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How to build a switched power supply for accessories

19K views 33 replies 13 participants last post by  onewizard 
#1 · (Edited)
I wrote the following article for another site and thought I would double post here for anyone interested.

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Why a relay?

Each pre existing circuit in your bike is fused. If you install a power hungry accessory like heated grips or tank mounted espresso maker it will start blowing fuses if connected to an existing circuit such as the head light circuit and if you up the fuse the original device is no longer protected.

With a high current accessory like heated grips they need to be powered directly from the battery on their own fused circuit. The only issue with a direct battery connection is the connected devices always have power so can run down the battery if left on. A switched power source that is only active when the engine is running is more desirable. This is where a relay comes in. A relay will allow you to take the power draw off the battery where it should be but only be active when the relay is turned on by the ignition.

Several bike accessory sources like Twisted Throttle sell a pre packaged bike accessory relay block for wiring in accessories for about $80. It does the exact same thing as the ~$10 circuit you can build yourself from an automotive relay.


How to build a relay controlled circuit

A relay is basically an electrically operated switch. Buy a generic 4 pin automotive relay. They're cheap and available at most auto parts stores. The switch circuit is very low power, while the switched circuit can handle high current flow. Energize the coil side of the relay from a source that is only on when the engine is running. For example pin 85 to Ground and pin 86 to the tail running light. Power the relay from the POS terminal of the battery (pin 30) via a fuse in an installed inline fuse holder of suitable capacity. You now have a high current power source, that is only energized when the ignition is ON, from pin 87. Optionally you can connect pin 87 to a wiring block to provide more than one connection point if you have multiple accessories.

Note some automotive relays will have five pins with the additional pin labeled 87a. The extra pin functions the same as pin 87 with the exception that it supplies power when the relay is not energized and switches off when the relay is energized, the opposite of pin 87. If you use a relay of this type remember to insulate the unused pin so it does not short out on the frame.


How to make a reliable electrical connection


There are several ways to connect wires together but several of them are problematic and the source of reliability issues. In particular tap connectors that punch through the insulation to tap into a connector are unreliable and should be avoided if possible. The best way to join wires is with solder. Twisting wires together creates another source of potential electrical problems.

To create a good solder joint twist the wires together as a first step. Next heat the twisted wires from the bottom (heat rises) with a hot soldering iron for a minute or two. If you hold the solder against the heating wires (from the top) it should melt into the wires when they get hot enough. Let the solder melt into the hot wires rather than melting the solder directly with the soldering iron. Remember to pre heat the soldering iron for 5 minutes or so before using - they don't heat up instantly.

A clean soldering iron tip will transfer heat much more effectively and just work better. Keep the tip of the soldering iron clean when it is hot, by brushing against a damp sponge or cloth, usually after every solder. Emery paper can be used to sand oxidation off a cold soldering iron tip that has not been used in a while. A clean tip will have a shinny coating of solder. If it is dull it needs cleaning.

A wire stripper like that pictured to the right is cheap (~$5-10) to purchase at a specialty electronics store like The Source and the best choice for removing insulation A dedicated wire stripper makes it easy and quick to remove insulation from wires with a high degree of precision. Knives, razor blades and in particular pliers like wire strippers perform poorly and often cut a portion of the wire as well as the insulation.

An electrical connection can be insulated with either electrical tape or a heat gun and shrink wrap. Do not use duct tape, scotch tape or other kinds of tape as the insulation properties and/or adhesive durability are can often be poor.
 

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#2 ·
I have a question about those fuse blocks they sell from Twistedthrottle (as well as others). It looks like they only have one relay. Wouldn't separate devices require separate relays?

I recently connected my heated grips and had planned on running some driving lights off the same relay. Then I started thinking about how I'd turn them off.

Nice write up, BTW.
 
#3 ·
Only one relay is needed for all your accessories but I would connect a fuse between pin 87 and each accessory. Most accessories like heated grips come with in line fuses anyway. Optionally you can buy a cheap terminal block to provide extra connection points to pin 87 of the relay or just connect the wires together at pin 87. Suggest 12guage wire.
 
#7 ·
This is what i understood, in graphical form. Correct me if i'm wrong.

