# Device used AC power supply, modifying circuit for DC - Fuse question

#### Mishael

##### Member
Hi all, got a question where I'm on the fence.

I have an audio system sequencer I purchased used that did not come with a power supply (they never do) and after looking at the PCB while looking for a replacement, I realized the power in went right into a bridge rectifier. Original PS we are sure was 18VAC, and we think the current rating was 2.5A.

After talking with the manufacturer, they were able to confirm that removing the diodes and connecting an 18VDC power source to some locations on the board would work. Positive to the flow side of D3 & D4, negative to the origin side of D1 & D2. After pulling the diodes out, I started thinking about the fuse that I would be effectively bypassing. So then I was thinking should I use jumpers where some of the diodes were and connect to the original power input pads?

It was then that I thought to check out the fuse. It is an AGC 1 1/2A 250V glass fuse. I have never changed something from AC to DC and since the fuse was on the AC side of the rectifier, I have no idea whether the fuse will work for DC. Theoretically it should be fine, but I don't have enough confidence in that to proceed without getting a second opinion. I know that in a lot of furnace control boards they operate on low voltage AC and use ATC fuses, but from my reading fuses seem to generally go DC fuse in AC ok, AC fuse in DC not so ok

Attached are two schematics:
one is the original plan for connection points that I confirmed with the manufacturer
the second is my new thought where I utilize the fuse and add a jumper

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#### gophert

##### Well-Known Member
You can leave everything as is, and run DC right through the bridge. You don't even have to care about polarity. The bridge will take care of that. An 18V x 2.5amp DC power supply should work (RMS). If it doesn't, (If you get some clipping), you may need to go up to 1.414 x 18v (24v) and try that (peak voltage). Either should work because the capacitors were likely charging to near peak voltage since those devices do not run much.

Then you have no worries and the original fuse also works.

Otherwise, be careful when selecting fuse holders. I've seen more problems caused by sloppy attempts to add a fuse being the problem. Also, most modern power supplies have over current protection (safe mode) but a fuse is always a good idea (if installed correctly).

I didn't answer your question but you created a problem that didn't need to be created because DC finds its way through a bridge just like AC does.

#### JimB

##### Super Moderator
I agree with gophert, but...

be aware that the circuit uses 24v for the relays.

In the original configuration with an 18v AC input, the 24v results from the rectification and smoothing of the 18vAC (18 x 1.4 = 25v give or take the diode drop).
When you are starting with 18vDC, after the bridge rectifier you will have about 16.8v dc available for the relays.

My best guess is that the 24v relays will work OK, (maybe a bit slowly), on 16 volts.

JimB

#### Mishael

##### Member
Well, I already removed the rectifier assy. because of the voltage drop and because that was the recommendation from the manufacturer. As far as the fuseholder goes, it is already there and I wasn't planning to modify anything to "add" a fuse.

The PS I purchased was a meanwell "tabletop" switching PS, 18V 60W (because of availability and cost at the time) M/N GST60A18-P1J so it has much more capacity than is needed.

I did not cut the diodes when I removed them so I could always add them back in

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#### Diver300

##### Well-Known Member
As you seem to have connected the power after the rectifier, there was no need to remove the rectifier. You have effectively bypassed it by connecting where you have.

If you have an ac device with a rectifier on dc, there are a few ways of dealing with it:-
1) Just connect the dc, either way round. There will be a voltage drop of two diodes. This is what Gophert suggested.
2) Replace two of the diodes with links, and use the original input. That will remove the voltage drop, but there will be no reverse polarity protection.
3) Remove three of the diodes and replace one of them with a link and use the original input. This will leave the voltage drop of one diode.
4) Connect the dc to the output of the bridge. There is no voltage drop, and no need to remove any diodes.

#### Mishael

##### Member
As you seem to have connected the power after the rectifier, there was no need to remove the rectifier. You have effectively bypassed it by connecting where you have.
I have not added any connections yet, i have just removed the diodes and the old pin connector for the AC power supply.

