Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: leroy on May 22, 2018, 09:02:39 pm

Title: Relay Supply Voltage - Getting to 24v from 50v
Post by: leroy on May 22, 2018, 09:02:39 pm

Hi all

So i have a transformer that has a spare 39v AC @ 250ma tap. It's original use was for bias and IC supply.
I however would like to incorporate two relays into my next build.
There is no centre tap on this supply and If i use a bridge rectifier i'll get around 50vdc @ 175ma after rectification.
The two relay coils require 24v and a power consumption of 750mw each. So total current consumption is 1.5 watts.

My query is what's the best way to get form 50v to 24v?

I've though about just using a 24 regulator as the 1.5watts should be well within it's capability. But they have a maximum input voltage of 40v.

I've also though about going, 4700uf cap - small resistor - 4700uf cap then a 5 watt 24v Zever to ground.  If i use a 280 ohm resistor it'll drop the voltage down to around 30v under load and then the Zener will keep it at or below 24v.

I've also thought about doing the above but using a regulator instead of the zener as the resistor will bring the regulator input voltage to 30v. But then I thought that the unloaded (relays switch off) voltage will still be 50v and that may fry the regulator.

oh, and I don't want to use the heater supply for the relays.

Any comments or suggestion welcome.
 
Thanks
Leroy

Title: Re: Relay Supply Voltage - Getting to 24v from 50v
Post by: PRR on May 22, 2018, 09:08:48 pm

Put a resistor in series with each relay coil.

Do math.

39V AC times 1.4 is 54V DC.

750mW @ 24V is 0.03125A.

You need to drop 54V-24V= 30V.

30V at 0.03125A is 960 Ohms 0.94 Watts.

Use 1K 2W.