Tyre inflator modification. Currently powered by cigarette lighter, want to add mains power option.

It's 12V (obviously, as it plugs in the 12V on the car), but it's fairly high current - a car socket is capable of about 10A.

How do I check that or do I assume 10A?

Would this do?

https://www.ebay.co.uk/itm/26619031...aZIty1hf6TjDrWoTP+8+zpRg==|tkp:Bk9SR7iJ6v_eYw


https://www.ebay.co.uk/itm/276394988336?itmmeta=01HVVZTGJ4TYHB80V181Y9SWWK&hash=item405a6cd330:g:~fkAAOSwMy1l~vhY&itmprp=enc:AQAJAAAAwB+sSZbNFETETyeFPGqFfyliCtGR2fkUf+RDa6PYjuEsYZf8I1b15oPhkjn1geDg+QbnKPEGzVkRQmzdTOjna1VXniL+hU/gTeGcC/y/PhQ3cy6C06TkEoPsV80RXspdROH4MeMpBp3LnJViKfPW3PQZgInc92P9oogoPatJSnSJsvWWesYi4Lwj6VfMb5cHqMHeqfzyY34w724YSshCdxJyzUjMM4q4Zq2IFGEd47xmSGxgpUnF4aRuMnecnCr76Q==|tkp:Bk9SR7CJ6v_eYw



Is there another way to do this without an adapter like adding components inside the inflator enclosure?

TheJay said:

Is there another way to do this without an adapter like adding components inside the inflator enclosure?

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That would be difficult unless you can fit the converter inside the inflator, since the components you would need would likely occupy a similar volume..

Just buy the external converter.
The bottom one would likely work.

Does the inflator label say how much current it takes?

Or just get a small air compressor that is designed to plug into AC

TheJay said:

How do I check that or do I assume 10A?

Would this do?

Click to expand...

Yes

TheJay said:

Is there another way to do this without an adapter like adding components inside the inflator enclosure?

Click to expand...

No. Power conversion to match the pump power required takes more space than the pump.
If you just want an AC pump, get an AC air pump.

Compressors for inflating car tyres can take around 5 times the running current as they start up. With a car battery or a linear unregulated power supply that current is less likely to be a problem as the compressors will start in around 200 ms.

However on a switch-mode supply or a regulated supply there is likely to be a current limit. The simple compressors are only single cylinder so the torque varies a lot as they rotate. If they are starting with the air pressure already high, they need a lot of current to start. With a current-limited supply or too much resistance in the wires they may fail to start at all unless the pressure is removed before starting.

I did some work on compressors designed to run from a cigar lighter supply. They took about 12A when running. Starting current was about 70A and 100 milli-Ohms of wiring resistance would make starting difficult.

Thanks for your reply. Do you think this would work?


I'd be interested in adding a two way switch so that where it takes power from is controlled by the switch. For example, left is mains and right is cigarette lighter.

Diver300 said:

I did some work on compressors designed to run from a cigar lighter supply. They took about 12A when running. Starting current was about 70A and 100 milli-Ohms of wiring resistance would make starting difficult.

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I second the suggestion of a linear, unregulated supply. Which I have been using for 25 years for the exact same purpose.
A 12V @ 15A transformer, a 20A bridge rectifier and a 10000 uF cap is all you require.
Before anyone complains that it would provide 16 Vdc which is quite high, let me say that at that current level the bridge rectifier drops almost 2 volts. The actual output voltage is 14 volts, which is very close to the 13.8V the car battery has when the engine is running. I always start the engine when I am pumping air into the tyres, the pump actually works better at that voltage.

In the end however, you will receive a dozen different options. Like any engineering solution, each will have its advantages and drawbacks.

What does an unregulated linear supply look like/what do I look for?

TheJay said:

What does an unregulated linear supply look like/what do I look for?

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https://www.eleccircuit.com/unregulator-power-supply/
This is quite a good guide.

High-current unregulated supplies aren't sold commercially much as the switch-mode supplies that you linked to are so much cheaper and better for just about every application unless a large inrush is needed. However unregulated supplies are really easy to make, as Schmitt Trigger suggested.

I would, however, suggest a much larger capacitor than 10,000 μF.