If you have an ac device with a rectifier on dc, there are a few ways of dealing with it:-
1) Just connect the dc, either way round. There will be a voltage drop of two diodes. This is what Gophert suggested.
I did not want to do this because of the voltage drop. The intent of this device is to allow for specifically timed sequencing on and off of audio equipment and any variance in its operation could damage equipment

2) Replace two of the diodes with links, and use the original input. That will remove the voltage drop, but there will be no reverse polarity protection.
3) Remove three of the diodes and replace one of them with a link and use the original input. This will leave the voltage drop of one diode.
RP protection was not something I had thought about but the final assembly of the device was going to have an IEC power inlet on the back of the unit and the power supply installed in a way to where the case would have to be opened to modify how it is wired.

4) Connect the dc to the output of the bridge. There is no voltage drop, and no need to remove any diodes.
This was what I was going to do (but still remove the diodes at the recommendation of the manufacturer) until I had started looking at the fuse's location in the circuit path which is what brought me here.

The question I still have is whether or not the fuse will be effective or if it should be changed to some sort of traditionally DC fuse like a 32V old auto style because I would like to keep a fusible link in the circuit. I understand that removing the diodes from an electrical standpoint is/was extraneous, but I had already removed them prior to asking for help here (which I absolutely appreciate)

Edit: I missed this
Then you have no worries and the original fuse also works.
So I think I am good to go

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#### Diver300

##### Well-Known Member
In the original circuit, the fuse is there to stop the transformer being damaged if there is a short. The transformer would have put out 5 - 10 times its normal current if it were shorted, and would burn out if the short continued for some time.

If your new power supply is a switch mode one, the fuse isn't really needed. The power supply should protect itself if shorted. If you post details of the supply that you are using, it should be possible to confirm what it does when shorted.

#### Mishael

##### Member
In the original circuit, the fuse is there to stop the transformer being damaged if there is a short. The transformer would have put out 5 - 10 times its normal current if it were shorted, and would burn out if the short continued for some time.

If your new power supply is a switch mode one, the fuse isn't really needed. The power supply should protect itself if shorted. If you post details of the supply that you are using, it should be possible to confirm what it does when shorted.
The PS I purchased was a meanwell "tabletop" switching PS, 18V 60W (because of availability and cost at the time) M/N GST60A18-P1J so it has much more capacity than is needed.
I had just assumed the fuse was there to protect the device as a whole, like in case of component failure causing a spike in current draw where the fuse would blow to prevent further damage to other parts and components of the circuit. Background of why I think this: I have seen similar protection in some arcade boards. There is one in particular that comes to mind ive come across on ticket eaters. under certain conditions (don't as what, its very rare and only happened twice in 10 years on 30-40 machines) a portion of the circuit would draw too much current and fry one of the main ICs, leading to a very costly repair and lots of machine downtime. the manufacturer added two solder pads in a trace, joined with a strand of wire (like a strand from 18awg, super tiny) so that if the conditions were met and that circuit would end up drawing too much, that single strand would be consumed and the machine would immediately stop functioning, but the repair was to remedy the causing conditions and replace the wire strand rather than RMA the whole board. Conversely I have seen the lack of a fuse where one is needed in games from china. We have a kids racing game that has LEDs and buttons that don’t do anything in the steering wheels. If the wire harness gets frayed and shorts (which would happen because its a kids racing game with an inadequate design and inferior materials to withstand aggressive jerking back and forth of the wheels, slamming them against their stops), there was no protection anywhere in the line and that short would kill the whole I/O board.

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#### Diver300

##### Well-Known Member
As the power supply is rated at more than twice the fuse rating, and the power supply might supply up to 150% of its rating, then you should leave the fuse in circuit unless you have carefully considered what could cause an over-current. However the fuse isn't needed to protect the power supply any more because the power supply won't be damaged by overload.

I agree that having a fuse will reduce the damage on the board if part of it fails, but that is only of use if the board can be repaired or something else removes the fault. If faults are permanent, and can only be fixed by changing the whole board, the fuse becomes fairly pointless.

#### KeepItSimpleStupid

##### Well-Known Member
back to fuses for a second. basically there is 32 V and 250 V fuses for the most part. if a 32 V rated fuse is used in a 250 V circuit, the fuse can arc after it blows thus the fuse is replaced by a "wire=arc".

DC and AC rated switches and relays have other issues. The contacts can have very different AC and DC ratings. They chey can also have switched and carry current values that are different.

