General > General Technical Chat
12v Relay with selectable 12 or 24v DC supply
Ditch:
Hi There,
I'm after a little guidance from you helpful people if you could.
I'm going to make a controller with 3x 12v relays onboard. This will be used within the fire and security where security systems work on 12vdc and fire systems work on 24vdc.
I don't want to be making 2 separate controllers for the 2 working voltages so my question is what is the approved way of driving the 12vdc relays from both voltages?
I want to use regulators. I know I can drop from 24v to 12v and then drive the relays from the 12v supply but a) this will cook the 12v regulator and b) I'll get a voltage drop through the regulator when at 12v which will be on the borderline of the operating voltage of the relay.
This controller will have a microchip (pic18f46k22 - I'll use another 5v regulator for this).
I don't want to go down the buck converter route and through past experience, solid-state relays can cause problems with the monitoring of the fire alarm inputs so these cant be used.
So, what is the easiest way to achieve this? Any pointers or guidance would be appreciated.
Ian
Vovk_Z:
If not buck converter then SEPIC?
madires:
In most cases 12V relays will also work fine at slightly lower voltages, e.g. 10V. Despite you don't like buck converters, they are the best choice to avoid hot linear regulators for a large input voltage range. If the circuit doesn't need much current an LDO could be an idea, i.e. it's a linear regulator with a low voltage drop and the power dissipation for a low current can be easily handled by a heatsink. For example, for 0.1A and 24V input it would be 12V * 0.1A = 1.2W.
Ditch:
Ok, thanks for your quick replies.
By your answers, the buck convertor or SEPIC (had to google that!)method is the answer to this.
I would like to possibly sell this commercially and would need these to pass the EU emissions which I assume would be difficult using a buck convertor for somebody like me!
Thanks for your help though.
Ian
Vovk_Z:
There are polarised type relays (they are usually low-current, like signal ones). They are more sensitive so they need less current to operate. So you may be fine with linear regulator (low-drop one). So you'll don't have a problem with emissions.
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