC if you get stuck. Picture...

Carling%20V%20Actuators.jpg
 
PaulMcC said:
Thanks VicS. I had the same thought and actually have a spare 1-2-both switch but it is about 6 inches across and made of orange plastic. I may end up resorting to that but I currently hoping I can find something small suitable to fit on, or next to, the main bank of switches above the chart table.
Click to expand...

BEP do a miniature 1-2-Bothe switch, the BEP701S. It's 69mm x 69mm. Any good?

71MdtgmesXL._SY355_.jpg


It's £33.99 from Amazon (http://www.amazon.co.uk/BEP-701S-Marine-Battery-Switch/dp/B000NI3EXY) which may not count as "cheap".
 
jfm said:
Not precisely what you asked for but easy and cheap: you could fit a BBM SPDT, then right beside it you could fit a BBM DPST NO non latching switch. If you take the 3 wires that you have connected to the SPDT switch (let's call them common, A and B) and
(i) link the common to one side of both poles of the momentary DPDT,
(ii) link A to the other side of one of the poles of the NO SPDT switch, and
(iii) link B to the other side of the other pole of the NO SPDT switch

then if you briefly press and hold the NO SPDT switch to "make" it, while you operate the BBM SPDT switch, you will cause the BBM SPDT to function as if it were a MBB. The NO SPDT switch basically joins all three wires together when you press it

This requires whatever equipment you're dealing with to tolerate an overlap measure in seconds not milliseconds. and it is not foolproof if someone else uses your boat, but if only you use the boat and the MBB function isn't life-or-death, this will meet your needs. A pair of Carlings to do this would be £12 or so

BTW if you want to be fussy, Axon will etch any legend you want onto a Carling actuator and you can backlight them if you get the Carlings with LEDs. I have all mine backlit red with the legends permanently illuminate during night time sailing. Would be handy in your case, because you could have a custom LED backlit legend in the switch set up I describe above, eg "HOLD WHILE OPERATING XXX SWITCH". Each engraved actuator is less than a tenner and Axon are very nice guys to deal with. Note that you would need a NO DPST switch with the LED on the top side of the switch so that it back lights the legend all the time, instead of the (more common) switch with LED at bottom that illuminates the "on" window only when the switch is pressed. However, Carling stock such a switch and I bought some a few weeks ago. I always overstock so I'll mail you one FOC if you get stuck.
Click to expand...

Like I said, only much, much better ;)
 
If there is a need to maintain continuity of supply to something while changing sources, you might be able to do it with a capacitor. If, for example, you need to drop no more than 0.5V while discharging 20A for 10ms, you could do it with a 20 *.01 / 0.5 = 0.4F supercapacitor. Easier just to find the right switch, though.
 
I had a look at rs but couldn't find anything with the current capacity that was MBB and small. I'll fire off a few emails to rs, aquafax and so on and see what they say but think it may end up not worth it.
The aim is to have two parallel routes for supplying power to my main distribution board, one of which is straight through and another which powers a set of diagnostics including an ammeter for the whole board. A break before make switch isn't a disaster, just means needing to reboot things whenever I want to switch over (not that often). Or I can have a single supply route and accept that the ammeter shunt is always powered, with other diagnostics on their own switches.

A MBB switch would be an elegant solution but it's definitely not a big problem!
 
I'm confused now. What is problem with having diagnostics switched as you say, and the shunt always "powered"? The shunt itself consumes no energy. The voltmeter you place across the shunt (the thing that is called an ammeter, but of course it is a voltmeter) can have a switch if you care about the power drain of that.
 
jfm said:
I'm confused now. What is problem with having diagnostics switched as you say, and the shunt always "powered"? The shunt itself consumes no energy. The voltmeter you place across the shunt (the thing that is called an ammeter, but of course it is a voltmeter) can have a switch if you care about the power drain of that.
Click to expand...

There will be a small volts drop across the shunt perhaps that's what the op is worried about but typically it will only be 50mV at the max reading of the ammeter!

To split hairs the shunt consumes energy at the rate of amps x PD across the it. Therefore a 20A/50mV shunt will consume 20x50/1000 watts = 1 watt (or 1 Joule per second) at the full 20 amps. The power consumed by the meter itself will be a fraction of that because only a very small current passes through it, so no point in fitting a switch in that circuit.
 