I'd vote for a 24 V supply with a +-10% adjustment. You would want 24+0.6*2 or 25.2 V. Leave the bridge in place. Polarity would not matter.
You could have audio noise issues with a switching power supply. I did not look closely at the circuit.

#### Diver300

##### Well-Known Member
The current rating of the supply comes from the need to power the relays, at 24 V. The 12 V to the electronics is much lower power. However you don't need to be too concerned about the exact voltage. Firstly, the original design was unregulated, so it can't be that important. Secondly, relays are not fussy about voltage. A 24 V relay would typically be guaranteed to operate at 18 V and wouldn't burn out until 36 V or more unless the ambient temperature was very high.

Both those reasons show that exact voltage isn't important, 18 V might be too low to work reliably.

#### Nigel Goodwin

##### Super Moderator
The current rating of the supply comes from the need to power the relays, at 24 V. The 12 V to the electronics is much lower power. However you don't need to be too concerned about the exact voltage. Firstly, the original design was unregulated, so it can't be that important. Secondly, relays are not fussy about voltage. A 24 V relay would typically be guaranteed to operate at 18 V and wouldn't burn out until 36 V or more unless the ambient temperature was very high.

Both those reasons show that exact voltage isn't important, 18 V might be too low to work reliably.
Yes, 18 was a bad choice - he should have asked her before he did anything.

For simplicity, and availability, a 24V DC PSU directly feeding the AC input socket would have worked perfectly, 18V is really too low - about 27V odd would probably be spot on, but 24V is fine.

#### Mishael

##### Member
I agree that having a fuse will reduce the damage on the board if part of it fails, but that is only of use if the board can be repaired or something else removes the fault. If faults are permanent, and can only be fixed by changing the whole board, the fuse becomes fairly pointless.
Its a fairly old device and everything is thru-hole or in a socket, easily servicable.

back to fuses for a second. basically there is 32 V and 250 V fuses for the most part. if a 32 V rated fuse is used in a 250 V circuit, the fuse can arc after it blows thus the fuse is replaced by a "wire=arc".
That is what I had been reading up on which brought me here. Using a low voltage ("DC") fuse in a high voltage ("AC") application would cause the fuse to blow in a way that would still pass current through. (I use quotes because those were the terms from my reading, not necessarily because the fuses are specific to one or the other). In this case, it is a 250V fuse in a "32V" circuit.

DC and AC rated switches and relays have other issues. The contacts can have very different AC and DC ratings. They chey can also have switched and carry current values that are different.
The outputs & relay contacts on the board are switching other relays in remote locations that handle the actual power switching.

You could have audio noise issues with a switching power supply. I did not look closely at the circuit.
This device does not directly handle audio signal, only power to signal handling components. When I have installed these in the past, I plug the device in a circuit that is not shared by audio components, usually my "courtesy" circuit with rack fans and lights

The current rating of the supply comes from the need to power the relays, at 24 V. The 12 V to the electronics is much lower power. However you don't need to be too concerned about the exact voltage. Firstly, the original design was unregulated, so it can't be that important. Secondly, relays are not fussy about voltage. A 24 V relay would typically be guaranteed to operate at 18 V and wouldn't burn out until 36 V or more unless the ambient temperature was very high.

Both those reasons show that exact voltage isn't important, 18 V might be too low to work reliably.
I did not know this. From my [12V] experience, relays don't tend to like lower voltages but in my minimal dabbling in 24V, mostly in LEDs, the tolerance range is much higher, so I guess that would make sense. you say 18V might be too low to work reliably, are you referring to the 18VDC? because the original supply was 18VAC and these devices are the category of "set it up once then don’t think about it for 15 years"
If there is any literature or writeups on this topic (relationship of low voltage AC to DC) you know of I would love to learn. I've got nothing but time on my hands

I'd vote for a 24 V supply with a +-10% adjustment. You would want 24+0.6*2 or 25.2 V. Leave the bridge in place. Polarity would not matter.
Yes, 18 was a bad choice - he should have asked her before he did anything.

