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Old 14th Mar 2017, 11:03 pm   #1
PsychMan
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Default Bluetooth Supply From Valve's Heater.

I was trying to get a power supply for a bluetooth module to fit into one of my 50s sets. I read about rectifying the heater supply and thought this sounded like the way forward. Strangely however I am seeing 8v DC on a digital multimeter, could this be due to the high ripple factor involved?

I have a 1000uf capacitor after a 1N4007 diode, and the same meter is seeing 6.3v AC from the supply I am half wave rectifying...
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Old 14th Mar 2017, 11:19 pm   #2
bikerhifinut
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Default Re: Bluetooth Supply From Heater

You have rectifed an AC waveform.
The meter is now measuring the PEAK volts which would be around 8.8V but subtract the forward voltage drop across your diode and 8V sounds about right.
The capacitor will smooth out the 50hz ripple a bit.
Frankly I think it's a waste of time and you could have issues with this as if the set uses something like an EZ81 rectifier fed from the same supply then it's likely to be a floating supply rather than referenced to earth. You run the risk of ht Volts being superimposed on the heater supply depending on how good the cathode to heater insulation is on the rectifier.
Or if its referenced to earth by simply connecting one side of the 6-3 to earth then you could end up having the positive supply connected to the ground rail of the circuit.
I dont think its a good idea just to save a few quid. why not just get one of those titchy 9V mains transformers and use a bridge rectifier and voltage regulator circuit to get 5V? I imagine the module won't draw masses of current?

A
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Old 14th Mar 2017, 11:27 pm   #3
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Default Re: Bluetooth Supply From Heater

Thanks for that, Im obviously forgetting how an AC wave form is measured, now you say it that makes perfect sense to me.

I was trying to keep components down as its extra parts to hide. Perhaps you're right and a tiny transformer is the way to go.

Cheers
Adam
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Old 14th Mar 2017, 11:42 pm   #4
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Default Re: Bluetooth Supply From Heater

I can't see anything wrong with doing it this way for a low current device, so long as you design it properly. If a rectifier fault causes HT on the heater supply then all the valve heaters will blow - I'm not aware of this ever having happened.

You could also take power from the output valve cathode.
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Old 14th Mar 2017, 11:43 pm   #5
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Default Re: Bluetooth Supply From Heater

No worries Adam.
I aint saying you cant do it, just that theres pitfalls to consider.
And I dont know for sure but i can imagine that your bluetooth my not like the "dirty" Dc from a halfwave rectifier. I will check back but I remember reading something in an old PW about pinching a bit of heater current to run a small MW tuner for a tape deck and I think that's where I picked up on checking how the heaters are referenced to earth.
just as an aside do you have a circuit diagram to hand of the set you are using? Because if the heaters are referenced to ground at one side of the 6.3V rather than a centre tap and no risk of Ht leakage onto the supply, then theoretically you could full wave rectify the lot, and then take a feed off via a couple of diodes to give a forward voltage drop of 1.2V or maybe a low drop out regulator to supply the bluetooth device?
Actually if the heaters were CT referenced, if you used DC you could ignore the CT and reference them on the negative leg. Just watch for overvoltage on the heaters but often because its a quite big current draw you get close to the 6.3V without having to resort to any adjustments.
A bit OTT perhaps.
A.
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Old 15th Mar 2017, 12:01 am   #6
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Default Re: Bluetooth Supply From Heater

I don't think I've ever encountered a domestic valve radio with CT heaters. That is hifi amp stuff.

The rectified supply won't be 'dirty' if it's smoothed properly.
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Old 15th Mar 2017, 12:17 am   #7
bikerhifinut
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Default Re: Bluetooth Supply From Heater

Acknowledged Paul.
I haven't seen any commercial radios that used CT heaters either, but I couldn't assume. Then again even the Leak troughline tuner doesn't use CT heaters so there you go.
I just found the article in the Sept 1965 Practical Wireless.
The writer shows a simple halfwave diode rectifier followed by a couple of RC filters to power up a small 2 transistor MW tuner. So on that basis Psychman you could get it running that way. And use an RC filter to reduce the supply ripple as well as dropping it to the desired 5V. Do you know what the current draw is? If so you can use Ohms law to calculate the resistor(s) needed.

A
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Old 15th Mar 2017, 12:25 am   #8
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Default Re: Bluetooth Supply From Heater

You could certainly use two smoothing caps and a resistor, just like a valve radio, but I suspect it won't be necessary with a large cap (2200uF would be a better choice than 1000). It's easy enough to experiment anyway, and part of the fun.

You could use a 7805 to derive exactly 5V, and that will reduce any ripple too. Any scrap computer PSU will contain several suitable regulators but they are only pennies if you need to buy one - 10 for 99p from Chinese eBay sellers.
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Old 15th Mar 2017, 8:34 am   #9
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Default Re: Bluetooth Supply From Heater

I'd be wary of using a 7805 in this application as it requires a minimum of 3V difference between input and output voltage to operate correctly. Once you start drawing current from the output, by connecting the Bluetooth module, the mean input voltage will fall which could cause ripple to appear on the 5V. You need a Low Dropout (LDO) regulator such as the LM317 or LM1085 but note that some variants of these regulators are adjustable types and will need a couple of resistors to set the required output voltage.

Keith
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Old 15th Mar 2017, 9:40 am   #10
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Default Re: Bluetooth Supply From Heater

Is this not what a buck module is designed to do? Pennies from China.

