# 42v to 12v

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#### SimonTHK

##### Member
Hello special people

I am to have a 12v relay used over my awesome lipo4 pack at 36v. Since I balance all cells, I dont want to use power from just few cell to power the 12v relay. (though does anyone know of a cheap 50amp latching relay?)...

How can I power up my annoying relay?

The relay consumes 0.15 amp over the coil.

Can I use this smart step up boost converter? http://www.ebay.com/itm/250W-10A-DC...707549?hash=item58df04105d:g:~U4AAOSwYSlXg4pU Which I have never used before. Seems like an awesome circuit!!!

Thank you

Best regards Simon

#### alec_t

##### Well-Known Member
A simple 180Ω dropper resistor would do the job of powering the relay coil, but waste 5W of power.
Why do you want a step-up converter?

#### spec

##### Well-Known Member
What are you switching. You may be able to use an N or P MOSFET which could possibly be configured to take very little power. But if you are switching 50 Amps the choice of MOSFET would be critical.

Otherwise you could use a step down switching regulator which would probably take around 45 Ma to drive your 12V, 110 mA relay. Suitable modules are available very cheaply on eBay.

I haven't found a 50 Amp latching relay at a reasonable price yet.

spec

#### SimonTHK

##### Member
Hi Alec
Thank you

Can you please explain me how the dropper works. I just need 180 ohm in series with the coil? And then ofcourse a high power resistor? There must be 24v over the resistor so it must be like 24v x 0,15 amp= 3,6watt and then you rounded up till 5watt?

It doesnt matter if it draws wattage from the whole pack. The relay will shut off when the voltage of the cells get too low.

I dont want step up, thought it could step down aswell. Maybe it can...!!! Doesnt matter for now, I should really just use dropper resistor. Problem is... i never get time enough to do the electronics I have some understanding about. I just...dont...have...the...time.....at..all!

Thank you in advance. Appreciate it very much.

Best regards Simon

#### SimonTHK

##### Member
What are you switching. You may be able to use an N or P MOSFET which could possibly be configured to take very little power. But if you are switching 50 Amps the choice of MOSFET would be critical.

Otherwise you could use a step down switching regulator which would probably take around 45 Ma to drive your 12V, 110 mA relay. Suitable modules are available very cheaply on eBay.

I haven't found a 50 Amp latching relay at a reasonable price yet.

spec

I will probably use those swithing regulators. Seems to be a good way to do it. This seems to be able to handle the input voltage at 42v and the output voltage and needed current.http://www.ebay.com/itm/DC-DC-LM259...df97fe&pid=100005&rk=5&rkt=18&sd=331327483066 Now I just need to know what to do to fix my issue till thursday, when I needed 15 devides done Why my boss sell stuff he knows too little about !!!!

The relay has to be able to handle peak ampere at 60 amps, since it drives a motor etc. 20-60 amps relay is needed.

#### alec_t

##### Well-Known Member
Personally, I'd avoid using a mechanical relay if I could. Have you considered a solid state relay (SSR)?

#### AnalogKid

##### Well-Known Member
A pair of 50 A or 75 A power MOSFETs (for redundancy) and a simple latching circuit will cost waaaaay less than an equivalent mechanical relay. You can use about 10 microamps of current from a battery to maintain the latch state, not nearly enough current to upset the stack charge balance. What is the voltage across the relay contacts now, 42 V?

ak

#### spec

##### Well-Known Member
The relay has to be able to handle peak ampere at 60 amps, since it drives a motor etc. 20-60 amps relay is needed.
If you use a 40A to 50A two pole latching relay just put put two contacts in parallel to give about 70A to 80A switching capacity.

How is the relay switching initiated: manual or electronic?

spec

#### spec

##### Well-Known Member
What is the voltage of the motor you are switching? Is it 36V?

spec

#### spec

##### Well-Known Member
Personally, I'd avoid using a mechanical relay if I could. Have you considered a solid state relay (SSR)?
I don't think most solid state relays switch DC; they are SCRs and won't turn off.

spec

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#### kubeek

##### Well-Known Member
Some do, they have mosfets inside instead of scrs.

#### spec

##### Well-Known Member
Some do, they have mosfets inside instead of scrs.
Yes, I agree, but most are SCR and tend to have zero volt switching. I would be interested if you know of any 50A DC switching solid state relays: I could not find any.

spec

#### spec

##### Well-Known Member
I should have said that I could not find any 50A DC SSR at a reasonable price. I did see those from Crydom.

spec

#### SimonTHK

##### Member
I do realæly want to look into more advanced solution´s, but I just doesnt have time

#### spec

##### Well-Known Member
A simple 180Ω dropper resistor would do the job of powering the relay coil, but waste 5W of power.
For a quick and simple solution Alec's suggestion can't be bettered.
(The resistor will dissipate 3.6W so use a 5 Watt, or more, resistor bolted to a heatsink of some sort, case of equipment for example.)

spec

#### bountyhunter

##### Well-Known Member
A pair of 50 A or 75 A power MOSFETs (for redundancy) and a simple latching circuit will cost waaaaay less than an equivalent mechanical relay. You can use about 10 microamps of current from a battery to maintain the latch state, not nearly enough current to upset the stack charge balance.
+1 good idea. The power resistor inline with the relay is cheapest but relays suck in general for reliability IMHO.

#### Diver300

##### Well-Known Member
If you use a buck regulator, the inductance of the coil itself will mean you don't need a separate inductor.

If you use something like an LM2576HV-ADJ, with an output load of an 8.2 Ω resistor, and have the relay coil in place of the inductor.

The LM2576HV-ADJ will control the voltage on its feedback pin to 1.23 V, so there will be 150 mA flowing in the load. All the load current comes through the inductor, so its average current will be 150 mA as well.

If you do that, you must not put any capacitors or diodes in parallel with the coil, and it's a good idea to have the coil close to the buck regulator. The resistor will generate a bit under 0.2 W of heat, so a 1/2 W or more resistor would be best.

The circuit diagram is on page 12 of http://www.ti.com/lit/ds/symlink/lm2576.pdf with L1 as the relay coil and "Load" the the 8.2 Ω resistor.

#### AnalogKid

##### Well-Known Member
I was surprised by the price, but most of my SSR experience is below 20 A. I guess its no real news that industrial-rated + 50 A = \$