For simplicity, and availability, a 24V DC PSU directly feeding the AC input socket would have worked perfectly, 18V is really too low - about 27V odd would probably be spot on, but 24V is fine.
Just because I have the 18V PS doesn't mean I have my heart set on using it. You're right, I should have come here first cause I had one hell of a time finding the 18V supply to begin with. If I had, i wouldn't have needed to even buy anything. I have 24V supplies out the wazoo anywhere from 24W to 350W.

If y'all think I should go with 24VDC passing thru the rectifiers as originally designed, I will do that.

#### Nigel Goodwin

##### Super Moderator
Its a fairly old device and everything is thru-hole or in a socket, easily servicable.

That is what I had been reading up on which brought me here. Using a low voltage ("DC") fuse in a high voltage ("AC") application would cause the fuse to blow in a way that would still pass current through. (I use quotes because those were the terms from my reading, not necessarily because the fuses are specific to one or the other). In this case, it is a 250V fuse in a "32V" circuit.
I think the chances of a 'low voltage' fuse continuing to pass current at higher voltages is pretty unlikely, particularly as they use the identical physical dimensions - and it's potential arcing across it that is the possible issue. Why would anyone even buy such a fuse?, and I suspect they are probably hard to come by?.

I suspect the main difference is just what it's tested and guaranteed at?.

There's been a fair amount of discussion about what the fuse is actually 'for' (i.e. what it protects), and it's almost certainly not what most people think it's for.

The outputs & relay contacts on the board are switching other relays in remote locations that handle the actual power switching.

This device does not directly handle audio signal, only power to signal handling components. When I have installed these in the past, I plug the device in a circuit that is not shared by audio components, usually my "courtesy" circuit with rack fans and lights

I did not know this. From my [12V] experience, relays don't tend to like lower voltages but in my minimal dabbling in 24V, mostly in LEDs, the tolerance range is much higher, so I guess that would make sense. you say 18V might be too low to work reliably, are you referring to the 18VDC? because the original supply was 18VAC and these devices are the category of "set it up once then don’t think about it for 15 years"
If there is any literature or writeups on this topic (relationship of low voltage AC to DC) you know of I would love to learn. I've got nothing but time on my hands

Just because I have the 18V PS doesn't mean I have my heart set on using it. You're right, I should have come here first cause I had one hell of a time finding the 18V supply to begin with. If I had, i wouldn't have needed to even buy anything. I have 24V supplies out the wazoo anywhere from 24W to 350W.

If y'all think I should go with 24VDC passing thru the rectifiers as originally designed, I will do that.
It makes it very simple to do, much closer to the original voltage, and you've no issues with reverse connection.

Historically, the only 'issues' with such changes were with clock radios - where commonly they used the incoming 50/60Hz to provide the accurate clock timing - and if you powered them with DC the clock didn't run Another obvious potential problem would be anything that uses zero crossing (such as a light dimmer, or zero crossing switch) - but thta's likely to be quite rare.

#### Mishael

##### Member
I think the chances of a 'low voltage' fuse continuing to pass current at higher voltages is pretty unlikely, particularly as they use the identical physical dimensions - and it's potential arcing across it that is the possible issue. Why would anyone even buy such a fuse?, and I suspect they are probably hard to come by?.

I suspect the main difference is just what it's tested and guaranteed at?.

There's been a fair amount of discussion about what the fuse is actually 'for' (i.e. what it protects), and it's almost certainly not what most people think it's for.

It makes it very simple to do, much closer to the original voltage, and you've no issues with reverse connection.

Historically, the only 'issues' with such changes were with clock radios - where commonly they used the incoming 50/60Hz to provide the accurate clock timing - and if you powered them with DC the clock didn't run Another obvious potential problem would be anything that uses zero crossing (such as a light dimmer, or zero crossing switch) - but thta's likely to be quite rare.
32V glass fuses (SFE) were the standard for vehicles (in north america) up until the 80s when ATO/ATC blade fuses became the norm. When compared to the AG fuses common now (AG-automotive glass) their diameters are the same (1/4") but their length was determined by their rating. The fuse in use here is an AGC (3AG series). SFE type are still used but not nearly as much as the various AG types, and it would seem that the fuse installed is fine.

