Hoffman regulated relay supply and heater CT

[CoffeeTones]

Hi, in the following circuit http://www.el34world.com/projects/images/5voltregulator.gif can either a true or artificial heater CT still be connected to ground as in the other circuits found on this http://www.el34world.com/projects/relay_switch.htm page?

Also the what is the recommended filter value for the extra radial cap slot on the Hoffman power supply board?

Would it be any trouble to run this off of an unused 5V supply?

Thanks

[kagliostro]

what is the recommended filter value for the extra radial cap slot

0.1uF is the value you can find in the datasheet

http://docs-europe.electrocomponents.com/webdocs/0daf/0900766b80daf4ad.pdf


K

[HotBluePlates]

... in the following circuit http://www.el34world.com/projects/images/5voltregulator.gif can either a true or artificial heater CT still be connected to ground as in the other circuits found on this http://www.el34world.com/projects/relay_switch.htm page?

...

Not if you follow the whole circuit shown, because it uses a bridge rectifier.

[sluckey]

Not if you follow the whole circuit shown, because it uses a bridge rectifier.

Actually you can. But you must float the negative side of the 5 volt supply. IOW, don't connect the "Grnd" turret of the Hoffman relay board to the heater centertap.

Would it be any trouble to run this off of an unused 5V supply?

5VAC is a little low for reliable operation of the regulator.

[CoffeeTones]

Thanks to all. Wasn't the relay / supply ground intended to stay floating anyway? As applying the -DC activates the relays?  Please give input on the following circuits. Heater CT can be actual or artificial.

Thanks again.

Attachments removed

[sluckey]

Thanks to all. Wasn't the relay / supply ground intended to stay floating anyway? As applying the -DC activates the relays?  Please give input on the following circuits. Heater CT can be actual or artificial.

Floating is only an issue if the 6.3 voltage source for the relay power supply is the same as your filament circuit. If you tap into the filament supply, you can still use either a real or artificial center tap, but float the power supply. And you can switch the +DC to energize the relay. Doesn't matter which side of the power supply is switched. If you use a separate 6.3VAC source, it's perfectly safe to connect the negative side of the 5VDC power supply to any ground (chassis, earth, whatever).

The reason behind floating the power supply has nothing to do with how you activate the relays. Nor is it peculiar to this 5VDC power supply circuit. The reason applies to using a bridge in any power supply. YOU ABSOLUTELY CANNOT CONNECT THE NEGATIVE SIDE OF THE BRIDGE TO THE SAME PLACE THAT A CENTER TAP FOR THAT PARTICULAR WINDING IS CONNECTED. Doing so will destroy two of the diodes in the bridge if you are lucky. If you are unlucky, the transformer will smoke also. If you draw a transformer winding with a CT and connect a bridge to the outside windings as usual, but also connect the negative side of the bridge to the center tap, you should see that there will be a diode directly across the top half of the winding and there will be another diode directly across the bottom half of the winding. These diodes will pop when voltage is applied. Hopefully they will pop open and no further damage will occur. But if they pop shorted, the transformer is at risk of smoking.

If this doesn't make sense to you, then float the power supply just to be safe, or better yet, use a totally separate transformer for the relay power supply.

[sluckey]

Please give input on the following circuits. Heater CT can be actual or artificial.

Ah, you added the circuits while I was replying. Neither of your circuits will work because you have a dead short across the 6.3VAC source (transformer winding). Furthermore, the second circuit will not work even when you remove the dead short because the negative side of the bridge must connect to the negative side of the regulator circuit.

[CoffeeTones]

Please give input on the following circuits. Heater CT can be actual or artificial.

Ah, you added the circuits while I was replying. Neither of your circuits will work because you have a dead short across the 6.3VAC source (transformer winding). Furthermore, the second circuit will not work even when you remove the dead short because the negative side of the bridge must connect to the negative side of the regulator circuit.

Ha Ha, yeah I was in a hurry and botched the CT connection / left out the 100 Ohm resistors for the artificial CT. Actual (real) CT would come from the winding center.

In the second circuit, why would the two diodes not work just as they do in a common mains supply? The other two diodes of the bridge package would not be in use.


Second circuit removed - It was incorrect

[sluckey]

In the second circuit, why would the two diodes not work just as they do in a common bias circuit? The other two diodes of the bridge package would not be in use.

