Relay Power From Unused Rectifier Filament

[dorrisant]

I have had this bouncing around my brain for a while... Iv'e used PTs in the past that had 5v filament wiring that never got used. I capped the leads off and tied them back.

Why not use them for the relay supply? Most 5v supplies are at least 3A. Seems like plenty of amperage there to do the job.

Here's a link to relays I'm pretty sure would work: https://www.mouser.com/datasheet/2/307/ ... 536510.pdf

My main questions are these:

1 - If using the circuit (voltage doubler) typically seen in HAD's circuits, should the relay control voltage be dropped to accommodate the 5v instead of the normal 6.3v? We normally see 12vdc here, but relays are available in lower control voltages - 9v, 6v and 5v. Seems to me that using these would be less strain on the circuit. Also, it doesn't seem the availability of one control voltage over another would be a deciding factor. From the link above (Mouser) 9v = $3.18 ea, 6v = $2.99 ea and 5v = $2.99 ea, all are in stock.

2 - How much current will this configuration support? Or, how many relays can be used with the power available?

3 - If using 5v, or 6v relays, the doubler may not be required. Maybe this is the route to go with. If so, how much current will this configuration support? Or, how many relays can be used with the power available?

There are lots of PTs in Edcors line that already have a 5v winding ready to be used for relay power... Doesn't it seem goofy to build and ODS with a dedicated filament TX? Why not pull it from the main PT? You aren't going to use a rectifier in every amp circuit anyway.

Full disclosure, I am trying to do this with my project here: https://tubeamparchive.com/viewtopic.php?f=4&t=31242

[pompeiisneaks]

There are plenty of 5V relays out there, so you don't even need a voltage doubler. You just need to rectify it and use it with a simple regulator. I 'think' this one that hoffman sells would work, it uses a 7805 regulator, http://hoffmanamps.com/MyStore/perlshop ... D=!ORDERID! look at his 'relay power supply board' it says to use the 6.3V input but the regulator is for 5V output, and it takes 5.0VDC input. Rectified out of a bridge rectifier, I'm pretty sure you'd be there. Someone may chime in if I'm smoking the good stuff

As for current rating, you'd just need to look up the datasheet for your relay, but the one I looked up (Omron) used like 50mA rated. This means it won't do much in the way of using up those 3A unless you do like 3000mA of current... I think your'e covered

The only part I'm not 100% sure of is if 5.0V AC through a full wave bridge, should be 1.4X at the output or 7VDC which is within that regulator's range, but I may be mistaken and it's .9 output which would be 'below' that regulator's input range.

~Phil

[sluckey]

Hoffman uses Omron 5V 40mA coil relays. Your 3 amp supply would operate about 3/.04 = 75 relays. Your 5V winding would produce about 7Vdc unloaded using a FWB rectifier and big cap. 7Vdc is not really enough voltage to use a simple 7805 regulator chip but two or three series connected diodes would drop that to a safe voltage for a 5V relay. That's probably what I'd do if I had a free 5VAC source and needed a simple supply for several relays. If only running two or three relays I'd consider just using a series resistor for each relay coil.

[pompeiisneaks]

sluckey, are you saying the 7805 won't be able to put out a consistent 5VDC with 7VDC input? it would be too high at output? I thought that was the primary job of a regulator, taking higher voltages and lowering them to expected levels? I'm definitely not amazing at them, only used them like 2 times, but just maybe misunderstood something?

~Phil

[sluckey]

These three terminal regulators require a certain "overhead" voltage to produce their stated regulated voltage. For example, the TI µA7805 requires the input voltage to be between 7V and 25V to reliably regulate at 5V. So, if your unregulated input is only 7V, maybe the output will be 5V and maybe it will be slightly less and not regulate if the line voltage fluctuates. The point being, to ensure a solid 5V output then use an unregulated input voltage that's high enough to ensure regulation. Probably not a good idea to use the bottom of the input range,

All this is probably a moot point because a relay does not need a regulated power source. Those 5V Omrons will work just as well with 4.5v or 6v. So, if you have unregulated 7v source, knock it down using diodes to get close to 5V rather than mess with a 7805 which would probably like to have a heatsink. Each series diode will produce a .6v or .7v consistent voltage drop.

[Colossal]

On the 5VDC regulator boards I used to offer here, they worked quite well off of an unused rectifier tap. A typical 5VAC is designed to deliver significant current, so when running a light load like a handful of relays, the winding will usually run a little hotter than 5VAC, a little cheat for a little more headroom. However, I also used very low forward voltage drop diodes to minimize losses. The 7805 typically needs 2VDC above the regulated output voltage to keep from cutting out.

[pompeiisneaks]

ahh okay makes sense, their own datasheets lie a bit then saying input voltage of 5v to something, Probably because it 'works' but as you've stated is flaky until you have surplus voltage. Thanks!

~Phil

[Colossal]

If you do decide to regulate, be advised (if you aren't so already) that the difference between the input voltage to the regulator and its regulated output voltage will be dissipated as heat, so the greater the difference, the more the heat the chip must dissipate. As Sluckey said, a heat sink is a good idea.

[Phil_S]

So, if you'd like to have some fun with voltage regulators and maybe some other stuff, look here: http://www.surplus-electronics-sales.co ... 4ac8318b26
They have a 5v regulator for 20¢ and another for 43¢. They have heat sinks for TO-220 packages and mica insulators. Other interesting stuff at similar price points. Postage is very reasonable. They don't have everything. They ship quickly. I give this place a thumbs up!

[bal704]

Those relays are pretty tolerant of voltage. I just run the 5V tap through a rectifier on mine. I don't bother with a voltage regulator.

[dorrisant]

Thank to all for the feedback!!

Gonna order a bunch of these: https://www.ebay.com/itm/25-PACK-Omron- ... 7714!US!-1

Brdge rectifier ->Filter -> Dropping resistor -> Filter... That's what I'm gonna do!

[mojotom]

A little late to the party but I’ve used the 5V tap for 12V relay without any issues.

I did power the pilot light then to a voltage doubler into a small resistor directly to the relays and everything hold up fine. Not the best solution but very easy to put together, no regulators, no noise either.