As far as the change logic, the only reason I had even considered the change was because finding an 18VAC power supply of appropriate capacity was proving to be impossible. Looking at the board, unable to find an AC supply, I noticed the bridge rectifier right after the power inlet which was what gave me the idea of changing to a DC supply. My reasoning was "well hey, i know what that assembly does, it changes AC to DC. If i take that out, then I could just use DC" and I knew meanwell made 18VDC versions of several model series, but what I did not know at the time was voltage drop over a diode, which was why I was looking for a direct substitute 18VAC->18VDC

#### Nigel Goodwin

##### Super Moderator
32V glass fuses (SFE) were the standard for vehicles (in north america) up until the 80s when ATO/ATC blade fuses became the norm. When compared to the AG fuses common now (AG-automotive glass) their diameters are the same (1/4") but their length was determined by their rating. The fuse in use here is an AGC (3AG series). SFE type are still used but not nearly as much as the various AG types, and it would seem that the fuse installed is fine.

As far as the change logic, the only reason I had even considered the change was because finding an 18VAC power supply of appropriate capacity was proving to be impossible. Looking at the board, unable to find an AC supply, I noticed the bridge rectifier right after the power inlet which was what gave me the idea of changing to a DC supply. My reasoning was "well hey, i know what that assembly does, it changes AC to DC. If i take that out, then I could just use DC" and I knew meanwell made 18VDC versions of several model series, but what I did not know at the time was voltage drop over a diode, which was why I was looking for a direct substitute 18VAC->18VDC
I've done just that many times, replaced an AC supply with a DC one - you just have to consider the voltage difference.

#### Diver300

##### Well-Known Member
A 24 V relay would typically be guaranteed to operate at 18 V
I did not know this. From my [12V] experience, relays don't tend to like lower voltages but in my minimal dabbling in 24V, mostly in LEDs, the tolerance range is much higher, so I guess that would make sense. you say 18V might be too low to work reliably, are you referring to the 18VDC? because the original supply was 18VAC and these devices are the category of "set it up once then don’t think about it for 15 years"
If there is any literature or writeups on this topic (relationship of low voltage AC to DC) you know of I would love to learn. I've got nothing but time on my hands
I just looked at some specifications for some 24 V relays at Farnell, and some have a "must operate" voltage of 18 V. That means that they are guaranteed to work at 18 V, but they would not be guaranteed at anything less, so if there is an 18 V supply, with any tolerance or voltage drop, or any losses in the transistors that drive the relays, the relays might not operate.

24 V nominal would be the usual supply to run a 24 V relay from, because even with a tolerance of +/- 20%, and some voltage drops, the voltage would always be over 18 V

An 18 V ac supply normally means 18 V rms. A pure sine wave at 18 V rms ac has a peak voltage of $$sqrt(2)$$ * 18 V, so about 25.4 V. The diode losses would take that down to very close to 24 V dc. The output voltage of an 18 V rms transformer could well be about 20 - 22 V with no load, so that would make it a bit higher. An 18 V transformer would be designed to give 18 V output at full load.

You can look up RMS at https://en.wikipedia.org/wiki/Root_mean_square

https://en.wikipedia.org/wiki/Mains_electricity says
Power distribution system voltage is nearly sinusoidal in nature. Voltages are expressed as root mean square (RMS) voltage.
The transformer specification sheets that I have found annoyingly do not say how the ac voltage is measured, but RMS is the normal way of measuring a voltage, and that is how all the transformers that I have ever come across are rated.

#### Nigel Goodwin

##### Super Moderator
The transformer specification sheets that I have found annoyingly do not say how the ac voltage is measured, but RMS is the normal way of measuring a voltage, and that is how all the transformers that I have ever come across are rated.
They don't need to say it, as it's always RMS.

#### KeepItSimpleStupid

##### Well-Known Member
People tend to look at AC and DC the same. e.g. 24 VAC and 24 VDC. They are not. See: http://www.hammondmfg.com/pdf/5c007.pdf

Converting 18 VAC to DC captures the peaks. The peak is 1.414*18 VAC. So, you get 25.2 VDC when you filter with a capacitor. You get less current though. If the 18 VAC source was 1A, the resulting 25.2VDC source could deliver 0.9A.

There are diode drops to contend with and Voltages dropped by the regulators, so it's just a start.