# help

#### rolexcomm

Mar 13, 2012
4
I have a car that we need to do a trick to the computer, it gets a low amperage 10v signal that we need to converte to a little higher 12v signal to turn on the relay, what could we use that is simple and quick???

#### duke37

Jan 9, 2011
5,364
What is the signal source?
What is the relay resistance?

Try the 12V relay on 10V, it may well work.
Could you use a 6V relay with series resistance?

#### rolexcomm

Mar 13, 2012
4
What is the signal source?
What is the relay resistance?

Try the 12V relay on 10V, it may well work.
Could you use a 6V relay with series resistance?

The signal source is 10v dc. but we tried using the signal source straight out and it won't move the relay, the relay we are turning on is just a regular automotive general purpose relay with 100 ohm coil... the signal source will not turn on a standard test light...

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
12,755
I don't think the voltage is the problem. Most 12 V relays will happily switch when operated from 10 V. A standard car battery can drop to 10 V during start of the engine.
I suspect that the voltage source is too weak to deliver the 100mA required by the relay. You can check by measuring the effective voltage at the relay coil.
In that case it will help to use another relay with less operating current (or higher coil resistance, which is equivalent). Just adding a few volts on top of the 10 V source you risk destroying the 10 V source because the relay will try to draw even more current.

Harald

#### duke37

Jan 9, 2011
5,364
The relay needs 12V and 120mA to work properly.
The signal source is 10V but you have given no indication of the current capability. You give no information on the current demand of the test light.

If the source has a very low current capability, at 10V it can still be used to switch a power FET to drive the relay.
A FET could perhaps switch the load directly, not using the relay. What is the load?

#### rolexcomm

Mar 13, 2012
4
The relay needs 12V and 120mA to work properly.
The signal source is 10V but you have given no indication of the current capability. You give no information on the current demand of the test light.

If the source has a very low current capability, at 10V it can still be used to switch a power FET to drive the relay.
A FET could perhaps switch the load directly, not using the relay. What is the load?

very low current capability is what i am guessing if it will not turn on a regular shop test light. it will turn on a led test lamp only. The fet would be best driving the relay because the consumer is an air conditioning compressor at say about 5 amps cold maybe even 10 amps cold.... I would just like to know what is the best way to hook this system up for reliability, so the fet will be long lasting and the system stays almost all original without making any major modifications.

#### duke37

Jan 9, 2011
5,364
An IRF540A has an on resistance of under 0.1 ohm and can take 5 or even 10A.

I show a circuit with some components to protect the FET from spikes.

#### Attachments

• CarRelay.png
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#### rolexcomm

Mar 13, 2012
4
An IRF540A has an on resistance of under 0.1 ohm and can take 5 or even 10A.

I show a circuit with some components to protect the FET from spikes.

Ok, that looks pretty simple, what size diodes would those be??? Also, will this be sending out power? the real signal that we are missing at the relay is ground. But we can work this out either way as we choose on the relay end. Also when you say 0 volts, do i need to use ground??

#### duke37

Jan 9, 2011
5,364
The 0V is the reference to which everthing else is measured. Car chassis (ground) probably.

The 15V zener diode is there to protect the base of the fet from spikes and will not normally conduct so a quarter watt diode should do.

The diode across the relay only conducts for a short period when the fet turns off. A 1N4001 to 1N4007 (1 Amp) should do.

If you drive your compressor directly, you may need a heat sink on the fet and a bigger diode.

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