 
#12 ·
not all years had auxiliary power take off
my personal preferrence would be to buy a premanufactured unit like the one sold by twisted throttle.
for me part of the fun of farkling is working it out myself even if it's not always the easy or cost effective route
 
#15 ·
come on... For the sake of correcting misinformation, point it out and share how you would've done it.
 
#17 · (Edited)
I noted from ebay there are relays that has built in 30-40 amps fuse.

**important** i found out that fuses should be placed BEFORE the relay.



So are you saying that if we are to use a mini relay, then we shoud be wiring multiple relays, one for each accessories that we are going to use, right? But how about the power that is used to activate the relay? Can we tap from a single source for 4-5 relays? What about if we run a direct 12V power from battery as the activation power and ground it back to chassis? The switch will both activate the relay and at the same time activates the accessory.



But that kinda defeats the purpose of having relay, no? Why not we just wire it direct? The switch will ensure the circuit is broken and hence no power running. So what is the purpose of having relay anyway?



Some basic reading.
http://www.gadgetjq.com/wiredrivinglights.htm
http://www.ripper1.com/tech/wiring.php
 
#25 ·
Nice drawings, too bad you don't have a complete understanding of relays and their purpose.

First, if you want to have something automatic but also have the choice of switching it off, use the control circuit and insert switch there, that is put your switch in series with the tail light, this way the relay handles the current, and in reality you could use a switch with a 1 amp rating.

Next, if you intend to use a switch and relay, but want to control with a switch only, irregardless if the bike is running or not, the switch should be shown downstream of the fuse, not line side of fuse tap, again the switch is controlling the relay. If your accessories all come on at the same time, you could also have a main fuse block controlled by the relay, fuse each device according to factory specs, this way if you get a short on your heated grips, you only lose that circuit.

It would be a lot easier if I new how to draw and upload what you did.:goodidea:
 
#18 ·
:sorry:I am NOT AN ELECTRICIAN and have not slept in a HolidayInn lately but...
My understanding is that the relay itself can be triggered by a smaller load (like a running light) but the circuit thru the relay that is feeding the accessory can handle a hgher load (heated gear for example) reducing the amount of heavy gage wire running all over the place. Not professing this to be factual or accurate just what my limited understanding tells me.
 
#19 ·
The ideal situation is to have multiple relays with the battery feed looped to each terminal 30. You then switch each relay terminal 85. The switch required to activate the relay is a small low current switch. Also the wiring can be much lighter. You can fit a fuse or circuit breaker between the battery and the first relay. If you are running lights a circuit breaker is good as it just interrupts the lighting circuit, switching on and off. It has a bi metal strip on one of the contacts. This way you still have enough lighting untill you can stop.
The reason why relays are run in vehicles is to reduce the wiring size = weight and cost, to run lighter switch gear and to reduce the amount of wiring. Running heavier loads through modern circuits ie fitting higher wattage bulbs in headlights, 1, you will get a voltage drop at the light ( the wire is to light to handle the current) and 2, your switch gear may fail. The switch contacts are not heavy enough to handle higher current. The arcing will destroy the switch contact. The ideal world is to run one relay per circuit with that relay being switched.
 
#20 ·
Youre missing the point. Only a single relay is needed. This sits next to the battery to supply a high current ,.switched source. If you have mult accessories you connect them to.a wiring block, each with their own fuse, that is energized by power from the relay. Using mult relays unneccesarly adds to cost, weight, and complexity.

Sent from my LG-P500h using Motorcycle.com App
 
#21 ·
I can see your point and understand it completely, especially in a restricted space. If you want to switch each item you just need switches that can take that load. The only switch that needs to be more robust is the extra headlight switch. The relay must be big enough to handle the complete load.
All i am saying is that one relay per circuit is the norm and ideal.
 
#22 ·
Hmm, that's true. One relay for each circuit that powers 1 single accessory does seem complex and adds to clutter. Not that we have much space under the seat anyway. So 1 single relay with 40A capacity should be able to handle 2-3 circuits of accessory, with the output from relay wired to a power block / fuse block / terminal / whatever you call it, to be split to each accessory.
 
#23 ·
So basically, this wiring setup would just use the relay to make sure all the accessories turn off when the bike is turned off? Beyond that, each accessory would need it's own switch. Think driving lights, heated grips, and Stebel air horn, although I could see some value to having the driving lights coming on when you blow the horn, though I can't see much valu on the heated grips coming on at that time.
 