First off, the negative side of the filter cap and regulator chip would have to be connected to the CT to get any voltage to the regulator circuit. But even if you do that, your two diode conventional rectifier only puts out half the voltage that the bridge does. IOW, you now have a rectifier fed by 3.15 - 0 - 3.15VAC. The DC output would only be 4.5v and that is not enough for the regulator to work. A 5v regulator (7805) needs at least 3 or 4 volts higher input to work reliably.

[CoffeeTones]

In the second circuit, why would the two diodes not work just as they do in a common bias circuit? The other two diodes of the bridge package would not be in use.

First off, the negative side of the filter cap and regulator chip would have to be connected to the CT to get any voltage to the regulator circuit. But even if you do that, your two diode conventional rectifier only puts out half the voltage that the bridge does. IOW, you now have a rectifier fed by 3.15 - 0 - 3.15VAC. The DC output would only be 4.5v and that is not enough for the regulator to work. A 5v regulator (7805) needs at least 3 or 4 volts higher input to work reliably.

I get what you are saying now. I was overlooking / misunderstanding the 3.15 - 0 - 3.15VAC and grounds part. I also meant mains supply, not bias supply using two diodes from the amp's PT HT and that now points me to the similar concept of the amps PT feeding ~345- 0 - 345 for example, just like you pointed out for the heater taps. Thanks for schooling and un - confusing me. LOL

[CoffeeTones]

what is the recommended filter value for the extra radial cap slot

0.1uF is the value you can find in the datasheet

http://docs-europe.electrocomponents.com/webdocs/0daf/0900766b80daf4ad.pdf


K

Thanks, but it appears another member here is using 330uF for extra filtration. What is the reasonable limits of the filter values, provided the cap fits the space? Anyone please.........

[CoffeeTones]

More on the unused 5 volt transformer taps. Is it best to keep it unregulated or go with something like these attachments show? Are there any other recommended circuits for using 5 volt AC taps?

Any input appreciated!

[sluckey]

0.1µF to 10,000µF. It ain't critical or even necessary in this application.

[sluckey]

More on the unused 5 volt transformer taps. Is it best to keep it unregulated or go with something like these attachments show? Are there any other recommended circuits for using 5 volt AC taps?

Any input appreciated!

You cannot reliably use Hoffman's relay PS board with 5VAC. If you want to use 5VAC and a 5V regulator chip such as 7805, you'll need to use a voltage doubler circuit such as the ones you just posted.

Do you have a specific project for this or is this just a general knowledge discussion?

[CoffeeTones]

Thanks for the reply. The project is a cascaded preamp with switching out the cascade and master for cleaner / Plexi sounds. Elevated heaters etc. I'm new to the relay switching, so trying to find out the best method while retaining the heater elevation. I will have the unused 5 volt tap although I don't think the 5 amp heater supply will have a problem with the relay supply and heaters combined. I calculated the power and preamp heater draw and had room left.

[sluckey]

I'm new to the relay switching, so trying to find out the best method while retaining the heater elevation.

The best method is to keep it simple and use a separate 6.3VAC or 12VAC transformer for the relay supply. All your concerns will evaporate by using a separate transformer. If you will elevate the heaters then definitely don't complicate things by tacking on a relay power supply.

You keep saying "unused 5 volt tap". If this is a totally separate 5 VAC winding with two leads out of the PT that are dedicated to powering a tube such as a 5Y3, then by all means, use it and a voltage doubler for your relay power supply. But if you are referring to a single 6.3VAC - 0 winding that just happens to have a single wire "tap" for 5VAC, and plan to elevate heaters, then don't use it because the dc elevating voltage that is applied to the 6.3VAC tap will also be felt on the 5VAC tap.

[CoffeeTones]

The 5 VAC I am referring to is the 5 VAC winding with two leads out of the PT, normally for a tube rectifier. So I should use that and if I don't, I should use a stand alone transformer for 6.3 - 12 VAC because the heater elevation will cause problems with the relay circuit, right?

I already ordered the Hoffman relay boards and parts so I will likely just get a $15, dedicated 6.3 VAC transformer. I'm trying to stay away from DC heaters for now and I want to keep the heater to cathode voltage at the cathode follower more reasonable.

[kagliostro]

Usually a 6v or 9v or 12v PCB transformer is cheaper than a 6.3v heater dedicated transformer

K

[CoffeeTones]

I wonder who makes the transformers sold at RadioShack.