If isolation is a problem, a small 1 to 1 or 1 to 1.5 transformer could be used.
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Old 15th Mar 2017, 10:30 am   #11
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Default Re: Bluetooth Supply From Heater

Is the 8V that you're getting higher than you want?

If so, a cheap and easy way of dropping it would be to connect extra 1N4007 diodes in series with the rectifier. As already discussed, you'd lose about 0.7V per diode.

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Old 15th Mar 2017, 10:37 am   #12
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Question Re: Bluetooth Supply From Heater

Thanks for the replies. I have some 7805s in my parts box so will try one of those and see how well it works.

This is mainly a fun experiment and I hope to refine the method and do this to some of my nicer sets. This is a tatty Stella ST234 which has been an ongoing project for me. The case is quite poor but it has some charm. I recently got FM working again with 2 new EF80s, so the 2 remaining tasks are to fix the tuning cord and get Bluetooth working on the gram input

I did consider a buck module but was wary of putting anything of the "switch mode" variety inside a radio.
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Old 15th Mar 2017, 10:42 am   #13
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Default Re: Bluetooth Supply From Heater

Quote:
Originally Posted by Hartley118 View Post
Is the 8V that you're getting higher than you want?

If so, a cheap and easy way of dropping it would be to connect extra 1N4007 diodes in series with the rectifier. As already discussed, you'd lose about 0.7V per diode.
The module operates 3.3v - 5v according to the little specs I have. With hindsight this was always going to be more than 2 parts when using the heater supply!

I suppose a voltage divider is another possibility to try
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Old 15th Mar 2017, 10:56 am   #14
G8HQP Dave
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Default Re: Bluetooth Supply From Heater

Quote:
Originally Posted by bikerhifinut
Because if the heaters are referenced to ground at one side of the 6.3V rather than a centre tap and no risk of Ht leakage onto the supply, then theoretically you could full wave rectify the lot,
If the heater supply is grounded one side (which would be normal for a radio) then you cannot full-wave rectify it. Even if you could, there is no advantage and some disadvantage in using DC heaters: risk of buzz, extra transformer loading.

Radios almost never use CT heaters, because this would double the amount of decoupling capacitors needed to prevent RF leaking along the heater wires.
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Old 15th Mar 2017, 11:10 am   #15
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Default Re: Bluetooth Supply From Heater

How about tapping a DC supply off the output valve cathode?

There are probably too many volts on the cathode itself, but you could split the cathode resistor into two to get what you want.

No rectifier needed of course but I'd still include that 1000uF decoupling capacitor.

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Old 15th Mar 2017, 12:59 pm   #16
PsychMan
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Default Re: Bluetooth Supply From Heater

That also sounds promising Martin. I thought Id try the heaters first due to room in the chassis. At the OTHER end of the EL84 there were some convenient spaces where I could attach some tag board and keep wiring to a minimum.

I did originally think I'd stick the Bluetooth module in the same space, then it occurred to me putting something that needs radio waves into an earthed metal box would probably not yield the best results!

So the placement of that will be decided when I have my supply sorted.
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Old 15th Mar 2017, 1:00 pm   #17
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Default Re: Bluetooth Supply From Heater

As this is presumably a low current device why not just tap off the HT line? Already DC and smoothed. A dropping resistor and a smoothing cap, or make a potential divider with 2 resistors for better regulation?
Peter
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Old 15th Mar 2017, 1:11 pm   #18
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Default Re: Bluetooth Supply From Heater

There are actually many, many ways to do this. I would use a current limiting resistor from the EL84 cathode and a 4.8 or 5.1V zener to clamp the voltage. You will still need a decoupling cap, but 220uF is likely to be adequate.

A couple of red or yellow LEDs in series will also act as a zener if an actual zener isn't to hand. You should obviously check the clamped voltage is in the correct range before connecting the device.
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Old 15th Mar 2017, 1:48 pm   #19
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Default Re: Bluetooth Supply From Heater

Quote:
Originally Posted by paulsherwin View Post
I don't think I've ever encountered a domestic valve radio with CT heaters. That is hifi amp stuff.

Have a look at sets with PX4 and the like- mostly they do it with a humdinger, but some use CT heater windings.


Ferranti Magna AC 1936 does it in a short superhet with DDP output.
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Old 15th Mar 2017, 10:35 pm   #20
bikerhifinut
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Default Re: Bluetooth Supply From Heater

Quote:
Originally Posted by G8HQP Dave View Post
Quote:
Originally Posted by bikerhifinut
Because if the heaters are referenced to ground at one side of the 6.3V rather than a centre tap and no risk of Ht leakage onto the supply, then theoretically you could full wave rectify the lot,
If the heater supply is grounded one side (which would be normal for a radio) then you cannot full-wave rectify it. Even if you could, there is no advantage and some disadvantage in using DC heaters: risk of buzz, extra transformer loading.

Radios almost never use CT heaters, because this would double the amount of decoupling capacitors needed to prevent RF leaking along the heater wires.
Ok given that there isn't any real advantage in DC heaters right through, I don't see why you can't full wave rectify the heater supply and reference the negative rail to ground because that is EXACTLY what the DC heater supplies on the small signal valves on my power amps and preamps do. As long as you don't ground it before the bridge rectifier. i.e. off the transformer secondary.

Just a caveat about pulling power off the cathode of an EL84, OK for a couple of mA but any substantial draw in parallel with the cathode resistor is likely to upset the DC operating conditions of the valve I would have thought. I think paul S might be alluding to that in his post. Do we know what the current draw of this module is? It's surely going to have a DAC in it and I wonder what the unit draws?
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