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VicS said:
There will be a small volts drop across the shunt perhaps that's what the op is worried about but typically it will only be 50mV at the max reading of the ammeter!
Click to expand...

If he is really worried about that watt he could fit a Hall-effect current sensor around the wire and lose nothing at all when the meter is off. Beat me why yachting people still use 'orrible crude shunts anyway.

MCS_SD2537_use.jpg
 
JumbleDuck said:
If he is really worried about that watt he could fit a Hall-effect current sensor around the wire and lose nothing at all when the meter is off. Beat me why yachting people still use 'orrible crude shunts anyway.
Click to expand...
that would be sensible
 
JumbleDuck said:
If he is really worried about that watt he could fit a Hall-effect current sensor around the wire and lose nothing at all when the meter is off. Beat me why yachting people still use 'orrible crude shunts anyway.
Click to expand...

Possibly because most cheap(ish) ammeters and battery monitors are designed around shunts.
 
VicS said:
There will be a small volts drop across the shunt perhaps that's what the op is worried about but typically it will only be 50mV at the max reading of the ammeter!

To split hairs the shunt consumes energy at the rate of amps x PD across the it. Therefore a 20A/50mV shunt will consume 20x50/1000 watts = 1 watt (or 1 Joule per second) at the full 20 amps. The power consumed by the meter itself will be a fraction of that because only a very small current passes through it, so no point in fitting a switch in that circuit.
Click to expand...
Vic, I'm rusty so I'll happily stand corrected, but I don't get that.

First, if there are no consumer loads then there is no current and the shunt (and the voltmeter, labelled as an ammeter) consumes nothing

Second, the 20A in your maths is the current through the circuit, the whole shunt, not the current thru just the high resistance leg of the shunt. It looks to me like your calculation is wrong therefore (though feel free to correct me if I'm wrong, as I said!). The high resistance leg of the shunt (the one you measure volt drop across, to get amps) might carry only say 1/100th of the 20Amps but for sure it always ADDS conductor x-section area AOTBE, so reducing resistnace of the whole shunt. So the shunt (if the main leg of the shunt is correctly sized so as not to be a bottleneck) itself adds no resistance and consumes no energy at all. The high resistance leg of the shunt is actually the opposite of a bottleneck (very slightly)
 
jfm said:
Second, the 20A in your maths is the current through the circuit, the whole shunt, not the current thru just the high resistance leg of the shunt. It looks to me like your calculation is wrong therefore (though feel free to correct me if I'm wrong, as I said!). The high resistance leg of the shunt (the one you measure volt drop across, to get amps) might carry only say 1/100th of the 20Amps but for sure it always ADDS conductor x-section area AOTBE, so reducing resistnace of the whole shunt. So the shunt (if the main leg of the shunt is correctly sized so as not to be a bottleneck) itself adds no resistance and consumes no energy at all. The high resistance leg of the shunt is actually the opposite of a bottleneck (very slightly)
Click to expand...

I don't recognise your description of a shunt. All the shunts I've ever seen on boats have all the current passing through, and don't have a "high resistance leg". The voltage drop is masured across the whole shunt, not just part of it.

200-50.png
 
jfm said:
Vic, I'm rusty so I'll happily stand corrected, but I don't get that.

First, if there are no consumer loads then there is no current and the shunt (and the voltmeter, labelled as an ammeter) consumes nothing

Second, the 20A in your maths is the current through the circuit, the whole shunt, not the current thru just the high resistance leg of the shunt. It looks to me like your calculation is wrong therefore (though feel free to correct me if I'm wrong, as I said!). The high resistance leg of the shunt (the one you measure volt drop across, to get amps) ...
Click to expand...

The high resistance thing is the meter, and the low resistance thing is the shunt. They are wired in parallel, so the voltage across them is the same (less a tiny, tiny drop in the wires from shunt to meter). A traditional moving coil meter is typically 1mA full scale deflection, so if you want it to measure 100A you need to arrange for 1mA to go through the meter when 99.999A is going through the shunt, so you need a shunt with a resistance 1/99999 times that of the meter.

In practice, the meter has no effect .... 99999 = 100000 for all practical purposes.
 
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