#24 ·
kinda like the relay controls the power to a fuse block then you branch off to the accesory items from there. most accessories come with their own controls but it simplifies the spaghetti bowl if they all get their power from one protected/switched source. think of the relay as the main cutoff to the fuse panel in our home. or am I being overly simple?
 
#26 · (Edited)
From the responses here I am not sure everyone understands the purpose of a relay in this context. It exists to supply a common switched source of high current that is ONLY active while the engine is running or the key is ON - depending what you activate the relay from. Kinda like the main switch on the fuse panel of your house that shuts the power on/off to all of your house. Relays are not normally used for individual control of items on a motorcycle, except for the starter motor.

The switched power source keeps items that will still suck small amounts of current when turned off (eg. electronic heat controllers) from draining the battery. It also keeps items that the rider forgets to switch off from draining the battery.

Each item still has it's own individual fuse, switch or controller (optional) and wiring as it would normally have. The POS wire just gets connected to pin 87 of the relay instead of directly to the POS terminal of the batter.

As for wire gauge size this has nothing to do with relays or switches but is dependent entirely on the current flow. See the chart.

 
#28 ·
I installed a PC-8 from Eastern Beaver. :thumb:
It provides 8 accessory circuits; 6 switched, 2 unswitched. There is also a choice of Relays, depending on anticipated loads. I went with the 30 amp. I have heated grips, heated vest plug, and GPS power, all switched; accessory plug for air pump and power for the phone charger unswitched. Fuses are provided, as well as the hardware for install. The instructions are clear, the build quality is great, and it has worked well for over 30k miles.
If your technical skills run more toward "bolt-ons" than "builds", this can be a good option.
 
#29 ·
t See the chart. [IMG said:
http://www.glasswolf.net/papers/wiringdiagram.jpg[/IMG]

At first I thought the chart looked like something from emergency lighting, I would be interested to know the source of the chart.

First, the wattage has nothing to do with the wire gauge, other than voltage drop. I can tell you if you put 40 amp on 14 gauge wire it will melt the insulation, I can also tell you it doesn't matter if it is 40 amp @ 12 VDC or 40 amp @ 2000 VDC, the only difference will be the arc flash when the wire fails.

A far better wire chart would be one from emergency lighting, it is wattage based and this is total wire length. You will notice the smallest wire gauge is 12 gauge.
http://www.emergencylighting.com/Pages/library/wiring distance chart.htm
 
#31 · (Edited)
Watts equals volts * amps (current)

Wire size is dependant on current, not wattage or voltage. For instance you can reduce the current, and wire size for a 100watt load by upping the voltage in a hypothetical circuit. This is why utilities transmit electricity at 12000+ volts.

Charts based on wattage are specific to a single voltage because of this.

Sent from my LG-P500h using Motorcycle.com App
 
#34 ·
Emergency Lighting chart / Wattage Based


Just going over this old post, what may have been misunderstood is my posting of wattage based chart. That is what the emergency lighting chart is displayed in. Take the above chart use the 20 amp 240 watt table, go across and it says 8 feet total wire length. Use the Emergency light chart and pick 250 watts at 12 VDC, go across it says #12 gauge at a total 8 feet .
My point is the Emergency light chart explains the 5% loss and has a few other pointers. For me it makes no difference, many ways to calculate wattage VA rating is largely associated with AC.
One thing that isn't considered is many of the LED units used for headlights have drivers that when the voltage is low the current goes up, therefore if your wire is undersized the losses will increase for the same total wattage outputted at the LED. At the same time, most incandescent headlight bulbs have a 12 volt rating, that is the current and wattage they will draw at 12 volts, the Versys under normal load conditions outputs 14.4 VDC, the current and wattage output will be higher for these incandescent loads, and this is a non linear increase.
 
#32 ·
I had the Eastern Beaver PC8 on an earlier bike, and it was great. However, it's a bit bulky for the Versys 650, which is space deprived... He also has a PC3 that doesn't use the fancy labeled box, and is much more space conscious. It has either 3 switched, or 2 switched/1 unswitched circuits. Great if you don't want to wire your own relay (comes w/ 40A relay, and 30A inline fuse).
 
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