Transformer:- https://cpc.farnell.com/multicomp/vtx-146-120-112/transformer-120va-2x-12v/dp/TF01609
Rectifier:- https://cpc.farnell.com/multicomp-pro/kbpc3506/diode-bridge-rect-1-ph-600v-module/dp/SC15622
Capacitor:- https://cpc.farnell.com/multicomp-pro/mclpr25v479m35x56/cap-47000-f-25v-alu-elec-snap/dp/CA08669

Notes:- the transformer is a little bit under-rated, so it will get hot but that will take some time. The next size up is a lot more money. You could use two 120 VA transformers for a lot less money than a single 225 VA transformer.

The rectifier doesn't need to be 600 V but it won't matter.

The capacitor should give about 2 V of ripple at full load. The capacitor is rated at about 10A of ripple so you should not use a smaller one that would have a lower ripple rating. A 10,000 μF one may overheat as it would only be rated at about 3 A of ripple.

On the transformer, wire blue and brown to live and neutral of the mains supply. That is the only mains-voltage part that has a risk of electric shock, so make sure that the connections are insulated. You can turn on when that is done and check that you are getting around 13 V AC on the other windings of the transformer.

Wire orange to black on the transformer, and connect those two to either of the connections labelled "~" on the bridge rectifier. Wire yellow to red on the transformer and connect those two to the other connection labelled "~".

Wire the "-" connection on the rectifier to the -ve of the capacitor. Wire the "+" connection on the rectifier to the +ve connection of the capacitor.

And that's all. The load (the compressor) can be connected to the two capacitor terminals.

Put it all in a box if you want to. The case of the rectifier can be connected to ground, but the case of the capacitor should not be connected to ground. Do not put any conductive items through the middle of the transformer that connecting to anything both sides.

Tell us if you get it all working.

Thank you very much for your reply. I read through that page and struggled to follow, a combination of the language and a lot of information.

A transformer converts 240v into a more useful/lower voltage. Mains power is DC. A transformer can only work with AC.

From what I could tell, the idea is to take only the positive current using diodes to control the direction. That leaves spikes between positive current. The problem with spikes is it changes the amount of energy constantly, so electrolytic capacitors (referred to as a filter) smooth out the peaks by filling in the gaps.

I feel like I am missing something, have I understood correctly?

Can you please show me photos of your unregulated linear supply? The transformers are 98mm x 44mm. How big is this going to be?

I really appreciate your detailed post.

TheJay said:

A transformer converts 240v into a more useful/lower voltage. Mains power is DC. A transformer can only work with AC.

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Mains power is AC. I assume that is a typo as the rest of the paragraph is absolutely correct.

TheJay said:

From what I could tell, the idea is to take only the positive current using diodes to control the direction. That leaves spikes between positive current. The problem with spikes is it changes the amount of energy constantly, so electrolytic capacitors (referred to as a filter) smooth out the peaks by filling in the gaps.

Click to expand...

The AC coming from the transformer is positive half the time and negative the other half. The arrangement of 4 diodes, known as a bridge rectifier, takes both the positive and negative currents from the transformer, but it connects the negative current the other way round, so what leaves the rectifier is always positive.

As you correctly say, there are gaps, and the capacitor is charged during the peaks and that charge partially fills in the gaps.

TheJay said:

Can you please show me photos of your unregulated linear supply? The transformers are 98mm x 44mm. How big is this going to be?

Click to expand...

I haven't made an unregulated supply recently. My suggestions are simply best guesses, but unregulated supplies are so simple that I would be very confident that it would work. All the clever stuff is done by the people who make the transformers, bridge rectifiers and capacitors. Like many others on this forum, designing an unregulated supply is something that I can do without drawing out circuit diagrams and making complicated calculations.

I did make an unregulated supply (to feed some existing regulators) for a repair, and that is outlined here:-https://www.electro-tech-online.com/threads/kef-psw2500-active-base-unit-repair.159244/#post-1380181 (The picture needs to be opened in a new tab)

For that I wound an additional secondary winding onto an existing toroidal transformer, which is easy if the additional winding only needs to be a small fraction of the power of the transformer. I would never wind a primary on a mains transformer, or expect much power from a home-wound secondary.

Whilst an unregulated power supply is an interesting idea, I have gone a different way with this.

I found an adapter that should work. My questions now relate to wiring it all up so it works.

The idea is to use a two way switch to alternate between power from the cigarette lighter and mains adapter. The two way switch will be mounted roughly where it has been placed within the casing and the DC jack on the right of the air hose.

Not sure how to wire this up to get it working.

There is no need for a switch to change between the to DC sources.

Just use a single DC input connector. Plug either power source into that connector, one at a time.